A Review: Ultrahigh-Vacuum Technology for Electron Microscopes 0128185732, 9780128185735

Table of contents :
Cover
A Review: Ultrahigh-Vacuum Technology for Electron Microscopes
Copyright
Contents
About the author
Profile of the author
Books by Nagamitsu Yoshimura
Preface
Acknowledgment
A review
Part 1
Part 1 Adsorption, desorption, diffusion, and outgassing/pumping
Introduction
Reviewed papers
[1-1] “Modeling of outgassing or pumping functions of the constituent elements such as chamber walls and high vacuum pumps”...
1 Introduction
2 Modeling of outgassing
3 Pressure analysis using equivalent vacuum circuit
4 Conclusion
References in the paper[1-1]
Reviewed paper
[1-2] “Testing performance of diffusion pumps” (Hablanian and Steinherz, 1962)
Reviewed paper
1
1 Microstructure and elemental features of stainless-steel surface
Reviewed papers
[1-1] “Outgassing characteristics and microstructure of an electropolished stainless steel surface” (Yoshimura, Sato, Adach...
2 Outgassing characteristics of an electropolished pipe wall
A After an in situ bakeout
3 Microstructure and elemental features of stainless-steel surfaces
A Microstructure
B Elemental features
4 Conclusion
References in the reviewed paper[1-1]
Reviewed paper
[1-2] “Outgassing characteristics and microstructure of a “vacuum fired” (1050°C) stainless steel surface” (Yoshimura, Hira...
2 Microstructure and elemental features
A Microstructure
B Elemental features
3 Vacuum characteristics
4 Conclusion
References in the reviewed paper[1-2]
Reviewed paper
[1-3] “Outgassing characteristics of electropolished stainless steel” (Tohyama, Yamada, Hirohata, Yamashina, 1990)
2 Samples and experimental method
3 Experimental results and discussions
Reviewed paper
Related paper
[1-4] “A review of the stainless steel surface” (Adams, 1983)
Related paper
2
2 Characteristics of outgassing from metal surfaces
Quoted book
[1-1] “Typical isotherms in the chemisorption of gases on metal-surfaces: equilibrium adsorption” (Redhead, Hobson, and Kor...
References in quoted book[1-1]
Quoted book
Reviewed papers
[1-2] “Relations between pressure, pumping speed and outgassing rate” (Dayton, 1960)
2 Semiempirical formulas
3 Relation among pressure, speed, and outgassing rate
References in the paper[1-2]
Reviewed paper
[1-3] “Outgassing rate of contaminated metal surfaces” (Dayton, 1962)
2 Equation for the outgassing rate
References in the paper[1-3]
Reviewed paper
[1-4] “The effect of bake-out on the degassing of metals” (Dayton, 1963)
2 Degassing time at constant temperature
3 Outgassing rate with variable temperature
References in the paper[1-4]
Reviewed paper
[1-5] “Advantage of slow high-vacuum pumping for suppressing excessive gas load in dynamic evacuation systems” (Yoshimura, ...
1 Introduction
2 Transitional phenomena of outgassing
3 Excessive gas load just after switching over the evacuation mode
3.1 Advantages of a small bypass valve
References in the paper[1-5]
Reviewed papers
[1-6] “The variation in outgassing rate with the time of exposure and pumping” (Rogers, 1964)
2 Analysis
4 First pump-down
5 Second pump-down
6 Summary
References in the paper[1-6]
Reviewed paper
[1-7] “Reduction of stainless-steel outgassing in ultra-high vacuum” (Calder and Lewin, 1967)
2 Theory
2.1 Effect of temperature on degassing
2.2 Outgassing after a normal bakeout
2.3 Permeation rate of atmospheric hydrogen
2.4 High-temperature bulk degassing in situ
2.5 High-temperature bulk degassing in a furnace with residual hydrogen pressure
5 Summary and conclusion
References in the paper[1-7]
Reviewed paper
[1-8] “Estimating the gas partial pressure due to diffusive outgassing” (Santeler, 1992)
2 Diffusive outgassing—Fick’s law
References in the paper[1-8]
Reviewed paper
[1-9] “Model for the outgassing of water from metal surfaces” (Li and Dylla, 1993)
4 Final remarks
References in the paper[1-9]
Reviewed paper
[1-10] “True and measured outgassing rates of a vacuum chamber with a reversibly adsorbed phase” (Akaishi, Nagasuga, and Fu...
Nomenclature
2 Modeling of pump-down
A Mass conservation equations
B Equilibrium and nonequilibrium adsorption isotherms
C Measured and true outgassing rates
3 Comparison between theory and experiment
4 Discussion
A Approximate expression of np
B g Dependence of K and p
References in the paper[1-10]
Reviewed paper
[1-11] “Recombination limited outgassing of stainless steel” (Moore, 1995)
1 Introduction
2 Method of analysis
A Measurements by Hseuh and Cui
B Numeric diffusion calculation
C Recombination limit
3 Results of analysis
A Recombination limited concentration profiles versus bake time
B Outgassing rate versus time
4 Discussion
A Bake efficiency as a function of temperature
B Vacuum furnace versus in situ bake efficiency
C Uncertainties in the estimate of outgassing rate
References in the paper[1-11]
Reviewed paper
Related papers
[1-12] “La désorption sous vide” (Schram, 1963)
Reviewed paper
[1-13] “Hydrogen pumping by austenitic stainless steel” (Zajec and Namenič, 2005)
Reviewed paper
3
3 Methods for measuring outgassing rates
Reviewed papers
[1-1] “Measurement of outgassing rates from materials by the differential pressure rise method” (Yoshimura, Oikawa, and Mik...
2 Principle of the differential pressure-rise method
Reviewed paper
[1-2] “A three-point-pressure method for measuring the gas-flow rate through a conducting pipe” (Hirano and Yoshimura, 1986)
1 Introduction
2 Three-point-pressure method
A Principle
B Optimization of the measuring system
C Measurement of gas-flow rates
3 Discussion
4 Conclusion
References in the paper[1,2]
Reviewed paper
[1-3] “Two-point pressure method for measuring the outgassing rate” (Yoshimura and Hirano, 1989)
1 Introduction
2 Two-point-pressure method
A Principle
B Measurement of the outgassing rate
C Validity of the one-point-pressure method
3 Measurement by the orifice method
4 Discussion and conclusion
References in the paper[1-3]
Reviewed paper
[1-4] “Speed measuring of ion getter pumps by the ‘three-gauge’ method” (Munro and Tom, 1965)
2 Method
3 Results and discussion
References in the paper[1-4]
Reviewed paper
[1-5] “Orifice method in which two pumping speeds can be selected”
1 Methods of measuring outgassing rates
References in the paper[1-5]
Reviewed paper
[1-6] “Corrections in outgassing rate measurements by the variable conductance method” (Berman, Hausman, and Roth, 1971)
2 Theory
3 Time influence
6 Conclusion
References in the paper[1-6]
Reviewed paper
[1-7] “Conductance modulation method for the measurement of the pumping speed and outgassing rate of pumps in ultrahigh vac...
1 Introduction
2 Conductance modulation method
5 Discussions and conclusion
References in the paper[1-7]
Reviewed paper
[1-8] Differential orifice method; “La désorption sous vide” (Schram, 1963)
Reviewed paper
[1-9] “Discussion on methods for measuring the outgassing rate” (Yoshimura, 1990)
Reviewed paper
4
4 Outgassing rates of system-component materials
[1] Outgassing Rates of Metallic Materials
Reviewed papers
[1-1] “Outgassing rates of stainless steel after different pretreatments” (Ishimori, Yoshimura, Hasegawa, and Oikawa, 1971)
Reviewed paper
[1-2] “Outgassing characteristics of stainless steel and aluminum with different surface treatments” (Young, 1969)
References in the paper[1-2]
Reviewed paper
[1-3] “Technology of low-pressure systems—establishment of optimum conditions to obtain low degassing rates on 316L stainle...
Conclusion
References in the paper[1-3]
Reviewed paper
[1-4] “Treatment of the wall materials of extremely high vacuum chamber for dynamical surface analysis” (Tsukui, Hasunuma, ...
2 Extremely high vacuum systems
3 Treatment of vacuum chamber wall
4 Surface topography
5 Surface composition
6 Outgassing rate
7 Conclusion
References in the paper[1-4]
Reviewed paper
[1-5] “Thin-wall vacuum chambers of austenitic stainless steel” (Moore, 1998[1-5-1], 2001)[1-5-2]
[1-5-1] “Atmospheric permeation of austenitic stainless steel” (Moore, 1998)
2 Analysis
A Eschbach experiment
B Extrapolation procedure
C Oxygen effects
D Quantitative effect of hydrogen recombination on outgassing and permeation
1 Overall
2 Bake in vacuum enclosure
3 Operation in atmosphere at room temperature
4 Bake in atmosphere, with chamber under vacuum
5 Residual permeation during bake in atmosphere
6 Residual permeation at room temperature
E Summary of hydrogen permeation corrections
F Effect of atmospheric water vapor
4 Summary
References in the paper[1-5-1]
Reviewed paper
Related paper
[1-5-2] “Thin-walled vacuum chambers of austenitic stainless steel” (Moore, 2001)
Related paper
[1-6] Outgassing characteristics of the thin-walled stainless steel cells
[1-6-1] “Outgassing in thin wall stainless steel cells”
Related paper
[1-6-2] “Experiments with a thin-walled stainless-steel vacuum chamber”
Related paper
[1-7] “Air bake-out to reduce hydrogen outgassing from stainless steel” (Bernardini, Braccini, De Salvo, Di Virgilio, Gaddi...
Related paper
[1-8] “An overview of methods to suppress hydrogen outgassing rate from austenitic stainless steel with reference to UHV an...
Related paper
Reviewed papers
[1-9] “Permeability measurements with gaseous hydrogen for various steels” (Eschbach, Gross, and Schulien, 1963)
6 Summary
References in the paper[1-9]
Reviewed paper
[1-10] Vapor-pressure data
[1-10-1] “Vapor-pressure data for the more common elements” (Honig, 1957)
Reviewed paper
[1-10-2] “Vapor-pressure data for some common gases” (Honig and Hook, 1960)
Reviewed paper
[2] Outgassing Rates of Nonmetallic Materials
[2-1] Data by Yoshimura et al. (1970) and Yoshimura (1985)
[2-1-1] “Measurement of outgassing rates from materials by differential pressure rise method” (Yoshimura, Oikawa, and Mikam...
[2-1-2] “A differential pressure-rise method for measuring the net outgassing rates of a solid material and for estimating ...
Reviewed papers
[2-2] Data by Dayton (1959)
References in the paper[2-2]
Reviewed paper
[2-3] “Water vapor permeation through Viton O-ring seals” (Yoshimura, 1989)
References in the paper[2-3]
Reviewed paper
[2-4] “The properties of Viton ‘A’ elastomers: Part V. The practical application of Viton ‘A’ seals in high vacuum” (de Cse...
1 Summary of previous results
2 Use of double O-rings
3 Contamination
4 Practical considerations
5 Conclusion
References in the paper[2-4]
Reviewed paper
[2-5] “Recent advances in elastomer technology for UHV applications” (de Chernatony, 1977)
3 Recent fluoro-elastomers: Kalrez ECD-006 and Viton E60C
4 Conclusion
References in the paper[2-5]
Reviewed paper
[2-6] “Practical selection of elastomer materials for vacuum seals” (Peacock, 1980)
2 Properties of polymer seal materials-chemical
3 Properties of polymer seal materials—mechanical
A Compression set
4 Outgassing
5 Permeation
6 Radiation damage of seal polymers
References in the paper[2-6]
Reviewed paper
Comments in Part-1
1 Recommended pretreatments for stainless-steel chamber walls
2 Effect of in situ baking
3 Effect of unit time exposure
4 Advantage of slow high-vacuum pumping for suppressing excessive gas load
5 Differential pressure rise method for measuring the outgassing rates of sample materials
6 Three-point-pressure, two-point-pressure, and one-point-pressure methods
7 Make use of mild-steel for scientific instrument chamber
Part 2
Part 2 Molecular-flow network
Introduction
Basic concept of molecular-flow networks
5
5 Molecular-flow conductance and gas-flow patterns
[1] Molecular-flow conductance
Quoted book
[1-1] “Conductance of orifice, long tube, and short tube” (Roth, 1990)
Quoted book
Reviewed papers
[1-2] “Monte Carlo calculation of molecular flow rates through a cylindrical elbow and pipes of other shapes” (Davis, 1960)
Reviewed paper
[1-3] “Optimization of molecular flow conductance” (Levenson, Milleron, and Davis, 1960)
Reviewed paper
Related paper
[1-4] “A review of the molecular flow conductance for systems of tubes and components and the measurement of pumping speed”...
Related paper
[2] Gas-flow patterns
Reviewed papers
[2-1] “Gas flow patterns at entrance and exit of cylindrical tubes”
Reviewed paper
[2-2] “Angular distributions of molecular flux from orifices of various thicknesses” (Nanbu, 1985)
Reviewed paper
[2-3] “A further discussion about gas flow patterns at the entrance and exit of vacuum channels” (Ji-Yuan, 1988)
2. Blade channels of the turbomolecular pump
References in the paper[2-3]
Reviewed paper
6
6 Total and partial pressure gauges for ultrahigh-vacuum use
[1] Bayard–Alpert gauge (BAG) and extractor gauge (EG)
Reviewed papers
[1-1] “New hot-filament ionization gauge with low residual current” (Redhead, 1966)
4 Summary and conclusion
References in the paper[1-1]
Reviewed paper
[1-2] “Practical guide to the use of Bayard–Alpert ionization gauges” (Singleton, 2001)
4 Safety precautions
7 Conclusion
References in the paper[1-2]
Reviewed paper
[1-3] “Comparison of the pressure indication of a Bayard–Alpert and an extractor gauge” (Beeck and Reich, 1972)
4 The extractor gauge
5 Comparison of pressure indicators between Bayard–Alpert gauge and extractor gauge
References in the paper[1-3]
Reviewed paper
[1-4] “Outgassing characteristics of an electropolished stainless-steel pipe with an operating extractor ionization gauge” ...
2 Experiment
References in the paper[1-4]
Reviewed paper
[1-5] Ion current characteristics of sputter-ion pump, compared with an extractor gauge (EG); “Ar-pumping characteristics o...
Reviewed paper
[1-6] “Starting delays in cold-cathode gauges at low pressures” (Kendall and Drubetsky, 1996)
8 Discussion
References in the paper[1-6]
Reviewed paper
[1-7] “Enhanced ignition of cold cathode gauges through the use of radioactive Isotopes” (Welch, Smart, and Todd)
5 Experiment trials
Reviewed paper
[1-8] “Measurement of ultra-high vacuum. Part 1. Total pressure measurements” (Weston, 1979)
4 Gauge calibration
5 Conclusion
References in the paper[1-8]
Reviewed paper
[1-9] “Review of pressure measurement techniques for ultrahigh vacua” (Lafferty, 1971)
Reviewed paper
[1-10] “Comparison of hot cathode and cold cathode ionization gauges” (Peacock, Peacock, and Hauschulz, 1991)
Reviewed paper
Related paper
[1-11] “Hot-cathode magnetron ionization gauge with an electron multiplier ion detector for the measurement of extreme ultr...
Related paper
[2] Partial pressure gauges
Reviewed papers
[2-1] “Perspectives on residual gas analysis” (Lichtman, 1984)
3 Current partial pressure analyzers
A Magnetic sector
B Quadrupole (monopole)
References in the paper[2-1]
Reviewed paper
[2-2] “Measurement of ultra-high vacuum: Partial pressure measurements” (Weston, 1980)
11 Calibration
12 Conclusion
References in the paper[2-2]
Reviewed paper
[2-3] “Determination of the ionization gauge sensitivity using the relative ionization cross-section” (Nakao, 1975)
1 Introduction
2 Sensitivity of the ionization gauge
3 Results and discussion
4 Conclusion
References in the paper[2-3]
Reviewed paper
[2-4] “Simplified methods for the calculation of partial pressure using the relative ionization cross-section” (Nakao, 1975)
1 Introduction
3 Experimental apparatus and procedures
3.1 Instruments
3.2 Calibration
3.3 Considerations of the ionization cross section
3.4 Determination of the correction factor
6 Conclusion
References in the paper[2-4]
Reviewed papers
7
7 Molecular-flow networks
[1] Resistor network simulation method
Reviewed paper
[1-1] “A differential pressure-rise method for measuring the net outgassing rates of a solid material and for estimating it...
1 Introduction
2 Characteristic values of a solid material as a gas source
3 Differential pressure-rise method
A Principle
B Measurement of net outgassing rates of solid materials
C Estimation of the characteristic values PX and K0 of Viton O-rings
4 Vacuum circuit composed of the characteristic values of constituent elements
5 Conclusion
References in the paper[1-1]
Reviewed paper
[1-2] “Resistor network simulation method for a vacuum system in a molecular flow region” (Ohta, Yoshimura, and Hirano, 1983)
1 Introduction
2 Concept of vacuum system
A New concept of a vacuum system and its components
3 Simulation of the high-vacuum system of electron microscope
A Procedures to design a simulator circuit
B The simulator circuit
C Application results
4 Discussion
5 Summary
Acknowledgments
References in the paper [1-3]
Reviewed paper
[1-3] “Matrix calculation of pressures in high-vacuum systems” (Hirano, Kondo, and Yoshimura, 1988)
1 Introduction
2 Linear vacuum circuit
5 Application to practical high-vacuum systems
A Pressure distribution along an outgassing pipe
B Pressure distribution in an electron microscope high-vacuum system
6 Conclusion
References in the paper[1-3]
Reviewed paper
[1-4] “Analysis of pressure Distribution, based on vacuum circuits” (Yoshimura and Hirano, 1988)
4 Pressure distribution in a complex-chambers-system with homogeneous walls
Reviewed paper
[2] Long history of works on molecular-flow networks
Reviewed papers
[2-1] “Comments on: ‘Resistor network simulation method for a vacuum system in a molecular flow region’” [J. Vac. Sci. Tech...
References in the paper[2-1]
Reviewed paper
[2-2] “Theoretical analysis of a two-pump vacuum system”* (Kendall, 1968)
Introduction
1 Equivalent networks
References in the paper [2-2]
Reviewed paper
[2-3] “An electrical analogue to a high vacuum system” (Aitkin, 1953)
Reviewed paper
[2-4] “Further applications of the electrical analogue to vacuum system” (Stops, 1953)
References in the paper[2-4]
Reviewed paper
[2-5] “Analogs for elements between the vacuum system and the electric circuit” (Degras, 1956)
Reviewed paper
[2-6] “Vacuum circuit for a parallel evacuation” (Ohta, 1962)
Reviewed paper
[2-7] “Numerical modelling of vacuum systems using electronic circuit analysis tools” (Wilson, 1987)
2 Model
A Transformation rules
References in the paper [2-7]
Reviewed paper
[2-8] “An analysis of a complex network of vacuum components and its application” (Horikoshi, Saito, and Kakihara, 1990)
2 Vacuum pump and conduit pipe as a network component
3 Kirchhoff’s law in a network of vacuum components
Reviewed papers
[2-9] “Vacuum calculations for large systems” (Reid, 1992)
5 Electrical analogues
6 Discussion
References in the paper[2-9]
Reviewed paper
Related papers
[2-10] “Theory of pulsed molecular-flow networks” (Kendall and Pulfrey, 1969)
Related paper
[2-11] “Computer design and analysis of vacuum systems” (Santeler, 1987)
Related paper
Comments in Part 2
Part 3
Part 3 Phenomena occurred in electron microscopes
Introduction
8
8 Electron-induced gas desorption
Reviewed papers
[1-1] “The effect of bakeout temperature on the electron and ion induced gas desorption coefficients of some technological ...
3 Temperature dependence of the desorption coefficients
5 Discussion and conclusion
References in the paper[1-1]
Reviewed paper
[1-2] “Temperature dependence of the electron induced gas desorption yields on stainless steel, copper, and aluminum” (Góme...
3 Results
A Temperature dependence
B Dependence of gas desorption yields on the temperature and the dose
1 Stainless steel
2 Oxygen-free high thermal conductivity copper
3 Aluminum
C Dependence of gas desorption yields on the temperature and the coverage
1 Stainless steel
2 Oxygen-free high thermal conductivity copper
3 Anticorodal aluminum
6 Summary and conclusion
References in the paper[1-2]
Reviewed paper
[1-3] “Photodesorption from stainless steel, aluminum alloy and oxygen free copper test chambers” (Ueda et al., 1990)
3 Results and discussion
4 Conclusion
Reviewed paper
[1-4] “Cleaning of metal parts in oxygen radio frequency plasma: Process study” (Korzec, Rapp, Theirich, and Engemann, 1994)
4 Conclusion
Reviewed paper
9
9 Phenomena induced by fine electron-probe irradiation
Reviewed papers
[1-1] “Mechanism of contamination build-up induced by fine electron-probe irradiation” (Yoshimura, Hirano, and Etoh, 1983)
1 Introduction
2 Experiments
2.1 Experimental conditions
2.1.1 Pretreatment of specimen and specimen cartridge
2.1.2 Three types of anticontamination devices
2.1.3 Procedures for irradiation and observation
2.1.4 Measurement of the contamination rate
2.2 Experimental results
2.2.1 Fine-probe irradiation without anticontamination device
2.2.2 Fine-probe irradiation with the types 1, 2, and 3 anticontamination device, individually
2.2.3 Other related experiments
3 Discussions
References in the paper[1-1]
Reviewed paper
[1-2] “The origin of specimen contamination in the electron microscope” (Ennos, 1953)
5 Replenishment of the hydrocarbon film
6 Experiments on surface film replenishment
7 Conclusion
References in the paper[1-2]
Reviewed paper
[1-3] “Observation of polymerized films induced by irradiation of electron beams” (Yoshimura and Oikawa, 1970)
Reviewed paper
[1-4] “The use of perfluoropolyether fluids in vapour stream pump”* (Holland, Laurenson, and Baker, 1972)
1 Introduction
2 Experimental
3 Results
3.1 Mass spectra
3.2 Electron bombardment
3.3 Infrared spectra
3.4 Pumping speed measurement
3.5 Critical backing pressures
References in the paper[1-4]
Reviewed paper
[1-5] “The behavior of perfluoropolyether and other vacuum fluids under ion and electron bombardment” (Holland, Laurenson, ...
2 Experimental systems and results
2.1 Cold cathode discharge study
2.3 RF plasma discharge
4 Conclusion
Reviewed paper[1-5]
[1-6] “The sources of electron-induced contamination in kinetic vacuum systems” (Ennos, 1954)
2 The vapor sources of contamination
References in the paper[1-6]
Reviewed paper
[1-7] “Formation of thin polymer films by electron bombardment”* (Christy, 1960)
4 Conclusion
References in the paper[1-7]
Reviewed paper
[1-8] “Contamination in the STEM at ultra-high vacuum” (Wall, 1980)
6 Discussion and conclusion
References in the paper[1-8]
Reviewed paper
[1-9] “The prevention of contamination without beam damage to the specimen” (Heide, 1962)
Reference in the paper[1-9]
Reviewed paper
[1-10] “Reduction of polymer growth in electron microscopes by use of a fluorocarbon oxide pump fluid” (Ambrose, Holland, a...
References in the paper[1-10]
Reviewed paper
[1-11] “Vapor pressures of vacuum pump oils” (Nakayama, 1965)
Reviewed paper
[1-12] “Technology and application of pumping fluids” (Laurenson, 1982)
Reviewed paper
Related papers
[1-13] “Contamination formed around a very narrow electron beam” (Knox, 1976)
Related paper
[1-14] “Contamination phenomena in cryopumped TEM and ultra-high vacuum field-emission STEM systems” (Fourie, 1976)
Related paper
[1-15] “Contribution to the contamination problem in transmission electron microscopy” (Reimer and Wächter, 1978)
Related paper
[1-16] “The elimination of surface-originating contamination in electron microscopes” (Fourie, 1978/79)
Related paper
[1-17] “A theory of surface-originating contamination and a method for its elimination” (Fourie, 1979)
Related paper
[1-18] “Elektronen-Mikroschreiber mit geschwindigkeitsgesteuerter Strahlführung. I” (Kahl-Heinz Müller, 1971)
Related paper
[1-19] “Herabsetzung der Kontaminationsrate im STEM bei einem Druck von 10−5Torr” (Bauer and Speidel, 1977)
Related paper
[1-20] “Perfluoroalkylpolyethers—a unique new class of synthetic lubricants” (Lawson, 1970)
Related paper
[1-21] “SEM vacuum techniques and contamination management” (Miller, 1978)
Related paper
[1-22] “Hydrocarbon contamination management in vacuum-dependent scientific instruments” (Bance et al., 1978)
Related paper
[1-23] “Secondary electron emission dependence on electron beam density dose and surface interaction from AES and ELS in an...
Related paper
[1-24] “Electric effects in contamination and electron beam etching” (Fourie, 1981)
Related paper
10
10 Micro-discharges in high vacuum
[1] Micro-discharges over insulator surfaces
Reviewed papers
[1-1] “Microdischarges on an electron gun under high vacuum” (Watanabe, Yoshimura, Katoh, and Kobayashi, 1987)
1 Introduction
2 Experiment
A Conditioning effect of an argon glow cleaning
B Micro-discharges over the insulator surface
C Outgassing from the insulator
3 Discussion
A Conditioning effect of argon glow cleaning
B Micro-discharges over the insulator surface
4 Conclusion
References in the paper[1-1]
Reviewed paper
[1-2] “Surface flashover of solid dielectric in vacuum” (Pillai and Hackam, 1982)
5 Flashover breakdown criteria
7 Conclusion
References in the paper[1-2]
Reviewed paper
[1-3] “Surface flashover of solid insulators in atmospheric air and in vacuum” (Pillai and Hackam, 1985)
3 Results and discussions
A Effect of conditioning on surface flashover
B Effect of external resistance
C Effect of gas pressure
D Effect of electrode material
E Dependence of the flashover voltage on electron impact energy and dielectric constant
F Effect of spacer diameter on the flashover voltage
G Dependence of flashover on length of solid insulator
References in the paper[1-3]
Reviewed paper
[1-4] “Mechanism of pulsed surface flashover involving electron-stimulated desorption” (Anderson and Brainard, 1980)
2 Surface flashover model
A Summary
B Secondary emission avalanche
C Observation of avalanche current
5 Conclusion
References in the paper[1-4]
Reviewed paper
[1-5] “Insulation of high voltage across solid insulators in vacuum” (Shannon, Philp, and Trump, 1965)
6 Discussion of results
A Influence of insulator design
B Ability to endure repeated sparking
C Conduction current and the influence of insulator material
7 Summary: attainable flashover performance
References in the paper[1-5]
Reviewed paper
[1-6] “Pulsed flashover in vacuum” (Watson, 1967)
3 Experimental results
Reviewed paper
[1-7] “Effects of corrugated insulator on electrical insulation in vacuum” (Yamamoto, Hara, Matsuura, Tanabe, and Konishi, ...
5 Discussions
6 Conclusion
References in the paper[1-7]
Reviewed paper
[1-8] “Solid insulators in vacuum: a review” (Invited Paper) (Hawley, 1968)
7 Hypotheses to explain the breakdown mechanism
8 Practical insulator and shields
9 Conclusion
References in the paper[1-8]
Reviewed paper
Related papers
[1-9] “Electrical breakdown over insulators in high vacuum” (Gleichauf, 1951)
Related paper
[1-10] “Surface flashover of insulators” (Miller, 1989)
Related paper
[1-11] “Mechanism of surface charging of high-voltage insulators in vacuum” (de Tourreil and Srivastava, 1973)
Related paper
[1-12] “DC electric-field modifications produced by solid Insulators bridging a uniform-field vacuum gap” (Sudarshan and Cr...
Related paper
[1-13] “The effect of cuprous oxide coatings on surface flashover of dielectric spacers in vacuum” (Cross and Sudarshan, 1974)
Related paper
[1-14] “The effect of chromium oxide coatings on surface flashover of alumina spacers in vacuum” (Sudarshan and Cross, 1976)
Related paper
[1-15] “Breakdown of alumina rf windows and its inhibition” (Saito, Anami, Michizono, Matuda, Kinbara, and Kobayashi, 1994)
Related paper
[2] Micro-Discharges Between High-Voltage Electrodes
Reviewed papers
[2-1] “The initiation of electrical breakdown in vacuum” (Cranberg, 1952)
2 Proposed hypothesis and supporting data
3 Conclusion
Acknowledgment
References in the paper[2-1]
Reviewed paper
[2-2] “Vacuum breakdown and surface coating of rf cavities” (Peter, 1984)
Acknowledgment
References in the paper[2-2]
Reviewed paper
[2-3] “Electron energy analysis of vacuum discharge in high-voltage accelerator tube” (Takaoka, Ura, and Yoshida, 1982)
2 Experimental setup
3 Experimental results and discussion
(1) Energy spectra for pulsed and continuous discharge
(2) Energy spectra in the case of secondary-ion suppression
References in the paper[2-3]
Reviewed paper
Related papers
[2-4] “New perspectives in vacuum high-voltage insulation: I. The transition to field emission” (Diamond, 1998)
Rerated paper
[2-5] “New perspectives in vacuum high-voltage insulation: II. Gas desorption” (Diamond, 1998)
Related paper
[2-6] “Electrical breakdown between stainless-steel electrodes in vacuum” (von Oostrom and Augustus, 1982)
Related papers
[2-7] “Mechanisms of electrical discharges in high vacuum at voltages up to 400,000V” (Prichard Jr., 1973)
Related paper
[2-8] “Influence of gap length on the field increase factor β of an electrode projection (whisker)” (Miller, 1984)
Related paper
[2-9] “Small-aperture diaphragms in ion-accelerator tubes” (Cranberg and Henshall, 1959)
Related paper
[2-10] “Pre-breakdown conduction in continuously-pumped vacuum systems” (Mansfield, 1960)
Related papers
[2-11] “Prebreakdown conduction between vacuum insulated electrodes” (Powell and Chatterton, 1970)
Related paper
[2-12] “Processes involved in the triggering of vacuum breakdown by low velocity microparticles” (Chatterton, Menon, and Sr...
Related papers
[2-13] “The initiation of electrical breakdown in vacuum – a review” (Davies, 1973)
Related paper
[2-14] “Microparticle-initiated vacuum breakdown – some possible mechanisms” (Menon and Srivastava, 1973)
Related papers
[2-15] “Emission of electrode vapor resonance radiation at the onset of dc breakdown in vacuum” (Davies and Biondi, 1977)
Related paper
[2-16] “The source of high-β electron emission sites on broad-area high-voltage alloy electrodes” (Allen, Cox, and Latham, ...
Related papers
[2-17] “Electrical breakdown strength of oxygen-free copper electrodes under surface and bulk treatment conditions” (Kobaya...
Related papers
Comments in Part-3
1 Electron-induced/photon-induced gas desorption
Reference
Part 4
Part 4 Ultrahigh-vacuum systems of electron microscopes
Introduction
11
11 Development of diffusion pump, baffle with a cold cap, and liq. N2 trap
[1] Development of JEOL diffusion pump stack
Reviewed papers
[1-1] “Advances in diffusion pump technology” (Hablanian and Maliakal, 1973)
Introduction
6 Back-streaming
References in the paper[1-1]
Reviewed paper
[1-2] “Diffusion pump back-streaming” (Harris, 1977)
Reviewed paper
[1-3] “Backstreaming in diffusion pump systems” (Rettinghaus and Huber, 1974)
5 Results of back-streaming measurements; response to variation of conditions
References in the paper[1-3]
Reviewed paper
[1-4] Development of JEOL diffusion pump stack (DP, baffle with cold cap)
Quoted book
[1-4-1] “Developing new DP stack” (Book; Yoshimura, Springer Briefs, 2014, p. 56–58)
References in the book [1-4-1]
Quoted book
Reviewed paper
[1-5] “Thermal loss of a cold Trap” (Hirano and Yoshimura, 1981)
Reviewed papers
12
12 Cascade diffusion pump systems for electron microscopes
[1] Performance of cascade diffusion-pump systems
Reviewed papers
[1-1] “A new vacuum system for an electron microscope” (Yoshimura, Ohmori, Nagahama, and Oikawa, 1974)
3 A new vacuum system for an electron microscope
Reviewed paper
[1-2] “A cascade diffusion-pump system for an electron microscope” (Yoshimura, Hirano, Norioka, and Etoh, 1984)
1 Introduction
2 Discussions on dynamic diffusion pump systems for electron microscope
A Typical diffusion pump systems tolerant of excess gas load in transitional evacuation
B Differential evacuation systems for obtaining clean vacuum in the column
C Safety systems to protect the vacuum system in an emergency
D Rough evacuation systems for clean vacuum
3 The cascade diffusion pump system of an electron microscope
4 Conclusion
References in the paper[1-2]
Reviewed paper
[1-3] “Practical advantages of a cascade diffusion-pump system of a scanning electron microscope” (Norioka and Yoshimura, 1991)
2 Experiment
A Static performance of a cascade diffusion pump system
B Cascade diffusion pump system of scanning electron microscope
C Turbomolecular pump systems for scanning electron microscope
3 Conclusion
References in the paper[1-3]
Reviewed paper
[1-4] “The influence of fore-vacuum conditions on ultra-high vacuum pumping systems with oil diffusion pumps” (Hengevoss an...
2 Experimental apparatus
3 The influence of back diffusion
4 Turning off the booster diffusion pump
7 Conclusion
Reviewed paper
[1-5] “Prevention of overload in high-vacuum systems” (Hablanian, 1992)
2 Volume flow and mass flow
3 The crossover pressure?
References in the paper[1-5]
Reviewed paper
[1-6] “Advantage of slow high-vacuum pumping for suppressing excessive gas load in dynamic evacuation systems” (Yoshimura, ...
1 Introduction
2 Transitional phenomena of outgassing
3 Excessive gas load just after switching over the evacuation mode
3.1 Advantages of a small bypass valve
3.2 High-resolution electron microscope, equipped with a small bypass valve
Low-conductance bypass valve, applicable to the turbo-molecular pump or cryogenic pump systems
References in the paper[1-6]
Reviewed paper
[1-7] “The effect of the inlet valve on the ultimate vacua above integrated pumping groups” (Dennis, Laurenson, Devaney and...
3 Operation of the inlet valve
4 Summary
References in the paper[1-7]
Reviewed paper
[2] Roughing systems with an oil-sealed rotary pump
Reviewed paper
[2-1] “Vacua: How they may be improved or impaired by vacuum pumps and traps” (Holland, 1971)
5. Effects on back-streaming rate of pipe conductance, gas flow and pressure
Reviewed paper
Related papers
[2-2] “Backstreaming from rotary pumps” (Fulker, 1968)
Related paper
[2-3] “The development of an adsorption fore-line trap suitable for quick cycling vacuum systems” (Baker and Staniforth, 1968)
Related paper
[3] Analytical electron microscopes with a cascade-diffusion pump system
Quoted book
[3-1] “Memorial Book-1: History of JEOL (1); 35 years from its birth” (JEOL Ltd., March 1986)
Analytical electron microscopes with a cascade-diffusion pump system[3-1-1]
Quoted book
13
13 Sputter-ion pumps for ultrahigh-vacuum use
[1] Development of sputter ion pumps for ultrahigh-vacuum electron microscopes
Reviewed papers
[1-1] “Pumping characteristics of sputter ion pumps with high-magnetic-flux densities in an ultrahigh-vacuum range” (Ohara,...
1 Introduction
2 Pump-design parameters
3 Pumping characteristics
4 Conclusion
References in the paper[1-1]
Reviewed paper
[1-2] “Ar-pumping characteristics of diode-type sputter ion pumps with various shapes of ‘Ta/Ti’ cathode pairs” (Yoshimura,...
1 Introduction
2 Diode-type sputter ion pumps with various shapes of Ti/Ta cathode pair
3 Pumping speed characteristics
4 Conclusion
References in the paper[1-2]
Reviewed paper
[1-3] “Ar-pumping characteristics of diode-type sputter ion pumps with various shapes of ‘Ta/Ti’ cathode-pairs” (Yoshimura,...
Reviewed paper
[2] Review: Papers on the physical basis of sputter-ion pumps and Ti-sublimation pumps
Reviewed papers
[2-1] “The physics of sputter-ion pumps” (Jepsen, 1968)
4 The “energetic-neutrals” hypothesis
5 Conclusion
References[2-1]
Reviewed paper
[2-2] “The development of sputter-ion pumps” (Andrew, 1968)
5 Pump configurations for improved inert-gas pumping
6 Stability of inert-gas pumping
References in the paper[2-2]
Reviewed paper
[2-3] “Sputter-ion pumps for low pressure operation” (Rutherford, 1963)
4 Experimental results
References in the paper[2-3]
Reviewed paper
[2-4] “Stabilized air pumping with diode type getter-ion pumps” (Jepsen, Francis, Lutherford, and Kietzmann)
3 The argon problem
5 Stabilized diode pumps
A Motivations
B Experimental results
References in the paper[2-4]
Reviewed paper
[2-5] “Pumping mechanisms for the inert gases in diode Penning pumps” (Baker and Laurenson, 1972)
3 Experimental results
A Pump stability
B Pump speeds
C Pump down and readmission curves
References in the paper[2-5]
Reviewed paper
[2-6] “Enhancement of noble gas pumping for a sputter-ion pump” (Komiya and Yagi, 1969)
Reviewed paper
[2-7] “Comparison of diode and triode sputter-ion pumps” (Denison, 1977)
Reviewed paper
[2-8] “Pumping of helium and hydrogen by sputter-ion pumps. I. Helium pumping” (Welch, Pate, and Todd, 1993)
10 Conclusion
Reviewed paper
[2-9] “Pumping of helium and hydrogen by sputter-ion pumps. II. Hydrogen pumping” (Welch, Pate, and Todd, 1993)
7 Conclusion
Reviewed paper
[2-10] “Review of sticking coefficients and sorption capacities of gases on titanium films” (Harra, 1976)
4 Conclusion
References in the paper[2-10]
Reviewed paper
[2-11] “Methane outgassing from a Ti sublimation pump” (Edwards, Jr., 1980)
4 Methane outgassing rate reduction
5 Discussion
References in the paper[2-11]
Reviewed paper
14
14 Ultrahigh-vacuum systems of electron microscopes
[1] Ultrahigh vacuum systems of JEOL electron microscopes
Reviewed papers
[1-1] “Significance of Vacuum Technology in Electron Microscope” (Harada and Yoshimura, 1987)
Reviewed paper
[1-2] “Ultrahigh-vacuum scanning electron microscope (STEM)” (Tomita, 1990)
Reviewed paper
[1-3] “Design and development of an ultrahigh vacuum high-resolution transmission electron microscope” (Kondo et al., 1991)
2 Design of vacuum system
2.1 Ultrahigh vacuum high column construction of the microscope
2.2 Construction of pumping system and its control system
2.3 Evaluation of vacuum system by computer simulation and measurement
References in the paper[1-3]
Reviewed paper
[2] Progress of electron microscopes
Reviewed papers
[2-1] Review paper: “High resolution, high speed ultrahigh vacuum microscopy” (Poppa, 2004)
6 Future ultrahigh vacuum–high-resolution transmission electron microscope systems
A Ultrahigh vacuum e-cell instruments
B Real-time defocus image modulation processing approach
C Aberration-corrected ultrahigh vacuum high-resolution transmission electron microscopes
7 Conclusion
References in the paper[2-1]
Reviewed paper
[2-2] “Progress of electron microscope technology in Japan”
Reference in the paper[2-2]
Reviewed papers
[3] JEOL analytical electron microscopes with a field-emission electron source
[3-1] Memorial Book-2: History of JEOL (2); “Creation and development: 60 years from its birth”, (JEOL Ltd., May 2010)[3-1]...
References in the paper[3-1]
Reviewed papers
Comments in Part-4
1 “Diffusion pump with thick-wall, mild-steel pump vessel, works well with pump fluid, Santovac-5”
2 Clean roughing system without sorption trap
Acknowledgments
Index
Back Cover

Citation preview

A Review: Ultrahigh-Vacuum Technology for Electron Microscopes

A Review: Ultrahigh-Vacuum Technology for Electron Microscopes

Nagamitsu Yoshimura Retired from the Japan Electron Optics Laboratory (JEOL), Tokyo, Japan

Academic Press is an imprint of Elsevier 125 London Wall, London EC2Y 5AS, United Kingdom 525 B Street, Suite 1650, San Diego, CA 92101, United States 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-818573-5 For Information on all Academic Press publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Mara Conner Acquisitions Editor: Fiona Geraghty Editorial Project Manager: Charlotte Rowley Production Project Manager: Selvaraj Raviraj Cover Designer: Hitchen, Miles Typeset by MPS Limited, Chennai, India

Contents About the author Preface Acknowledgment A review

xxi xxiii xxv xxvii

2 Samples and experimental method 3 Experimental results and discussions Reviewed paper

Part 1 Adsorption, desorption, diffusion, and outgassing/pumping 1. Microstructure and elemental features of stainless-steel surface

[1-3] “Outgassing characteristics of electropolished stainless steel” (Tohyama, Yamada, Hirohata, Yamashina, 1990) 19 20 25

Related paper [1-4] “A review of the stainless steel surface” (Adams, 1983)

7

Related paper

25

Reviewed papers

2. Characteristics of outgassing from metal surfaces

[1-1] “Outgassing characteristics and microstructure of an electropolished stainless steel surface” (Yoshimura, Sato, Adachi, and Kanazawa, 1990) 2 Outgassing characteristics of an electropolished pipe wall A After an in situ bakeout 3 Microstructure and elemental features of stainless-steel surfaces A Microstructure B Elemental features 4 Conclusion References in the reviewed paper Reviewed paper

Quoted book

7 8 11 11 12 14 14 14

[1-2] “Outgassing characteristics and microstructure of a “vacuum fired” (1050 C) stainless steel surface” (Yoshimura, Hirano, Sato, Ando, and Adachi, 1991) 2 Microstructure and elemental features A Microstructure B Elemental features 3 Vacuum characteristics 4 Conclusion References in the reviewed paper Reviewed paper

27

[1-1] “Typical isotherms in the chemisorption of gases on metal-surfaces: equilibrium adsorption” (Redhead, Hobson, and Kornelsen, 1968) References in quoted book

28

[1-2] “Relations between pressure, pumping speed and outgassing rate” (Dayton, 1960) 2 Semiempirical formulas 3 Relation among pressure, speed, and outgassing rate References in the paper Reviewed paper

29 29 32 32

[1-3] “Outgassing rate of contaminated metal surfaces” (Dayton, 1962) 15 15 16 16 18 18 18

2 Equation for the outgassing rate References in the paper Reviewed paper

32 37 37

[1-4] “The effect of bake-out on the degassing of metals” (Dayton, 1963) 2 Degassing time at constant temperature

38 v

vi

Contents

3 Outgassing rate with variable temperature References in the paper Reviewed paper

43 45 45

[1-5] “Advantage of slow high-vacuum pumping for suppressing excessive gas load in dynamic evacuation systems” (Yoshimura, 2009) 1 Introduction 2 Transitional phenomena of outgassing 3 Excessive gas load just after switching over the evacuation mode 3.1 Advantages of a small bypass valve References in the paper Reviewed paper

46 46 49 49 51 51

[1-6] “The variation in outgassing rate with the time of exposure and pumping” (Rogers, 1964) 2 Analysis 4 First pump-down 5 Second pump-down 6 Summary References in the paper Reviewed paper

51 52 53 54 54 54

[1-7] “Reduction of stainless-steel outgassing in ultra-high vacuum” (Calder and Lewin, 1967) 2 Theory 2.1 Effect of temperature on degassing 2.2 Outgassing after a normal bakeout 2.3 Permeation rate of atmospheric hydrogen 2.4 High-temperature bulk degassing in situ 2.5 High-temperature bulk degassing in a furnace with residual hydrogen pressure 5 Summary and conclusion References in the paper Reviewed paper

55 55 56 57 58

59 60 60 60

61 62

[1-10] “True and measured outgassing rates of a vacuum chamber with a reversibly adsorbed phase” (Akaishi, Nagasuga, and Funato, 2001) Nomenclature 2 Modeling of pump-down A Mass conservation equations B Equilibrium and nonequilibrium adsorption isotherms C Measured and true outgassing rates 3 Comparison between theory and experiment 4 Discussion A Approximate expression of np B g Dependence of K and p References in the paper Reviewed paper

64 64 64 65 67 68 69 69 70 70 70

[1-11] “Recombination limited outgassing of stainless steel” (Moore, 1995) 1 Introduction 2 Method of analysis A Measurements by Hseuh and Cui B Numeric diffusion calculation C Recombination limit 3 Results of analysis A Recombination limited concentration profiles versus bake time B Outgassing rate versus time 4 Discussion A Bake efficiency as a function of temperature B Vacuum furnace versus in situ bake efficiency C Uncertainties in the estimate of outgassing rate References in the paper Reviewed paper

71 72 72 72 72 73 73 74 74 74 74 74 75 75

[1-12] “La de´sorption sous vide” (Schram, 1963) Reviewed paper

75

[1-13] “Hydrogen pumping by austenitic stainless steel” (Zajec and Namenic, ˇ 2005)

[1-9] “Model for the outgassing of water from metal surfaces” (Li and Dylla, 1993) 4 Final remarks

63 63

Related papers

[1-8] “Estimating the gas partial pressure due to diffusive outgassing” (Santeler, 1992) 2 Diffusive outgassing—Fick’s law References in the paper

References in the paper Reviewed paper

63

Reviewed paper

76

Contents

3. Methods for measuring outgassing rates

77

Reviewed papers [1-1] “Measurement of outgassing rates from materials by the differential pressure rise method” (Yoshimura, Oikawa, and Mikami, 1970) 2 Principle of the differential pressure-rise method Reviewed paper [1-2] “A three-point-pressure method for measuring the gas-flow rate through a conducting pipe” (Hirano and Yoshimura, 1986) 1 Introduction 2 Three-point-pressure method A Principle B Optimization of the measuring system C Measurement of gas-flow rates 3 Discussion 4 Conclusion References in the paper Reviewed paper

81 81 81 83 83 85 88 89 89

[1-3] “Two-point pressure method for measuring the outgassing rate” (Yoshimura and Hirano, 1989) 1 Introduction 2 Two-point-pressure method A Principle B Measurement of the outgassing rate C Validity of the one-point-pressure method 3 Measurement by the orifice method 4 Discussion and conclusion References in the paper Reviewed paper

89 90 90 91 93 94 95 96 96

[1-5] “Orifice method in which two pumping speeds can be selected” 1 Methods of measuring outgassing rates References in the paper Reviewed paper

1 Introduction 2 Conductance modulation method 5 Discussions and conclusion References in the paper Reviewed paper

101 103 103

109 110 112 113 113

[1-8] Differential orifice method; “La de´sorption sous vide” (Schram, 1963) Reviewed paper

114

[1-9] “Discussion on methods for measuring the outgassing rate” (Yoshimura, 1990)

4. Outgassing rates of system-component materials

115

117

[1] Outgassing Rates of Metallic Materials Reviewed papers [1-1] “Outgassing rates of stainless steel and mild steel after different pretreatments” (Ishimori, Yoshimura, Hasegawa, and Oikawa, 1971) Reviewed paper

96 98 101 101

104 107 107 108 108

[1-7] “Conductance modulation method for the measurement of the pumping speed and outgassing rate of pumps in ultrahigh vacuum” (Terada, Okano, and Tuji, 1989)

Reviewed paper

[1-4] “Speed measuring of ion getter pumps by the ‘three-gauge’ method” (Munro and Tom, 1965) 2 Method 3 Results and discussion References in the paper Reviewed paper

[1-6] “Corrections in outgassing rate measurements by the variable conductance method” (Berman, Hausman, and Roth, 1971) 2 Theory 3 Time influence 6 Conclusion References in the paper Reviewed paper

77 80

vii

121

[1-2] “Outgassing characteristics of stainless steel and aluminum with different surface treatments” (Young, 1969) References in the paper Reviewed paper [1-3] “Technology of low-pressure systems—establishment of optimum conditions to obtain low degassing rates on 316L stainless steel by heat treatments” (Nuvolone, 1977)

123 123

viii

Contents

Conclusion References in the paper Reviewed paper

124 124 124

[1-4] “Treatment of the wall materials of extremely high vacuum chamber for dynamical surface analysis” (Tsukui, Hasunuma, Endo, Osaka, and Ohdomari, 1993) 2 Extremely high vacuum systems 3 Treatment of vacuum chamber wall 4 Surface topography 5 Surface composition 6 Outgassing rate 7 Conclusion References in the paper Reviewed paper

125 125 126 127 130 130 131 131

[1-7] “Air bake-out to reduce hydrogen outgassing from stainless steel” (Bernardini, Braccini, De Salvo, Di Virgilio, Gaddi, Gennai, Genuini, Giazotto, Losurdo, Pan, Pasqualetti, Passuello, Popolizio, Raffaelli, Torelli, Zhang, Bradaschia, Del Fabbro, Febbro, Ferrante, Fidecaro, La Penna, Mancini, Poggiani, Narducci, Solina, and Valentini, 1998) Related paper

137

[1-8] “An overview of methods to suppress hydrogen outgassing rate from austenitic stainless steel with reference to UHV and EXV” (Ishikawa and Nemanic, ˇ 2003) Related paper

138

Reviewed papers

[1-5] “Thin-wall vacuum chambers of austenitic stainless steel” (Moore, 1998, 2001)

[1-9] “Permeability measurements with gaseous hydrogen for various steels” (Eschbach, Gross, and Schulien, 1963)

[1-5-1] “Atmospheric permeation of austenitic stainless steel” (Moore, 1998)

6 Summary References in the paper Reviewed paper

2 Analysis A Eschbach experiment B Extrapolation procedure C Oxygen effects D Quantitative effect of hydrogen recombination on outgassing and permeation E Summary of hydrogen permeation corrections F Effect of atmospheric water vapor 4 Summary References in the paper Reviewed paper

131 131 132 132

134 134 134 134 134

Related paper

135

[1-6] Outgassing characteristics of the thin-walled stainless steel cells [1-6-1] “Outgassing in thin wall stainless steel cells” Related paper

136

141

[1-10-2] “Vapor-pressure data for some common gases” (Honig and Hook, 1960) Reviewed paper

143

[2] Outgassing Rates of Nonmetallic Materials

[2-1-1] “Measurement of outgassing rates from materials by differential pressure rise method” (Yoshimura, Oikawa, and Mikami, 1970) [2-1-2] “A differential pressure-rise method for measuring the net outgassing rates of a solid material and for estimating its characteristic values as a gas source” (Yoshimura, 1985) Reviewed papers

[1-6-2] “Experiments with a thin-walled stainless-steel vacuum chamber” Related paper

[1-10-1] “Vapor-pressure data for the more common elements” (Honig, 1957)

[2-1] Data by Yoshimura et al. (1970) and Yoshimura (1985)

[1-5-2] “Thin-walled vacuum chambers of austenitic stainless steel” (Moore, 2001) Related paper

[1-10] Vapor-pressure data

Reviewed paper 133

140 140 140

144

[2-2] Data by Dayton (1959) 137

References in the paper

149

Contents

[2-3] “Water vapor permeation through Viton O-ring seals” (Yoshimura, 1989) References in the paper

154

[2-4] “The properties of Viton ‘A’ elastomers: Part V. The practical application of Viton ‘A’ seals in high vacuum” (de Csernatony and Crawley, 1966) 1 Summary of previous results 2 Use of double O-rings 3 Contamination 4 Practical considerations 5 Conclusion References in the paper

154 156 157 158 159 159

179

Quoted book [1-1] “Conductance of orifice, long tube, and short tube” (Roth, 1990) Quoted book

180

Reviewed papers [1-2] “Monte Carlo calculation of molecular flow rates through a cylindrical elbow and pipes of other shapes” (Davis, 1960)

160 164 164

[2-6] “Practical selection of elastomer materials for vacuum seals” (Peacock, 1980) 2 Properties of polymer seal materials-chemical 3 Properties of polymer seal materials—mechanical A Compression set 4 Outgassing 5 Permeation 6 Radiation damage of seal polymers References in the paper

5. Molecular-flow conductance and gas-flow patterns

175

[1] Molecular-flow conductance

[2-5] “Recent advances in elastomer technology for UHV applications” (de Chernatony, 1977) 3 Recent fluoro-elastomers: Kalrez ECD-006 and Viton E60C 4 Conclusion References in the paper

Part 2 Molecular-flow network

ix

Reviewed paper

182

[1-3] “Optimization of molecular flow conductance” (Levenson, Milleron, and Davis, 1960) Reviewed paper

187

Related paper 164 165 165 166 167 168 168

[1-4] “A review of the molecular flow conductance for systems of tubes and components and the measurement of pumping speed” (Steckelmacher, 1966) Related paper [2] Gas-flow patterns

Comments in Part-1

Reviewed papers

1 Recommended pretreatments for stainless-steel chamber walls 2 Effect of in situ baking 3 Effect of unit time exposure 4 Advantage of slow high-vacuum pumping for suppressing excessive gas load 5 Differential pressure rise method for measuring the outgassing rates of sample materials 6 Three-point-pressure, two-point-pressure, and one-point-pressure methods 7 Make use of mild-steel for scientific instrument chamber

[2-1] “Gas flow patterns at entrance and exit of cylindrical tubes”

169 170 170

171

171 172 173

188

Reviewed paper

191

[2-2] “Angular distributions of molecular flux from orifices of various thicknesses” (Nanbu, 1985) [2-3] “A further discussion about gas flow patterns at the entrance and exit of vacuum channels” (Ji-Yuan, 1988) 2. Blade channels of the turbomolecular pump References in the paper

192 193

x

Contents

6. Total and partial pressure gauges for ultrahigh-vacuum use

195

[1] Bayard Alpert gauge (BAG) and extractor gauge (EG)

5 Experiment trials Reviewed paper

Reviewed papers [1-1] “New hot-filament ionization gauge with low residual current” (Redhead, 1966) 4 Summary and conclusion References in the paper Reviewed paper

196 196 196

[1-2] “Practical guide to the use of Bayard Alpert ionization gauges” (Singleton, 2001) 4 Safety precautions 7 Conclusion References in the paper

197 197 198

198

199 201 201

202 205

[1-5] Ion current characteristics of sputter-ion pump, compared with an extractor gauge (EG); “Ar-pumping characteristics of diode-type sputter ion pump with various shapes of “Ti/Ta” cathode-pairs” (Yoshimura, Ohara, Ando, and Hirano, 1992) Reviewed paper

4 Gauge calibration 5 Conclusion References in the paper Reviewed paper

210 213 214 214

[1-9] “Review of pressure measurement techniques for ultrahigh vacua” (Lafferty, 1971) Reviewed paper

215

216

[1-11] “Hot-cathode magnetron ionization gauge with an electron multiplier ion detector for the measurement of extreme ultra-high vacua” (Lafferty, 1963) Related paper

217

[2] Partial pressure gauges Reviewed paper [2-1] “Perspectives on residual gas analysis” (Lichtman, 1984) 3 Current partial pressure analyzers A Magnetic sector B Quadrupole (monopole) References in the paper Reviewed paper

218 218 219 221 221

[2-2] “Measurement of ultra-high vacuum: Partial pressure measurements” (Weston, 1980) 206

[1-6] “Starting delays in cold-cathode gauges at low pressures” (Kendall and Drubetsky, 1996) 8 Discussion References in the paper Reviewed paper

[1-8] “Measurement of ultra-high vacuum. Part 1. Total pressure measurements” (Weston, 1979)

Reviewed paper

[1-4] “Outgassing characteristics of an electropolished stainless-steel pipe with an operating extractor ionization gauge” (Yoshimura, Hirano, Ohara, and Ando, 1991) 2 Experiment References in the paper

208 210

[1-10] “Comparison of hot cathode and cold cathode ionization gauges” (Peacock, Peacock, and Hauschulz, 1991)

[1-3] “Comparison of the pressure indication of a Bayard Alpert and an extractor gauge” (Beeck and Reich, 1972) 4 The extractor gauge 5 Comparison of pressure indicators between Bayard Alpert gauge and extractor gauge References in the paper Reviewed paper

[1-7] “Enhanced ignition of cold cathode gauges through the use of radioactive Isotopes” (Welch, Smart, and Todd)

207 207 207

11 Calibration 12 Conclusion References in the paper

222 225 226

[2-3] “Determination of the ionization gauge sensitivity using the relative ionization cross-section” (Nakao, 1975) 1 Introduction 2 Sensitivity of the ionization gauge

226 227

Contents

3 Results and discussion 4 Conclusion References in the paper Reviewed paper

227 232 233 233

[2-4] “Simplified methods for the calculation of partial pressure using the relative ionization cross-section” (Nakao, 1975) 1 Introduction 3 Experimental apparatus and procedures 3.1 Instruments 3.2 Calibration 3.3 Considerations of the ionization cross section 3.4 Determination of the correction factor 6 Conclusion References in the paper Reviewed paper

7. Molecular-flow networks

234 234 234 235 236 236 238 238 239

241

[1] Resistor network simulation method Reviewed paper

241

[1-1] “A differential pressure-rise method for measuring the net outgassing rates of a solid material and for estimating its characteristic values as a gas source” (Yoshimura, 1985) 1 Introduction 2 Characteristic values of a solid material as a gas source 3 Differential pressure-rise method A Principle B Measurement of net outgassing rates of solid materials C Estimation of the characteristic values PX and K0 of Viton O-rings 4 Vacuum circuit composed of the characteristic values of constituent elements 5 Conclusion References in the paper Reviewed paper

251 252 253 254 255 256 256 256 256

[1-3] “Matrix calculation of pressures in high-vacuum systems” (Hirano, Kondo, and Yoshimura, 1988) 1 Introduction 257 2 Linear vacuum circuit 257 5 Application to practical high-vacuum systems 258 A Pressure distribution along an outgassing pipe 258 B Pressure distribution in an electron microscope high-vacuum system 260 6 Conclusion 262 References in the paper 262 Reviewed paper 262 [1-4] “Analysis of pressure Distribution, based on vacuum circuits” (Yoshimura and Hirano, 1988)

241 242 243 243 243

4 Pressure distribution in a complex-chambers-system with homogeneous walls Reviewed paper

263 264

[2] Long history of works on molecular-flow networks Reviewed papers

243 245 246 247 248

[1-2] “Resistor network simulation method for a vacuum system in a molecular flow region” (Ohta, Yoshimura, and Hirano, 1983) 1 Introduction 2 Concept of vacuum system A New concept of a vacuum system and its components

3 Simulation of the high-vacuum system of electron microscope A Procedures to design a simulator circuit B The simulator circuit C Application results 4 Discussion 5 Summary Acknowledgments References in the paper Reviewed paper

xi

[2-1] “Comments on: ‘Resistor network simulation method for a vacuum system in a molecular flow region’” [J. Vac. Sci. Technol. A 1, 84 (1983)] (Kendall, 1983) References in the paper Reviewed paper

265 265

[2-2] “Theoretical analysis of a two-pump vacuum system”* (Kendall, 1968) 248 249 249

Introduction 1 Equivalent networks References in the paper Reviewed paper

266 266 267 267

xii

Contents

[2-3] “An electrical analogue to a high vacuum system” (Aitkin, 1953) Reviewed paper

269

[2-4] “Further applications of the electrical analogue to vacuum system” (Stops, 1953) References in the paper Reviewed paper

272

[2-6] “Vacuum circuit for a parallel evacuation” (Ohta, 1962) Reviewed paper

273

[2-7] “Numerical modelling of vacuum systems using electronic circuit analysis tools” (Wilson, 1987) 2 Model A Transformation rules References in the paper Reviewed paper

274 274 277 277

[2-8] “An analysis of a complex network of vacuum components and its application” (Horikoshi, Saito, and Kakihara, 1990) 2 Vacuum pump and conduit pipe as a network component 278 3 Kirchhoff’s law in a network of vacuum components 278 Reviewed papers 281 [2-9] “Vacuum calculations for large systems” (Reid, 1992) 5 Electrical analogues 6 Discussion References in the paper Reviewed paper

282 282 283 283

Related papers

283

[2-11] “Computer design and analysis of vacuum systems” (Santeler, 1987) Related paper Comments in Part 2

8. Electron-induced gas desorption

289

[1-1] “The effect of bakeout temperature on the electron and ion induced gas desorption coefficients of some technological materials” (Achard, Calder, and Mathewson, 1979) 3 Temperature dependence of the desorption coefficients 5 Discussion and conclusion References in the paper Reviewed paper

289 293 294 294

[1-2] “Temperature dependence of the electron induced gas desorption yields on stainless steel, copper, and aluminum” (Go´mez-Gon˜i and Mathewson, 1997) 3 Results A Temperature dependence B Dependence of gas desorption yields on the temperature and the dose C Dependence of gas desorption yields on the temperature and the coverage 6 Summary and conclusion References in the paper Reviewed paper

295 295 296 300 302 303 303

[1-3] “Photodesorption from stainless steel, aluminum alloy and oxygen free copper test chambers” (Ueda et al., 1990) 3 Results and discussion 4 Conclusion Reviewed paper

303 307 307

[1-4] “Cleaning of metal parts in oxygen radio frequency plasma: Process study” (Korzec, Rapp, Theirich, and Engemann, 1994) 4 Conclusion Reviewed paper

[2-10] “Theory of pulsed molecular-flow networks” (Kendall and Pulfrey, 1969) Related paper

287

Reviewed papers 270 270

[2-5] “Analogs for elements between the vacuum system and the electric circuit” (Degras, 1956) Reviewed paper

Part 3 Phenomena occurred in electron microscopes

309 309

9. Phenomena induced by fine electronprobe irradiation 311 Reviewed papers

284

[1-1] “Mechanism of contamination build-up induced by fine electron-probe irradiation” (Yoshimura, Hirano, and Etoh, 1983)

Contents

1 Introduction 2 Experiments 2.1 Experimental conditions 2.2 Experimental results 3 Discussions References in the paper Reviewed paper

311 312 312 313 317 318 318

[1-2] “The origin of specimen contamination in the electron microscope” (Ennos, 1953) 5 Replenishment of the hydrocarbon film 6 Experiments on surface film replenishment 7 Conclusion References in the paper Reviewed paper

318 320 321 322 322

[1-3] “Observation of polymerized films induced by irradiation of electron beams” (Yoshimura and Oikawa, 1970) Reviewed paper

324

[1-4] “The use of perfluoropolyether fluids in vapour stream pump”* (Holland, Laurenson, and Baker, 1972) 1 Introduction 2 Experimental 3 Results 3.1 Mass spectra 3.2 Electron bombardment 3.3 Infrared spectra 3.4 Pumping speed measurement 3.5 Critical backing pressures References in the paper Reviewed paper [1-5] “The behavior of perfluoropolyether and other vacuum fluids under ion and electron bombardment” (Holland, Laurenson, Hurley, and K. Williams, 1973) 2 Experimental systems and results 2.1 Cold cathode discharge study 2.3 RF plasma discharge 4 Conclusion Reviewed paper

4 Conclusion References in the paper Reviewed paper

330 330 332 332 333

6 Discussion and conclusion References in the paper Reviewed paper

340 340 340

[1-9] “The prevention of contamination without beam damage to the specimen” (Heide, 1962) Reference in the paper Reviewed paper

343 343

[1-10] “Reduction of polymer growth in electron microscopes by use of a fluorocarbon oxide pump fluid” (Ambrose, Holland, and Laurenson, 1972) 345 345

[1-11] “Vapor pressures of vacuum pump oils” (Nakayama, 1965) Reviewed paper

346

[1-12] “Technology and application of pumping fluids” (Laurenson, 1982) Reviewed paper

347

Related papers [1-13] “Contamination formed around a very narrow electron beam” (Knox, 1976) 348

[1-14] “Contamination phenomena in cryopumped TEM and ultra-high vacuum field-emission STEM systems” (Fourie, 1976) Related paper

349

[1-15] “Contribution to the contamination problem in transmission electron microscopy” (Reimer and Wa¨chter, 1978) Related paper

333 338 338

339 339 339

[1-8] “Contamination in the STEM at ultra-high vacuum” (Wall, 1980)

Related paper

[1-6] “The sources of electron-induced contamination in kinetic vacuum systems” (Ennos, 1954) 2 The vapor sources of contamination References in the paper Related paper

[1-7] “Formation of thin polymer films by electron bombardment” (Christy, 1960)

References in the paper Reviewed paper 324 325 326 326 326 327 327 328 329 329

xiii

349

[1-16] “The elimination of surface-originating contamination in electron microscopes” (Fourie, 1978/79) Related paper

350

xiv

Contents

[1-17] “A theory of surface-originating contamination and a method for its elimination” (Fourie, 1979) Related paper

350

[1-18] “Elektronen-Mikroschreiber mit geschwindigkeitsgesteuerter Strahlfu¨hrung. I” (Kahl-Heinz Mu¨ller, 1971) Related paper

351

[1-19] “Herabsetzung der Kontaminationsrate im STEM bei einem Druck von 1025 Torr” (Bauer and Speidel, 1977) Related paper

351

[1-20] “Perfluoroalkylpolyethers—a unique new class of synthetic lubricants” (Lawson, 1970) Related paper

352

[1-21] “SEM vacuum techniques and contamination management” (Miller, 1978) Related paper

353

354

[1-23] “Secondary electron emission dependence on electron beam density dose and surface interaction from AES and ELS in an ultrahigh vacuum SEM” (Le Gressus, Massignon, Mogami, and Okuzumi, 1979) Related paper

355

[1-24] “Electric effects in contamination and electron beam etching” (Fourie, 1981) Related paper

10. Micro-discharges in high vacuum

356

357

[1] Micro-discharges over insulator surfaces Reviewed papers [1-1] “Microdischarges on an electron gun under high vacuum” (Watanabe, Yoshimura, Katoh, and Kobayashi, 1987) 1 Introduction 2 Experiment

359 360 362 363 363 364 365 365 365

[1-2] “Surface flashover of solid dielectric in vacuum” (Pillai and Hackam, 1982) 5 Flashover breakdown criteria 7 Conclusion References in the paper Reviewed paper

366 367 367 367

[1-3] “Surface flashover of solid insulators in atmospheric air and in vacuum” (Pillai and Hackam, 1985)

[1-22] “Hydrocarbon contamination management in vacuum-dependent scientific instruments” (Bance et al., 1978) Related paper

A Conditioning effect of an argon glow cleaning B Micro-discharges over the insulator surface C Outgassing from the insulator 3 Discussion A Conditioning effect of argon glow cleaning B Micro-discharges over the insulator surface 4 Conclusion References in the paper Reviewed paper

357 358

3 Results and discussions 368 A Effect of conditioning on surface flashover 368 B Effect of external resistance 369 C Effect of gas pressure 370 D Effect of electrode material 371 E Dependence of the flashover voltage on electron impact energy and dielectric constant 371 F Effect of spacer diameter on the flashover voltage 372 G Dependence of flashover on length of solid insulator 373 References in the paper 374 Reviewed paper 375 [1-4] “Mechanism of pulsed surface flashover involving electron-stimulated desorption” (Anderson and Brainard, 1980) 2 Surface flashover model A Summary B Secondary emission avalanche C Observation of avalanche current 5 Conclusion References in the paper Reviewed paper

376 376 376 377 378 378 378

[1-5] “Insulation of high voltage across solid insulators in vacuum” (Shannon, Philp, and Trump, 1965) 6 Discussion of results

379

Contents

A Influence of insulator design B Ability to endure repeated sparking C Conduction current and the influence of insulator material 7 Summary: attainable flashover performance References in the paper Reviewed paper

379 380 381 382 382 382

[1-6] “Pulsed flashover in vacuum” (Watson, 1967) 3 Experimental results Reviewed paper

383 384

385 386 386 386

387 388 388 388 388

Related papers [1-9] “Electrical breakdown over insulators in high vacuum” (Gleichauf, 1951) Related paper

389

[1-10] “Surface flashover of insulators” (Miller, 1989) Related paper

390

[1-11] “Mechanism of surface charging of high-voltage insulators in vacuum” (de Tourreil and Srivastava, 1973) Related paper

390

[1-12] “DC electric-field modifications produced by solid Insulators bridging a uniform-field vacuum gap” (Sudarshan and Cross, 1973) Related paper

392

[1-15] “Breakdown of alumina rf windows and its inhibition” (Saito, Anami, Michizono, Matuda, Kinbara, and Kobayashi, 1994) Related paper

393

[2-1] “The initiation of electrical breakdown in vacuum” (Cranberg, 1952) 2 Proposed hypothesis and supporting data 3 Conclusion Acknowledgment References in the paper

394 397 397 398

[2-2] “Vacuum breakdown and surface coating of rf cavities” (Peter, 1984) Acknowledgment References in the paper

400 400

[2-3] “Electron energy analysis of vacuum discharge in high-voltage accelerator tube” (Takaoka, Ura, and Yoshida, 1982) 2 Experimental setup 3 Experimental results and discussion (1) Energy spectra for pulsed and continuous discharge (2) Energy spectra in the case of secondary-ion suppression References in the paper

401 404 404 404 409

Related papers [2-4] “New perspectives in vacuum high-voltage insulation: I. The transition to field emission” (Diamond, 1998) Rerated paper

410

[2-5] “New perspectives in vacuum high-voltage insulation: II. Gas desorption” (Diamond, 1998) 391

[1-13] “The effect of cuprous oxide coatings on surface flashover of dielectric spacers in vacuum” (Cross and Sudarshan, 1974) Related paper

Related paper

Reviewed papers

[1-8] “Solid insulators in vacuum: a review” (Invited Paper) (Hawley, 1968) 7 Hypotheses to explain the breakdown mechanism 8 Practical insulator and shields 9 Conclusion References in the paper Reviewed paper

[1-14] “The effect of chromium oxide coatings on surface flashover of alumina spacers in vacuum” (Sudarshan and Cross, 1976)

[2] Micro-Discharges Between High-Voltage Electrodes

[1-7] “Effects of corrugated insulator on electrical insulation in vacuum” (Yamamoto, Hara, Matsuura, Tanabe, and Konishi, 1996) 5 Discussions 6 Conclusion References in the paper Reviewed paper

xv

392

Related paper [2-6] “Electrical breakdown between stainless-steel electrodes in vacuum” (von Oostrom and Augustus, 1982)

411

xvi

Contents

Related papers

411

[2-7] “Mechanisms of electrical discharges in high vacuum at voltages up to 400,000 V” (Prichard Jr., 1973) Related paper

412

[2-8] “Influence of gap length on the field increase factor β of an electrode projection (whisker)” (Miller, 1984) Related paper

413

[2-17] “Electrical breakdown strength of oxygen-free copper electrodes under surface and bulk treatment conditions” (Kobayashi, Hashimoto, Maeyama, Saito, and Nagai, 1996) Related papers

418

Comments in Part-3 Electron-induced/photon-induced gas desorption Reference

419 419

[2-9] “Small-aperture diaphragms in ion-accelerator tubes” (Cranberg and Henshall, 1959) Related paper

413

[2-10] “Pre-breakdown conduction in continuously-pumped vacuum systems” (Mansfield, 1960) Related papers

414

[2-11] “Prebreakdown conduction between vacuum insulated electrodes” (Powell and Chatterton, 1970) Related paper

414

415

416

[2-14] “Microparticle-initiated vacuum breakdown some possible mechanisms” (Menon and Srivastava, 1973) Related papers

416

Introduction 6 Back-streaming References in the paper Reviewed paper

423 424 426 427

[1-2] “Diffusion pump back-streaming” (Harris, 1977) Reviewed paper

428

5 Results of back-streaming measurements; response to variation of conditions References in the paper Reviewed paper

429 431 431

[1-4] Development of JEOL diffusion pump stack (DP, baffle with cold cap) 417

[2-16] “The source of high-β electron emission sites on broad-area high-voltage alloy electrodes” (Allen, Cox, and Latham, 1979) Related papers

Reviewed papers

[1-3] “Backstreaming in diffusion pump systems” (Rettinghaus and Huber, 1974)

[2-15] “Emission of electrode vapor resonance radiation at the onset of dc breakdown in vacuum” (Davies and Biondi, 1977) Related paper

423

[1-1] “Advances in diffusion pump technology” (Hablanian and Maliakal, 1973)

[2-13] “The initiation of electrical breakdown in vacuum a review” (Davies, 1973) Related paper

11. Development of diffusion pump, baffle with a cold cap, and liq. N2 trap

[1] Development of JEOL diffusion pump stack

[2-12] “Processes involved in the triggering of vacuum breakdown by low velocity microparticles” (Chatterton, Menon, and Srivastava, 1972) Related papers

Part 4 Ultrahigh-vacuum systems of electron microscopes

Quoted book [1-4-1] “Developing new DP stack” (Book; Yoshimura, Springer Briefs, 2014, p. 56 58)

418

References in the book

434

Quoted book

434

Contents

Reviewed paper

[1-4] “The influence of fore-vacuum conditions on ultra-high vacuum pumping systems with oil diffusion pumps” (Hengevoss and Huber, 1963)

[1-5] “Thermal loss of a cold Trap” (Hirano and Yoshimura, 1981) Reviewed papers

12. Cascade diffusion pump systems for electron microscopes

437

439

[1] Performance of cascade diffusion-pump systems

2 Volume flow and mass flow 3 The crossover pressure? References in the paper Reviewed paper

[1-1] “A new vacuum system for an electron microscope” (Yoshimura, Ohmori, Nagahama, and Oikawa, 1974) 440 442

[1-2] “A cascade diffusion-pump system for an electron microscope” (Yoshimura, Hirano, Norioka, and Etoh, 1984) 1 Introduction 2 Discussions on dynamic diffusion pump systems for electron microscope A Typical diffusion pump systems tolerant of excess gas load in transitional evacuation B Differential evacuation systems for obtaining clean vacuum in the column C Safety systems to protect the vacuum system in an emergency D Rough evacuation systems for clean vacuum 3 The cascade diffusion pump system of an electron microscope 4 Conclusion References in the paper

442 443

444 446 447 448 449 452 452

[1-3] “Practical advantages of a cascade diffusion-pump system of a scanning electron microscope” (Norioka and Yoshimura, 1991) 2 Experiment A Static performance of a cascade diffusion pump system B Cascade diffusion pump system of scanning electron microscope C Turbomolecular pump systems for scanning electron microscope 3 Conclusion References in the paper Reviewed paper

2 Experimental apparatus 3 The influence of back diffusion 4 Turning off the booster diffusion pump 7 Conclusion Reviewed paper

460 461 462 463 464

[1-5] “Prevention of overload in high-vacuum systems” (Hablanian, 1992)

Reviewed papers

3 A new vacuum system for an electron microscope Reviewed paper

xvii

464 466 469 469

[1-6] “Advantage of slow high-vacuum pumping for suppressing excessive gas load in dynamic evacuation systems” (Yoshimura, 2009) 1 Introduction 2 Transitional phenomena of outgassing 3 Excessive gas load just after switching over the evacuation mode 3.1 Advantages of a small bypass valve 3.2 High-resolution electron microscope, equipped with a small bypass valve References in the paper Reviewed paper

470 470 473 473 473 475 476

[1-7] “The effect of the inlet valve on the ultimate vacua above integrated pumping groups” (Dennis, Laurenson, Devaney and Colwell, 1992) 3 Operation of the inlet valve 4 Summary References in the paper Reviewed paper

476 477 478 478

[2] Roughing systems with an oil-sealed rotary pump Reviewed paper

453 453 456 458 459 459 459

[2-1] “Vacua: How they may be improved or impaired by vacuum pumps and traps” (Holland, 1971) 5. Effects on back-streaming rate of pipe conductance, gas flow and pressure Reviewed paper

479 481

Related papers [2-2] “Backstreaming from rotary pumps” (Fulker, 1968) Related paper

482

xviii

Contents

[2] Review: Papers on the physical basis of sputter-ion pumps and Ti-sublimation pumps

[2-3] “The development of an adsorption fore-line trap suitable for quick cycling vacuum systems” (Baker and Staniforth, 1968) Related paper

Reviewed papers 482

[3] Analytical electron microscopes with a cascade-diffusion pump system

4 The “energetic-neutrals” hypothesis 5 Conclusion References Reviewed paper

Quoted book [3-1] “Memorial Book-1: History of JEOL (1); 35 years from its birth” (JEOL Ltd., March 1986) Analytical electron microscopes with a cascade-diffusion pump system Quoted book

13. Sputter-ion pumps for ultrahigh-vacuum use

483 483

485

[2-3] “Sputter-ion pumps for low pressure operation” (Rutherford, 1963) 501 504 504

[2-4] “Stabilized air pumping with diode type getter-ion pumps” (Jepsen, Francis, Lutherford, and Kietzmann)

[1-1] “Pumping characteristics of sputter ion pumps with high-magnetic-flux densities in an ultrahigh-vacuum range” (Ohara, Ando, and Yoshimura, 1992) 485 486 487 490 490 490

[1-2] “Ar-pumping characteristics of diode-type sputter ion pumps with various shapes of ‘Ta/Ti’ cathode pairs” (Yoshimura, Ohara, Ando, and Hirano, 1992) 491 491 492 493 494 494

3 The argon problem 5 Stabilized diode pumps A Motivations B Experimental results References in the paper Reviewed paper

505 506 506 508 509 509

[2-5] “Pumping mechanisms for the inert gases in diode Penning pumps” (Baker and Laurenson, 1972) 3 Experimental results A Pump stability B Pump speeds C Pump down and readmission curves References in the paper Reviewed paper

510 510 511 512 513 513

[2-6] “Enhancement of noble gas pumping for a sputter-ion pump” (Komiya and Yagi, 1969) Reviewed paper

[1-3] “Ar-pumping characteristics of diode-type sputter ion pumps with various shapes of ‘Ta/Ti’ cathode-pairs” (Yoshimura, Ohara, Ando, and Hirano, 1992) Reviewed paper

5 Pump configurations for improved inert-gas pumping 499 6 Stability of inert-gas pumping 500 References in the paper 501

4 Experimental results References in the paper Reviewed paper

Reviewed paper

1 Introduction 2 Diode-type sputter ion pumps with various shapes of Ti/Ta cathode pair 3 Pumping speed characteristics 4 Conclusion References in the paper Reviewed paper

496 498 498 498

[2-2] “The development of sputter-ion pumps” (Andrew, 1968)

[1] Development of sputter ion pumps for ultrahigh-vacuum electron microscopes

1 Introduction 2 Pump-design parameters 3 Pumping characteristics 4 Conclusion References in the paper Reviewed paper

[2-1] “The physics of sputter-ion pumps” (Jepsen, 1968)

514

[2-7] “Comparison of diode and triode sputter-ion pumps” (Denison, 1977) Reviewed paper 495

[2-8] “Pumping of helium and hydrogen by sputter-ion pumps. I. Helium pumping” (Welch, Pate, and Todd, 1993)

514

Contents

10 Conclusion Reviewed paper

515 515

[2-9] “Pumping of helium and hydrogen by sputter-ion pumps. II. Hydrogen pumping” (Welch, Pate, and Todd, 1993) 7 Conclusion Reviewed paper

516 516

2.3 Evaluation of vacuum system by computer simulation and measurement References in the paper Reviewed paper

xix

526 527 527

[2] Progress of electron microscopes Reviewed papers [2-1] Review paper: “High resolution, high speed ultrahigh vacuum microscopy” (Poppa, 2004)

[2-10] “Review of sticking coefficients and sorption capacities of gases on titanium films” (Harra, 1976) 4 Conclusion References in the paper Reviewed paper

517 517 517

[2-11] “Methane outgassing from a Ti sublimation pump” (Edwards, Jr., 1980) 4 Methane outgassing rate reduction 5 Discussion References in the paper Reviewed paper

518 519 519 519

6 Future ultrahigh vacuum high-resolution transmission electron microscope systems A Ultrahigh vacuum e-cell instruments B Real-time defocus image modulation processing approach C Aberration-corrected ultrahigh vacuum high-resolution transmission electron microscopes 7 Conclusion References in the paper

528 528 528

529 529 530

[2-2] “Progress of electron microscope technology in Japan”

14. Ultrahigh-vacuum systems of electron microscopes

521

Reference in the paper Reviewed paper

[1] Ultrahigh vacuum systems of JEOL electron microscopes

[3] JEOL analytical electron microscopes with a field-emission electron source

Reviewed papers

[3-1] Memorial Book-2: History of JEOL (2); “Creation and development: 60 years from its birth”, (JEOL Ltd., May 2010) (in Japanese)

[1-1] “Significance of Vacuum Technology in Electron Microscope” (Harada and Yoshimura, 1987) Reviewed paper

522

[1-2] “Ultrahigh-vacuum scanning electron microscope (STEM)” (Tomita, 1990) Reviewed paper

533 533

Comments in Part-4 524

[1-3] “Design and development of an ultrahigh vacuum high-resolution transmission electron microscope” (Kondo et al., 1991) 2 Design of vacuum system 2.1 Ultrahigh vacuum high column construction of the microscope 2.2 Construction of pumping system and its control system

References in the paper Reviewed paper

532 532

525 525 525

1 “Diffusion pump with thick-wall, mild-steel pump vessel, works well with pump fluid, Santovac-5” 533 2 Clean roughing system without sorption trap 534 Acknowledgments 535 Index

537

About the author Profile of the author 1965

1985

1998 2003 2000 1965 2003

Graduated from Osaka Prefecture University, Engineering Division, Electronics Entered the Division for Development and Research Laboratory in JEOL Ltd. Engaging in research and development of vacuum technology in electron microscopes for about 30 years at JEOL Ltd. Take a doctorate in engineering from Osaka Prefecture University Doctoral dissertation: Research and development of the high-vacuum system of electron microscopes (written in Japanese) I took the readership to product the sputter-ion pumps in a JEOL-related company Retired at the age limit from JEOL Ltd. Membership of AVS (American Vacuum Society), Vac. Soc. Japan, and JVIA (Japan Vacuum Industry Association).

Books by Nagamitsu Yoshimura 1. “Vacuum Technology in Micro-Nano Electron-Probe Analytical Instruments” (in Japanese). Nagamitsu Yoshimura, Supervised by Tatsuo Okano, December 19, 2003, NTS Co. Tokyo, Japan. 2. “Vacuum Technology: Practice for Scientific Instruments.” Nagamitsu Yoshimura, January 09, 2007, SpringerVerlag, Berlin, Heidelberg. 3. “Historical Evolution Toward Achieving Ultrahigh Vacuum in JEOL Electron Microscopes.” Nagamitsu Yoshimura, 2014, Springer Briefs in Applied Science and Technology, Springer, Tokyo, Heidelberg, New York, Dordrecht, London. 4. “Review: New Evolution in Ultra-High Vacuum Technology: Make use of molecular-flow networks for system analysis” (in Japanese). Nagamitsu Yoshimura, April 24, 2017, NTS Co. Tokyo, Japan.

xxi

Preface Several our papers on “ultrahigh-vacuum systems of JEOL electron microscopes” are reviewed in this book. One apparent feature of this book is the detail description of “desorption,” “adsorption,” “diffusion (inside the wall),” and “outgassing/pumping.” The title of Part I is “Adsorption, desorption, diffusion, and outgassing/pumping.” And another feature is the detailed description of “molecular-flow networks for analyzing ultrahigh-vacuum systems.” The first reviewed paper (in “Introduction” of Part I) is “Modeling of outgassing or pumping functions of the constituent elements such as chamber walls and high-vacuum pumps” (Yoshimura, 2001). The physical basis of the molecular-flow networks is clearly described in Abstract of the first reviewed paper, as follows: Chamber walls, subjected to “in situ” baking, sometimes show a pumping function for a high vacuum, while a high-vacuum pump sometimes shows outgassing in ultrahigh vacuum. Such functions of the system elements can be represented by using the internal pressure PX. All the system elements, such as chamber walls, pumps, and pin holes through a pipe wall, can be replaced by a pressure generator with the internal pressure PX and the internal flow impedance RX in the equivalent vacuum circuit. The internal pressure PX of the chamber wall varies depending on the wall history under high vacuum.

“Resistor-network-simulation method,” that is “the molecular-flow network technology” has been applied to the ultrahigh-vacuum electron-microscope systems, widely. It is useful to present the relationship among desorption, adsorption, and outgassing/pumping of the chamber wall surfaces. For the chamber wall surfaces, when Qdesorption rate . Qadsorption rate , Qoutgassing rate 5 Qdesorption rate
Qadsorption rate and when Qdesorption rate , Qadsorption rate , Qpumping rate 5 Qadsorption rate
Qdesorption rate : Similarly, for the device called “vacuum pump”, when the ultimate pressure of the pump, Pultimate, is lower than the pressure Pchamber in the chamber to which the pump is attached, that is, Pultimate , Pchamber, for the pump with pumping speed Spump, Qpumping rate 5 Spump 3 ðPchamber
Pultimate Þ; and Pultimate . Pchamber, Qoutgassing rate of pump 5 Spump 3 ðPultimate
Pchamber Þ; And, Spump is usually Cpump opening. Therefore Qpumping rate 5 Cpump opening 3 ðPchamber
Pultimate Þ: When discussing Qoutgassing rate of the chamber wall or Qpumping rate, we must understand individual process, adsorption, desorption, diffusion, outgassing/pumping, and ultimate pressure of the pump.

xxiii

Acknowledgment I greatly appreciate JEOL’s permission to present the book, which contains know-how technology concerning to JEOL electron microscope. Due thanks are given to the co-workers of JEOL Ltd., especially to Mr. Haruo Hirano. It is a nice memory that we all did our best in working on ultrahigh vacuum system technology related to the JEOL electron microscopes.

xxv

A review Ultrahigh-Vacuum Technology for Electron Microscope

Nagamitsu Yoshimura

xxvii

Part 1

Adsorption, desorption, diffusion, and outgassing/pumping Introduction It is well known that the anodic oxide layer of aluminum shows extremely large outgassing rate when placed inside the high-vacuum chamber. And the anodic oxide layer of aluminum is well known to have thick, porous layers, showing large outgassing rate in high vacuum. Porous surfaces work as outgassing sources in high vacuum. And, when such porous surfaces have been degassed at high temperature, they show an effective pumping function under high vacuum.

Reviewed papers [1-1] “Modeling of outgassing or pumping functions of the constituent elements such as chamber walls and high vacuum pumps” (Yoshimura, 2001) Yoshimura presented the paper,[1-1] “Modeling of outgassing or pumping functions of the constituent elements such as chamber walls and high-vacuum pumps.” In this book the paper[1-1] is fully introduced. Abstract Sometimes, chamber walls, subjected to “in situ” baking, show a pumping function for a high vacuum, while a highvacuum pump shows outgassing in an ultrahigh vacuum. Such functions of the system elements can be represented by the internal pressure PX. All the system elements, such as chamber walls, pumps, and pinholes through a pipe wall, can be replaced by a pressure generator with the internal pressure PX and the internal flow impedance RX in the equivalent vacuum circuit. The internal pressure PX of the chamber wall varies depending on the wall history under high vacuum. The equivalent vacuum circuit comprises many characteristic values (PX and RX) and flow impedances R can represent the gas flows in the original high-vacuum system. Keywords: Internal pressure; pressure generator; vacuum circuit; outgassing; pumping

1

Introduction

In an ultrahigh-vacuum system the chamber wall, subjected to “in situ” baking, sometimes shows a pumping effect, and the vacuum pump sometimes shows an outgassing phenomenon. Alpert[1] reported on an interesting ultrahigh-vacuum system, where 2 3 10210 Torr was kept in the “sealed-off” portion for a long period of time after the portion is isolated from the diffusion pump (DP), while the pressure in the portion, connected to the DP, rose gradually and reached a saturated pressure of 1 3 1027 Torr, as shown in Fig. 1. Alpert[1] attributed the pressure rise with time to the gradual deterioration of the pumping function of the ion gauge, IG-B.

A Review: Ultrahigh-Vacuum Technology for Electron Microscopes. DOI: https://doi.org/10.1016/B978-0-12-818573-5.00021-9 © 2020 Elsevier Inc. All rights reserved.

1

2

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

FIG. 1 Pressure versus time (A) in the sealed-off portion of vacuum system and (B) in the ion gauge in contact with the DP DP, Diffusion pump. From Alpert D. J Appl Phys 1953;24(7):860
76.

I.G.B Valve

Manifold

To pumps I.G.A

2 B

10–7 5

Pressure (mm Hg)

2 10–8 5 2 10–9 5 A

2 10–10

1

2

5

10

1 Day

30 Days

20 50 100 200 Time (h)

500 1000

10–5 Al 6063-T6 (new treatment) Bag P( )

10–6

Bag P( )

FIG. 2 Pressure variation at 130 C, 24 h bake. (x) Pressures far from the TMP; (K) pressures near the TMP. QMF, Quadrupole mass filter; TMP, turbo-molecular pump. From Narushima K, Ishimaru H. J Vac Soc Jpn 1982;25(4):172
5.

TMP

QMF

P (Torr)

10–7

10–8 130ºC Bake 10–9 QMF On 10–10 40

60

80

100 120 140 160 180 200 Time (h)

Narushima and Ishimaru[2] reported the “inverted pressure distribution” in an aluminum alloy pipe system, as shown in Fig. 2. That is, after “in situ” baking, the pressure at the position far from the turbo-molecular pump (TMP) became lower than the pressure near the TMP. They attributed the “inverted pressure distribution” to the pumping function of the pipe wall.[2] We shall here represent the “footprint” of pressure in Fig. 1[1] and the “inverted pressure distribution” in Fig. 2[2] by using the characteristic values of the system elements.

Adsorption, desorption, diffusion, and outgassing/pumping Part | 1

2

3

Modeling of outgassing

Yoshimura[3] defined the characteristic values of the outgassing source (the internal pressure PX and the free outgassing rate[4] per unit surface area K0) and developed the theory of the equivalent vacuum circuit composed of the characteristic values of the constituent elements.[3] A high-vacuum system is recognized as a system composed of various walls. The chamber wall prevents molecules in the atmosphere from flowing into the high-vacuum field. Besides this shielding function the chamber wall makes outgassing in high vacuum. This is another function of the wall. A pin hole through the pipe wall causes the leakage of gas, and this resembles outgassing. Outgassing can be represented by a pressure generator with the internal pressure PX and flow impedance RX under high-vacuum pressure P. A high-vacuum pump is recognized as a kind of wall in a high vacuum. The gas compression function of the transfer-type pump is recognized as a wall function, because it shields the gases in the fore-vacuum side. The getter pump has ultimate pressure PU, which varies according to the pump history under vacuum. The pumping (or outgassing) function of a vacuum pump (so-called) is represented by a pressure generator with internal pressure PU and internal flow impedance 1/S (the reciprocal of its pumping speed). The pressure in the vacuum chamber is influenced by various walls, such as the conduit and the pump used. The chamber wall is subjected to the incidence of residual gas molecules, resulting in a functional correlation appearing among many constituent elements. When the internal pressure PX of the chamber wall is higher than the pressure P in the chamber, the chamber wall shows outgassing. However, when PX is lower than P, the chamber wall shows pumping. The vacuum pump generally has a very low internal pressure PU and a small internal flow impedance 1/S, compared with the corresponding respective characteristic values of the chamber wall. Pressures and gas flows in the high-vacuum system can be analyzed by the equivalent vacuum circuit composed of the characteristic values of the system elements.[5,6] There are many gas species in high-vacuum field. Internal pressure PX is the sum of the partial pressures. Therefore pressure and gas flows in a high-vacuum system should preferably be analyzed for the respective gas species.

3

Pressure analysis using equivalent vacuum circuit

The pressure distributions shown in Figs. 1 and 2 can be described by the internal pressures of the system elements, which vary according to the history of the elements under vacuum. Consider the “sealed-off” portion isolated from the DP in Fig. 1.[1] The manifold wall and the pipe wall were made of the same kind of glass and were both treated by in situ baking at 420 C, and so the internal pressures of the respective walls would be likewise 2 3 10210 Torr after in situ baking. The internal pressure PIG of the ion gauge scarcely varied with time in the “sealed-off” portion, which might be a little lower than 2 3 10210 Torr. The equivalent vacuum circuit for the “sealed-off” portion is presented in Fig. 3A. Next, consider the portion connected to the DP. At an elapsed time after in situ baking (420 C), the internal pressure PX of the pipe wall must be as low as 2 3 10210 Torr. However, residual gas molecules, mainly coming from the DP, were adsorbed onto the pipe wall, and the PX of the pipe wall gradually rose to a saturated pressure of 1 3 1027 Torr, that is, the ultimate pressure PU of the DP used. The equivalent vacuum circuit corresponding to the portion connected to the DP, under the saturated condition, is presented in Fig. 3B, where the internal pressure PX of the pipe and IG is the same as the PU of the DP. There is not a gas flow in part B, which is recognized merely as a vacuum box of pressure PU of 1 3 1027 Torr. Now, consider the “inverted pressure distribution” of Fig. 2.[2] Assume that the pumping function of the Bayard
Alpert gauge was negligibly small. According to the proposed model of outgassing, the internal pressure of the pipe wall, during “in situ” baking, was higher than the ultimate pressure (the same as the internal pressure) of the TMP, and so the pressure at the position far from the TMP was higher than the pressure near the TMP. On the other hand, after “in situ” baking, the internal pressure in the pipe become lower than the ultimate pressure of the TMP, resulting in the pressure far from the TMP lower than that near the TMP. The equivalent vacuum circuit corresponding to the system of Fig. 2 is presented in Fig. 4.

4

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

FIG. 3 Equivalent vacuum circuits corresponding to the systems of Fig. 1 at the saturated condition. In(A) R 5 1/F, where F is the flow conductance of the manifold, and R0 5 1/F0 , where F0 is the flow conductance of the pipe. RIG 5 1/ SIG, where SIG is the pumping speed of IG-A. In(B) PU and 1/S correspond to the characteristic values PX and RX of the pump, respectively.

(A) R/2

R ′/2

R/2

RX

R ′/2

R ′X

1/SIG

PX

PIG

PX

(B)

R ′X

1/SIG

PU

R/2

PU

PU

R/2

RX

PX

4

1/S

R ′/2

FIG. 4 Vacuum circuit corresponding to the system of Fig. 2. (A) PX ; P0X . PU , during “in situ” baking. (B) PX , P0X , PU , variable with time, after in situ baking.

R ′/2

R ′X

P ′X

1/S

PU

Conclusion

Functions of all the constituent elements in a high-vacuum system as an outgassing source can be represented by a pressure generator with an internal pressure PX and flow impedance RX. A high-vacuum system is defined as a system with more than two different walls with different internal pressures PX from each other. Pressures and gas flows in the highvacuum system can be analyzed by the equivalent vacuum circuit.[5,6] Lewin[7] discussed on the theme, “how to characterize the vacuum environment,” where he claimed what is visualized is the particle flux F. He proposed that the gas density n and the flux F should be employed instead of the pressure P and the throughput Q. On the contrary, the present author claims that the pressures including the internal one PX of elements could be regarded as kinds of potential governing net gas flows in the entire system under high vacuum.[3] This paper demonstrates that the pressures characterize the vacuum environment.

References in the paper[1-1] [1] [2] [3] [4] [5] [6] [7]

Alpert D. J Appl Phys 1953;24(7):860
76. Narushima K, Ishimaru H. J Vac Soc Jpn 1982;25(4):172
5. Yoshimura N. J Vac Sci Technol A 1985;3(6):2177
83. Dayton BB. Proceeding of the AVS 6th. Vacuum symposium transactions; 1959. p. 101. Ohta S, Yoshimura N, Hirano H. J Vac Sci Technol A 1983;1(1):84
9. Hirano H, Kondo Y, Yoshimura N. J Vac Sci Technol A 1988;6(5):2865
9. Lewin G. Vacuum 1990;41(7
9):2048
9.

Reviewed paper [1-1] Yoshimura N. Modeling of outgassing or pumping functions of the constituent elements such as chamber walls and high-vacuum pumps. Appl Surf Sci 2001;196/170:685
8.

Adsorption, desorption, diffusion, and outgassing/pumping Part | 1

5

[1-2] “Testing performance of diffusion pumps” (Hablanian and Steinherz, 1962) Hablanian and Steinherz (1962) presented the paper,[1-2] “Testing performance of diffusion pumps.” This paper comprises the following sections: 1. 2. 3. 4. 5. 6. 7.

Introduction Ultimate vacuum Fore-pressure tolerance Back-streaming Throughput Test consistency Conclusions

In this book Fig. 1 in Section 2. Ultimate vacuum is introduced. Abstract Measurement techniques and experimental results are presented for the major performance criteria of oil diffusion pumps. Special emphasis is placed on new developments including the new diffusion pump oils and zeolite traps. Statistical variations observed in testing groups of pumps of the same design are presented. Comments by Yoshimura: Fig. 1 presents that porous zeolite balls, when degassed sufficiently, work as an ultrahigh-vacuum pump. However, zeolite balls work as gas sources in the extreme high-vacuum chamber evacuated by the well-designed sputter-ion pump.

Gauge

FIG. 1 Diffusion pump (using DC704 oil) with zeolite trap and test dome.

Oven

Zeolite trap

Water cooled baffle

4″ Pump

Reviewed paper [1-2] Hablanian MH, Steinherz HA. Testing performance of diffusion pumps. In: Trans. 8th vac. symp. and 2nd inter. congr. 1962. p. 333.

Chapter 1

Microstructure and elemental features of stainless-steel surface Reviewed papers [1-1] “Outgassing characteristics and microstructure of an electropolished stainless steel surface” (Yoshimura, Sato, Adachi, and Kanazawa, 1990) Yoshimura et al. presented the paper,[1-1] “Outgassing characteristics and microstructure of an electro-polished stainless-steel surface.” The paper comprises the following sections: 1. 2. 3. 4.

Introduction Outgassing characteristics of an electro-polished (EP) pipe wall Microstructure and elemental features of stainless-steel surfaces Conclusions

In this book, Sections 2, Outgassing characteristics of an EP pipe wall, 3, Microstructure and elemental features of stainless-steel surfaces, and 4, Conclusions, are introduced. Abstract Outgassing characteristics of an EP stainless-steel (SS) (SS304) pipe wall were investigated by an isolation method. The free outgassing rate after an in situ bakeout (150
C, 20 h) was estimated as low as 1.6 3 10212 Pa L (s cm2)1. After the in situ bakeout, H2 molecules were steadily evolved from the pipe wall, whereas most of CO, C, CH4, and CO2 molecules were emitted from the operating mass spectrometer (MS) and BayardAlpert gauge with incandescent filaments. Auger depth-profile analysis revealed that the oxide layer of an EP surface was cleaner, thinner, and finer in microstructure than that of a buff-polished surface. This is the reason why an EP surface showed a very low outgassing rate.

2

Outgassing characteristics of an electropolished pipe wall

Outgassing rates of SS304 plates (2 mm thickness), finished with different surface polishing treatments (belt-polishing, buff-polishing, and electro-polishing), were recently measured using throughput methods by Yoshimura et al.[10,11] Their outgassing rates K as a function of pumping time are presented in Fig. 1, which were already reported elsewhere.[11] The outgassing rate of an EP plate [0.7 S (μmÞof surface finish] was much lower than the rates of mechanically polished plates. The outgassing rate after an in situ bakeout (100
C, 5 h) was as low as the detectable limit. This limit was caused by the outgassing from a turbo-molecular pump (TMP) that had not been baked at high temperature. The outgassing from a BAG might influence the outgassing data for EP plates.

A Review: Ultrahigh-Vacuum Technology for Electron Microscopes. DOI: https://doi.org/10.1016/B978-0-12-818573-5.00001-3 © 2020 Elsevier Inc. All rights reserved.

7

8

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

–6

Log K, Pa L (s cm2)–1

–7

Bake

–8

–9

–10

0

10

20 30 Time, h

40

50

FIG. 1 Outgassing rates of the belt-polished (2K 2 ), buff-polished (2x 2 ), and EP (2" 2 ) plates. The rate of the EP plate was measured by the orifice method,[11] and the rate of other plates was measured by the two-point-pressure method.[10] Note: The bakeout temperatures for the EP plate and for other plates were 100
C and 200
C, respectively. EP, Electropolished.

Outgassing characteristics of an EP pipe wall were measured by an isolation method: The direct effect of the outgassing from a TMP was naturally eliminated in an isolation period. The experimental setup is given in Fig. 2. The innerwall of the pipe (820 cm2 inner surface area, 1 L vol.) had been EP to 0.7 S by Taiyo Sanso Ltd. The pipe had already been degassed a few times (100
C, 5 h of unit baking time) in a previous experiment.[11]

MS

BAG

FIG. 2 Experimental setup for the isolation test. The inner wall of the pipe was electropolished (0.7 S).

A After an in situ bakeout The pipe and quadrupole MS were in situ baked (150
C, 20 h), while the metal valve was baked at about 100
C. The MS with a W filament was first operated at 5 mA of emission current for 1 h, and then operated at 0.5 mA continuously. The BAG (glass tube type, W filament) was degassed by an electron bombardment (30 mA/700 V, 20 min) and then operated at 1 mA. Mass numbers of residual gases analyzed at 2.6 3 1027 Pa of the base pressure after approximately 50 h pumping following baking were 2 (H2), 18 (H2O), 28 (CO and N2), 44 (CO2), and 16 (CH4), in order of concentration. First, the metal valve was closed at 2:6 3 1027 Pa, thus the outgassing molecules being built-up inside the isolated pipe. The mass spectrum at 2.6 3 1025 Pa in the isolation period is presented in Fig. 3. This mass spectrum pattern was quite different from that analyzed under evacuation. Most of the H2O molecules detected under evacuation must be evolved from the TMP (blades and body walls), which had not been degassed at high temperature. Most of the CO (28), CO2 (44), C (12), and CH4 (16) molecules might be emitted from the operating MS (0.5 mA) and BAG (1 mA) through chemical changes by incandescent filament[12] and electron-stimulated desorption (ESD).[13]

Microstructure and elemental features of stainless-steel surface Chapter | 1

9

FIG. 3 Mass spectrum analyzed at 2:6 3 1025 Pa in the isolated pipe. The valve was closed at 2:6 3 1027 Pa.

–11 28

Log I, A

–12

–13

12

2

16 14

27 29 44

18 –14

Mass

Next, partial pressure-rise characteristics for individual masses were measured. The BAG was switched off at 6 3 1027 Pa just before isolation and again switched on 3 min after isolation. Building-up characteristics of masses 28 and 12 are presented in Fig. 4A, and those of masses 16, 2, 18, and 44 are presented in Fig. 4B. The characteristics of masses 28 and 12 in the earlier 3 min after isolation are again presented in Fig. 4B by chain lines for comparison.

(A)

(B)

30

FIG. 4 Building-up characteristics of individual masses after the in situ bakeout (150
C, 20 h). (A) Masses 28 (mostly CO) and 12 (C); and (B) masses 16 (CH4), 2 (H2), 18 (H2O), and 44 (CO2). The BAG was switched on 3 min after isolation.

15 2

28 16

16

20

10

5

2 44

44

28

1

2 3 Time, min

4

5

18

18 16

12

44 16

(28)

12

0 0

2

2

2

I, 10–14 A

I, 10–13 A

10

0

0

(12)

1

44

2 3 Time, min

4

5

Comments on individual masses are as follows: 2 (H2): The peak rose steadily with time and jumped up appreciably when the BAG was switched on. Then, it was soon lowered in a short time and again rose slowly. Most of the H2 molecules must be evolved from the EP pipe-wall. 12(C): The peak amplitude kept constant after isolation. The peak jumped up greatly when the BAG was switched on, and then it rose steadily with further time. The C molecules must be produced due to dissociation of CO, CO2, and CH4 in the MS and BAG with an incandescent filament. 16(CH4): The peak jumped up from 2:0 3 10215 to 1:1 3 10213 A when the metal valve was closed. It rose slowly until the BAG was switched on, and then it was lowered at a considerably high rate. CH4 molecules may be partly dissociated into C and H2 molecules in contact with an incandescent filament. 18 (H2O): The peak rose very slowly. The variation of the peak amplitude occurring when the BAG was switched on was very small. The peak amplitude was kept almost constant after that. The outgassing rate was negligibly low.

10

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

28 (mostly CO): The peak jumped up from 7:0 3 10215 to 2:5 3 10214 A when the metal valve was closed, and then it rose steadily with tome. It jumped up from 2:5 3 10214 to 1:5 3 10212 A when the BAG was switched on. The gas species must be mostly CO, which is known to be emitted through ESD.[13] Most of the CO molecules must be emitted from the operating MS and BAG. 44 (mostly CO2): The peak jumped up from 5:0 3 10215 to 3:5 3 10214 A when the metal valve was closed, and then it rose slowly. The peak jumped up to its double amplitude when the BAG was switched on, and then its amplitude was kept almost constant. Furthermore, pressure-rise characteristics were measured with the MS kept on (0.5 mA) and off, which are presented in Fig. 5A.

(A)

(B)

6

5 Pressure, 10–6 Pa

Pressure, 10–6 Pa

5

6

4 3 2

4 3 Px 2

1

1

0

0

ΔP/Δt

0

10

20 30 40 Time, min

50

0

10

20 30 40 Time, min

50

60

FIG. 5 (A) Pressure-rise curves measured with the MS kept on (2K 2 , 0.5 mA) and off (2x 2 ), (B) natural exponential curve for estimating the characteristic values Px and K0, which was drawn from a lower pressure part of the curve (2x 2 ) of (A).

The pressure rose rather slowly while the MS was off; it rose rapidly while the MS was on. The outgassing rate of the operating MS was much higher than that of the whole pipe-wall after several days’ pumping following the in situ bakeout. Conventionally, the outgassing characteristics have been represented by the net outgassing rate K per unit surface area measured under a high vacuum, though it does not give any information about the pressure dependence of outgassing. Recently, the characteristic values of a gas source (Px and K0) have been introduced,[14]
by which the net outgassing rate K under a pressure of high vacuum P could be expressed as K 5 K0 1 2 P=Px . A vacuum circuit for representing pressures and net gas flows in a high vacuum system comprises the characteristic values of the system elements.[10,1416] We have estimated the characteristic values Px and K0 of the EP pipe wall after the in situ bakeout (150
C, 20 h) using the curve (2x 2 ) of Fig. 5A. The curve is fairly distorted from a natural exponential curve given by a broken line in Fig. 5A. The characteristics values should be estimated using a lower pressure part of the curve because a constant sorption probability is assumed[10,1416] for the gas molecules impinging upon the surface. A natural exponential curve, extended to zero pressure, is presented in Fig. 5B. The value Px is easily estimated as 2:4 3 1026 Pa from the curve of Fig. 5B. The value K0, called the free outgassing rate by Dayton,[17] is estimated from the gradient of 1:3 3 1029 Pa s21 at zero pressure as K0 5 1:3 3 1029 ðPa s21 Þ 3 1ðLÞ=820ðcm2 Þ 5 1:6 3 10212 ðPa L ðs cm2 Þ21 Þ: It should be noted that the curve (2x 2 ) of Fig. 5A was still affected by an additional outgassing of the operating BAG (1 mA). It can be said that 2:4 3 1026 Pa of Px and 1:6 3 10212 Pa L (s cm2)21 of K0 are indeed low and that most of the built-up molecules of the spectrum of Fig. 3 must be emitted from the operating MS (0.5 mA) and BAG (1 mA) with incandescent filaments.

Microstructure and elemental features of stainless-steel surface Chapter | 1

3

11

Microstructure and elemental features of stainless-steel surfaces

As Bhasavanich et al.[18] have reported, SSs with different prevacuum surface conditions have quite different surface microstructure. A SEM micrograph in Fig. 6 shows the typical microstructure of an as-received SS304 surface with deep grain boundaries that function as gas reservoirs such as surface fissures. Such surfaces are believed to be covered with porous oxide layers.[14] Gas molecules trapped in deep grain boundaries and porous oxide layers will be evolved into the vacuum due to diffusion. FIG. 6 SEM micrograph of the as-received surface.

Three kinds of SS304 plates (2 mm thickness) were provided, belt-polished (5.0 S), buff-polished (2.5 S), and EP (0.7 S) plates. The belt-polished and buff-polished plates had been finished and cleaned by Nippon Steel Co. Ltd., and some of the buff-polished plates had been additionally EP and cleaned by Taiyo Sanso Co. Ltd. Their outgassing rates are presented in Fig. 1. Microstructure and elemental features of their surfaces were investigated with a SEM and an AES, respectively. The sample pieces had been held in the moist atmosphere of our laboratory for more than 6 months before their surfaces were observed and analyzed.

A Microstructure Three kinds of sample surfaces were observed by a scanning electron microscopy (SEM) and backscattering electron microscopy (BEM) modes using a scanning electron microscope (JSM-T330, JEOL Ltd.). The belt-polished surface had many fissures whose sizes and structures varied depending on the area selected. SEM micrographs of high magnification revealed that some of the fissures contained dust, in which a large number of gas molecules must be trapped. SEM micrographs of the selected areas of the belt-polished surface are presented in Fig. 7A and B.

FIG. 7 SEM micrographs of the selected area of the belt-polished surface: (A) Area with wide fissures and (B) fissure containing dust.

12

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

The buff-polished surface also has many fissures along the polishing direction, whose sizes are smaller than those of the belt-polished surface. SEM micrographs of high magnification also revealed some fissures containing dust. Such micrographs are presented in Fig. 8A and B.

FIG. 8 SEM micrographs of the buff-polished surface: (A) typical area and (B) fissure containing dust.

The microstructure of the EP surface was almost independent of the area selected. SEM and BEM micrographs of the EP surface are presented in Fig. 9A and B, respectively. There were no fissures on the surface. The BEM micrograph clearly shows the difference among backscattering electron yields of individual crystal faces, which means the EP surface was extremely flat, smooth, and clean.

FIG. 9 (A) SEM micrograph and (B) BEM micrograph of the EP surface. BEM, Backscattering electron microscopy; EP, electropolished.

B

Elemental features

SEM micrographs of the belt-polished and buff-polished surfaces showed that both surfaces were almost the same in microstructure in essence, though the sizes of the fissures were different from each other. AES analysis was conducted for the as-received, buff-polished, and EP surfaces to clarify their elemental features, by concentrating the interest on the O and C intensities. AES spectra on the outer surfaces and AES depth profiles for sample surfaces were analyzed using an Auger electron spectrometer (JAMP-30, JEOL Ltd.). An AES spectrum and AES depth profile of the as-received surface are presented in Fig. 10A and B, respectively. Those of the buff-polished and EP surfaces are presented in Figs. 11 and 12, respectively.

Microstructure and elemental features of stainless-steel surface Chapter | 1

(A)

FIG. 10 (A) AES spectrum and (B) AES depth profile of the as-received surface. Notes: Sputtering conditions were as follows: sputter ion; Ar1, energy; 3 keV, normal incidence. The sputtering rate was unknown for both sample surfaces, though it was known as 10 nm min21 for SiO2.

(B)

O Ca

Ni

Intensity

Cr

Intensity

Cl S

Fe

13

Fe C

O

Cr

C 0

200

Cr

S Ni 400 600 Kinetic energy, mV

800

0

1000

(A)

S

C

1 2 Sputtering time, min

FIG. 11 (A) AES spectrum and (B) AES depth profile of the buff-polished surface. Notes: Sputtering conditions were as follows: sputter ion; Ar1, energy; 3 keV, normal incidence.

(B)

S

O

Ni

Intensity

Intensity

Cr

Fe

Fe C

Cr

O Ni

C 0

200

400 600 Kinetic energy, mV

800

1000

(A)

Ni

Cr O

S

C 1 2 Sputtering time, min

0

FIG. 12 (A) AES spectrum and (B) AES depth profile of the EP surface. Notes: Sputtering conditions were as follows: sputter ion; Ar1, energy; 3 keV, normal incidence. EP, Electropolished.

(B)

S

Intensity

Intensity

O

Ni Cr

Fe

Fe

C C C S 0

200

O 400 600 Kinetic energy, mV

800

1000

0

Cr Ni 1 2 Sputtering time, min

14

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

By comparing the spectra of Figs. 10 and 11, one can recognize that the as-received and buff-polished surfaces were almost the same in elemental features. Elemental features of the EP surface, in comparison with the buff-polished surface, are as follows: 1. The O intensity on the outer surface was much higher than that on the buff-polished surface, which means that the oxide layer of the EP surface was finer in microstructure than that of the buff-polished surface. The O intensity was reduced much more rapidly to a negligible level with sputtering, in comparison with that of the buff-polished surface, which means that the oxide layer of the EP surface was much thinner than that of the buffpolished surface. 2. The C intensity on the outer surface was much lower than that on the buff-polished surface; it rapidly reduced to zero with sputtering. The EP surface was cleaner than the buff-polished surface, in respect of the C-based contamination. 3. The Cr and S intensities on the outer surface were both higher than those on the buff-polished surface, respectively.

4

Conclusion

Outgassing rates of an EP pipe wall were estimated by an isolation method, by minimizing the effect of the outgassing from the quadrupole MS and BAG. After an in situ bakeout (150
C, 20 h), the free outgassing rate K0 was very low as 1.6 3 10212 Pa L (s cm2)21, and the outgassing species was mostly H2. Most of the CO, C, CH4, and CO2 molecules analyzed as the built-up molecules must be emitted from the operating MS and BAG with incandescent filaments. Microstructure and elemental features of an EP surface were investigated with SEM and AES, respectively, in comparison with those of an as received, belt-polished, and buff-polished surfaces. No fissures were observed on the EP surface by SEM. The oxide layer of the EP surface was cleaner and thinner, and finer in microstructure than those of mechanically polished surfaces. This is the reason why the EP surface showed an extremely low outgassing rate.

References in the reviewed paper[1-1] [1] [2] [3] [4] [10] [11] [12] [13] [14] [15] [16] [17] [18]

Schram A. AVS ninth vacuum symposium transactions; 1962. p. 301. Dayton BB. AVS eighth vacuum symposium transactions; 1961. p. 42. Rogers KW. AVS 10th vacuum symposium transactions; 1963. p. 84. Achard MH, Calder R, Mathewson A. Vacuum 1979;29:53. Yoshimura N, Hirano H. J Vac Sci Technol A 1989;7:3351. Yoshimura N, Hirano H, Kanazawa T. Proceedings of seventh meeting of UHV techniques for accelerators and storage rings. KEK, March 2728, 1989. Redhead PA. AVS seventh vacuum symposium transactions; 1960. p. 108. Redhead PA, Hobson JP, Kornelson EV. The physical basis of ultrahigh vacuum (electron-impact desorption). London: Chapman and Hall Ltd.; 1968. p. 290. Yoshimura N. J Vac Sci Technol A 1985;3(6):2177. Ohta S, Yoshimura N, Hirano H. J Vac Sci Technol A 1983;3(6):84. Hirano H, Kondo Y, Yoshimura N. J Vac Sci Technol A 1988;6(5):2865. Dayton BB. AVS sixth vacuum symposium transactions; 1959. p. 101. Bhasavanich D, Williams EM. Vacuum 1980;30:91.

Reviewed paper [1-1] Yoshimura N, Sato T, Adachi S, Kanazawa T. Outgassing characteristics and microstructure of an electropolished stainless steel surface. J Vac Sci Technol A 1990;8(2):9249.

Microstructure and elemental features of stainless-steel surface Chapter | 1

15

[1-2] “Outgassing characteristics and microstructure of a “vacuum fired” (1050
C) stainless steel surface” (Yoshimura, Hirano, Sato, Ando, and Adachi, 1991) Yoshimura et al. (1991) presented the paper,[1-2] “Outgassing characteristics and microstructure of a “vacuum fired” (1050
C) stainless steel surface.” The paper comprises the following sections 1. 2. 3. 4.

Introduction Microstructure and elemental features Vacuum characteristics Conclusion.

In this book, Sections 2, Microstructure and elemental features, 3, Vacuum characteristics, and 4, Conclusions, are introduced. Abstract “Vacuum fired” (1050
C) SS304 surfaces were investigated with scanning electron microscopy and Auger electron spectroscopy. The grain boundaries of the surface were vague and shallow, which occurred due to elemental diffusion at high temperature in vacuum. The newly formed oxide layer of the vacuum fired surface was much thinner and could be said to be finer in microstructure than the native layer of an “as-received” surface. A vacuum fired (1050
C) SS304 chamber was evacuated by a sputter ion pump (SIP) whose vessel was pretreated by vacuum firing (1050
C). An extremely high vacuum (XHV) of 1.5 3 1029 Pa (N2 equiv pressure) was indicated by an extractor ionization gauge after a mild bake (170
C) following an air exposure. The outgassing rate of the chamber wall at the elapsed time of 1 day after a mild bake (170
C) was roughly estimated as low as 2 3 10211 Pa L (s cm2)21 (N2 equiv value) by an orifice method. Vacuum firing has the effects of degassing the gas molecules which are dissolved in the interior of SS wall, and of forming a new, thin, and fine (in microstructure) oxide layer on the surface.

2

Microstructure and elemental features

Several pieces of SS304 disk plates (as received, 10 mm diam, 2 mm thickness) were vacuum fired (1050
C, 30 min) in a vacuum furnace. The pressure during the vacuum firing process was presumably around 1021 Pa. The as-received surface showed a dull color, which must be covered with a thick oxide layer. Vacuum fired surface, on the other hand, was brightened, showing a silver color. Microstructure and elemental features of the vacuum fired surface were investigated with scanning electron microscopy (SEM) and Auger electron spectroscopy (AES), in comparison with those of the as-received surfaces. The vacuum fired sample plates had been held in the moist atmosphere of our laboratory for more than 1 year before their surfaces were observed and analyzed.

A Microstructure The SEM micrographs of the as-received and vacuum fired surfaces are presented in Fig. 1A and B, respectively. The as-received surface (Fig. 1A) had deep grain boundaries with sharp edges, in which contaminants could be entrapped. The surface was believed to be covered with thick, porous oxide layers, as Dayton discussed.[2] On the other hand, the SEM micrograph of the vacuum fired surface (Fig. 1B) is fairly different from the micrograph of (Fig. 1A). The grain boundaries of the vacuum fired surface were vague and shallow, which might be the result of elemental diffusion at high temperature.[22,25] The surface is apparently cleaned and slightly roughened due to reduction and elemental evaporation at high temperature in vacuum.[22]

16

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

FIG. 1 SEM micrographs: (A) as-received surface and (B) vacuum-fired surface.

B

Elemental features

The AES depth profiles of the as-received and vacuum fired surfaces are presented in Fig. 2A and B, respectively. The sputtering rates for the sample surfaces were unknown, though the sputtering parameters for both surfaces are presented in the figure caption. The depth profiles of the vacuum fired surface (Fig. 2B) is fairly different from that of the asreceived surface (Fig. 2A), that is, the intensities of “O” and “Cr” on the surface of the vacuum fired sample are much higher, and those of “O” and “C” in the interior of the vacuum fired sample are much lower, in comparison with the respective intensities of the as-received surface. The solute carbon in the interior was considered to be diffused out and exhausted in reduction in the holding period (30 min) at 1050
C in vacuum. The oxide layer (maybe Cr2O3) of the vacuum fired surface was considered to be newly formed when the vacuum fired surface was again exposed to the atmosphere. The oxide layer of the vacuum fired surface was thinner and could be said to be finer in microstructure than the oxide layer observed on the as-received surface. It is also noted that the intensity of “S” on the vacuum fired surface (Fig. 2B) is much higher, in comparison with that on the as-received surface (Fig. 2A).

(A)

(B)

FIG. 2 AES depth profiles: (A) as-received surface and (B) vacuum fired surface. Sputtering conditions were as follows: sputter ion; Ar1, energy; 3 keV, normal incidence. The sputtering rate was unknown for both sample surfaces, though it was known as 10 nm min21 for SiO2.

3000 O

3000 Fe Fe

Raw intensity

Raw intensity

C

O

2000

Cr

1000

2000 C

Cr

1000 S

C

Ni

C

Ni

S

0

0

10 20 Sputtering time, min

30

0

0

10 20 Sputtering time, min

30

Very recently, the present authors reported microstructure and elemental features of an EP SS304 surface and described that the oxide layer of the EP surface was thinner and finer in microstructure than those of mechanically polished surfaces.[14] The AES depth profile of the vacuum fired surface of Fig. 2B did indeed resemble the profile of the EP surface, especially as to Cr and O intensities.[14]

3

Vacuum characteristics

A vacuum fired (1050
C) SS304 system was manufactured using the procedure described next. Vacuum brazing (Ni-based brazing alloy, 1050
C) was applied to the chamber joints with an exception of Conflat flange joints which were attached by argon-arc welding. Consequently, most of the chamber walls were vacuum fired (1050
C, 30 min) in a vacuum furnace. The density of H2 molecules dissolved in the interior of SS304 walls of 4 mm thickness should be much reduced[3,16] during the vacuum firing period.

Microstructure and elemental features of stainless-steel surface Chapter | 1

17

A SIP with Ti-sublimation pump filaments (conventional ones) was newly designed and manufactured for XHV pumping. The nominal pumping speed of the SIP was 75 L s21 for N2, and the N2 pumping speed of the Ti-sublimation pump (just after fresh getter films were deposited) was estimated 400 L s21. The (d 3 BÞ product value (d, anode cylinder diameter; B, magnetic flux) of the Penning discharge cells was designed as high as 3.1 kG cm on the average for effective pumping in the ultrahigh vacuum range. This value (3.1 kG cm) of ðd 3 BÞ product is much higher than those of commercial SIPs. As Rutherford discussed,[26] a high value of ðd 3 BÞ product of the Penning cells is very effective for XHV. Indeed, the SIP has a considerably high measured pumping speed in the 10261027 Pa range. For instance, the pumping speeds for N2 at 1026 and 1027 Pa were measured to be 70 and 45 L s21, respectively. The chamber was equipped with a conventional quadrupole MS (tungsten filament) in addition to a BayardAlpert ionization gauge (BAG, tungsten filament, nude type) and an extractor ionization gauge (EXG, iridium filament coated with thorium oxide, nude type). Eleven Conflat-type flanges (mostly 70 mm diam, blind type) were additionally equipped to the chamber. The SIP was connected to the chamber directly, and a conventional TMP was connected through a conventional all-metal valve. The volume and surface area of the chamber (including those of the SIP vessel and MS head) were approximately 8 L and 4000 cm2, respectively. Pressures of 1028 Pa range were indicated by the BAG and EXG in the chamber which had been in situ baked (about 300
C1 day and 200
C—1 day). The residual gas spectrum is typical, in which the H2 ion intensity was the highest. And it was found that the major source was the MS head itself with an incandescent tungsten filament. It was also found that the BAG indicated a pressure (N2 equiv) higher than the pressure indicated by the EG. The pressure indication by the BAG was not lower than 1028 Pa even when the EG indicated a low 1029 Pa. The pressure indication by the EXG is believed to be more reliable, as Beeck and Reich[27] discussed. The pumping characteristics were measured by the EG in a long pumping time after the chamber (including the SIP vessel) was exposed to the moist atmosphere for 15 min. The pressure (N2 equiv) in the chamber with the MS being off was approximately 2 3 1029 Pa just before the air exposure, 10 days after the in situ bakeout (200
C and 300
C). After 2 days’ evacuation by the TMP following the air exposure, the pressure indicated by the EG was 9 3 1027 Pa. Then, the chamber (including the SIP vessel and MS head) was in situ baked at 80
C for 20 h. After 60 h evacuation by the SIP with the metal valve closed, the pressure was 5:4 3 1028 Pa. And, the chamber (including the pump vessel and MS head) was further in situ baked at 170
C for 1 day. The EG was degassed with electron bombardment (19 W, 3 min), and three Ti filaments were degassed (40 A) for 4 min each. The Ti getter was flashed (50 A, 90 s each) three times in the 1029 range. The pressure just before flashing Ti getter was about 2:1 3 1029 Pa. On the next day an XHV of 1:5 3 1029 Pa was indicated by the EG with the MS and BAG being off, as an ultimate pressure. The pressure would not significantly change with elapsed pumping days, though the pressure varied a little around 1:5 3 1029 Pa with the variation of the room temperature. The dominant residual gas molecules must be H2. It should be noted that the indicated pressure of 1:5 3 1029 Pa was a N2 equiv pressure. In order to measure the outgassing rate of the chamber wall, an oxygen-free copper disk plate (121 mm diam) with an aperture of 3.3 mm diam was used as a gasket for the inlet flange of the SIP. The pumping conductance of the orifice of 3.3 mm diam was calculated as 1.0 L s21 for N2 and 3.7 L s21 for H2 at the room temperature. The MS head and BAG were removed, and blind Conflat flange (70 mm diam) was installed. Thus the surface area of the chamber walls, which separated the orifice plate, was reduced to 2500 cm2. The chamber with the SIP (kept off) was evacuated by the TMP through the metal valve following reassembling the chamber system. After 17 h evacuation, the pressure indicated by the EG was 1:6 3 1026 Pa. Then, the chamber and the SIP vessel were in situ baked at 170
C for 1 day. The SIP was switched on soon after the baking heater was switched off. In an interval of 1 h after cessation of bakeout, the following treatments were sequentially conducted: three Ti filaments of the Ti sublimation pump were sequentially degassed (40 A, 3 min each), and the Ti getter was flashed (50 A, 90 s each) three times, in sequence. Then, the EG was degassed with electron bombardment (19 W, 3 min). The metal valve was closed at about 4:0 3 1026 Pa, 1 h after cessation of bakeout. The pressure jumped up to about 5:0 3 1025 Pa when the valve was closed, and it went down smoothly into the 1028 Pa range. The chamber walls were still hot when the metal valve was closed. The pump-down characteristics, after the metal valve closed, were measured by the EG. The pressure indicated on the next day was about 5:0 3 1028 Pa. We can estimate the outgassing rate of the chamber wall after an in situ bakeout (170
C, 1 day) as a N2 equiv value. The outgassing rate as a N2 equiv value, as a function of the elapsed time t after the metal valve closed, was roughly estimated from the pressure P(t) (N2 equiv) as K (N2 equiv) 5 P(t) (N2 equiv) (Pa) 3 1(L s21)/2500 (cm2), by neglecting the outgassing of the copper disk plate (121 mm diam).

18

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

The characteristics of the outgassing rate K (N2 equiv) thus calculated are presented in Fig. 3. The outgassing rate K (N2 equiv) at the elapsed time of 15 h was estimated as 2:2 3 10211 Pa L (s cm2)21, by neglecting the outgassing of the copper disk plate (121 mm diam). FIG. 3 Outgassing rate K (N2 equiv) of the vacuum-fired SS304 chamber wall, as a function of the elapsed time after the metal valve closed. The rate was calculated from the pressure in the chamber evacuated through an orifice (3.3 mm diam 1 L s21 for N2). The pressure was measured by the EG as N2 equiv pressure.

K (N2 equiv), Pa L (s cm2)–1

10–7

10–8

10–9

10–10

10–11 1

10 102 Time, min

103

It was natural to assume that the species of residual gas molecules was mostly H2. The outgassing rate for the actual gas species must be considerably higher than the calculated outgassing rate as a N2 equiv value.

4

Conclusion

Vacuum firing (1050
C) is a useful treatment for reducing the thick oxide layers of mechanically polished, or asreceived SS surfaces. Apparently, a new, thin, and fine (in microstructure) oxide layer of enriched in Cr2O3 forms on the surface. Vacuum firing has a beneficial degassing effect for SS304, as indicated by very low net outgassing rates after removal of absorbed H2O vapor. Vacuum firing (1050
C) SS surface was slightly roughened due to elemental evaporation. However, such roughened surfaces did not show any adverse effect on the net outgassing and pumping characteristics. Note added in proof: A report describing microstructure and elemental features of vacuum fired SS surfaces has been published elsewhere,[28] which is written in Japanese.

References in the reviewed paper[1-2] [2] [3] [14] [16] [22] [25] [26] [27] [28]

Dayton BB, AVS eighth vacuum symposium transactions; 1961. p. 42. Calder R, Lewin G. Br J Appl Phys 1967;18:1459. Yoshimura N, Sato T, Adachi S, Kanazawa T. J Vac Sci Technol A 1990;8(2):924. Ando I, Yoshimura N. J Vac Soc Jpn 1990;33:185. Ishigami I, Tsunasawa E, Yamanaka K. J Jpn Inst Metals 1979;43:392. Achard MH, Calder R, Mathewson A. Vacuum 1979;29:53. Rutherford SL. Transactions of the tenth American vacuum society symposium; 1963. p. 185. Beeck U, Reich G. J Vac Sci Technol 1972;9:126. Yoshimura N, Ando I, Sato T, Adachi S. J Vac Soc Jpn 1990;33:525.

Reviewed paper [1-2] Yoshimura N, Hirano H, Sato T, Ando I, Adachi S. Outgassing characteristics and microstructure of a “vacuum fired” (1050
C) stainless steel surface. J Vac Sci Technol A 1991;9(4):232630.

Microstructure and elemental features of stainless-steel surface Chapter | 1

19

[1-3] “Outgassing characteristics of electropolished stainless steel” (Tohyama, Yamada, Hirohata, Yamashina, 1990) Tohyama et al. (1990) presented the paper,[1-3] “Outgassing characteristics of electropolished stainless steel” (in Japanese). The paper comprises the following sections: 1. Introduction 2. Sample and experimental method 3. Experimental results and discussions In this book, figures and tables in Sections 2, Samples and experimental method, and 3, Experimental results and discussions, are introduced. Abstract To keep abreast of the exceptionally rapid advances of leading industries, technologies need to be developed for an XHV process. Because of its outstanding corrosion resistance, strength, and outstanding characteristics, SUS304(L) and 316(L) SS are used for ultra-high vacuum equipment. Also, electropolishing technique has been adopted by industries because it makes the surface roughness as small as possible by minimizing the actual areas. This is a future trend of XHV equipment. The outgassing characteristics of the EP surface of SS and effects of gas contents and inclusions of steel itself on the outgassing rate controlled by diffusion have been studied. SUS316L SSs, which have various contents of gas and the amounts of nonmetallic inclusions, have been melted in various conditions and rolled to plates. The samples of 8 mm 3 8 mm 3 1 mm were prepared from these plates, and all surfaces were finally EP. The amounts of outgassing gas by thermal desorption and the outgassing rate of H2, H2O, CO, CO2 after baked at 1123K in a vacuum of 1027 Pa were measured by a quadrupole MS. The molecular amount of H2 of all the samples was larger by 10102 times, than those of H2O, CO, and CO2, and there was no correlation between the H2 molecular amounts of outgassing by heating and the H2 content in steel. But the outgassing rate of H2 was reduced with decreasing both the H2 content and the amounts of nonmetallic inclusions. Especially, the outgassing rate of H2 was attributed to the inclusions because the hydrogen was found to be entrapped by them on the EP surface. Clean steel, which has been produced in a special cleaned melt, showed two orders of magnitude lower in rate than normal steel. The outgassing rates of CO and CO2 were reduced with decreasing C content in steel. Keywords: Austenitic stainless steel, outgassing rate, thermal desorption, quadruple mass spectrometer, electropolishing, nonmetallic inclusion

2

Samples and experimental method

Nine kinds of samples (see Table 1) are prepared. The results of nonmetallic inclusions are presented in Table 2. TABLE 1 Chemical composition (mass%). Steel

C

Si

Mn

P

S

Ni

Cr

Mo

N

Oa

Ha

K

0.015

0.45

1.58

0.026

0.005

12.86

16.39

2.19

0.022

33

6.5

1A

00.08

0.15

0.17

0.003

0.006

14.41

16.88

2.33

0.005

78

1.8

2A

0.005

0.15

0.15

0.002

0.004

14.37

16.84

2.34

0.005

34

1.1

3A

0.008

0.11

0.18

0.004

0.005

14.58

16.98

2.34

0.005

48

1.7

4A

0.007

0.12

0.17

0.004

0.003

14.43

16.93

2.35

0.005

29

1.8

5A

0.005

0.09

0.16

0.003

0.003

14.35

16.88

2.34

0.004

16

1.3

6A

0.004

0.12

0.27

0.001

0.0006

14.96

16.80

2.36

0.004

18

0.9

F

0.011

0.47

0.98

0.027

0.001

12.45

17.64

2.25

0.028

36

0.5

C

0.007

0.13

0.28

0.005

0.001

14.88

16.98

2.32

0.007

10

1.0

b

# 0.03

# 1.00

# 2.00

# 0400

# 0.030

12.0016.00

16.0018.00

2.003.00







SUS316L

a

PPM. JIS.

b

20

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

TABLE 2 Results of nonmetallic inclusion measurement. Steel

Count method (1/mm2)

Inclusion parameter

Spherical inclusion

a

Linear inclusion

a

SS

SM

SL

BS

BM

BL

I (1/mm2)

K

10.8

0.3

0.1

3.0

1.5

0

22.9

1A

21.6

0.4

0.1

0

0

0

22.1

2A

10.5

0.5

0.1

0

0

0

11.1

3A

11.5

0.6

0.2

0

0

0

12.3

4A

10.8

0.4

0.1

0

0

0

11.3

5A

8.6

0.2

0

0

0

0

0.8

6A

2.5

0.2

0

0

0

0

2.7

F

31.0

0.7

0.1

3.9

1.5

0

37.2

C

0.3

0

0

0

0

0

0.3

BL, Linear inclusion; BM, linear inclusion; BS, linear inclusion; SL, spherical inclusion; SM, spherical inclusion; SS, spherical inclusion. a SS , ϕ5 µm, BS ,15 µm, SM , 510 µm, BM , 510 µm, SL . 10 µm, BL . 10 µm.

Experimental apparatus of TDS (thermal desorption spectroscopy) apparatus is presented in Fig. 1.

Vacuum gauge

Sample preparation chamber

TMP

RP

Gate valve

TDS analysis chamber

TMP

RP

Q.M.S Vacuum gauge Infrared light furnace Sample

FIG. 1 Schematic diagram of the experimental apparatus of TDS analysis. QMS, Quadrupole mass spectroscopy; RP, rotary pump; TDS, thermal desorption spectroscopy; TMP, turbo-molecular pump.

3

Experimental results and discussions

Some experimental data in the paper[1-3] are presented here. The bright annealed surface has many fissures and grain boundaries. On the other hand, the EP surface is extremely flat. In this book the experimental results, Figs. 2, 3, 4, 5, 6, 7, 8, 9, and 10 and Tables 35, are presented.

Microstructure and elemental features of stainless-steel surface Chapter | 1

21

FIG. 2 SEM micrographs of bright annealed and electropolished surfaces.

Experimental results of surface roughness measurement for the bright anneal sample and the EP sample, are presented in Tables 35. TABLE 3 Results of surface roughness measurement. Treatment

Bright anneal (μm)

Electropolished (μm)

R max

2.489

0.385

0.172

0.057

0.301

0.070

1.299

0.243

Ra

ðn 5 20Þ ðn 5 20Þ

RMS max Rz

ðn 5 20Þ

ðn 5 20Þ

The AES (Auger electron spectroscopy) data of the before and after electropolishing of sample K are presented in Fig. 3. Bright anneal

50

Fe

Fe

40

Concentration (at.%)

Concentration (at.%)

40

30

20 Cr

30

20 Cr

Ni

10

10

C

C

0

FIG. 3 Auger analysis of bright annealed and electropolished surfaces.

Electropolishing 50

0

60 120 180 Sputtering time (t), s

240

0

0

Ni

60 120 180 Sputtering time (t), s

240

22

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Fig. 4 shows the example of H2-, H2O-, CO-, and CO2-TDS curves for the sample K.

10–4

QMS intensity (Iq), A m–2

H2

10–5

CO

H2O

10–6

CO2

10–7

300

500 700 900 1100 Temperature (T ), K

1300

FIG. 4 Example of TDS curves (TD. No. K). TDS, Thermal desorption spectroscopy.

10–3

1A 2A 5A F

QMS intensity (Iq), Am–2

10–4

10–5

10–6

300

500 700 900 1100 Temperature (T), K

FIG. 5 Example of H2 TDS curves. TDS, Thermal desorption spectroscopy.

Microstructure and elemental features of stainless-steel surface Chapter | 1

TABLE 4 Molecular amounts of outgassing gas. Sample (steel)

Gas H2O

H2

CO

CO2

K

9.559 3 10

3.4 3 10

2.0 3 10

6.0 3 1018

1A

3.554 3 1020

4.2 3 1019

2.6 3 1019

2.7 3 1018

2A

1.070 3 1020

1.6 3 1019

9.3 3 1018

1.4 3 1018

3A

2.986 3 10

2.0 3 10

19

5.6 3 10

1.8 3 1019

4A

3.738 3 1020

4.3 3 1019

2.7 3 1019

6.8 3 1018

5A

4.412 3 1020

6.3 3 1018

3.5 3 1019

4.8 3 1018

6A

2.900 3 1020

3.6 3 1019

1.7 3 1019

1.9 3 1018

F

1.578 3 1020

5.0 3 1019

1.5 3 1019

3.5 3 1018

C

8.613 3 1020

4.1 3 1019

2.3 3 1019

4.2 3 1018

20

20

19

20

19

Values are in mol m22 for gases.

TABLE 5 Layer amounts of outgassing gas. Sample (steel)

Gas H2

H2O

CO

CO2

K

95.59

3.4

2.0

0.6

1A

35.54

4.2

2.6

0.3

2A

10.70

1.6

0.9

0.1

3A

29.86

19.7

5.6

1.8

4A

37.38

4.3

2.7

0.7

5A

44.12

0.6

3.6

0.5

6A

29.00

3.6

1.7

0.2

F

15.78

5.0

1.5

0.4

86.13

4.1

2.3

0.4

C 22

Values are in layers m

for gases.

23

24

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

K

The ratio of practical H2 outgassing rate, Ra

1 1A

F

3A

5A

4A

10–1

2A

6A

10–2 C

0.5

1 5 10 H2 content in steel, ppm

FIG. 6 Relationship between the ratio of practical H2 outgassing rate and H2 content in steel.

FIG. 7 Example of nonmetallic inclusion on the electropolished surface.

20

Microstructure and elemental features of stainless-steel surface Chapter | 1

25

FIG. 8 The comparison of H2 outgassing rate between practical and expected. K

The ratio of practical H2 outgassing rate, Ra

1 1A 3A

F

5A 4A

10–1

2A

6A

10–2 C

10–2 10–1 1 The ratio of expected H2 outgassing rate, Re* * Re = 2.157×10–2 [H] + 3.755×10–2 [I]

Reviewed paper [1-3] Tohyama A, Yamada T, Hirohata Y, Yamashina T. Outgassing characteristics of electropolished stainless steel. J Jpn Inst Met 1990;54 (3):24754 (in Japanese).

Related paper [1-4] “A review of the stainless steel surface” (Adams, 1983) Adams (1983) presented the paper,[1-4] “A review of the stainless steel surface.” This review refers to 98 pieces of papers. This paper comprises the following sections: 1. 2. 3. 4. 5. 6.

Introduction Adsorptiondesorption of gases Oxidation Oxide film Surface segregation Summary

Abstract The characteristics of the surface of various SSs are reviewed. The property of these alloys that makes them stainless is the formation of Cr2O3 on the surface. It has been found that this protective layer can be modified relatively easily by heating, abrading, chemical treatment, or ion bombardment. Modification can be changed in the chemical composition of the surface layer or the formation of a layer of segregated material on the surface. These changes may alter the protective nature of the surface films. The outgassing characteristics of SS surfaces also vary depending upon the treatment these surfaces receive.

Related paper [1-4] Adams RO. A review of the stainless-steel surface. J Vac Sci Technol A 1983;1(1):1218.

Chapter 2

Characteristics of outgassing from metal surfaces Quoted book [1-1] “Typical isotherms in the chemisorption of gases on metal-surfaces: equilibrium adsorption” (Redhead, Hobson, and Kornelsen, 1968) Outgassing occurs when the surfaces of the chamber walls are exposed to high vacuum, affecting the pressures in high vacuum systems. Therefore outgassing rates of various construction materials must be estimated to simulate the pressures in high-vacuum systems. The outgassing rate Q can be expressed by the following equation: Qoutgassing 5 Qdesorption 2 Qadsorption : Redhead et al. introduced many adsorption isotherms in the famous book.[1-1] The important three adsorption isotherms in the chemisorption of gases on metals are known as the Langmuir isotherm, the Freundlich isotherm, and the Temkin isotherm, respectively. The Langmuir isotherm is based on the simplest model and assumes that the rate of adsorption is proportional to the number of empty states. For nondissociative adsorption θ5

Ap ; 1 1 Ap

(1)

where A is a constant. If adsorption is dissociative and immobile then θ5

ðApÞ1=2 1 1 ðApÞ1=2

:

(2)

The Freundlich isotherm is θ 5 Bp1=n ;

(3)

where B and n are constants at a given temperature, can be derived if it is assumed that the heat of adsorption decreases exponentially with coverage. The Temkin isotherm is θ5

RT 3 lnðcpÞ q0 α

(4)

can be derived from the Langmuir isotherm if the heat decreases linearly with coverage, namely, q 5 q0 ð1 2 αθÞ. In Eq. (4), c 5 a expðq0 =RTÞ. These isotherms, and methods of testing whether experimental data fit the isotherms, are discussed by Hayward and Trapnell (1964).[1] Redhead et al. (1968)[1-1] have introduced many adsorption isotherms in the famous book[2-1]. Fig. 1 shows the test Temkin Isotherm for H2 on a Mo film at 78K.[1] A Review: Ultrahigh-Vacuum Technology for Electron Microscopes. DOI: https://doi.org/10.1016/B978-0-12-818573-5.00002-5 © 2020 Elsevier Inc. All rights reserved.

27

28

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

FIG. 1 (Fig. 2.16 in [1-1], page 53). Test of the Temkin isotherm for H2 on a Mo film at 78K.

10–5

Pressure (torr)

10–6

10–7

10–8

90 91 Coverage (mol cm–2)

92

×1014

References in quoted book[1-1] [1] Hayward DO, et al. (1966).

Quoted book [1-1] Redhead PA, Hobson JP, Kornelsen EV. The physical basis of ultrahigh vacuum. Chapman and Hall Ltd.; 1968.

Reviewed papers [1-2] “Relations between pressure, pumping speed and outgassing rate” (Dayton, 1960) Dayton (1960) presented the paper[1-2] titled “Relation between size of vacuum chamber, outgassing rates, and required pumping speed.” This long paper comprises the following sections: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Methods of measuring outgassing rates Semiempirical formulas Relation among pressure, speed, and outgassing rate The sorption coefficient Outgassing rate for solids Desorption of gas from exposed surfaces The ultimate pressure Formula for estimating the pressure in the chamber The time of evacuation Tables of data

In this book, Sections 2, Semiempirical formulas, and 3, Relation between pressure, speed and outgassing rate, are introduced.

Characteristics of outgassing from metal surfaces Chapter | 2

2

29

Semiempirical formulas

The experimental data obtained from the modified Zabel apparatus used by the writer (Dayton), from the publications cited above by Jeackel and Schittko,[3] Geller,[5] Santeler,[9] and others and from unpublished pump-down curves on industrial vacuum systems available to the writer (Dayton), all indicate that the partial pressure p, due to outgassing and permeability of a vacuum system as measured inside a chamber near the opening to the pumping system at time t after the beginning of pumping with a high vacuum pump of relatively constant speed, is given approximately by p 5 pu 1

C ðB; 1 tα Þ

(1)

where pu is the ultimate pressure, C is a constant which is roughly proportional to the exposed area of outgassing material divided by the net pumping speed, B is a constant which is negligible compared with t after a few minutes, and α is an exponent which may increase slowly with time in an irregular way from values as low as 0.5 to values greater than one. Eq. (1) is similar to a formula recently published by Kraus.[19] The values of pu , C, B, and α may be used to characterize the expected pump-down behavior of a given system which has been allowed to come to equilibrium at atmospheric pressure and at a specified humidity. Methods of estimating these constants are given in the following. The outgassing rate is preferably expressed in Torr L (s cm2)21 at a specified temperature and a specified time after the beginning of pumping. When outgassing rate is plotted against time on loglog graph paper, the resulting curve frequently has a nearly constant slope for the first few hours. As shown by the data of Santeler,[9] Geller,[5] and others, this initial part of the curve can often be represented by logðKm 2 Ku Þ 5 log K1 2 α log t

(2)

where Km is the outgassing rate at the time t, Ku , and K1 are constants, and the slope of the curve, α, frequently has values between 0.5 and 0.8. When experimental data on the outgassing rate are not available, Ku , K1 , and α can sometimes be estimated from other known physical properties of the material by the formulas given in the following.

3

Relation among pressure, speed, and outgassing rate

Assuming that there are n different volatile constituents (hereafter referred to as “gases”) and m different outgassing surfaces, the principle of material balance requires that the net throughput of each of the n gases across the inlet to the pumping system be equal to the net throughput of that gas with respect to the combined exposed surfaces plus the rate of reduction of the quantity (in pressurevolume units) of that gas in the free space bounded by these surfaces. When the partial pressure in Torr of the nth gas is pn at the temperature T (K) in the gas phase inside the vacuum chamber, the rate at which n-type molecules strike a plane surface of area Am (cm2) in throughput units of Torr L s21 at the temperature T is Qn 5 fn Am pn

(3a)

2 21

where fn is the “effusion law” factor (L (s cm )

as given by
 R0 T 1=2 3 1023 fn 5 2πMn

(3b)

in which R0 is the molar gas constant in cgs units (R0 5 8:31 3 107 ), and Mn is the molecular weight of the nth gas. If enm is the fraction of these molecules which are sorbed (the word “sorbed,” including both adsorption and absorption processes), then the rate pf sorption for the exposed surface is Qnm 5 fn enm Am pn

(4)

At any instant of time during pump-down, n-type molecules will be escaping from the mth surface at a rate depending on the temperature, Tm, of the surface and the concentration of these molecules at the surface. The rate of desorption from and through the surface of area Am is Q0nm 5 Knm Am

(5)

30

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

where Knm is defined as the “free outgassing rate” in Torr L (s cm2)21 for the nth gas from the mth surface as measured at the temperature T of the gas phase within the chamber. The “free outgassing rate” of the mth material for all volatile matter is defined by X Km 5 Knm (6) n

and it is this quantity which is usually reported in the published literature on outgassing rates of materials. The net throughput of n-type molecules with respect to the mth surface is then qnm 5 Q0nm 2 Qnm

(7a)

qnm 5 Am ðKnm 2 fn enm pn Þ

(7b)

or The outgassing rate in Torr L (s cm2)21 for the nth gas from the mth surface is given by qnm knm 5 Am

(8a)

or using (7b) knm 5 Knm 2 fn enm pn

(8b)

The net pumping speed, Sn , for the nth gas at the inlet to the pumping system in liters per second is given by

 1 1 1 5 1 Sn Spn Un

(9)

where Spn is the speed of the pump for the nth gas, and Un is the conductance of the passage between the pump and the vacuum chamber in liters per second. The net throughput of the nth gas across the inlet to the pumping system will be qn 5 Sn pn The principle of material balance then gives qn 5

X m

(10)


dpn qnm 2 V dt

 (11)

where V is the volume of the vacuum chamber in liters. The net “speed of exhaust” of the system for the nth gas in liters per second is defined by

 V dpn (12) En 5 2 pn dt and this should not be confused with the pumping speed, Sn . From (7b), (10), and (11) the partial pressure, pn , in Torr is given by P Knm Am 2 Vðdpn =dtÞ pn 5 mP m fn enm Am 1 Sn The total pressure, p, in the vacuum chamber is given by X pn p5

(13)

(14)

n

and the total throughput of the pumping system at the pressure p is X q5 Sn pn

(15)

n

The total net pumping speed is defined by S5

q p

(16)

Characteristics of outgassing from metal surfaces Chapter | 2

and the total net speed of exhaust is E52



 V dp p dt

31

(17)

Eq. (13) determines the proper design for Zabel-type apparatus for measuring outgassing rates. When a sample of only one material is placed in the apparatus and the outgassing of the blank system is negligible, Eqs. (6), (13)(16) give
 dp S 3 p 5 Km Am 2 Fm Am 2 V (18a) dt where Km is the free outgassing rate and Fm is the “free sorption rate” as given by X fn enm pn Fm 5

(18b)

n

when Sn $ 100fn enm Am

(19a)

for all the values of n, then Fm may be neglected in comparison with Km and the free outgassing rate is given by Km 5

Sp Am

(19b)

provided Vðdp=dtÞ is also negligible. In general, enm is so small for the common materials studied in the Zabel apparatus that condition (19a) is fulfilled even when speeds of only a few-tenths of a liter per second are used with sample areas of square centimeters. However, it is also important to determine enm . This can be done by measuring pn with a mass-spectrometer-type vacuum gauge while varying the ratio Sn =Am between the limits 10fn enm and 0:1fn enm . When Km has been measured and is not too large compared to Fm , the value of Fm can be determined by closing the opening to the pumping system and measuring the rate of rise of pressure as given by dp Am 5 ðKm 2 Fm Þ dt V Dayton (1960)[1-2] presented Fig. 3.

FIG. 3 Typical outgassing curve for an elastomer.

10–5

10–6 Knm

10–7

10–8 1

tm 10

100 Time th (h)

1000

(20)

32

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

References in the paper[1-2] [3] [5] [9] [19]

Jeackel R, Schittko FJ. Forschungberichte des Wirtschafts-und Verkehrsministeriums Nordrhein-Westfalen Nr. 369. Geller R. Le Vide 1958;13(74):71. Santeler DJ. General Electric Co. Report No. 58GL303 Vacuum symposium transactions. London: Pergamon Press; 1958. Kraus T. Vakuum-Technik 1959;8:39. Vacuum symposium transactions, p. 3840, Pergamon Press, London (1959).

Reviewed paper [1-2] Dayton BB. Relations between size of vacuum chamber, outgassing rate, and required pumping speed. 1959 6th national symposium on vacuum technology transactions.. Pergamon Press;; 1960. p. 10119.

[1-3] “Outgassing rate of contaminated metal surfaces” (Dayton, 1962) Dayton (1962) presented the review paper,[1-3] “Outgassing rate of contaminated metal surfaces.” This long paper comprises the following sections: Introduction 1. 2. 3. 4. 5. 6.

The desorption time parameter Equation for the outgassing rate The total initial gas content of the contaminating layer The distribution function for the diffusion time parameter Variation of outgassing rate with temperature Summary of published experimental data

In Section 2 “Equation for the outgassing rate”, the interesting description on the persistency of chemisorbed water, are presented. Abstract Successive evaporation of various molecules from a heterogeneous layer of surface contamination can result in an outgassing rate that is inversely proportional to the time of pumping. A relatively uniform distribution of the activation energies of desorption will result in a sum of the individual outgassing curves for constant desorption energy that appears as a straight line with a slope of 21 on the loglog plot of outgassing rate versus time of pumping. This line forms an “outgassing barrier” that limits the ultimate pressure in unbaked metal vacuum systems. Comment by Yoshimura: “A heterogeneous layer of surface contamination” means an oxide layer of metal surface.

2

Equation for the outgassing rate

Assume that the parameter τ is a constant for the fraction FðτÞdτ of the contaminating layer on the metal surface, where FðτÞ is a function of τ such that ð τm FðτÞdτ 5 1 (2.1) τ0

the integration being carried out from the minimum value, τ 0 , to the maximum value, τ m . Then from Eqs. (13) and (41) in the 1959’s paper[1] (replacing tnm by τ and t by th ) the contribution to the total free outgassing rate [in Torr L (s cm2)21] by the fraction FðτÞdτ of the contaminating layer will be given approximately by 
  C0 Bnm 3 τ 1=2 Knm 5 (2.2) 1 thα 3600 60 where th is the pumping time in hour, and

Characteristics of outgassing from metal surfaces Chapter | 2

thα


1=2 h  t i τ h 2 1 2 exp 2 2τ
 760 TLm s0 P0 3 1023 C0 5 273 4

1=2 5 th

33

(2.3) (2.4)

in which s0 is the solubility at an initial temperature T0 of the gas (assumed to be mainly H2O) in the contaminating (oxide) layer expressed in units of cm3 of gas at standard atmospheric pressure of 760 Torr and standard temperature of 273K dissolved in 1 cm3 of the solid when the layer is in equilibrium with the gas (water vapor) at the partial pressure of 1 Torr, while P0 is the actual initial partial pressure (in Torr) of the gas (water vapor) which would be in equilibrium with the layer at the time th 5 0 and at an initial temperature T0 , T being the temperature at which the pressure of the evolved gas is measured within the vacuum chamber in determining the outgassing rate Knm in Torr L (s cm2)21, and where Bnm is a constant that is negligible after a few seconds of pumping but for th close to zero may be evaluated from Bnm 5

λnm ðπDnm Þ1=2

(2.5)

The factor ð1 2 λnm =Lm Þ in Eq. (41b) in the 1959’s paper[1] is omitted because this factor only applies when the outgassing material forms a wall of chamber through which permeation can occur. The function thα as given by (2.3) is a semiempirical formula that the author has found to be a useful approximation to the infinite series of exponential functions of time corresponding to the exact solution of the Fick law equation for the diffusion of gas through a solid with finite thickness Lm . The exact solution gives in place of (2.2) and (2.3)   N ðπ=2ÞC0 X ð2m11Þ2 π3 th Knm 5 exp 2 (2.6) 3600τ m50 64τ The exponential terms in the above series can be simplified by substituting in (2.6)
3 π τ5 τe 32

(2.7)

where τ e then becomes the desorption time parameter in place of τ. Since π3 =32 5 0:97, the value of τ e is approximately equal to τ. There is no particular reason for preferring τ to τ e except that τ is based on Eq. (1.2) that corresponds to the time at which the van Liempt “apparent outgassing depth”[28] becomes equal to Lm . Combining Eqs. (1.5), (1.8), and (2.7) gives τe 5

ð8=π2 ÞðLm =λnm Þ2 tα 3600

The average free outgassing rate for the whole surface will be ð τm Kn 5 Knm FðτÞdτ

(2.8)

(2.9)

τ0

The distribution function FðτÞ is determined by the separate distribution functions for the variables Lm , dm , and tα upon which τ depends according to Eq. (1.8). The constant tα for the adsorption time of the H2O molecules on the walls of the capillary pores will in general depend on the capillary diameter, dm , and on the fraction, θ, of the sites occupied at time th . When tα depends on θ, the diffusion coefficient, Dnm , varies with concentration and hence with time. In that case the solution corresponding to Eqs. (2.9), (2.2), (2.3), and (1.8) is not applicable since these equations were derived for the case of a constant diffusion coefficient. However, as an approximation we can still use these equations by including the effect of the change in θ in the evaluation of the distribution function FðτÞ. To illustrate the significance of Eq. (2.9), we assume that FðτÞ is a constant equal to Fðτ 1 Þ over the range τ i 2 0:5Δτ to τ i 1 0:5Δτ and that the whole oxide film can be divided into four regions, each region corresponding to the fraction Fðτ i ÞΔτ 5 0:25

34

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Then the total outgassing rate will be given by a sum, i54 X

Kn 5

Ki

(2.10)

i51

where Ki is the outgassing rate for the fraction Fðτ i ÞΔτ as given by 0:25Ci

Ki 5

(2.11)

thαi τ i

1=2

where Bnm in (2.2) is neglected and Ci 5

C0 3600

(2.12)

in which C0 is given by (2.4), while αi is defined by tααi

1=2

1=2 5 th

τi 2

!


1 2 exp

th 2τ i

 (2.13)

in which τ i is the diffusion time constant in hours as given by τi 5

π
L 2 m

4

λnm

ti

(2.14)

where ti is the adsorption time in hours. Using the typical values T 5 300K, P0 5 10 Torr, Lm 5 5 3 1025 cm, and assuming that s0 5 8 for reasons given later, then Eqs. (2.4) and (2.12) give C1 5 24 3 1027 . Assuming for simplicity that the oxide film thickness, Lm , is constant, then C1 will be constant as the parameter τ i varies from a fraction of an hour to several hours. As the parameter, τ i , takes various constant values, Eq. (2.11) gives a family of curves, as shown in Fig. 1 plotted on a loglog graph. In this graph, we have chosen τ 1 5 0:25 h, τ 2 5 1 h, τ 3 5 4 h, and τ 4 5 16 h in anticipation of the requirement that F ðτ Þdτ must be proportional to dðlnτÞ in order that the integral shall give a total outgassing rate which decays as t21 in the region from τ 0 to τ m . This corresponds to FðτÞ proportional to τ 21 .

FIG. 1 Total outgassing rate as a sum of curves for constant values of τ.

Outgassing rate Kn [Torr L (s cm2)–1]

10–6

0.25

10–7 Total

1.0

4

10–8

16

10–9

10–10 0.1

1

10 Time (h)

100

1000

Characteristics of outgassing from metal surfaces Chapter | 2

35

Comments by Yoshimura: Diffusion time constants, τ i , have various values depending on the oxide layer. In Fig. 1, Dayton has chosen τ 1 5 0:25 h, τ 2 5 1:0 h, τ 3 5 4 h, and τ 4 5 16 h in anticipation of the requirement that decays as t21 in the region from τ 0 to τ m . The total outgassing rate will be given by the sum of the four curves and is shown by the dashed line. In the region from τ 1 5 0:25 to τ 4 5 16 h, this dotted line is approximately a straight line with a slope of 21 on the loglog graph. The total outgassing rate at 1 h is about 1:6 3 1027 Torr L (s cm2)21 in agreement with the experimental data. As Δτ approaches dτ, both the envelopes of the family of curves and the integral sum will be curves parallel to the curves for the distribution function, FðτÞ, in the region from τ 0 to τ m . In order for the outgassing rate, Kn , to vary as th21 , the distribution function FðτÞ must be proportional to τ 21 . This will be true regardless of the exact form of the function thα as long as the latter corresponds to α , 1 when th {τ and increases exponentially with time when th cτ. The envelope and the sum are then determined mainly by the form of the curve in the neighborhood of th 5 τ, and consequently by the distribution function for the parameter τ. Therefore in order to obtain agreement with the empirical Eq. (1.1) (Kn 5 K1 =thα ) with α 5 1, we must assume that as an approximation over the range τ 0 to τ m FðτÞ 5

k1 τ

(2.15)

where k1 is a constant. Substituting (2.15) in (2.1) then gives k1 5

1  ln τ m =τ 0

(2.16)

It should be noted that Eq. (1.1) (Kn 5 K1 =thα ) is quite general and that α may have values from about 0.7 to more than 2 depending on the pretreatment of the contaminated metal surface. The distribution function could similarly be made more general by replacing τ in (2.15) by τ α . However, since α appears to be nearly equal to 1 in many cases, for simplicity we shall assume α 5 1 in the following analysis. Substituting (2.2), (2.3), (2.12), and (2.15) in (2.9), assuming T, Lm , and s0 constant so that C1 can be brought outside the integral sign, we have
2 h ð τm

 t i21 τ h 1=2 3=2 Kn 5 k1 C1 th τ 2 dτ (2.17) 12exp 2 2τ τ0 where C1 5 1:94 3 1027 TP0 s0 Lm An approximate solution of the integral in (2.17) can be obtained by noting that the integrand reduces to 2t  h I1 5 2τ 22 exp 2τ

(2.18)

(2.19)

in the region τ 0 , τ , 0:1th and reduces to 21=2 23=2

I3 5 th

τ

(2.20)

in the region 4th , τ , τ m , while in the intermediate region, 0:1th , τ , 4th , the integrand is approximately equivalent to  t  h I2 5 1:5τ 22 exp 2 (2.21) 2τ as can be seen by the substitution of numerical values in the two expressions. Integrating (2.17) over the separate regions in which Eqs. (2.19)(2.21) are valid, and neglecting small terms, we obtain Kn 5 3:66k1

C1 th

where 10τ 0 , th , τ m =4. When th . 10τ m , the integrand reduces to I1 , and in this region

(2.22)

36

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping


 th Kn 5 4k1 C1 th21 exp 2 2τ m

(2.23)

neglecting exp ð2 th =2τ 0 Þ in comparison with expð2 th =2τ m Þ. Instead of integrating (2.7) the exact solution may be obtained by substituting (2.6), (2.12), and (2.15) in (2.9). The resulting series expansion is readily integrated term by term giving



N X exp 2 ð2m11Þ2 th =2τ m 2 exp 2 ð2m11Þ2 th =2τ 0 32k1 C1 Kn 5 3 (2.24) π2 th ð2m11Þ2 m50 where use has been made of Eq. (2.7) so that τ m in (2.24) is the maximum value of τ e , and τ 0 is the minimum value of τ e , the latter being given by Eq. (2.8). When 10τ 0 , th , τ m =4, Eq. (2.24) becomes equal to
 32 β (2.25) Kn 5 2 k1 C1 π th where β lies between 0.925 and N X

π2 5 1:23 8

ð2m11Þ22 5

m50

(2.26)

depending on the value of th =τ m . The exact solution therefore varies between Kn 5 4k1

C1 th

(2.27)

Kn 5 3k1

C1 th

(2.28)

and

as th =τ m varies from nearly zero to 1=4. Eqs. (2.22), (2.27), and (2.28) may be compared with the empirical Eq. (1.1) with α 5 1. As indicated by (2.23) or (2.24), the outgassing rate eventually decays exponentially as th becomes much larger than τ m . As a reasonably good approximation over the range from th . 2τ 0 to th 5 τ m , it can be assumed that
 th 21 Kn 5 4k1 C1 th exp 2 (2.29) τm and over the range 0:5τ m , th , N Kn 5



 32 th 21 C t exp 2 k 1 1 h π2 2τ m

(2.30)

At th 5 τ m =4 Eq. (2.29) becomes Kn 5 3:1k1

C1 th

(2.31)

which is close to the correct value in (2.28). Persistency of chemisorbed water (presented at the closing part of Section 5): (From 5, Variation of outgassing rate with temperature) At bakeout temperature above 200 C the physically adsorbed water will be desorbed from the oxide film in less than 10 min, since τ m 5 1:46 3 1023 h when Hm 5 20; 000 cal mol21 , Lm 5 1 3 1025 cm, λnm 5 5 3 1028 cm, and Tm 5 473K. Fig. 3 shows a family of outgassing curves for water vapor for various values of Tm , using Hm 5 20,000 cal mol21, H0 5 10,000 cal mol21, Lm 5 1 3 1025 cm, dm 5 8 3 1028 cm, T 5 Tm , e0 5 0:1, and θ0 5 1:3. When the temperature is suddenly raised above 150 C, some of the physically adsorbed water will become chemically adsorbed but most of the physically adsorbed H2O should escape from the oxide film within a few minutes, either be pumped away or condensed on cooler portions of the vacuum system (such as the walls of a gauge tube with envelope at ambient room temperature). The partial pressure of H2O in the chamber will rise rapidly to some peak value depending on the quantity of H2O evolved and the volume of the chamber as well as the pumping speed and the rate of

Characteristics of outgassing from metal surfaces Chapter | 2

37

increase in temperature. The pressure will then fall rapidly to a value determined by the rate of evaporation of the H2O sorbed on the cool portions of the system. The pressure of H2O should then decrease with time according to Eq. (91) in the 1959’s paper[1] until limited by the rate of evolution of chemisorbed H2O from the heated surfaces. As the temperature is raised above 100 C, the chemisorbed water begins to desorb at an appreciable rate, but a temperature of more than 500 C is required to remove all the chemisorbed water from oxides according to the data cited by Gregg.[40] Chemisorption of H2O on oxides is believed to be associated with the formation of hydroxide groups (OH2 ions). Data are given in the book edited by Garner[40] that indicate that most of the chemisorbed water is located on the surface of the oxide (including the walls of the pores). FIG. 3 Theoretical curves for desorption of water at various temperatures.

10–5

100ºC 50ºC 25ºC

Oougassing rate [Torr L (s cm–2)–1]

10–6

Water

Vapor

Hm = 20,000 cal mol–1

10–7

10–8

10–9

10–10

150ºC

100ºC

50ºC

25ºC

10–11 10–2

10–1

1 10 Time (h)

102

103

References in the paper[1-3] [1] Dayton BB. 1959 Vacuum symposium transactions. New York: Pergamon Press; 1960. 101119. [28] Barrer RM. Diffusion in and through solids. C.U.P.; 1941. p. 215. [40] Garner WE, editor. Chemisorption. New York: Academic Press; 1957 (loc. cit. pp. 59-75).

Reviewed paper [1-3] Dayton BB. Outgassing rate of contaminated metal surfaces. 1961 transactions of the eighth national vacuum symposium.. Pergamon Press;; 1962. p. 4257.

[1-4] “The effect of bake-out on the degassing of metals” (Dayton, 1963) Dayton (1963) presented the paper,[1-4] “The effect of bake-out on the degassing of metals.” This paper comprises the following sections: 1. Introduction 2. Degassing time at constant temperature 3. Outgassing rate with variable temperature

38

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

In this book, Sections 2, Degassing time at constant temperature, and 3, Outgassing rate with variable temperature, are introduced. Abstract Equations are derived for the effects of a bakeout period on the rate of outgassing of hydrogen and other gases dissolved in metals. It is shown that van Liempt’s formula for the time to remove 95% of the gas is incorrect, and a corrected formula is derived. The equations are compared with available experimental data.

2

Degassing time at constant temperature

In the following, y will represent the fraction of the original total gas content that has been removed by degassing for the time t (in seconds). It can be shown that the equation derived by van Liempt[5] for y , 0:5 is a good approximation to the result given by the exact solution of the Fick law equations for diffusion of gas out of a plane sheet. However, on page 782 of his paper he made the incorrect assumption that for y . 0:5 the “apparent outgassing depth” would continue to move inward from each surface with a velocity proportional to ðDm =tÞ1=2 , where Dnm is the diffusion coefficient (in cm2 s21), even when the concentration in the middle of the sheet has fallen below the initial concentration, c0 . This results in the incorrect equation t5

πL2m ; 256Dnm ð12yÞ2

ðy . 0:5Þ;

(1)

where Lm is the thickness of the sheet (in cm). In van Liempt’s method the distribution curve for the gas concentration as a function of distance χ from the exposed surface is approximated by a straight line through the origin with a slope c0 =anm , where c0 is the initial concentration, and anm is the “apparent outgassing depth” that moves inward with time. In his first paper,[10] van Liempt derives the formula
 4 anm 5 1=2 ðDnm tÞ1=2 (2) π by equating the area under the curve of concentration as a function of the dimensionless parameter x=2ðDnm tÞ1=2 for the special case of diffusion in a semiinfinite medium with the area of the right triangle whose altitude is the initial concentration, c0 , and whose base is anm =2ðDnm tÞ1=2 . This is equivalent to defining anm so that the total quantity of gas removed in time t during degassing into a high vacuum through unit area of the plane surface of a semiinfinite medium is accurately given by c0 anm =2. However, there is another method of defining anm that gives a slightly different result but follows logically from van Liempt’s assumption that the distribution curve for the gas concentration in the solid can be approximated by a straight line with slope c0 =anm . The outgassing rate will always be given by

 G0 dy K5 ; (3) dt A where G0 is the total initial gas content and A is the exposed surface area (in cm2). From Fig. 1, which shows the assumed linear distribution of concentration as a function of the distance, it is evident that for anm , Lm =2 the total gas removed through unit area of one exposed surface will be equal to the triangular area c0 anm =2. Hence, yG0 anm 5 c0 : A 2 From (3) and (4) the outgassing rate through this surface is c 
da  0 nm K5 ; 2 dt

(4)

(5)

as can be seen from Fig. 1 where the moving line with slope c0 =anm sweeps out a triangular area ðc0 =2Þdanm in the time dt. The outgassing rate will also be given by
 @c K 5 Dnm ; (6) @x x50

Characteristics of outgassing from metal surfaces Chapter | 2

39

where ð@c=@xÞx50 is the concentration gradient at the exposed surface. From Fig. 1 it is evident that this concentration gradient is c0 =anm in van Liempt’s diagram, and therefore
 c0 K 5 Dnm : (7) anm Combining (5) and (7) gives anm 5 2ðDnm tÞ1=2 :

(8)

Since 4=π1=2 5 2:26, Eq. (8) gives a slightly lower value of anm than Eq. (2). However, the correct equation for the outgassing rate of a semiinfinite solid is obtained by substituting (2) rather than (8) in Eq. (5). The advantage of determining anm by the method that led to Eq. (8) will become apparent later when we consider the case of y . 0.5. From Fig. 1 it is evident that the fraction of gas removed is anm y5 ðy , 0:5Þ (9) Lm when the sheet is exposed on both sides. Substituting (9) in (2) gives van Liempt’s formula for the degassing time for y , 0.5: t 5 πy2

(10)

For y 5 0.5 we have t1=2 5

πL2m : 64Dnm

(11)

We have no criticism of Eqs. (10) and (11), for it can be shown that they give correct results.

FIG. 1 Inward motion of the “apparent outgassing depth.”

}

danm anm

c0

Lm

When anm . Lm =2 corresponding to y . 0.5 for a sheet exposed on both sides, Eq. (5) must be replaced by

 Lm db K5 dt 4

(12)

and Eq. (7) by ðc0 2 bÞ ; K 5 Dnm  Lm =2

(13)

where b is the difference between the initial concentration, c0 , and the actual concentration at the middle of the sheet (x 5 Lm =2) as shown schematically by Fig. 2 (cf. Fig. 4 in van Liempt’s paper[5]). The area of the shaded triangle in Fig. 2 is (ðLm =4Þdb), and the concentration gradient is ðc0 2 bÞ=ðLm =2Þ. Combining (12) and (13) gives

40

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

8Dnm L2m or

ðt t1=2




dt 5

L2m t 5 t1=2 1 8Dnm



ðb

db ; c 0 02b

 c0 ln : ðc0 2 bÞ

(14)



(15)

FIG. 2 Decline of concentration at midpoint.

anm b

c0

db

Lm

From Fig. 2 it can be shown that the fraction of gas removed in time t is y5

c0 Lm 2 ðLm =2Þðc0 2 bÞ ; c0 Lm

(16)

ðc0 2 bÞ 5 2ð1 2 yÞ: c0

(17)

Thus

Substituting (11) and (17) in (15) gives the correct equation for the degassing time (in seconds) for y . 0:5: t5

πL2m L2 2 : 1 m ln 64Dnm 8Dnm 2ð1 2 yÞ

(18)

Instead of (18) van Liempt obtained the incorrect Eq. (1) because he assumed incorrectly that Eq. (2) would hold for anm . Lm =2. From Fig. 2 it is evident that
 Lm c0 : (19) anm 5 2 ðc0 2 bÞ Combining (15) and (19) gives the correct formula for anm when anm . Lm =2:  
 
Lm 8Dnm  anm 5 t 2 t1=2 : exp 2 L2m

(20)

Eq. (20) is quite different from (2), and the quantity anm no longer has any important physical significance when anm . Lm =2. We may now compare the correct Eq. (18) with the exact result as obtained from Eq. (4.18) in Crank’s book.[4] The fraction y becomes Mt =MN in Crank’ symbols. The time required to remove the fraction y of the gas dissolved in a sheet exposed on both sides can also be determined from Fig. 4.7 on page 57 of Crank’s book where Crank’s symbol l equals Lm =2. When the outgassing rate is not limited by surface phenomena but depends only on the diffusion coefficient, the curve marked N in Crank’s Fig. 4.7 should be used. This curve shows that for y 5 0.5

Characteristics of outgassing from metal surfaces Chapter | 2


 4Dnm t 1=2 5 0:45 L2m

41

(21)

or t 5 0:0506

L2m ; Dnm

(22)

where t is in seconds. This is in close agreement with Eq. (18) which for y 5 0:5 reduces to Eq. (11), corresponding to van Liempt’s formula, where π=64 5 0:0491. For y 5 0:5 Crank’s Fig. 4.7 gives
 4Dnm t 1=2 5 1:09 (23) L2m or L2m ; Dnm

(24)

L2m ; Dnm

(25)

L2m Dnm

(26)

t 5 0:297 whereas the incorrect Eq. (1) for y 5 0:95 gives t 5 4:91 while the correct Eq. (18) gives

t 5 0:337

which is much closer to the exact Eq. (24). Thus van Liempt’s approximate formulas are not very accurate above y 5 0:5, and in this region Crank’s Fig. 4.7 or Eq. (18) should be used. For a sheet exposed on only one side the right side of Eqs. (10), (11), (18), (22), (24)(26) should be multiplied by 4. Similarly, it can be shown that van Liempt’s equation for the degassing time of a long cylindrical rod of radius R (in cm) for the case anm . R or y . 2=3 is not correct. His equation for y , 2=3 is also not a good approximation except for t , 0:1R2 =Dnm . By applying Eqs. (3) and (6) the author has derived the following corrected equations for the degassing time of a long cylindrical rod at constant temperature:
 %

$ 3πR2 y 4y 1=2 2 t5 (27a) 1 2 12 ; y, 32Dnm 3 3
 

 πR2 4 2 ln3ð1 2 yÞ ; y. : (27b) 12 t5 π 3 24Dnm Calculations based on these equations agree very well with the more complicated exact formulas corresponding to Eq. (5.23) on page 66 and Eq. (5.49) on page 73 of Crank’s book.[4] From (27b)
  
 1 π 6Dnm t y512 exp 2 ; (28) 3 4 R2 which is a convenient for estimating the fraction of gas removed at time t when y . 2=3. Substituting (28) in (3) and using G0 Rc0 5 ; A 2 the outgassing rate for t . ðπ=24ÞR2 =Dnm will be  

 Dnm π 6Dnm t K 5 c0 2 exp 4 R2 R

(29)

(30)

42

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

in units of cm3 (stp) (s cm2)21, or


Knm 5

 
  760 Dnm π 6Dnm t 2 1023 c0 3 exp 4 R2 273 R

(31)

in unit of Torr L (s cm2)21 at the temperature T (K) of exposed surface, where c0 is the initial gas concentration in cm3 (stp) cm23 of solid. When 0 , t , ðπ=24ÞR2 =Dnm , the following approximate formula can be used for the outgassing rate of a wire: $

 
% 760T Dnm 1=2 Dnm 23 2 Knm 5 10 c0 3 : (32) 273 πt 2R Euringer[11] and Edwards[12] have measured the degassing rate as a function of time for a nickel wire that had been previously degassed and then saturated with hydrogen so that the initial gas content was uniformly distributed. The degassing ratetime curves plotted on a loglog graph were in agreement with the theoretical curve for diffusion of hydrogen out of a metal wire without any interference by rate-determining interactions at the surface. The theoretical curve was based on the formula

 N Dnm X 2 Dnm β 2i t K 5 2c0 exp ; (33) R2 R i51 where K is in units of cm3 (stp) (s cm2)21 and β i represent the roots of the equation J0 ðxÞ 5 0

(34)

in which J0 ðxÞ is the Bessel function of the first kind of order zero. The first four roots of (34) are β 1 5 2:405, β 2 5 5:520, β 3 5 8:654, β 4 5 11:792. When t . R2 =3Dnm , the terms for i . 1 in the series in Eq. (33) are negligible compared to the first term, and (33) reduces to 
2 
 Dnm β 1 Dnm t K 5 c0 : (35) exp ln2 2 R2 R Eq. (35) is nearly identical to (30) since ln 2 5 0:69 while π=4 5 0:78 and β 21 5 5:77. Integration of (33) with respect to time and multiplying by A=G0 as given by (29) yields the exact formula for the fraction, y, of gas removed from a long cylinder in time t:
 N X 2 Dnm β 2i t 22 β i exp y5124 ; (36) R2 i51 where use is made of the relation N X 4 5 1: 22 i51 β i

Lawson[13] checked the validity of Eq. (36) for the degassing of nickel wire but found discrepancies attributable to the fact that the initial gas content was not uniformly distributed but was concentrated near the surface of the wire. Eborall and Ransley[14] present the following formula for the fraction y of gas removed in t seconds by diffusion from a cylinder of finite length Lc (in cm) and radius R (in cm): (
) (
 X  ) N 2 X 2 Dnm β 2i t 8 N 22 2π t 22 β i exp ð2j21Þ exp 2Dnm ð2j21Þ 2 y512 4 3 ; (37) R2 π2 j51 Lc i51 where β i represents the successive roots of Eq. (34). In Eq. (37) we have corrected a misprint in Eborall and Ransley’s article[14] where π2 t=L2c is given as πt=L2c . Eborall and Ransley measured the accumulated hydrogen evolved from cylindrical samples of an aluminummagnesium alloy at temperatures in the range from 300 C to 500 C. They obtained good agreement between the experimental curve and the theoretical curve based on Eq. (37) when allowance was made for the initial time lag for the specimen to attain the temperature of the furnace.

Characteristics of outgassing from metal surfaces Chapter | 2

3

43

Outgassing rate with variable temperature

When the outgassing rate is not limited by processes occurring at the exposed surface but depends only on the diffusion coefficient, Dnm , as indicated by Eq. (6), the effect of suddenly raising the temperature, Tm , of a metal plate from room temperature, Ta , to a bakeout temperature, Tb , at the time tb will be a sudden increase in the outgassing rate by the factor Db =Da where Db is the value of Dnm at Tb and Da is the value at Ta . This follows from Eq. (6) when it is noted that the concentration gradient @c=@x is not changed by a sudden change in temperature. However, when the metal is then maintained at the bakeout temperature, the concentration gradient at x 5 0 begins to decrease rapidly from its value at time th 5 tb to a value at th . tb given by
 @c c0 5 ; (38) @x x50 xnm where xnm is given by xnm 5 60π1=2 ½Da tb 1Db ðth 2tb Þ1=2

(39)

in which th is the total pumping time in hours and th , t1=2 =3600 where t1=2 is given by (11). This follows from the procedure explained in Section 9.1 on page 147 of Crank’s book[4] since

and Eq. (9.3) in Crank’s book

[4]

Dnm 5 Da

ðth , tb Þ

(40a)

Dnm 5 Db

ðth . tb Þ

(40b)

becomes tx 5 3600½Da tb 1 Db ðth 2 tb Þ;

(41)

where tx (in seconds) is equivalent to Crank’s symbol T, while th is the pumping time in hours. Corresponding to Crank’s Eq. (9.4) we have @c @2 c 5 2: @tx @x For a semiinfinite slab, Eq. (42) gives

" c 5 c0 erf

and

x

(42) # (43)

ð4tx Þ1=2


 @c c0 5 : @x x50 ðπtx Þ1=2

(44)

Substituting (41) in (44) leads to (38) and (39). Substituting (38), (39), and (40b) in (6) gives an equation for the outgassing rate after bakeout begins ðth . tb Þ: K5

ðDb =3600πÞ1=2 c0

(45)

½th 2ð12Da =Db Þtb 1=2

where K is in units of cm3 (stp) (s cm2)21 and c0 is the initial concentration in cm3 (stp) of gas per cm3 of metal. In units of Torr L (s cm2)21 the outgassing rate for th . tb is Knm 5

1023 ð760 T=273ÞðDb =3600 πÞ1=2 c0 ½th 2ð12Da =Db Þtb 1=2

:

(46)

44

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

If bakeout does not begin until after th 5 1 h, then the outgassing rate at th 5 1 h is

1=2 Da 23 760T K1 5 10 c0 ; 273 3600π

(47)

and after bakeout begins ðth . tb Þ Knm 5

ðDb =Da ÞK1 ½ðDb =Da Þðth 2tb Þ1tb 1=2

:

(48)

This equation indicates that the outgassing rate at th 5 tb , just after the temperature has been quickly raised to Tb , will be Db =Da times the rate just before the increase in temperature, and as th increases beyond tb the outgassing rate falls rapidly and approaches asymptotically a rate which is ðDb =Da Þ1=2 times what the rate would have been if the temperature had been maintained at Ta corresponding to no bakeout. This is shown graphically inFig. 3 by the solid line curve in the region 110 h (and the dotted curve beyond th 5 10) where K1 has been chosen equal to 1028 Torr L (s cm2)21 and Db =Da 5 100 as convenient values to illustrate the equation. Eqs. (45)(48) apply only to the case in which the thickness, Lm , of the plate is so large that the outgassing rate at th is equivalent to that calculated for a semiinfinite solid. The van Liempt outgassing depth is given by
 4 anm 5 60 1=2 ½Da tb 1Db ðth 2tb Þ1=2 (49) π and a plate of thickness Lm exposed to vacuum on only one side will behave like a semiinfinite solid until anm 5 Lm , while for a plate exposed on both sides the outgassing depth reaches the midpoint and exponential decay of the outgassing rate begins when anm 5 Lm =2. If th 5 tnm when anm 5 Lm , then Eq. (49) gives for a plate exposed on only one side 
 πL2m Da 1 12 (50) tnm 5 tb : 16 3 3600 Db Db Hence, for a plate exposed on only one side, Eqs. (46) and (48) hold for the time period tb , th , tnm , where tnm is given by (50). When the metal is maintained at the steady bakeout temperature, Tb , from th 5 tb to th 5 tc and then suddenly cooled to room temperature, Ta , and maintained at Ta after th 5 tc , then for th . tc we have in the place of (41) tX 5 3600½Da tb 1 Db ðtc 2 tb Þ 1 Da ðth 2 tc Þ;

(51)

and Dnm in (6) is again replaced by Da so that by combining (6), (40a), (44), and (51) we have 1023 ð760T=273ÞðDa =3600πÞ1=2 c0 Knm 5    1=2 th 1 ðDb =Da Þ21 ðtc 2tb Þ

(52)

in units of Torr L (s cm2)21. Using (47) we can also write Knm 5

K1 fth 1½ðDb =Da Þ21ðtc 2tb Þg1=2

:

(53)

Comparing (48) and (53) it may be noted that for th 5 tc the outgassing rate just after reducing the temperature suddenly from Tb to Ta will be Da =Db times the rate just before this decrease in temperature. When th is small compared to  ðDb =Da Þ 2 1 ðtc 2 tb Þ, Eq. (53) indicates that the outgassing rate is nearly constant for a while after the temperature has been lowered to room temperature. As th increases and becomes large compared to ðDb =Da Þ 2 1 ðtc 2 tb Þ, the outgassing rate for a “semiinfinite” slab of metal approaches as a limit the ordinary rate for room temperature outgassing without bakeout, Knm 5 This behavior is illustrated in Fig. 3.

K1 th 1=2

:

(54)

Characteristics of outgassing from metal surfaces Chapter | 2

FIG. 3 Theoretical curve for effect of bakeout. Ta , ambient temperature and Tb , bakeout temperature.

10–6

Outgassing rate [Torr L (s cm2)–1]

45

Tb

10–7 Ta 10–8

10–9

Ta

10–10 tb

tc

1

10

10–11 0

(tnm)b 102

103

(tnm)a 104

105

Time (h)

References in the paper[1-4] [4] [5] [10] [11] [12] [13] [14]

Crank J. The mathematics of diffusion. London: Oxford University Press; 1957. van Liempt JAM. Recl Trav Chim Pays-Bas 1938;57:87182. van Liempt JAM. Recl Trav Chim Pays-Bas 1932;51:114. Euringer G. Z Phys 1935;96:3752. Edwards AG. Br J Appl Phys 1957;8:4069. Lawson RW. Br J Appl Phys 1962;13:11521. Eborall R, Ransley CE. J Inst Met 1945;LXXI:52552.

Reviewed paper [1-4] Dayton BB. The effect of bake-out on the degassing of metals. Transactions of the ninth national vacuum symposium, 1962. New York: Macmillan; 1963. p. 293300.

[1-5] “Advantage of slow high-vacuum pumping for suppressing excessive gas load in dynamic evacuation systems” (Yoshimura, 2009) Yoshimura (2009) presented the paper,[1-5] “Advantage of slow high-vacuum pumping for suppressing excessive gas load in dynamic evacuation systems”, which is written in Japanese. In this paper, Yoshimura analyzed a pressure-rise curve, which is seen as a straight line, by considering the relation among the desorption rate, adsorption rate, and diffusive outgassing. This paper[1-5] was written in relation to the paper by Redhead (1996),[1-5-1] “Effects of re-adsorption on outgassing rate measurements.” The paper[1-5] comprises the following sections: 1. 2. 3. 4. 5.

Introduction Transitional phenomena of outgassing Excessive gas load just after switching over the evacuation mode Advantage of slow, short high vacuum pumping before high-speed, high-vacuum pumping Conclusion

46

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

In this book, Sections 1, Introduction, 2, Transitional phenomena of outgassing, and 3, Excessive gas load just after switching over the evacuation mode, are translated by Yoshimura and introduced, in view of “desorption, adsorption, and diffusive outgassing.” Abstract In dynamic diffusion pump (DP) systems, such as vacuum evaporators and sputter coating systems, where the chamber is frequently evacuated from atmospheric pressure to high vacuum, the oil-vapor back-streaming problem sometimes occurs owing to the excessive gas load flowing into the high-vacuum pump just after switching the evacuation mode from lowspeed roughing to high-speed, high-vacuum pumping. Two kinds of excessive gas load exist just after crossover: (1) gas molecules in the vacuum chamber space and (2) temporarily increased outgassing from the chamber wall surfaces. The outgassing rate from the chamber wall surface becomes very large with the rapid reduction of pressure owing to high-speed, high-vacuum pumping, because the time constant of diffusion of gas molecules in the wall surfaces is much larger compared with the time constants of pumping down and the resultant reduction of absorption rate of impinging gas molecules. Slow, high-vacuum pumping, followed by high-speed pumping, is very effective to suppress the temporarily increased outgassing load and the adverse effect of the space gas load, and to meet the maximum throughput capacity of the DP. Providing with a low-conductance bypass valve makes it possible to use a small-volume buffer tank and a low-speed RP as a backing pump, leading to a reduction in the cost of high-vacuum evacuation systems.

1

Introduction

Various types of vacuum evaporators are utilized in industrial field, which require a clean ultrahigh vacuum (UHV), in order to improve the quality of the film deposited. The high-resolution electron microscope requires UHV for the specimen chamber and the gun chamber, provided with the large camera chamber of large gas load. For such systems the large chamber such as the bell-jar or the camera chamber must be frequently exposed to atmosphere. Therefore the dynamical, clean evacuation system is essentially important, in considering the cost performance. For such systems, oil DPs or turbo-molecular pumps (TMPs) are generally utilized, both of which show the gas-compressing function. In dynamic DP evacuation systems, serious back-streaming of oil vapor sometimes occurs due to the temporarily increased gas-load just after crossover (crossover: switching the evacuation mode from low-speed roughing to highspeed, high-vacuum pumping). Two types of gas load exist just after crossover: the gas molecules in the large chamber and outgassing from the surfaces of the chamber walls and the parts inside the chamber. The outgassing load temporarily much increased just after crossover. This phenomenon is complicated, leading to the misunderstanding sometimes. In this paper, this temporarily increased outgassing phenomenon is examined by referring to the dynamic outgassing characteristics reported. In designing a vacuum system, we must understand the outgassing process, as well. The surfaces of a vacuum chamber evolve gas molecules at net rates, the difference between “diffusion followed by desorption” and “reabsorption of impinging gas molecules,” as well as desorption of the adsorbed gases in shallow internal surfaces, occurred at the same time.

2

Transitional phenomena of outgassing

The inner surfaces of the chamber wall of stainless steel (SS) are large outgassing sources. There are many pits and grain boundaries on the machined SS surfaces (belt-polished and buff-polished), which are covered with porous oxide layers, containing a large amount of gas molecules. Such trapped gas molecules diffuse out gradually when such surfaces are exposed to high vacuum. That is, the main process of outgassing is “diffusion” of gas molecules inside the materials, followed by “desorption” from the top-most surface. Dayton defined the outgassing rate Qoutgas as follows[1]: Qoutgas 5 Qdesorption 2 Qadsorption

(1)

As seen in Eq. (1), the outgassing rate Qoutgas depends on the gas sorption rate Qadsorption, which is strongly dependent on the gas impinging rate. Dayton (1960)[1] explained the outgassing rate depending on the pumping speed S of the system and on the ratio of S/A (A: sample area). The outgassing-rate measuring system used by Dayton has two pumping routes selectable, the pumping speed of 0.3 and 1.3 L s21, respectively.

Characteristics of outgassing from metal surfaces Chapter | 2

47

The present author (Yoshimura) would like to discuss the dependence of outgassing rate on such parameters. Engineers in around 1960 might considered as follows. “When the sample area is evacuated by a larger pumping speed, the outgassing rate of the sample increases compared with being evacuated by a smaller pumping speed” “When the chamber is evacuated by a large pumping speed, the evolved gas molecules will be pumped out from the chamber, the number of the impinging gas molecules will be reduced. Therefore the ratio of S/A (A: area of the chamber wall) must be an important parameter for the outgassing rate of the wall. The balanced pressure inside the chamber decreases with the increase of the pumping speed, so the impinging gas molecules reduced with the increase of pumping speed, leading to the reduction of sorption rate. That is, the outgassing rate of the chamber wall is considered to be dependent of the pumping speed. However, strictly speaking, in the UHV region, the pressure in the chamber keeps almost constant, when varying the opening of the valve between the chamber and the UHV pump. This fact means that the pump evolves gases effectively. Additionally speaking, the gas molecules in the pump vessel have a chance of entering the chamber and being absorbed onto the chamber wall again. From the discussion above, the outgassing rate of the chamber wall depends upon the pressure to which the chamber wall is exposed, not upon the pumping speed for the chamber. The outgassing rate of the sample material is often measured by the isolation method, namely the pressure-rise method. It is recommended that the pressure-rise rates are measured as low as possible in order to reduce the error involved in the measured rates. Transitional outgassing is complicated. Rapid pumping-down means the rapid reduction of impinging gas molecules, resulting in rapid reduction of sorption (sorption means adsorption and absorption, altogether). This means the rapid increase of outgassing. At the same time, pumping out of residual gas molecules will gradually reduce outgassing because the total quantity of gas molecules inside the wall of chamber is gradually reduced. Generally speaking, the outgassing rate first increases, then decreases gradually with pumping time. Yoshimura et al. (1991)[4] measured the pressure-rise curves repeatedly in sequence with an extractor gauge (EG), as follows. Let us consider the SS (SS304) chamber, which is isolated repeatedly by the metal valve from the TMP. The base pressure just before the first isolation test was 5.8 3 1028 Pa (N2 equivalent pressure). The electropolished (EP) SS304 pipe-chamber was in situ baked (150 C, 20 h), and the metal valve was baked at about 100 C. An EG in the water-cooled adapter was degassed by electron bombardment (19 W, 3 min), then operated at 0.59 mA. The base pressure was 5.8 3 1028 Pa (N2 equiv.). The isolation test was repeated four times in sequence. For the first two tests, the valve was closed 60 min each, while for the last two tests, 90 min each. The evacuation period between successive isolations was 10 min. The pressure-rise curves in the form of time sequence are presented in Fig. 2. FIG. 2 Pressure-rise curves and pump-down curves for the pipe chamber in series of isolation test. The inner surface of the SS304 chamber is EP. Tangents ① to ④ on the pressure-rise curves at 1 3 1025 Pa are drawn for comparing the outgassing rate under the same impinging rate. EP, Electropolished.

Pressure (×10–6 Pa)

20 4 3 2 1

10

0 0

100

200 Time (min)

300

The pressure-rise rate and the attained pump-down pressure were both increased when isolation and evacuation were repeated. This result shows that the quantity of gas molecules absorbed into the shallow oxide layer increased with repeated isolation tests, resulting in increased outgassing.

48

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Fig. 2 provides us the following information: 1. The tangential angle of the first pressure-rise curve around 1 3 1027 Pa is very large, one reason of which may be additional outgassing from the metal valve when pressing the valve wedge into the copper nose. The first curve bends considerably at around 5 3 1026 Pa, then pressure rises almost linearly with time. 2. The tangents ① to ④ at 1 3 1025 Pa are drawn for comparing the outgassing rates with each other under the same impinging rate. The outgassing rate becomes larger when the isolation test (without any venting) is repeated. 3. In spite of small time-constant of evacuation, pumping down is considerably slow, which means the outgassing rate is much increased after each isolation test. The attained pressure at the end of each 10-min evacuation becomes higher and higher with repeated cycles. The pressure after 10-min evacuation following the fourth isolation test is as high as 9 3 1026 Pa, which is more than 100 times greater than the base pressure of 5.8 3 1028 Pa. This means the outgassing rate becomes more than 100 times larger than the rate just before the first isolation test. 4. The pressure under isolation seems to rise almost linearly in higher pressure range. The sorption rate will increase with pressure rise, and so the desorption rate will also increase with the increase of sorption rate, resulting in almost constant outgassing during the long isolation period. Redhead (1996)[1-5-1] introduced an interesting pressure-rise curve as follows. “Jousten (1994, 1996)[5,6] measured the pressure rise in the 316LN chamber that had been vacuum fired and then baked in situ for 48 h at 250 C. After pumping down to 2.0 3 1028 Pa (1.5 3 10210 Torr) (H2 equiv.) it was sealed off and the p(t) curve was measured with the spinning rotor gauge (SRG) for 4 weeks; the curve was linear and the pressure increased by a factor of about 106.” Redhead discussed as follows[7]: “Experimental measurements by the pressure-rise method of the outgassing of austenitic stainless steels show that the gas is hydrogen, that the pressure rise after isolation is linear for as long as 103 h or more, and that the ratio of final to initial pressure may be as high as 106. A model to explain these results must address two matters: the constancy of the outgassing rate, and the apparent absence of any re-adsorption.” Let us discuss on the almost straight pressure-rise characteristics. The measured pressure-rise curve might be an earlier part of the whole curve approaching the saturated pressure smoothly. The whole curve might be logarithmic, as discussed by Yoshimura (1985).[2] An earlier part of the whole logarithmic curve is almost linear. Many adsorption isotherms presented in textbooks[6],[7] show that typical residual gas species such as H2, CO, and H2O are all absorbable onto various metal surfaces. Let us further discuss the almost constant outgassing rate during the long isolation period by referring to Fig. 4,

FIG. [1-5-1]-7 In Redhead.[1-5-1] Pressure rise after seal-off of a stainless-steel chamber, previously vacuum-fired and baked at 250 C for 48 h, measured with the spinning rotor gauge. The chamber was sealed off at a pressure of 2 3 1028 Pa, the first point taken at 33 min at a pressure of 1.52 3 1025 Pa. From Jousten K. Shinku 1994;37 (9):67885 with permission; Redhead PA. Effects of re-adsorption on outgassing rate measurement. J Vac Sci Technol A 1996;14(4): 2599609.

0.020

Hydrogen pressure →

Pa

Pressure rise method with SRG

0.015

0.010

0.005

0.000 0

100

200

300 400 Time →

500

h

700

where the pressure-rise line is the same as the line introduced by Redhead (1996).[1-5-1] Four points (a)(d) along the line are selected for discussion. The pressure-rise line is indeed almost linear, which means almost the same outgassing rates at points (a) to (d). The rate qoutgas is expressed as qoutgas 5 qdesorption 2 qsorption . The impinging rate of gas molecules at point (d) is six times

Characteristics of outgassing from metal surfaces Chapter | 2

FIG. 4 Relationship among desorption rate qdesorption , sorption rate qsorption and outgassing rate qoutgas during the long isolation period, showing almost constant outgassing rate qoutgas . The pressure-rise line is the same as the line of Fig. [1-5-1]-7.

Pressure rise curves

Hydrogen pressure (Pa)

0.020

↑ qdesorption ↓ qsorption ↑ qoutgas

0.015

49

(d)

0.010

(c)

0.005 (b) (a) 0.000 0

100

200

400 300 Time (h)

500

600

700

that at point (a), so the rate qsorption at point (d) would be six times that at point (a), assuming a constant sorption coefficient. The surface oxide layer containing a large quantity of gas molecules in shallow positions, will show a large desorption rate. That is, the gas molecules absorbed into the surface oxide layer will desorb, keeping an almost constant outgassing rate qoutgas . This relationship among qdesorption , qsorption , and qoutgas is shown by arrows m（qdesorption ), k (qsorption ) and ↥ (qoutgas ) at points (a)(d). Suppose that the valve is opened to evacuate the chamber that has been isolated for 700 h. The pressure will fall down very slowly, which is controlled by the greatly increased outgassing rate. Also, suppose that the second isolation test is conducted by closing the valve again following supposed evacuation of a few minutes. The pressure will rise more quickly owing to the increased outgassing, which could be predicted from the pressure-rise curves presented in Fig. 2.

3

Excessive gas load just after switching over the evacuation mode

In dynamic evacuation systems, a serious oil-vapor back-streaming sometimes occurs due to a temporarily increased gas load just after crossover (crossover: switching the evacuation mode from low-speed roughing to high-speed, highvacuum pumping). Particularly in DP systems, an excessive gas load causes serious oil-vapor back-streaming when the backing pressure exceeds the critical value. Two kinds of excessive gas load exist just after crossover: (1) the space gas and (2) the outgassing gas load. Hablanian (1992)[12] discussed how to prevent overload as follows: The overloading of the high vacuum pump due to the space gas can be prevented by opening the high vacuum valve slowly or by using a parallel low-conductance bypass. However, the overload due to the outgassing rate can only be prevented by following the golden rule of mass flow limitation. (Golden rule: the crossover must be performed when the gas mass flow from the vacuum chamber is less than the maximum throughput capacity of the high-vacuum pump.) An immediate corollary of matching mass flows is that the larger the roughing pump, the lower the crossover pressure must be.

The present author (Yoshimura) could not agree on the underlined part of the discussion by Hablanian (1992).[12]

3.1 Advantages of a small bypass valve In dynamic DP systems, the chamber is frequently evacuated from atmospheric pressure to high vacuum. In such systems the oil-vapor back-streaming problem sometimes occurs owing to the excessive gas load flowing into the highvacuum pump just after crossover (crossover means switching the evacuation mode from low-speed roughing to high-speed, high-vacuum pumping). Two kinds of excessive gas loads exist just after crossover (1) gas molecules in the vacuum chamber space and (2) temporarily increased outgassing from the chamber wall surface. The outgassing rate from the chamber wall surfaces become very large with the rapid reduction of pressure owing to high-speed, high-vacuum pumping, because the time constant of diffusion of gas molecules inside the wall surface is

50

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

much larger compared with the time constant of impinging gas molecules, resulting in adsorption of impinging gas molecules. Slow high-vacuum pumping, followed by high-speed high-vacuum pumping, is very effective to suppress the temporarily increased outgassing load and the adverse effect of the space gas load, and to meet the maximum throughput capacity of the DP. Equipping with a low-conductance bypass valve makes it possible to use a smallvolume buffer tank and a low-speed RP as a backing pump, leading to a reduction in the cost of high-vacuum evacuation system. Suppose the DP system provided with a bypass valve (3 L s21) as shown in Fig. 6. The chamber (B100 L) is first evacuated to a switching pressure Ps (10 Pa) by a roughing RP) (2 L s21), then evacuated through a bypass valve (3 L s21) for about 30 s, followed by high-speed DP pumping (300 L s21). The comparatively low pumping speed of the roughing RP is further reduced when the pressure goes down to the range of 10 Pa and much reduced when approaching the switching pressure of 1 Pa. FIG. 6 DP system equipped with a bypass valve and the supposed pumping-down and pressure-rise curves in isolation tests. The supposed pumping-down curves and the supposed pressure-rise curves for the conventional DP system, not equipped with a bypass valve, are drawn with broken lines. The followings are supposed: conductance of bypass valve; 3 L s21, chamber volume; 100 L, rated pumping speed of RP; 2 L s21, rated speed of DP system; 300 L s21, switching pressure; 10 Pa. Supposed bypass pumping period, 30 s. ①, ②, ③; supposed pressure-rise curves; ① at crossover, ② at the end of bypass pumping, and ③ at an early period of high-speed pumping. DP, Diffusion pump.

≈

Pressure (Pa)

1

10 2

3

0

0

10 Time (min) Ph Bypass valve

Pb

BT

By the help of the low-conductance bypass valve, the outgassing rate of the chamber-wall surface could be controlled not to exceed the rate just before crossover. Furthermore, the maximum pressure at the inlet port of DP could be suppressed below 0.1 Pa just after crossover because the ratio of bypass conductance (3 L s21) to DP pumping speed (300 L s21) is so small as 0.01. Slow evacuation through a bypass valve is very effective to suppress the temporarily increased outgassing load and the adverse effect of the space gas load, and to meet the Golden rule of Hablanian.[12] Slow evacuation resolves four restrictive conditions for DP systems; ① roughing RP oil-vapor backstreaming, ② DP oil-vapor backstreaming, ③ critical backing-pressure problem, and ④ working fluid flow-out. Using a low-conductance bypass valve for a short time makes it possible to use a small-volume buffer tank and a low-speed backing RP, leading to cost reduction of the evacuation system. Next, suppose the DP system not equipped with a bypass valve. The pressure in the chamber goes down rapidly to high-vacuum after crossover as shown by a broken line in Fig. 6. The outgassing rate of the chamber-wall surface becomes very large as shown by the predicted pressure-rise curves drawn by broken lines, because the time constant of diffusing gas molecules in oxide layers is much larger compared to the time constant of the reduction of pressure and that of the reduction of sorption rate. Note: As for the straight line of pressure rising, P.A. Redhead presented the paper [1-5-1], [1-5-1*].

Characteristics of outgassing from metal surfaces Chapter | 2

51

References in the paper[1-5] [1] Dayton BB. 1959 sixth national symposium on vacuum technology transactions. Pergamon Press; 1960. p. 10119. [2] Yoshimura N. J Vac Sci Technol A 1985;3(6):217783. [4] Yoshimura N, Hirano H, Ohara K, Ando I. J Vac Sci Technol A 1991;9(4):231518. [5] Jousten K. Shinku 1994;37(9):67885. [6] Jousten K. Vacuum 1996;47:325. [12] Hablanian MH. J Vac Sci Technol A 1992;10(4):262932. [1-5-1] Redhead PA. Effects of re-adsorption on outgassing rate measurement. J Vac Sci Technol A 1996;14(4):2599609. [1-5-1*] Redhead PA. Erratum: effects of re-adsorption on outgassing rate measurements [J. Vac. Sci. Technol. A 14, 2599 (1996)]. J Vac Sci Technol A 1997;15(4):2455.

Reviewed papers [1-5] Yoshimura N. Advantages of slow high-vacuum pumping for suppressing excessive gas load in dynamic evacuation systems. J Vac Soc Jpn 2009;52(2):928 (in Japanese).

[1-6] “The variation in outgassing rate with the time of exposure and pumping” (Rogers, 1964) Rogers (1964) presented the paper,[1-6] “The variation in outgassing rate with the time of exposure and pumping.” The paper[1-6] comprises the following sections: 1. 2. 3. 4. 5. 6.

Introduction Analysis Typical history First pump-down Second pump-down Summary In this book, Sections 2, Analysis, 4, First pump-down, 5, Second pump-down, and 6, Summary, are introduced.

Abstract Past experiments have shown that the pump-down time of a vacuum chamber varies with the exposure to moist air. This paper presents a theoretical analysis of the variation. It is shown that when the vapor outgassing dominates the pump-down, the slope of the log pressure versus log pumping time curve will vary from 1/2 to 3/2 for a semiinfinite plate. This is in contrast to the theoretical slope of 1/2 that is found for uniformly distributed gas in a semiinfinite plate.

2

Analysis

In this analysis it will be assumed that the wall outgassing is governed by the usual diffusion equations, with a constant diffusion coefficient. @C @2 C 5D 2 @t @X
 @C N 52D @X X50

(1) (2)

where D: Length2/time C: Molecules/length3 N: Molecules/length2  time. It is first necessary to determine suitable initial conditions and boundary conditions for use in solving these equations. The initial conditions will depend upon the exposure history of the chamber walls. If the chamber has just been thoroughly outgassed, there will be a negligible concentration of molecules in the wall and the appropriate initial

52

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

condition will be C 5 0 throughout the wall. If the chamber has just been exposed to a moist atmosphere for a long period of time, the concentration will be nearly uniform at a value C0 which will depend upon the wall temperature and the partial pressure of the water vapor in the moist atmosphere. Due to surface condensation, the value of C0 to be associated with the wall surface may be many times the concentration of water molecules in the atmosphere. This occurs even when the surface temperature is well above the temperature at which the pure water vapor would condense and results from the greater attraction between the water molecule and the wall than between water molecules. While these represent two simple initial conditions, in the general case the initial conditions will be more complex. In the typical operation of a chamber, the walls may be exposed alternately to both moist atmospheres and vacuums. If these exposure times are short compared to the time required to establish a uniform concentration of molecules within the sorption layer, the outgassing rate will depend upon the exposure history of the chamber over several of the preceding cycles. Since there are wide variations of possible exposure histories, it will be useful to analyze a typical history that demonstrates the effects of these variations.

4

First pump-down

Considering only the pump-down immediately following the initial moist atmosphere exposure, two limiting conditions can be noted. If the equation is evaluated when the pumping time is short compared to the exposure time, the ðt2tm1 Þ21=2 term dominates. Since t 2 tm1 is the pumping time, this equation is the usual diffusion equation. On the other hand, if the equation is evaluated after long pumping times compared to the exposure time, a ðt2tm1 Þ23=2 term dominates and the equation no longer resembles the usual diffusion equation. This result is shown in Fig. 1 where the relative outgassing rate has been calculated for a series of exposure times. In this and the following figures, the outgassing rate has been ratioed to the parameter ðD=πÞ1=2 C0 . FIG. 1 Variation in outgassing rate with exposure time for an initially outgassed system (any consistent time units).

1. Ref. 2 1 1

Exposure time 1 MIN 3 HRS 17 HRS

Outgassing rate × 1 C0

π D

1 2 3

.1 Exposure .1 time

1.

10

1000

100

∞

1 2

.01

.001 .1

1.

10 Pumping time

100

3

1000

Included in this plot are some experimental data from Hayashi[1] and Power and Crawley.[2] These results were determined by calculating the average wall outgassing rate from the ratio of the chamber wall area to the pump mass flow, and using a value near the start of pump-down to evaluate C0 ðD=πÞ1=2 . While the average outgassing rate should depend on the type of “O” ring seals, etc., and the parameter C0 ðD=πÞ1=2 would be expected to vary from chamber to chamber, for the three chambers considered the variation was only about a factor of 4 and the level was about 5 3 1015 molecules (cm2 s1/2)21. In Fig. 1, the experimental level was adjusted to coincide with the theory at one point, so the method can only be used to check the general slope of the experiment and theory. The range of the slope in the Power and Crawley data[2] results from determining the outgassing rate by two methods. In the first method the pumping speed was assumed constant and in the second method the speed was varied according to their estimated pumping speed as a function of pressure. The resulting variation in outgassing rate shows the importance of accurately determining the pumping speed curve. The variation in slope of the Hayashi[1] data resulted from changes in wall temperature. While the experimental and theoretical slopes show the approximate agreement, it appears specific experiments in a single chamber will be required to determine the validity of the theoretical approach. Power and Crawley[2] did present

Characteristics of outgassing from metal surfaces Chapter | 2

53

data for a short exposure time along with the data for the 17 h exposure. The pump-down time required after the short exposure was much longer than the theoretical time, but since that experimental pump-down time was comparable to the time required after an extended dry air exposure, it is concluded that other factors contributed to the long pumpdown time. If the system has a pumping speed that is independent of pressure, Fig. 1 can be used to estimate the relationship between the exposure time and the pumping time required to reach a specific pressure level. When this is done for a typical range of pimping and exposure times, it is found that the required pumping time is proportional to the exposure time raised to the 1/2 to 2/3 power. Thus doubling the exposure time should increase the pump-down time by about 50%.

5

Second pump-down

If the initial moist atmosphere exposure time was very long compared to the time of interest, the concentration can be considered uniform, and the first pump-down will exhibit the square root relationship between outgassing rate and pumping time. If the system is now exposed to a moist atmosphere, and again pumped down, the resulting outgassing variation will depend upon both the previous pump-down time and the following moist atmosphere exposure time. Examples of the resulting outgassing variations are shown in Fig. 2 for a range of initial pumping times which have been followed by a unit exposure time. The general pattern of the outgassing curve, is to follow the thoroughly outgassed curve until the outgassing rate is of the same order of magnitude as that at the end of the prior pump-down at which point the curve flattens out approaching the prior pump-down curve, but always lagging behind it by a time interval equal to the sum of the exposure time and the present pumping time. This return to the square root relationship serves to emphasize that the unit exposure and subsequent pump-down represent only a relatively small variation in the general pump-down of a semiinfinite plate. This can be seen in Fig. 3 where the curve with a 100 unit initial pumpdown time has been replotted so the time scale includes the initial pump-down and exposure. Here it is seen that the unit exposure and the subsequent pump-down occupy only a small fraction of the logarithmic plot, and it is also evident that the pump-down will continue along the original curve, despite additional minor exposures. This illustrates the general problem in attempting to outgas the walls of a chamber when the slow diffusion rate makes the walls essentially semiinfinite. Each succeeding pump-down will produce an outgassing rate only slightly below that of the previous pump-down, and since the pressure is directly related to the outgassing rate, enormous pumping times would be required to significantly reduce the system base pressure. In practice this difficulty is overcome by baking the chamber, thus increasing the diffusion coefficient enough to remove the gas from the finite walls. Once the system has been baked and cooled, the slow diffusion rate will delay any significant in-gassing, and the analysis for the thoroughly outgassed system will apply. This emphasizes that the outgassing characteristics of a system are dependent upon the complete exposure history of the system, and not just the conditions immediately preceding the pump-down. FIG. 2 Variation in outgassing rate with prior pumping time for a system with a uniform initial concentration (any consistent time units).

1. 0

Outgassing rate × 1 C0

π D

10 .1

102

103

.01

104

∞ .001 1.0

10

105

100 Pumping time

Prior pumping time

1000

10,000

54

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

1. π D Outgassing rate × 1 C0

FIG. 3 Variation in outgassing rate with total elapsed time (any consistent time units).

Unit time exposure to moist atmosphere Following pump-down

Initial pump-down

.1

.01 1.0

10

100

1000

Elapsed time

Notes (by Yoshimura): In Figs. 13, C is the concentration in molecules/length3; C0 is the initial concentration; D is the diffusion coefficient in length2/time.

6

Summary

The results of a theoretical analysis have demonstrated that for a diffusion-controlled process, the outgassing rate depends upon the exposure history of the system. It has been shown that the slope of the log outgassing rate versus log pumping time curve will vary from nearly 0 to 3/2, if the layer is thick enough to satisfy the criterion L $ 2ðDtÞ1=2 .

References in the paper[1-6] [1] Hayashi S. Trans fourth Nat. Vac. Symp. 1957. New York: Pergamon Press; 1958. p. 13. [2] Power BD, Crawley JE. (Namur Congress), 1958 Advances in vacuum sciences and technology. Oxford: Pergamon Press; 1960. p. 20611.

Reviewed paper [1-6] Rogers KW. The variation in outgassing rate with the time of exposure and pumping. Transactions of the 10th national vacuum symposium, 1963. New York: Macmillan; 1964. p. 847.

[1-7] “Reduction of stainless-steel outgassing in ultra-high vacuum” (Calder and Lewin, 1967) Calder and Lewin (1967) presented the paper,[1-7] “Reduction of stainless-steel outgassing in ultra-high vacuum” The paper comprises the following sections: 1. Introduction 2. Theory 2.1. Effect of temperature on degassing 2.2. Outgassing after a normal bakeout 2.3. Permeation rate of atmospheric hydrogen 2.4. High-temperature bulk degassing in situ 2.5. High-temperature bulk degassing in a furnace with residual hydrogen pressure 3. Experimental details

Characteristics of outgassing from metal surfaces Chapter | 2

55

4. Results and discussions 4.1. Specimen A 4.2. Specimen B 4.3. Specimen C 4.4. Specimen D 5. Summary and conclusion Appendix In this book, Sections 2, Theory, and 5, Summary and conclusion, are introduced. Abstract The outgassing rate of a 2 mm thick SS sheet was measured in ultrahigh vacuum at constant pressure to avoid the readsorption occurring in a rate of pressure-rise determination. The rate was typically 10212 Torr L (cm2 s)21, and about 99% or more of the gas was hydrogen. Since SS usually contains large amounts of hydrogen and the diffusion coefficient of hydrogen in SS is high, it was suspected that the hydrogen diffuses to the surface from the interior of the metal and is released into the vacuum. Calculations show that the observed outgassing rate could be explained by such a process and should be reduced by several orders of magnitude by a high-temperature treatment. The effects of residual hydrogen in the treatment furnace and hydrogen permeation from the atmosphere are also considered in these calculations. The greater the thickness of the metal, the higher the temperature has to be. Measurements are in reasonable agreement with these calculations.

2

Theory

2.1 Effect of temperature on degassing The one-dimensional diffusion equation is D

@2 c @c 5 @x2 @t

(1)

where D is the diffusion coefficient, and c is the concentration. Eq. (1) is solved for a slab of unit cross section and thickness d. Initially the concentration is c0 and constant throughout. At time t 5 0 vacuum is applied to both faces. The initial and boundary conditions are c 5 c0

for 0 # x # d

at t 5 0

and c50 The solution is (Levin, 1965,

[6]

for x 5 0 and x 5 d

at t . 0:

p. 32)

(
 ) N 4X πð2n 1 1Þx xð2n11Þ 2 21 exp 2 ð2n11Þ sin Dt : cðx; tÞ 5 c0 π 0 d d

The instantaneous gas flow from one face of the slab is (

 2 ) N X @c 4c D πð2n11Þ 0 Q_ 5 D 5 exp 2 Dt : @x x50 d d 0 Values of N X 0

(2)

(3)

(
 ) πð2n11Þ 2 exp 2 Dt d

as a function of Dt=d are given in Fig. 1. For Dt=d 2 . 0:025, as is the case in many practical situations, we can write, to a sufficient approximation, 2

4 πx c 5 c0 sin expð2 π2 d22 DtÞ: π d

(4)

56

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

and Q_ 5 4c0 Dd21 expð2 π2 d22 DtÞ:

(5)

If a sheet has been degassed for t1 seconds at a temperature T1 , the outgassing rate (immediately) afterward at room temperature Tr is Q_ r 5 4c0 Dr d21 expð2 π2 d22 D1 t1 Þ: 10

∞

Σ exp – 0

1

π (2n + 1) d

2

(6)

FIG. 1 Plot of infinite series of Eq. (3) and its first term.

Dt

exp (–π2 d–2 Dt)

10–1

10–2 10–4

10–3

10–2

10–1

Dt/d2

It is shown in the Appendix that Q_ r is, for all practical purpose, time independent. Since the product D1 t1 appears in the exponent, an increase of the bakeout temperature, which renders D1 larger, has the same effect as an increase in the time of bakeout at a lower temperature. However, because of the exponential dependence of D on temperature[1] a large reduction in outgassing rate should be readily obtained by a high-temperature bakeout. For example, the diffusion constants for H2 in SS are (Eschbach et al., 1963) D300 5 3:5 3 1028 cm2 s21 and D1000 5 8:7 3 1025 cm2 s21 . Hence a 1-h bakeout at 1000 C is equivalent to 2500 h at 300 C. Both are calculated by Eq. (6) to decrease the subsequent roomtemperature outgassing rate, compared with that given after a normal bakeout, by more than a factor of 1030 for 2 mm thick material. Clearly this enormous potential of high-temperature degassing cannot be utilized. It could be nullified by any small leak. Furthermore, other gases will desorb at much higher rates. The above calculation neglected permeation of atmospheric hydrogen and the influence of residual hydrogen pressure in the bakeout furnace. It is pertinent to examine the importance of these limiting effects to determine what degree of degassing is likely to be useful. A high-temperature bakeout of vacuum-chamber components may, in principle, be executed either in situ or in a furnace prior to assembly, since diffusion at room temperature is negligible. The latter case would necessitate a final, in situ, normal bakeout to remove surface contamination. Hereafter we shall consider only 2 mm thick material unless it is explicitly stated otherwise. SS of 2 mm thickness is commonly employed in the fabrication of vacuum chambers. For material of other thicknesses the following calculations are easily modified.

2.2 Outgassing after a normal bakeout Chemical analysis of hydrogen from the supplier (Compagnie des Ate´liers et des Forges de la Loire, France) of our SS gave 5 standard cm3 per 100 g or, equivalently, c0 5 0:3 Torr L cm23 . The diffusion coefficient for hydrogen in SS at room temperature is[1] Dr 5 5:0 3 10214 cm2 s21 . The application of Eq. (6) for 2 mm sheet after a normal degas at 300 C for 24 h gives Q_ r 5 3:0 3 10213 expð2 0:75Þ 5 1:4 3 10213 Torr L cm22 s21 :

(6a)

Characteristics of outgassing from metal surfaces Chapter | 2

57

A second bakeout would reduce Q_ r by a further factor e20:75 5 0:47. The experimentally measured values were close to 10212 Torr L (cm22 s)21 and are not strongly dependent on further bakeout. There is some indication that c0 may have been higher. This calculation neglects the partial pressure of hydrogen at the outside wall. This pressure is supposed to be 4 3 1024 Torr.[3] We do not know how much it varies and what it was in our laboratory. At 300 C, the atmospheric hydrogen causes a surface concentration ca 5 1:4 3 1024 Torr L cm23 [see Eq. (9)]. After the bake, the maximum concentration in the center is, from Eq. (4), c0 ð4=πÞ0:47 5 0:2 Torr L cm23 . Hence the effect of the atmospheric hydrogen is negligible in this case. The effect of permeation of a thoroughly degassed slab is treated in the next section.

2.3 Permeation rate of atmospheric hydrogen It is assumed that the SS has been completely degassed. Eq. (1) is solved for a slab where c and P are initially zero. At time t 5 0, one face is exposed to a constant pressure P1 which produces a concentration c1 at this surface while vacuum is maintained at the other face. Hence Eq. (1) must be solved with the following initial boundary conditions: c 5 0 for 0 5 0 # x # d

at t 5 0

c50

for x 5 0

at t . 0

c 5 c1 for x 5 d

at t . 0

[6]

The solution is

   N c1 x 2c1 X ð21Þn nπx nπ 2 1 exp 2 sin cðx; tÞ 5 Dt : d d d π 1 n The instantaneous permeation rate at time t is

  
 N @c Dc1 2Dc1 X nπ 2 1 Q P1 5 D 5 ð21Þn exp 2 Dt : @x x50 d d d 1 n  N 2 o P Values of 1 1 2 ð21Þn exp 2 nπ=d Dt as a function of Dtd22 are given in Fig. 2. 1

1+2 1

∞

Σ (–1)n exp – 1

nπ d

2

Dt

10–1

10–2

10–3

10–4 10–2

10–1 Dt/d 2

1

FIG. 2 Plot of the infinite series of Eq. (8).

(7)

(8)

58

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

For t-N, the equilibrium permeation flow rate at room temperature is ðQ_ P1 Þr 5 Dr

c1 : d

For shorter times, such that Dr td22 {0:02, the permeation rate is ððQP1 Þr0 , 1024 ðQ_ P1 Þr : (For 2 mm thick SS this is true for t # 1010 s.) The concentration c1 of a diatomic gas dissolved in a metal at temperature T which is in equilibrium with an external gaseous pressure P1 is[6] c1 5

KT 1=2 P DT 1

(9)

where KT is the permeability constant and DT the corresponding diffusion constant. For H2 in SS Kr 5 1.2 3 10216 Torr1/2 L (cm s)21. Thus for 2 mm SS exposed to a partial pressure of 4 3 1024 Torr of hydrogen 1=2

c1 Dr K r P1 ðQ_ P1 Þr 5 5 D1 3 10217 Torr L ðcm2 sÞ21 : d d

(10)

and for practical times ðQ_ P1 Þr0 {1 3 10221 Torr L ðcm2 sÞ21 . Hence concentration changes at room temperature are negligible. However, the normal in situ bakeouts, by virtue of the greatly enhanced diffusion constant at the bakeout temperature, will modify the above conclusion to the extent of establishing near-equilibrium permeation rates. In fact, because K=D is a function increasing with temperature, the apparent permeation rate at room temperature after an in situ bakeout at temperature T will, in the limit, be increased above the value given by Eq. (10) to ðQ_ P1 ÞTr 5


 K Dr 1=2 P : D T d 1

(11)

Taking a temperature of 300 C where K300 5 2:3 3 10210 Torr1=2 L ðcm sÞ21 and D300 5 3:5 3 1028 cm2 s21 , Eq. (11) gives for the limiting value of hydrogen permeation 217 ðQ_ P1 Þ300 Torr L ðcm2 sÞ21 : r D3:3 3 10

The flow rate is 13% of this amount after 24 h since D300 t24 d22 5 7:5 3 1022 [see Eq. (8) and Fig. 2]. It is worth noting that during a prolonged 300 C bakeout the hydrogen permeation rate would reach a limiting value of K300 1=2 P D2:3 3 10211 Torr L ðcm2 sÞ21 : ðQ_ P1 Þ300 5 d 1 Determination of hydrogen permeation through a suitable metal, for example, palladium, at elevated temperatures may be a practical method to measure the partial pressure of hydrogen in the atmosphere.

2.4

High-temperature bulk degassing in situ

It is clear from the above considerations that any attempt to reduce the outgassing rate below about 10216 Torr L (cm2 s)21 is pointless. This, however, already represents a theoretical improvement factor of 103 on that obtained via a normal bakeout. We may calculate the bakeout requirements necessary to yield a Q_ r 5 10216 Torr L ðcm2 sÞ21 for 2 mm SS with an initial concentration of 0.3 Torr L cm23 of hydrogen. It follows from Eq. (6) that Q_ r 5 10216 5 3 3 10213 expð2 π2 d22 DtÞ or Dt 5 3:25 3 1022 cm2 . Table 1 is based on data from Eschbach et al. (1963).[1]

(12)

Characteristics of outgassing from metal surfaces Chapter | 2

59

_ r 5 10216 Torr L ðcm2 sÞ21. TABLE 1. Bakeout times of 2 mm thick sheet at various temperatures for Q t (s) 1:0 3 106 8:6 3 104 1:1 3 104 3:6 3 103

(11 days) (24 h) (3 h) (1 h)

T (cm2 s21)

T ( C)

3:5 3 1028 3:8 3 1027 3:0 3 1026 9:0 3 1026

300 420 570 635

In situ bakeouts up to temperatures of 420 C are feasible. Application of Eq. (11) at T 5 420 C where K420 5 3:2 3 1029 Torr1=2 L ðcm sÞ21 gives 217 Torr L ðcm2 sÞ21 ðQ_ P1 Þ420 r 5 4:2 3 10

a value which is still quite acceptable.

2.5 High-temperature bulk degassing in a furnace with residual hydrogen pressure If the degassing is performed at higher temperatures in a furnace, it is necessary to consider the partial pressure of hydrogen in the residual furnace atmosphere. A pressure of PF Torr will give rise to an equilibrium concentration in the material being degassed of
 K 1=2 P 5 cF : D T F The result will be to modify Eqs. (2) and (4) to give

(
 ) N 4X πð2n 1 1Þx πð2n11Þ 2 21 exp 2 ð2n11Þ sin Dt c 5 cF 1 ðc0 2 cF Þ π 0 d d
 4 πx 2 π2 Dt DcF 1 ðc0 2 cF Þ sin exp : π d d2

(13)

(14)

Immediately after removal from the furnace the uniform concentration component would give a very high outgassing rate not controlled by diffusion and not easily calculated. However, the normal degassing operation, which would always be necessary to remove surface contamination received during assembly of the vacuum chamber, will reduce the outgassing component from cF to a value again controlled by the diffusion mechanism. In practice, after a furnace treatment, the approximation (14) will always be valid and, using the same reasoning as in the Appendix, with subscripts f and n for the furnace and subsequent normal bakeouts, the net effect will be to give c5

    4 πx sin exp 2π2 d22 Dn tn cF 1 ðc0 2 cF Þexp 2π2 d22 Df tf π d

(15)

and Q_ r 5 4Dr d21 fcF 1 ðc0 2 cF Þexpð2 π2 d22 Df tf Þgexpð2 π2 d22 Dn tn Þ: Even a rather moderate vacuum furnace ought to achieve a hydrogen partial pressure as low as 10 highest furnace temperatures contemplated this would give
 1=2 ðcF Þ700 5 KD PF

(16) 24

Torr. At the

700

1:26 3 1027 5 3 1022 D1:1 3 1024 Torr L cm23 : 1:16 3 1025 It follows from Eq. (12) that c0 expð2 π2 d22 Df tf ÞDcF . Hence the furnace vacuum requirement is not critical. It appears from the above considerations that outgassing rates of hydrogen from SS down to approximately 10216 Torr L (cm2 s)21 ought to be obtainable. This represents an improvement factor of about 104 on rates normally

60

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

measured. The improvement obtainable appears to be limited only by diffusion of atmospheric hydrogen or by the quality of the vacuum in the furnace. The generalization of the above considerations to materials of various thicknesses is easy—thicker material must be degassed for a longer time, approximately proportional to the square of the thickness (or, alternatively, at a correspondingly higher temperature), to give the same outgassing rate and will exhibit a lower (inversely proportional to the thickness) permeation rate for atmospheric hydrogen. Thinner material may be more easily degassed but will suffer from an increased permeation rate.

5

Summary and conclusion

After a normal bakeout hydrogen was observed to outgas at a rate of a few times 10212 Torr L (cm2 s)21 from a 2 mm thick SS wall and to constitute at least 99% of the desorbed gas. Calculations based on a diffusion model suggest that it should be possible to achieve outgassing rates for H2 as low as 10216 Torr L (cm2 s)21 by using higher bakeout temperatures. The limiting factor is the absorption of atmospheric hydrogen during the bake. Experimentally, it was found that after degassing 2 mm SS sheet for 3 h at 1000 C the hydrogen outgassing rate was at least as low as 1 3 10214 Torr L (cm2 s)21 and, because of apparatus limitations, is estimated to be considerably lower. As indicated by the diffusion-model theory, it has been found that relatively thin SS is thoroughly degassed by normal bakeout procedures at 300 C. The value of such material in normal vacuum systems as liners, which may be degassed in situ, has been discussed. Finally, it has been demonstrated that it is useful to raise the sample temperature when measuring extremely low outgassing rates. This not only produces a higher gas flow but also permits the separation of specimen outgassing from that of the measuring equipment. The diffusion coefficients, permeability constants, and hydrogen content of the atmosphere may differ from those values employed in our calculations. The treatment at 1000 C for 3 h is probably rather drastic and adequate degassing could be obtained at considerably lower temperatures. Experiments are now in progress to determine the minimum temperature-time conditions necessary. It is hoped that, when these conditions are determined, it will possible to include this treatment in the final stages of SS sheet or tube manufacture. This would be a convenient method because it has been shown that such material, after high-temperature degassing, is not readily recontaminated.

References in the paper[1-7] [1] Eschbach HL, Gross F, Schulien S. Vacuum 1963;13:5437. [3] Gleuckauf E. In: Malone TF, editor. Compendium of metrology. Boston, MA: American Meteorological Society; 1951. p. 310. [6] Levin G. Fundamentals of vacuum science and technology. New York: McGraw-Hill; 1965.

Reviewed paper [1-7] Calder R, Lewin G. Reduction of stainless-steel outgassing in ultra-high vacuum. Br J Appl Phys 1967;18:145972.

[1-8] “Estimating the gas partial pressure due to diffusive outgassing” (Santeler, 1992) Santeler (1992) presented the paper,[1-8] “Estimating the gas partial pressure due to diffusive outgassing.” The paper comprises the following sections: 1. Introduction 2. Diffusive outgassing—Fick’s law 3. Graphical solutions In this book, Section 2, Diffusive outgassing—Fick’s law, are introduced. Abstract The operating pressure of a leak-tight vacuum system is usually limited by the outgassing of materials in the system. A major source of the outgassing is frequently water, carbon oxides, and hydrocarbons that are bound to the surface

Characteristics of outgassing from metal surfaces Chapter | 2

61

and that may be partially removed with a relatively low-temperature bakeout. Diffusive outgassing from the interior of various materials may then become the major residual gas source particularly if the system is to be operated at an elevated temperature. Hydrogen outgassing from iron-based metals such as SS, and water outgassing from plastic or polymeric materials are typical examples where diffusive outgassing processes have been demonstrated to be major gas sources. The diffusive transport of gas through materials is governed by the second-order differential equation Dδ2 C=δX 2 5 δC=δt, known as Fick’s law [Jost W. Diffusion in solids, liquids, gases. New York: Academic; 1960[4]; Barrier RM. Diffusion in and through solids. Cambridge: Cambridge University; 1941[5]; Carslaw HS, Jaeger JC. Conduction of heat in solids. 2nd ed. Oxford: Clarendon; 1959[6]]. Two alternate solutions of this equation are available, which provide for simplified approximations for both the short- and long-term behavior, and for the exact numerical answer. An important aspect of true diffusive outgassing is its excellent predictability as compared to experiment. This permits calculating the outgassing rate following a known timetemperatureenvironment history. A simple graphical solution is demonstrated to accomplish this for the example of hydrogen outgassing from a thick-walled SS vessel operating at an elevated temperature following various bakeout schedules.

2

Diffusive outgassing—Fick’s law

Diffusive outgassing refers to that gas which originates in the interior of a material and moves to the surface by diffusion through a concentration gradient. At the surface, the gas enters the atmosphere or the vacuum space at a rate dependent on the abundance of the gas in both the solid and the gas phases, the material temperature, and the binding energy of the respective gas to the material. Diffusive outgassing is partial-pressure dependent for each gas type that is soluble in the material. In true diffusive outgassing, the process is dominated by the diffusion process rather than by any chemical or desorption reactions at the surface. This is the assumed condition for the present analysis, that is, the diffusing gas is assumed to leave the material as fast as it arrives at the surface rather than building up a local concentration that mediates the diffusion process. We also assume that the material had reached a uniform concentration throughout its bulk during manufacture, that is, there are no initial concentration gradients present. The outgassing process begins immediately after manufacture at the ambient temperature condition and at the prevailing partial pressure of the hydrogen gas. Typically, the partial pressure of hydrogen in the environment is of the order of 4 3 1024 Torr. For the purpose of this analysis, we will assume that after manufacture the material is always in an environment of low hydrogen partial pressure—either a vacuum or our low hydrogen—concentration environment. Thus we will assume that both sides of the chamber walls are in a continuing outgassing condition with temperature and the initial hydrogen concentration being the sole controlling factors on the diffusive outgassing rate for any particular set of hydrogen/metal diffusive properties. We are thus assuming that neither side of the wall is exposed (for any appreciable time), to a high-temperature/high-hydrogen-partial-pressure combination that would put hydrogen gas back into the material. For this set of conditions, the outgassing status at any time will only be a function of the time-temperature history of the material. For the purpose of this analysis, we will also assume that the time equaling zero occurs at the end of the manufacturing process when the material is first cooled down in the atmosphere. The low partial pressure of hydrogen adjacent to the SS instigates hydrogen outgassing. As time passes, a concentration gradient develops in the bulk of the material. The outgassing rate decays according to Fick’s law of diffusion[46] given by the second-order differential equation: D

δ2 C δC : 5 δx2 δt

One standard solution to this typical vacuum-wall boundary-value problem is #
0:5 " n5N X D n 2n2 l2 =Dt q 5 C0 112 ð21Þ 3 e Torr L ðs cm2 Þ21 ; π n51

(1)

(2)

where C0 is the initial concentration in Torr L cm23, D is the temperature-dependent diffusion coefficient in cm2 s21 (both C0 and D depend on the material and the gas type), t is the time in seconds, and l is the material thickness in centimeters. Eq. (2) as applied to vacuum systems is discussed in Refs. [7,8]. An alternate solution of Eq. (1) for a vacuum wall is discussed in Refs. [2,9] where the timetemperature outgassing is given as ( 
" N  )# 4C0 D X πð2n11Þ 2 q5 2 exp 2 Dt 3 T L ðs cm2 Þ21 : (3) l l n20

62

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

The two solutions given by Eqs. (2) and (3) appear completely different but both give the same numerical answers. Note that while both equations are summations of exponential terms, the solutions are effectively summed from opposite directions. As a result, the two solutions have different single-term approximations and for different time periods. For outgassing times that are short relative to Dt=l2 , the summation of the exponential terms in the bracketed term in Eq. (2) is negligible relative to the unity term and the entire bracketed term is approximately equal to 1.0. Eq. (2) can then be approximated by
0:5 D q 5 C0 Torr L ðs cm2 Þ21 : (4) πt Conversely, for times which are long compared to 0.025 l2 Eq. (3) can be approximated by the first exponential term       4C0 D π 2 q5 Dt Torr L ðs cm2 Þ21 : (5) exp 2 l l It is important to note that these outgassing processes are partial pressure dependent; thus, with a low partial pressure of hydrogen on both sides of the wall, the material outgasses in both directions. For this condition, the half thickness should be used for l. The concentration gradient that is moving into the material from both sides will meet at the center. Each half side may be treated as a separate material that is outgassing through one face and sealed on the other face. Since the two sides are symmetric, no gas flows across the interface. Diffusive outgassing is extremely temperature dependent through the diffusivity D which is given by D 5 D0 e2Ed=RT cm2 s21 2 21

where D0 is the diffusivity at infinite temperature, in cm s , Ed is the activation energy for diffusion in cal (g mol) R is the gas constant [1.987 cal (g mol K)21], and T is the absolute temperature in K. Typical values of these parameters for hydrogen in SS are D0 5 0.012 cm2 and Ed 5 13,100 cal (g mol)21.

References in the paper[1-8] [2] [4] [5] [6] [7] [8] [9]

Calder R, Lewin G. J Appl Phys 1967;18:1459. Jost W. Diffusion in solids, liquids, gases. New York: Academic; 1960. Barrier RM. Diffusion in and through solids. Cambridge: Cambridge University; 1941. Carslaw HS, Jaeger JC. Conduction of heat in solids. 2nd ed. Oxford: Clarendon; 1959. Pagano F. 13th national vacuum symposium; 1966. p. 103. Santeler DJ, et al. Vacuum technology and space simulation. In: NASA SP-105; 1966. p. 188, 204. Lewin G. Fundamentals of vacuum science and technology. New York: McGraw-Hill; 1965. p. 25.

Reviewed paper [1-8] Santeler DJ. Estimating the gas partial pressure due to diffusive outgassing. J Vac Sci Technol A 1992;10(4):187983.

[1-9] “Model for the outgassing of water from metal surfaces” (Li and Dylla, 1993) Li and Dylla (1993) presented the paper,[1-9] “Model for the outgassing of water from metal surfaces.” The paper comprises the following sections: 1. 2. 3. 4.

Introduction Outline of the model Experiment Final remarks In this book, Section 4, Final remarks, is introduced.

(6) 21

,

Characteristics of outgassing from metal surfaces Chapter | 2

63

Abstract Numerous measurements of outgassing from metal surfaces show that the outgassing obeys a power law of the form Q 5 Q10 t2α , where α is typically near unity. For unbaked systems, outgassing is dominated by water. This work demonstrates that α is a function of the water vapor exposure during venting of the system, and the physical properties of the passivation oxide layer on the surface. An analytic expression for the outgassing rate is derived based on the assumption that the rate of water diffusing through the passivation oxide layer to the surface governs the rate of its release into the vacuum. The source distribution function for the desorbing water is assumed to be a combination of a Gaussian distribution centered at the interior surface driven by atmospheric exposure, and a uniform concentration throughout the bulk. We have measured the outgassing rate from a clean SS (type 304) chamber as a function of water exposure to the chamber surface from ,1 to 600 monolayers. The measured outgassing rate data show that α tends to 0.5 for low H2O exposures and tends to 1.5 for high H2O exposures as predicted by the model.

4

Final remarks

Our model for the outgassing from metal surfaces (or more precisely, the outgassing of water from metals with passivation oxide layers) predicts that the outgassing rate is proportional to the number of H2O molecules adsorbed onto the surface or absorbed within the near surface region of the metal. The model assumes that the source distribution function for water outgassing comprises two terms: (1) a uniform distribution, presumably formed as the metal was cooled from a liquid melt in ambient conditions and (2) a distribution centered on the metal surface as a result of atmospheric pressure exposure to humid air. The model quantitatively predicts the observed power law dependence for outgassing, where Q 5 Q10 t2α , with α near unity for typical ambient air exposures. We have shown that the value of α tends toward 0.5 for the limit of dry gas exposures and tends toward 1.5 for large water exposures ( . 600 ML). The model is presently too simplistic to predict much about the dependence of outgassing on the properties of the passivation oxide layer, since this layer is characterized in the model simply as a thin layer (with a thickness small compared to the macroscopic dimensions of the system), and a diffusion constant D, which is spatially invariant within the oxide layer. Nonetheless, the present model should predict the outgassing behavior for times less than a characteristic time given by l2 =D, where l is the oxide thickness. The model predicts that the total number of absorbed H2O molecules, and hence the resulting outgassing load at ambient conditions, can be reduced by (1) reducing the water content of the venting gas; (2) reduction of the adsorption area (surface roughness factor); (3) reduction of the diffusion constant (i.e., by producing a defect-free, nonporous, passivation oxide layer). Surfaces with low surface roughness, resulting from sophisticated surface treatments, such as mirror polishing and electropolishing, should show comparatively lower outgassing rates if the comparison is made with equivalent passivation oxide layer properties and thickness, such as the recent study of polished aluminum surfaces by Suemitsu et al.[10] However, a more dramatic effect on outgassing at ambient conditions is affected by significantly reducing the water content of the venting gases. This latter observation is consistent with long-standing practice in vacuum technology that has been quantitatively verified by the measurements in this study, and a recent measurement by Ishimaru et al.[12] that shows pumping times reduced to minutes for a vacuum system vented to extremely dry (,ppb H2O) N2 gas.

References in the paper[1-9] [10] Suemitsu M, Shimoyamada H, Miyamoto N, Tokai T, Moriya Y, Ikeda H, Yamada H. J Vac Sci Technol A 1992;10:570. [12] Ishimaru H, Itoh K, Ishigaki T, Furutate M. J Vac Sci Technol A 1992;10:547.

Reviewed paper [1-9] Li M, Dylla HF. Model for the outgassing of water from metal surfaces. J Vac Sci Technol A 1993;11(4):17027.

64

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

[1-10] “True and measured outgassing rates of a vacuum chamber with a reversibly adsorbed phase” (Akaishi, Nagasuga, and Funato, 2001) Akaishi et al. (2001) presented the paper,[1-10] “True and measured outgassing rates of a vacuum chamber with a reversibly adsorbed phase.” The paper comprises the following sections: 1. 2. 3. 4. 5.

Introduction Modeling of pump-down Comparison between theory and experiment Discussion Summary

In this book, Sections 2, Modeling of pump-down, 3, Comparison between theory and experiment, and 4, Discussion, are introduced. Abstract The modeling for the pump-down of a vacuum chamber with a reversibly adsorbed phase is carried out and the outgassing equation that predicts variations of surface coverage and gas density with time is derived. When it is assumed that the wall surface of a vacuum chamber consists of oxide layer and this layer involves weakly chemisorbed (nondissociate) H2O with desorption energies from 20 to 30 kcal mol21, it is shown that the calculated outgassing rate from the outgassing equation well explains the experimental result that the outgassing rate measured by the orifice pumping method in the unbaked SS chamber is dependent on pumping speed.

Nomenclature n number density of gas phase molecules σ number of adsorbed molecules per unit area σm number of adsorbed molecules per unit area in one monolayer coverage θ relative coverage defined as a fraction of occupied sites with adsorbed molecules on the wall surface of a vacuum chamber, defined as

θ5

σ σm

(1)

V volume of a vacuum chamber A area of the wall surface of a vacuum chamber τ mean residence time of adsorbed molecules on a solid surface, defined as

τ 5 τ 0 eE=RT ;

(2)

where τ 0 is the nominal period of a vibration of an adsorbed molecule on a solid surface, E is the activation energy of desorption, R is the gas constant and T is the absolute temperature s0 sticking probability of a molecule on empty sites s effective sticking probability defined as

s 5 s0 ð1 2 θÞ

(3)

nðu=4Þ arrival rate of gas phase molecules at a wall surface per unit area per unit time where u is the average velocity of molecules in gas phase a effective area of the pumping orifice, defined as a 5 S=ðu=4Þ, where S is the pumping speed of pumping orifice

2

Modeling of pump-down

A Mass conservation equations The total number of gas molecules in a vacuum chamber N is Vn 1 Aσm θ 5 N;

(4)

Characteristics of outgassing from metal surfaces Chapter | 2

65

where Vn , Aσm θ are the number of gas molecules in the gas phase and at the wall surface, respectively. When the pump-down of the vacuum chamber is started, from Eq. (4) the variations of the gas density and the surface coverage with time are described by the following two equations: u dN dn dθ 5V 1 Aσm 52a n; dt dt dt 4  s0 ð1 2 θÞ u=4 n dθ θ 5 2 2 ; σm dt τ

(5) (6)

where the term of the right-hand side of Eq. (5) is the pumping rate.

B

Equilibrium and nonequilibrium adsorption isotherms

In the equilibrium adsorption state when the pump-down of the vacuum chamber is cut off, the equilibrium state between desorption and adsorption at the wall surface of the chamber is described by the following conditions: a 5 0 and

dN=dt 5 dn=dt 5 dθ=dt 5 0;

by denoting the gas density in Eq. (6) as n-neq we obtain
 θ 5 Bð1 2 θÞneq ðθÞ; τ θ

(7)

(8)

where B5

s0 ðu=4Þ : σm

(9)

If we introduce the Temkin isotherm for a nonuniform surface, as shown in Appendix A, τðEÞ and neq in Eq. (8) can be expressed as a function of θ as follows:
 τðE0 Þ θ τðEÞ 5 bθ  τðθÞ; (10) e 21 12θ and Bneq ðθÞτðE0 Þ 5 ðebθ 2 1Þ;

(11)

where b is the constant when the activation energy of desorption distributes from the minimum E1 to the maximum E0 , defined as b5

E0 2 E1 ; RT

(12a)

and τðE0 Þ is the mean residence time at E 5 E0 , defined as τðE0 Þ 5 τ 0 eE0 =RT :

(12b)

Eq. (11) represents the equilibrium adsorption isotherm of the vacuum chamber. Next, let us consider  a nonequilibrium adsorption state when the pump-down of the vacuum chamber is started from a point of θ; neq ðθÞ on the equilibrium adsorption isotherm at t 5 0. If we set the conditions of the transient phase after the start of the pump-down as follows:    
 dθ dn θ     Aσm   , V   and 5 constant; (13) dt dt τ θ then by replacing the gas density as n-np we can express Eq. (5) as

 V dnp θ 1 Bð1 2 θ 1 γÞnp 5 ; Aσm dt τ θ

(14)

66

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

where γ in the brackets of the second term on the left-hand side is the pumping parameter, defined as a : γ5 As0

(15)

Eq. (14) shows that only the gas density in the gas phase decreases due to pumping, while the gas release rate from the source term of desorption is kept constant. The solution of Eq. (14) is given by
 1 θ 2t=τ p 1 ð1 2 e2t=τ p Þ; (16a) np ðtÞ 5 neq ðθÞe Bð1 2 θ 1 γÞ τ θ where τ p is the time constant for pump-down, defined as τp 5

V ; ð1 2 θ 1 γÞAs0 ðu=4Þ

(16b)

using Eq. (8), Eq. (16a) is expressed as np ð0Þ 5 neq ðθÞ at and at t 5 t

t50


np ðtÞ 5 neq ðθÞ


 γ 12θ 2t=τ p 1 e : ð1 2 θ 1 γÞ 12θ1γ

(16c)

If the gas release rate of the wall surface is still kept constant, while the pumping time is in the length: 2τ p {t;

(17)

 12θ np ðθÞ 5 neq ðθÞ; ð1 2 θ 1 γÞ

(18)

Eq. (16c) yields the following gas density:




then by combining Eqs. (8) and (18) we may express the nonequilibrium adsorption isotherm of the pumped chamber as follows:
 12θ Bnp ðθÞτðE0 Þ 5 (19) ðebθ 2 1Þ: ð1 2 θ 1 γÞ Fig. 1 shows the adsorption isotherms of Eqs. (11) and (19) depicted on the θ 2 logn plane proposed by Kanazawa,[3] where two lines of PQ and P0 Q0 are the equilibrium and the nonequilibrium adsorption isotherms, respectively. Because in the transient phase of the pump-down the gas density in the gas phase changes from the point A to B0 , the nonequilibrium adsorption isotherm (i.e., the line P0 Q0 ) is expressed mathematically as the parallel shift of the line PQ by Δθ from the point B to B0 along with the horizontal axis. As a result, the equivalent relationship between neq and np for the coverage shift may be expressed as np ðθÞ 5 neq ðθ 2 ΔθÞ. Because the term ð1 2 θÞ=ð1 2 θ 1 γÞ in Eq. (18) is always smaller than 1 for 0 , γ, we can see that the nonequilibrium adsorption isotherm sits in the region of ½θ; n slightly below the equilibrium adsorption isotherm. Log (n)

FIG. 1 Equilibrium and the nonequilibrium adsorption isotherm in the θ 2 log n plane, where PQ and P0 Q0 lines are adsorption isotherms of Eqs. (11) and (19), respectively.

P P′ A A′ B

B′

Q

Q′ θ–Δθ

θ

θ+Δθ

θ

Characteristics of outgassing from metal surfaces Chapter | 2

C

67

Measured and true outgassing rates

When the pumping time becomes longer than around 2τ p (i.e., 2τ p , t), we may expect that the pump-down of the vacuum chamber reaches to a quasi-steady state. As discussed in Appendix B, the condition of the quasi-steady state may be expressed by
0,

2 dθ dt

    dn dθ V  {Aσm  : dt dt

 and

(20)

Under the condition, we may expect that the gas releasing rate of the source term reaches a quasi-steady state. If we consider that the starting point of the quasi-steady state is point B0 in Fig. 1, this point will move down to the region of smaller coverage with time on the line P0 O0 . In addition we notice that when the pumping point shifts from the point A to B0 , the coverage decreases by Δθ from the point B0 to B. This means that the decrease of the source term of desorption actually occurs in the transient phase of the pump-down. Therefore we may interpret that the measured outgassing rate will be generated by the decrease of surface coverage of Δθ until the pump-down reaches the quasi-steady state. If we consider that the deviation of coverage Δθ occurs in a time interval Δt in the transient phase of the pump-down, Δθ may be expressed as
 
  θ θ 2Δθ 5 2 Δt; τ θ τ θ2Δθ

(21a)

where the second term of the brackets of the right-hand side represents the source term of desorption at the point B on the line PQ in Fig. 1 and is given by
 θ 5 Bð1 2 θ 1 ΔθÞneq ðθ 2 ΔθÞ: τ θ2Δθ

(21b)

Because the gas density at the point B0 is given by Eq. (18), using Eqs. (8), (19), and the relation np ðθÞ 5 neq ðθ 2 ΔθÞ, we can rewrite Eq. (21a) as follows: 2

Δθ Bð1 2 θÞðν 2 ΔθÞneq ðθÞ 5 ; Δt 12θ1γ

then by taking the limit of, Δt-0, we obtain the time derivative of coverage as follows:

 dθ γð1 2 θÞ 2 5 Bneq ðθÞ: dt 12θ1γ

(22)

(23)

If one denotes the measured outgassing rate as
qm 5 σ m

 dθ 2 ; dt

from Eq. (23) we can define the measured outgassing rate in the quasi-steady state as

 γð1 2 θÞ qm 5 σ m Bneq ðθÞ 5 σm Bγnp ðθÞ: 12θ1γ Using Eq. (11) the variable separation form Eq. (25) is given as


bθ  dθ γð1 2 θÞ e 21 5 2 : dt 12θ1γ τðE0 Þ

(24)

(25)

(26)

In addition if we define the source terms of desorption of Eqs. (8) and (21b) as the true outgassing rate qt ðθÞ and qt ðθ 2 ΔθÞ, using Eq. (25) we obtain
 12θ1γ (27a) qt ðθÞ 5 qm ; γ

68

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

and


qt ðθ 2 ΔθÞD

 12θ qm : γ

(27b)

The same expression as Eq. (27a) is also shown by Redhead.[5]

3

Comparison between theory and experiment

To numerically solve Eq. (26), we need to know proper values of parameters E0 , E1 , and s0 . In an earlier paper,[2] by taking into account that the main desorbing gas in an unbaked vacuum chamber is usually water vapor and by referring the measured result of the sticking probability of a SS chamber by Tuzi et al.,[6] we assumed that the activation energy of desorption for chemisorbed H2O at a SS surface is distributed in the range from E1 5 15 to E0 5 27 kcal mol21 and the sticking probability of H2O for the zero coverage surface is in the range from s0 5 0.001 to 0.1. However, concerning the activation energy of desorption, Dayton[7] suggested in the review in AVS, 46th International Symposium at Seattle that weakly chemisorbed water molecules with desorption energies in the range from 20 to 30 kcal mol21 are responsible for the observed slow outgassing of water molecules in the oxide layer on metallic surfaces. Therefore we should use E0 5 30 kcal mol21 and b 5 13.5 (corresponding to ΔE 5 8 kcal mol21). In addition, he also suggests that the real surface area involves an extended surface that includes the wall of the capillary pores and grain boundaries in a porous oxide layer on the metal. Thus, for the estimation of outgassing rate of water vapor σm should be replaced by Ar σm in which Ar 5

Ae A

(28)

is the ratio of the extended area Ae , which includes the walls of the capillary pores in the oxide layer, to the geometric surface area A. Therefore Ar is an adjustable parameter then Eq. (26) can be put in integral form and integrated numerically using a computer program to obtain pairs of values for θ at a given time t in s. From Eq. (26),  ð  τðE0 Þ θ0 12θ1γ dθ; (29) t 2 t0 5 bθ γ θt ð1 2 θÞð2 2 1Þ where the lower limit θt is the value of θ at the time t, and the upper limit θ0 is the value of θ at time t0 which can be set equal to 0. Because θ0 must be less than one in order to avoid a pole during integration, we use θ0 5 0:8. The outgassing rate in molecules (s cm2)21 of a nonuniform surface with σm replaced by σm Ar :   ð1 2 θÞγ ebθ 2 1 qm 5 σ m Ar : (30) 1 2 θ 1 γ τðE0 Þ The outgassing rate in Torr L (s cm2)21 as measured at temperature T is then
 Rv T T 220 qm 5 3:108 3 10 K5 qm ; Na 300

(31)

where Rv 5 62:36 is the molar gas constant in vacuum engineering units [Torr L (mol K)21] and Na is Avogado’s number. From Eq. (25) the gas density np in number of molecules cm23 is given by   σ m Ar ð1 2 θÞ ebθ 2 1 ; (32) np 5 s0 ðu=4Þ 1 2 θ 1 γ τðE0 Þ then the pressure p in Torr at T 5 297K is calculated by p 5 3:0766 3 10217 np . To obtain values that can be compared with the experimental data[1] as Figs. 2 and 3, we use T 5 296:4K, σm 5 1015 , Ar 5 100, s0 5 0:1, b 5 13:5, E0 5 30 kcal mol21 so that from Eq. (12b), τðE0 Þ 5 1:319 3 109 s for T 5 296:4K. We may define the pumping parameter S=A in the experimental data as g5

S a 5 14:85 5 14:85 s0 γ; A A

(33)

where 14.85 is the value of ðu=4Þ in L (s cm2)21 for water vapor at room temperature. As in Ref. [1] pressures and outgassing rates at the pumping time t 5 72 h are shown, the theoretical values for outgassing rates and pressures were also calculated for t 5 72 h ( 5 2:593 3 105 s). Figs. 2 and 3 show the comparisons between the theory and the experiment for the outgassing rate and the pressure as a function of g at the pumping time t 5 72 h.

Outgassing rate [Torr L (s cm2)−1]

Characteristics of outgassing from metal surfaces Chapter | 2

FIG. 2 Theoretical and the experimental outgassing rates as a function of pumping parameter g at pumping time t 5 72 h for an unbaked 304 stainless-steel chamber. The theoretical outgassing rates are calculated with Eq. (30) by setting as θ 5 0:8 at t 5 0, τðE0 Þ 5 1:319 3 109 s for E0 5 30 kcal mol21 , b 5 13:5, and s0 5 0:1.

10–9

10–10 Experim.[1] Eq. (31) 10–11 0.0001

0.1 0.001 0.01 Pumping parameter g [L (s cm2)−1]

1

FIG. 3 Theoretical and experimental pressures as a function of g at pumping time t 5 72 h for an unbaked 304 stainless steel chamber. The theoretical pressures are calculated with Eq. (32) by setting as θ 5 0:8 at t 5 0, τðE0 Þ 5 1:319 3 109 s for E0 5 30 kcal mol21 , b 5 13:5, and s0 5 0:1.

10–6

Pressure (Torr)

Experim.[1] Eq. (32) 10–7

10–8

10–9 –5 10

4

69

–3

0 10 10–1 10–4 10 10–2 2 −1 Pumping parameter g [L (s cm ) ]

Discussion

A Approximate expression of np In Section 2 (Modeling of pump-down) we considered the generation of outgassing rate as the change of the source term of desorption. If we replace the difference of the source terms of desorption on the right-hand side of Eq. (21a) by the total derivative Δðθ=τÞθ , we may estimate the magnitude of the deviation of coverage from the following equation:
   θ DBð1 2 θÞ neq ðθÞ 2 np ðθÞ ; (34) Δ τ θ because the left-hand side is expressed as
 5 Δ τθ θ

   1 Δ ð1 2 θÞ ebθ 2 1 τðE0 Þ 2 3 0 1 bθ bΔθðe 2 1Þ 4 1 A 15 5 ð1 2 θÞ@1 1 bθ 2 ; τðE0 Þ e 21 b

and from Eq. (18) the right-hand side is given by




neq ðθÞ 2 np ðθÞ 5

 γ neq ðθÞ; 12θ1γ

thus Eq. (34) yields bΔθ 5

γ 12θ1γ

for

1{ðebθ 2 1Þ;

and

1 {ð1 2 θÞ: b

(35)

70

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Then using the relationship neq ðθ 2 ΔθÞ 5 np ðθÞ if we rewrite Eq. (11) as Bnp ðθÞ

5

ebðθ2ΔθÞ 2 1 τðE0 Þ

2 3 e2bΔθ ðebθ 2 1Þ 4 ð1 2 e2bΔθ Þ5 12 ; D τðE0 Þ ðebθ 2 1Þ from Eq. (35) this is approximated as Bnp ðθÞDBneq ðθÞe2γ=12θ1γ :

(36)

Thus another expression of Eq. (19) may be given by Bnp ðθÞτðE0 ÞDe2γ=12θ1γ ðebθ 2 1Þ:

(37)

It is proved that Eq. (36) is equal to Eq. (18), because the exponential term of the right-hand side of Eq. (36) can be expanded as e2γ=12θ1γ D1 2

γ 12θ 5 12θ1γ 12θ1γ

for

γ , 1: ð1 2 θ 1 γÞ

By the numerical examination, it is found that we can use Eq. (36) instead of Eq. (18) by allowing the error within 10% when γ # 0:1.

B

g Dependence of K and p

In Fig. [1-11]-2 we see that the plotting of the calculated outgassing rates gives a curve very close to the experimental outgassing data. Theoretical curve indicates that qm becomes equal to qt for large γ as predicted byEq. (27a), while qm reaches 0 for very small γ as depicted by Eq. (18). Thus, there is no reason to doubt the validity of the experimental values because the theoretical equations with the proper values of E0 and b for weakly chemisorbed H2O give curves almost matching the experimental curve. When we use E0 5 27 kcal mol21 and b 5 20 to estimate the value of K, we fail to explain theoretically the dependence of the experimental outgassing rate on γ because it is shown that the values of K are independent of γ. This means that it is very important to properly evaluate parameter values for the water adsorption on the real surface. On the other hand, although in Fig. 3 the plotting of calculated pressures gives a curve very close to the experimental data, the disagreement between the experiment and the theory is emphasized for large and small values of g. The reason that the pressure in the experiment does not decline more rapidly for g . 0:01 might be that there is a limiting ultimate pressure of the order of 1028 Torr due to contamination in the pressure gauge. The fact that the experimental pressure increases as g decreases below 0.001 may be due to the presence of gases other than H2O that are not adsorbed by the oxide layer. It is shown that from the gas analysis in the recent outgassing measurement[8] of an unbaked SS chamber by the pressure rise method, the desorption of H2 contributes to the pressure rise.

References in the paper[1-10] [1] [2] [3] [5] [6] [7] [8]

Akashi K, Yubota Y, Motojima O, Nakasuga M, Funato T, Mushiaki M. J Vac Sci Technol A 1997;15:259. Akashi K. J Vac Sci Technol A 1999;17:229. Kanazawa K. J Vac Sci Technol A 1986;7:3361. Redhead PA. J Vac Sci Technol A 1996;14:2599. Tuzi Y, Tanaka T, Takeuchi K, Saito Y. J Vac Soc Jpn 1997;40:377. Dayton BB. AVS, 46th international symposium, VT-ThM3. Seattle; October 28, 1999. Akashi K, Ezaki K, Yubota Y, Motojima O. J Vac Soc Jpn 1999;42:204.

Reviewed paper [1-10] Akaishi K, Nagasuga M, Funato Y. True and measured outgassing rates of a vacuum chamber with a reversibly adsorbed phase. J Vac Sci Technol A 2001;19(1):36571.

Characteristics of outgassing from metal surfaces Chapter | 2

71

[1-11] “Recombination limited outgassing of stainless steel” (Moore, 1995) Moore (1995) presented the paper,[1-11] “Recombination limited outgassing of stainless steel.” The paper comprises the following sections: 1. 2. 3. 4. 5.

Introduction Method of analysis Results of analysis Discussion Summary.

In this book, Sections 1, Introduction, 2, Method of analysis, 3, Results of analysis, and 4, Discussion, are introduced. Abstract Those who need a better vacuum, either in existing systems or for new construction, should be interested in this discussion, since there is at present no method of analysis to predict the bake time and temperature required to achieve a specific low outgassing rate from SS. It has been known for a generation that atomic hydrogen can be baked out of SS to achieve an outgassing rate of 10212 Torr L (s cm2)21. The time and temperature required are in reasonable agreement with measured diffusion constants and diffusion theory. However, the diffusion theory completely fails when predicting lower outgassing rates. The time and temperature required greatly exceed those predicted. It was observed two generations ago that at low pressures the permeation of hydrogen through metals is no longer a function of the square root of pressure but approaches zero. This has been attributed to surface effects, specifically including the recombination limit; the atomic hydrogen outgassed to the surface must recombine into molecular hydrogen before it can escape into the vacuum. The recombination rate is a function of the square of the surface coverage by atomic hydrogen. The concentration at the surface cannot be zero as assumed in conventional diffusion theory but must remain at finite levels consistent with the outgassing rate. In this paper a finite-difference analysis is made of a reported 2 h bake at 950 C; the diffusion of hydrogen to the surface is modified to include a recombination limit. The outgassing rates with time are found together with the concentration profiles in the steel. The recombination coefficient is adjusted to match the observed postbake outgassing rate. It is concluded that the addition of the recombination limit to diffusion theory may be a viable analysis method to predict the results of specific time and temperature bake patterns.

1

Introduction

There is no method of analysis available today, which can predict the bake time and temperature profile needed for austenitic SS to reach specific outgassing rates much below 10212 Torr L (s cm2)21. It is well known that hydrogen enters certain metals as atoms, not molecules; that the atoms cannot leave the metal until they have recombined into molecular hydrogen at the surface; and that the residual outgassing of baked SS is almost all hydrogen. Diffusion theory[1,2] has been successfully applied to predict the higher outgassing rates.[35] However, it fails to predict the lower rates by a large margin.[4] The measured rates are far greater than the predicted. In this method of analysis the atomic hydrogen concentration at the bulk surface is assumed to be zero,[35] the hydrogen is outgassed as rapidly as it can diffuse to the surface. This concept will be referred to as “diffusion limited outgassing.” Permeation of hydrogen through metals is sometimes considered to be[35] a simple function of the square root of the pressure. However, it was reported in 1937[6] that at low pressures the permeation rate is greatly reduced and approaches zero. This more complex function was attributed by Barrer[7] to surface effects. He specifically pointed out that atomic hydrogen diffusing to the surface must recombine into molecular hydrogen before it can escape into the vacuum, that the rate of this process must depend on the square of the number of hydrogen atoms available on the surface, and that this rate must also depend on a recombination coefficient. Later he[8] concluded that “. . . the theory seems ready for quantitative application . . ..” The recombination rate of deuterium implanted in thin foils of 304 SS has been extensively investigated.[9] It appears to be extremely sensitive to surface cleaning;[10] an ion-sputtered surface gave up to 1000 times higher rates

72

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

than an EP surface. The recombination rate is here[10] defined as a constant relating the outgassing rate to the bulk concentration of hydrogen just inside the surface. Hseuh and Cui[11] measured the postbake outgassing rate of 304LN and of 316L SS s, as a function of temperature. These measurements are used in the present paper to find a recombination constant for 304LN SS. The 316L differs slightly.

2

Method of analysis

A Measurements by Hseuh and Cui Hydrogen-outgassing measurements were made on beam tube sections. The SS analyzed in the present paper is 304LN, 1.9 mm thick, chemically cleaned in “. . . hot trichloroethane with ultrasonic agitation . . .” with subsequent washing and rinsing. The tube was fired in a vacuum furnace at 950 C for 2 h. There was a 4 h ramp-up in temperature, and a 1 h cooldown to 500 C or less. Postbake measurements of outgassing were made at room temperature and at several higher temperatures up to 300 C. The rates were unchanged with time, indicating that the surface concentration was constant during these measurements. The data shown on an Arrhenius plot fall on a straight line with very little scatter. Extrapolating this plot, the postbake outgassing rate at 950 C is approximately 114,000 times that at 25 C.

B

Numeric diffusion calculation

Crank[2] has an entire chapter on numeric analysis of diffusion equations, “. . . replacing derivatives by finite-difference approximations.” The first and the simplest method Crank describes, Schmidt’s method, “. . . is a satisfactory one in that it is not subject to any cumulative error . . .,” Schmidt’s method was used for the calculations in this paper. Crank gives some advice on choosing the parameters for the numeric analysis to avoid divergence. In these calculations, the 1.9 mm thickness of steel is described by 161 points from edge to edge. The initial uniform concentration is assumed to be 0.3 Torr L at 0 C cm23 (as used by Calder and Lewin[4]). The concentration profile is calculated each 0.01 s. The diffusion coefficient was assumed to be 6.254 3 1025 cm2 s21 (extrapolated[4]). The calculation proceeds like a two-step dance. The set of concentrations at the beginning of a time interval is used to find a set of changes in concentration during the time interval. Then the set of concentrations is revised for the beginning of the next time interval. The change in concentration at any point depends only on its concentration and that at the two adjacent points: dðKÞ 5 f½cðK 2 1Þ 2 cKÞ 2 ½cðK 1 1ÞgT;

(1)

where K is the identification number of a point. K varies from 0 at the center of the steel to S at the steel surface; d(K) is the change in concentration at point K during a time interval; c(K) is the concentration at point K at the beginning of the time interval, (stp) cm3 cm23; T 5 DC 3 NS=WZ 2 ; DC is the diffusion constant at the bake temperature, cm2 s21; NS is the seconds per time interval; WZ is the width of zone, or separation between points, cm. The flow of hydrogen atoms out to one surface, (stp) cm3 (s cm2)21 is QD 5 ½cðS 2 1Þ 2 cðSÞT

WZ : NS

(2)

In the diffusion limited method of calculation, cðSÞ is set to zero.

C Recombination limit Instead of assigning the value of zero to cðSÞ, the concentration at the surface, this calculation finds a definite value at each interval of time. This concept will be referred to as “recombination limited outgassing.” The first step is to select a value for the recombination coefficient KL; this remains constant during the calculation of the 2 h bake. It establishes the outgassing rate for any given concentration at the surface, following the expression[10]: Q 5 KLCðSÞ2 ; 2 21

(3) 23

where Q is the hydrogen outgassing rate, atom (s cm ) ; C(S) is the hydrogen concentration at the surface, atom cm ; KL is the recombination coefficient, [atom (s cm2)21]/[atom cm23]2 5 cm4 (atom s)21.

Characteristics of outgassing from metal surfaces Chapter | 2

73

Of course, the finite concentration at the surface CðSÞ will affect the rate of diffusion from within the bulk out to the surface. During each increment of time it is assumed that an equilibrium has been reached where the flow to the surface from the bulk just equals the outgassing rate. The value of CðSÞ where this equilibrium occurs is found by solving a quadratic equation followed by numeric iteration for more accuracy. The only variable during this process is CðSÞ; the concentration at the adjacent point within the bulk is temporarily held constant. To convert the concentration from cm3 (stp) cm23 to atom cm23, multiply by 2 atoms molecule21 and by 2.686763 3 1019 molecules cm23. After the 2 h bake is completed, the steel is cooled to room temperature, reducing the outgassing rate by the factor 114,000 described above.[11] This room-temperature outgassing rate is compared with the measured rate. From the difference, a new value for KL is manually selected and the entire bakeout calculation repeated until the desired matching accuracy is achieved.

3

Results of analysis

A Recombination limited concentration profiles versus bake time

H2 concentration (cm3(STP) cm–3)

The recombination limited concentration of hydrogen atoms through the thickness of the metal is shown in Fig. 1 for increasing bake times. The finite concentrations at the surfaces are a major departure from diffusion limited analyses that assume the surface concentration to be zero. As time increases, the distributions approach a sine function plus a constant, instead of the sine function found with diffusion limited outgassing.[4] FIG. 1 Concentration profiles of atomic hydrogen calculated by recombination limited outgassing, within a 1.9-mm-thick 304LN stainless-steel sheet, vacuum furnace baked at 950 C. The concentration is shown as a function of the cross-sectional position measured from the center of the thickness of the steel sheet. A number of bake seconds are labeled on the profiles. The recombination coefficient assumed is 6 3 10222 cm4 (atom s)21. Initial concentration assumed is 0.3 Torr L at 0 C cm23.

0.5 0.4 1

10

20

0.3 0.2 100 0.1 200 0 –1.0 –0.8

–0.6 –0.4 –0.2 0 0.2 Steel thickness (mm)

0.4

0.6

0.8

1.0

H2 concentration (cm3(STP) cm–3)

In Fig. 2, the same data are shown with the ordinate scale increased 100 times. The sine component continues to decrease with time. After 2000 s of the bake the ratio of minimum-to-maximum concentration is greater than 0.95; after 7200 s it is almost 0.99. The concentration has become practically uniform. 0. 005 0.004

FIG. 2 The same as Fig. 1 except the scale of the y axis is expanded 100 times.

1000 1100

0.003 0.002

2000

0.001 0 –1.0 –0.8 –0.6 –0.4 –0.2 0 0.2 0.4 Steel thickness (mm)

0.6

0.8

1.0

74

B

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Outgassing rate versus time

H2 outgassing [Torr L (s cm2)−1]

Diffusion limited outgassing as a function of time is compared with recombination limited in Fig. 3. The outgassing rate is shown in Torr L at 0 C (s cm2)21. For the first few seconds of the bake the diffusion limited outgassing is slightly greater, but within a few hundred seconds the recombination limited outgassing is far higher. At the end of the bake, the slope of the recombination limited outgassing approaches 22 on the loglog plot; increasing the time by a factor of 10 would reduce the outgassing rate by a factor of 100. The cooldown time after the completion of the bake is neglected, so the outgassing drops vertically by a factor of 114,000, the ratio scaled from Hseuh and Cui.[11] Beyond this time there was no change reported in outgassing rate. –2 –4 –6 –8 –10 –12 –14 0

1

2 3 Log of bake time (s)

4

5

FIG. 3 Two methods to calculate postbake outgassing rates from one surface of a 1.9 mm-thick sheet of 304LN SS are compared with experimental results[11]. The widely used “diffusion limited” method[4] (dashed line) gives rates even during the bake which become far less than the postbake room-temperature measurement of about 10213 Torr L at 0 C (s cm2)21. The solid line is a combination of calculated and measured data. For the duration of the 2 h vacuum furnace bake at 950 C (the 4 h warm-up ramp is neglected), the “recombination limited” method is used with a recombination coefficient assumed to be 3 3 10222 cm4 (atom s)21. After the end of the bake, the solid line represents experimental data. The cooldown from 950 C to 25 C causes a reduction in outgassing rate by a factor of 114,000; this number is extrapolated from the measured postbake changes up to 300 C.[11] After this, at room temperature, there were no further changes in rate.[11] It appears that this recombination coefficient predicts a postbake outgassing rate near that observed. A number of uncertainties limit the utility of this result: (1) the initial concentration in the experimental beam tubes is not known, it is only assumed to be 0.3 Torr L at 0 C cm23 to be consistent with Calder and Lewin.[4] (2) The calculations are one dimensional; they assume that the steel is a flat sheet, semiinfinite in extent so that the only variations in concentration are normal to the surface. However, the experimental samples were finite beam tubes, cylindrical in shape. (3) The calculation could be refined. The outgassing during warm-up and cooldown could be included.

4

Discussion

A Bake efficiency as a function of temperature The diffusion constant at 950 C is about 109 times that at room temperature. With diffusion limited analysis, the outgassing rates are increased by the same factor, so that the additional costs of a high-temperature vacuum furnace seem reasonable. With recombination limited outgassing the same temperature increase will only raise the postbake outgassing rates by 105; the relative costs and effectiveness of various bake temperatures need to be reconsidered.

B

Vacuum furnace versus in situ bake efficiency

Existing vacuum systems may be baked in situ at relatively low temperatures for extended periods to achieve desired outgassing rates. Permeation of atmospheric hydrogen through the SS chamber wall will not limit the outgassing rates achieved on the vacuum side; this has been established by reports of the low outgassing rates of air-baked surfaces.[12] The availability of this option suggests that vacuum furnace bakes might in some circumstances be less attractive when compared to in situ baking.

C Uncertainties in the estimate of outgassing rate The initial concentration of hydrogen in the SS beam tube measured[11] is not known but is likely to be within a factor of 2 of the value assumed for consistency with Calder and Lewin.[4]

Characteristics of outgassing from metal surfaces Chapter | 2

75

The warm-up and cooldown portions of the bake have been neglected, as is customary; however, it would be desirable to include these in a more refined analysis. The calculations are one dimensional; they assume that the steel is a flat sheet, semiinfinite in extent so that the only variations in concentration are normal to the surface. However, the experimental samples were finite beam tubes, cylindrical in shape. The analysis of additional time and temperature profiles would make the method more credible. Despite these uncertainties, recombination limited analysis produces predictions of outgassing rates that are far more useful than the widely used diffusion limited analysis.[4]

References in the paper[1-11] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Barrer RM. Diffusion in and through solids. Cambridge: Cambridge University Press; 1941. Crank J. The mathematics of diffusion. New York: Oxford University Press; 1956. Lewin G. Fundamentals of vacuum science and technology. New York: McGraw-Hill; 1965. Calder R, Lewin G. Br J Appl Phys 1967;18:1459. Lewin G. J Vac Sci Technol 1967;6:420. Smithells C, Ransley C. Proc R Soc 1937;CLVII:292. Barrer RM. Philos Mag 1939;28:253. Ash R, Barrer RM. Philos Mag 1959;4:1197. Moller W. Nucl Instrum Methods 1983;209/210:773. Myers. SM, Wampler WR, Besenbacher F. J Appl Phys 1984;56:1561. Hseuh HC, Cui Xiahua. J Vac Sci Technol A 1989;7:2418. Nuvolone R. J Vac Sci Technol 1977;14:1210.

Reviewed paper [1-11] Moore BC. Recombination limited outgassing of stainless steel. J Vac Sci Technol A 1995;13(3):5458.

Related papers [1-12] “La de´sorption sous vide” (Schram, 1963) Schram (1963) presented the paper,[1-12] “La de´sorption sous vide,” written in French. In the paper the important concept on outgassing, “true surface area,” is described. Abstract A general survey is given of our present knowledge in this field. Important discrepancies between the experimental results and the theoretical explanations put forward by the different authors are pointed out. The complexity of the phenomena is emphasized and the relative importance of the different elementary processes is discussed. A limited choice of research is proposed for desorption in vacuum. Theoretical calculations based on simplified models is compared with some experimental results. The knowledge of the true surface area is found necessary and a simple experimental method used to measure this surface is described. The experimental curves of desorption are discussed taking in account the first results of true surface area measurements. In unbaked systems the absorbed gas layers at the surface are proved to yield the most important part of the total desorption rate. The residual desorption of baked systems in UHV techniques seems more likely to be a bulk (diffusion, permeation), rather than a surface phenomenon. Analysis by such mass-spectrometers as the omegatron becomes a necessity.

Reviewed paper [1-12] Schram A. La desorption sous vide. Le Vide 1963;103:5568 (in French).

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PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

[1-13] “Hydrogen pumping by austenitic stainless steel” ˇ 2005) (Zajec and Namenic, Zajec and Nameniˇc (2005) presented the paper,[1-13] “Hydrogen pumping by austenitic stainless steel.” Abstract In the present study, hydrogen sorption and desorption kinetics close to equilibrium were investigated in a pinched-off AISI 316 cell by a sensitive pressure-rise method. The pressure was monitored with an SRG just before the pinch-off and after it for 6 months at two stabilized temperatures: 25 C and 55 C. The preprocessing of the cell (of uniform wall thickness 0.15 mm volume 125 cm3, and inner surface 460 cm2) consisted of baking at 200 C for 109 h with several evaluation cycles to UHV. The quantity of released hydrogen during the bakeout procedure equaled the average concentration change ΔC 5 2:8 3 1017 atom H cm23 . After the pinch-off intentionally done in the high vacuum range where hydrogen represented the residual atmosphere, surprisingly the hydrogen pressure slowly declined from the initial pð328KÞ 5 3:7 3 1024 mbar, with an initial rate dp=dt 5 2 5:5 3 10211 mbar s21 and later attained a stable value, which could be termed the equilibrium. In similar reported experiments, where valving-off began in the UHV, the dp/dt was always positive and constant over several orders of magnitude in pressure. During 6 months of measurements, a sudden temperature jump from 25 C and 55 C or back was applied a few times to investigate the stability of the equilibrium or the impact on the pressure course. The most plausible explanation of the results is given along with discussion whether hydrogen permeate through the cell wall or if it was absorbed in the cell wall.

Reviewed paper [1-13] Zajec B, Nemaniˇc V. Hydrogen pumping by austenitic stainless steel. J Vac Sci Technol A 2005;23(2):3229.

Chapter 3

Methods for measuring outgassing rates Reviewed papers [1-1] “Measurement of outgassing rates from materials by the differential pressure rise method” (Yoshimura, Oikawa, and Mikami, 1970) Yoshimura et al. (1970) presented the paper,[1-1] “Measurement of outgassing rates from materials by the differential pressure rise method,” written in Japanese. The paper comprises the following sections: 1. 2. 3. 4. 5.

Introduction Principle of the differential pressure-rise method Experimental apparatus Pressure-rise characteristics and outgassing rates of sample materials Discussion on error in measurement data In the paper, the advantages of the differential pressure-rise method are described in detail. In this book, Section 2, Principle of the differential pressure-rise method, is introduced.

Abstract This paper deals with the differential pressure-rise method employed to measure the outgassing rates of the sample materials. With this method, measurement errors due to the outgassing from the test-dome surface, the gas absorption of the test-dome surface, and the pumping of the ionization gauge used can be considerably reduced. Auxiliary experiments were carried out to determine the pumping speed of the gauge used. Under some assumptions, errors, introduced in the outgassing rates measured with the differential pressure-rise method, were discussed. For the examples of the outgassing data measured with this method, ones of Viton “A,” Teflon, steatite, and porous alumina are presented in this paper. For the standpoint of simplicity and accuracy, it can be concluded that the differential pressure-rise method is highly suitable for outgassing measurement at room temperature in respect of nonmetallic materials.

2

Principle of the differential pressure-rise method

Fig. 1 shows the test-dome system schematically, where both domes, TD1 and TD2, are made of homogeneous glass, including greaseless cocks C1 and C2. TD2 with a sample with surface area AS has its volume V2 and area A2 , both of which are much smaller than volume V1 and area A1 of the empty dome TD1, respectively. Typical pressure-rise curves in measuring procedures are presented in Fig. 2.

A Review: Ultrahigh-Vacuum Technology for Electron Microscopes. DOI: https://doi.org/10.1016/B978-0-12-818573-5.00003-7 © 2020 Elsevier Inc. All rights reserved.

77

78

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Furnace

GC2

GC1 Heater

BAG Sample

T1 HP

FIG. 1 Schematic diagram of the differential pressure-rise method. V1 5 0.6 L, V2 5 0.06 L, A1 5 1000 cm2, A2 5 100 cm2. C2, Cock; I, gas inlet; VG, vacuum gauge.

Pressure

C1 opened

(dP/dt)2

(dP/dt)1 Pr

C2 closed

Pr

C1 closed C1 closed Time

FIG. 2 Procedures for measuring the curves ðdP=dtÞ1 and ðdP=dtÞ2 .

The test-dome system with a sample is evacuated to a high vacuum. After an evacuation time t1 , the cocks C1 and C2 are sequentially handled to measure a pressure-rise rate ðdP=dtÞ1 at a reference pressure Pr for combined domes of TD1 and TD2, including the sample and another rate ðdP=dtÞ2 at the same pressure Pr for the empty dome TD1, as shown in Fig. 2. The total time required to measure the successive two pressure-rise rates, ðdP=dtÞ1 and ðdP=dtÞ2 , is only a few minutes, which is negligibly short comparing with the evacuation time t1 . This process is repeated after scheduled evacuation times t2 , t3 , and so on. Denote net outgassing rates per unit surface area of the domes TD1 and TD2 at a reference pressure Pr for the measurement of ðdP=dtÞ1 as q11 and q21 , respectively, and a net rates per unit surface area of the domes TD1 at Pr for the measurement of ðdP=dtÞ2 as q12 . And denote pumping speeds of the vacuum gauge (VG) at Pr for the measurements of ðdP=dtÞ1 and ðdP=dtÞ2 as Sg1 and Sg2 , respectively. Then, the following equations are obtained:
 dP 5 QS 1 q11 A1 1 q21 A2 2 Pr Sg1 ; (1) ðV1 1 V2 Þ dt 1
 dP V1 5 q12 A1 2 Pr Sg2 ; (2) dt 2 where QS is the net outgassing rate of the sample at Pr . Assume that q11 5 q21 5 q12 5 q and Sg1 5 Sg2 5 Sg , then Eqs. (1) and (2) reduce to
 dP 5 QS 1 ðA1 1 A2 Þq 2 Pr Sg ; ðV1 1 V2 Þ dt 1

(3)

Methods for measuring outgassing rates Chapter | 3

V1


 dP 5 A1 q 2 Pr S g ; dt 02

79

(4)

respectively. The assumption introduced previously would be acceptable because the histories of both domes under vacuum would almost the same with each other and the same gauge VG is used for measuring the pressure-rise rates ðdP=dtÞ1 and ðdP=dtÞ2 .   When the left-hand side of Eq. (1), ðV1 1 V2 Þ dP=dt 1 , is calculated as QS , as usually done in conventional pressure-rise methods, a large error is involved due to the outgassing of both domes and the pumping function of the gauge VG. Now, the outgassing rate QS can be calculated more accurately by two differential methods, as described later. Method 1: The following equation is derived from Eqs. (3) and (4):

 dP dP 2 V1 5 QS 1 A2 q: (5) ðV1 1 V2 Þ dt 1 dt 2 That is, QS is calculated from the left-hand side of Eq. (5), though an error A2 q, the outgassing rate of TD2, is involved in the calculated value. The error A2 q is much smaller than the error involved in the value conventionally calculated from the left-hand side of Eq. (1). Method 2: The following equation is derived from Eqs. (3) and (4):

 dP A1 1 A2 dP A2 2 V1 5 QS 1 Pr Sg : (6) ðV1 1 V2 Þ dt 1 dt 2 A1 A1   an error Pr Sg A2 =A1 due to the pumping speed That is, QS is calculated from the left-hand side of Eq. (6), though  of VG is involved in the calculated value. The error Pr Sg A2 =A1 becomes small because the ratio A2 =A1 is selected as small as 0.1. Now, compare the errors involved in the values QS calculated by the two differential methods described previously. The parameters related to the errors are A1 , 1000 cm2, A2 , 100 cm2; Pr , 10281025 Torr; q, 102111028 Torr L (s cm2)21. A BayardAlpert (BA) gauge (BAG) with an emission of 0.5 mA was used to measure pressure-rise rates. The pumping speed Sg of the BAG has been estimated as about 0.01 L s21 as described later. Therefore the error A2 q involved in the value QS calculated by the method 1 is estimated as 10291026 (Torr L) s21, while the error  Pr Sg A2 =A1 involved in the value QS calculated by the method 2 is estimated as 102111028 (Torr L) s21. As a result, the method 2 is superior to method 1 from the view point of error. Measurement of pumping speed of a B-A gauge: Fig. 3 shows the apparatus for measuring pumping speeds of a BAG. Furnace

GC2

GC1 Heater

BAG FT T1 HP FIG. 3 Apparatus for measuring pumping speeds of a BAG. HP, T1, GC1, GC2, BAG with emission current of 0.55 mA. The apparatus is made of glass. BAG, BayardAlpert gauge; FT, filament tube; GC1, greaseless cock 1; GC2, greaseless cock 2; HP, Hickman pump; T1, U-type liquid nitrogen trap.

80

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Procedure for measuring: Evacuate the chamber to a high vacuum with the greaseless cocks GC1 and GC2 being opened. Flash the filament tube (FT) for a few minutes under high vacuum until an equilibrium high vacuum is obtained. Then, stop flashing the FT and close the GC1 to adsorb the gas molecules in the chamber on the filament surfaces sufficiently. And then, flash the FT a few second to increase the pressure rapidly, and just after flashing the cocks GC1 and GC2 are closed to eliminate the pumping function of the FT and the pump HP. The pressure is pumped down by the pumping functions of the BAG and chamber walls between two cocks. The pumping-down curves are measured. Using the measured pumping-down curves, the pumping speeds of the closed chamber with the BAG (0.55 mA) are calculated from the initial gradient of pumping-down curves, which are presented in Fig. 4.

(l s–1) 0.04

(B) 0.03

0.02

(C) (A)

0.01

0

0

1

2

3

4

5

6

7

8 (h)

FIG. 4 Pumping speeds of the closed chamber with the BAG (0.55 mA). (A) After bakeout at 100 C for 3 h, (B) after bakeout at 300 C for 3 h, and (C) after bakeout at 300 C for 3 h and subsequent exposure to the air introduced through a charcoal trap for 30 min. BAG, BayardAlpert gauge.

As seen in Fig. 4, pumping speeds vary depending on the condition of the chamber walls, which is due to the variation of the pumping speed of the chamber walls. On the other hand, the pumping speed of the gauge operating a constant emission current of 0.55 mA is considered to be constant. As a result, the pumping speed of a BAG with 0.55 mA is about 0.01 L s21, which is the base speed for three different conditions. The pumping speed of the glass-tube-type BAG with an emission current of 1 mA may be about 0.02 L s21.

Reviewed paper [1-1] Yoshimura N, Oikawa H, Mikami O. Measurement of outgassing rates from materials by differential pressure rise method. J Vac Soc Jpn 1970;13(1):238 [in Japanese].

Methods for measuring outgassing rates Chapter | 3

81

[1-2] “A three-point-pressure method for measuring the gas-flow rate through a conducting pipe” (Hirano and Yoshimura, 1986) Hirano and Yoshimura (1986) presented the paper,[1-2] “A three-point-pressure method for measuring the gas-flow rate through a conducting pipe.” In this book the paper[1-2] is fully introduced. Abstract A new method for measuring the gas-flow rate through an outgassing pipe has been introduced, in which the pressures at three different points in the pipe are measured to calculate the real gas-flow rate. This method was applied to measuring the flow rate of nitrogen through a pipe. The gas-flow rates calculated using three pressures were compared with those calculated using two pressures under the same condition. The method based on three pressures gave real rates which were higher than those based on two pressures, especially in lower pressure regions, thus indicating the presence of outgassing.

1

Introduction

The accurate measurement of the gas-flow rate into a test dome is essential to high vacuum technology in such areas as the measurement of the pumping speed of a high-vacuum pump or the conductance of a pipe. Much work has been reported on this subject.[17] Two methods for measuring the flow rate have been generally used; one is the orifice method[3,4,8] and the other the conducting pipe method.[4,7,9,10] In the former the measured rate involves errors due to the angular distribution[1,11,12] of the density of gas molecules near the orifice. In the latter the outgassing from the pipe wall has generally been neglected. This outgassing induces errors, especially in low pressure ranges, and VGs must be located at a distance from the ends of the pipe in order to be in the linear conductance portion. In the pipe method the gas molecules introduced into the pipe are assumed to scatter diffusely at the pipe wall. To make this assumption acceptable, pressures in the pipe are measured after they come into an equilibrium with a sufficiently large leak rate of the introduced gas. Actually, some of the introduced gas molecules have first been sorbed on or into the pipe wall and are then evolved during the period of the flow rate measurement. As a result, the outgassing from the pipe wall may become appreciable during the measurement period, even if the pipe has been degassed before the measurement. The present report first introduces a new method for calculating the gas-flow rate through a pipe, which includes the outgassing rate of the pipe wall. The flow rates calculated by the proposed methods are then compared with the rates calculated by a conventional pipe method.

2

Three-point-pressure method

The gas-flow rate Q through a pipe, which is composed of the introduced leak rate QL and the outgassing rate QW of the pipe wall, can be calculated using the pressures measured at three points in the pipe in the free molecular flow region.

A Principle The following three assumptions are introduced to express pressures in the pipe: 1. Gas molecules are scattered diffusely at the molecularly rough surface of the pipe wall,[13,14] which is needed for classical gas-flow equations. 2. The outgassing from the pipe wall is uniform[15] over the whole pipe during measurement. 3. The species of the gas evolved from the pipe wall is identical with the gas leaked into the pipe. The first assumption applies when the outgassing from the pipe wall is in a steady state. The second one applies when the pretreatment of the pipe wall under high vacuum is almost identical regardless of position. The third one applies if the pipe has been degassed sufficiently to remove alien species, leaving only the introduced gas molecules which had sorbed on or into the pipe wall.

82

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Now, let us examine the pressures in a portion of a long outgassing pipe where the net gas flow, including the leak rate QL , directs to the left, as shown in Fig. 1. The conductance C of tube length L is calculated using the long tube formula assuming a linear conductance for the pipe portion. Pk Pl

PX PX + ΔP

P0

PL QW,C

Q x

FIG. 1 Pipe portion in a long outgassing tube, where gas effectively flows from the right to the left. Gas-flow rate Q is composed of the leak rate QL and the outgassing rate QW .

QL

Δx l L k(=1/L)

0

1

Consider a very short element with a length Δx and a conductance CL=Δx. Since the total gas-flow rate at a position x is calculated as QL 1 ð1 2 x=LÞQW , the pressure drop ΔP across the element is ΔP 5

½QL 1 ð1 2 x=LÞQW Δx : CL

That is, dP=dx 5

QL 1 ð1 2 x=LÞQW : CL

(1)

Integrating Eq. (1) from 0 to l with respect to x: ð Pl ðl QL 1 ð1 2 x=LÞQW dx: dP 5 CL P0 0 and so Pl 5 P0 1

fQL 1 ½1 2 l=ð2LÞQW gl ; CL

where P0 and Pl denote the pressures at the positions x 5 0 and l, respectively. Substituting a fraction k for l=L and rewriting Pl to Pk , the pressure Pk at a position indicated by the fraction k is given by Pk 5 P0 1

k½QL 1 ð1 2 k=2ÞQW  : C

(2)

Eq. (2) shows that Pk varies quadratically[1517] as a function of k. Eq. (2) contains three unknown factors QL , QW , and P0 . These three factors can be calculated from Eq. (2) using three pressures Pk1 , Pk2 , and Pk3 measured at three different points in the pipe identified by their fractional positions ki . Gas-flow rates QL and QW are then expressed as QL 5 C

ð2 2 k2 2 k3 Þðk2 2 k3 ÞPk1 1 ð2 2 k3 2 k1 Þðk3 2 k1 ÞPk2 1 ð2 2 k1 2 k2 Þðk1 2 k2 ÞPk3 ðk2 2 k3 Þk2 k3 1 ðk3 2 k1 Þk3 k1 1 ðk1 2 k2 Þk1 k2 QW 5 2 2C

ðk2 2 k3 ÞPk1 1 ðk3 2 k1 ÞPk2 1 ðk1 2 k2 ÞPk3 ðk2 2 k3 Þk2 k3 1 ðk3 2 k1 Þk3 k1 1 ðk1 2 k2 Þk1 k2

where 0 # ki # 1. As a result, the total gas-flow rate Q 5 QL 1 QW is expressed as  2      k2 2 k32 Pk1 1 k32 2 k12 Pk2 1 k12 2 k22 Pk3 Q52C ðk2 2 k3 Þk2 k3 1 ðk3 2 k1 Þk3 k1 1 ðk1 2 k2 Þk1 k2

(3)

(4)

(5)

Methods for measuring outgassing rates Chapter | 3

83

This is a good solution as long as the gauges are kept sufficiently far from the tube ends in the linear pressure section. We call this gas-flow rate measuring method based on Eq. (5), the “three-point-pressure method” (or simply the 3PP method). The process for driving Eqs. (3) and (4) is given in the Appendix.

B

Optimization of the measuring system

For accurate calculations of QL and QW , the fractions k1 , k2 , and k3 should be selected so that the differences between the pressures at the successive two points become nearly equal, that is, Pk1 2 Pk2 DPk2 2 Pk3 . As an example, when, QW {QL the desirable value of k2 is easily found to be 0.5 for k1 5 0:1 and k3 5 0:9. On the other hand, when QL {QW , the desirable value of k2 is calculated to be 0.36 using the following equation, which is reduced from Eq. (2) by neglecting the value QL : Pk 5 P0 1 ð1 2 k=2ÞkQW =C: In some cases, QW and QL would be comparable, so k2 was actually selected as 0.4 for k1 5 0:1 and k3 5 0:9 in our measuring system. Substituting 0.1, 0.4, and 0.9 for k1 , k2 , and k3 of Eqs. (3)(5), respectively, QL 5 ð5C=12Þ 3 ð7P0:1 2 16P0:4 1 9P0:9 Þ;

(6)

QW 5 ð2 5C=3Þ 3 ð5P0:1 2 8P0:4 1 3P0:9 Þ;

(7)

Q 5 ð2 5C=12Þ 3 ð13P0:1 2 16P0:4 1 3P0:9 Þ:

(8)

C Measurement of gas-flow rates Flow rates of nitrogen were calculated by the 3PP method and then compared with those simultaneously calculated by a conventional pipe method for the same system. A conducting pipe with three BAGs G1, G2, and G3 was connected to a typical test dome with an ion pump (60 L s21, triode type). Three gauges were located at the positions indicated by the fractions k of 0.1, 0.4, and 0.9 as shown in Fig. 2. G1(P1)

G2(P2) QW,C

k = 0.1

G4(P4)

G3(P3)

k = 0.4

QL

k = 0.9 k=1

Pump FIG. 2 Gas-flow rate measuring system with three gauges installed on a conducting pipe. G1, G2, G3, and G4 are BA gauges; QL is the leak rate of the introduced gas; QW is the outgassing rate of the tube portion from k 5 0 to 1 of the pipe; and C is the conductance of the same portion. BA, BayardAlpert.

A pipe of 43 mm diameter and 800 mm length (corresponding to the length L) with a conductance of 12 L s21 was actually selected to make the pressures P01 , P04 , and P09 be in the same pressure range. The gauge G3 is located at a distance of 240 mm from the variable leak valve, so the ratio of the distance 240 mm to the diameter of the pipe 43 mm is as high as B6. The ratio is considered to be enough for the gauge G3 being in the linear portion of the arrival rate distribution.[18] Substituting 12 for C and rewriting P01 , P04 , and P09 to P1, P2, and P3, respectively, Eqs. (6), (7), and (8) become QL 5 5ð7P1 2 16P2 1 9P3Þ;

(9)

QW 5 2 20ð5P1 2 8P2 1 3P3Þ;

(10)

Q 5 2 5ð13P1 2 16P2 1 3P3Þ:

(11)

84

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

On the other hand, gas-flow rates are conventionally calculated using two pressures in the pipe. The gas-flow rate Q0123 calculated using the pressures P1 and P3 is given by Q0123 5 15ðP3 2 P1Þ;

(12)

where 15 is the conductance (given in L s21) between the gauges G1 and G3. We call this method the two-pointpressure method (or simply the 2PP method). The following pretreatment was conducted: 1. The system was baked under a high vacuum created by a turbomolecular pump. The conducting pipe was “uniformly” baked at 200 C for one day with the temperatures at three points of the pipe being controlled, during which the pump and the dome were baked at 300 C. 2. The electrodes of the ion pump were cleaned by an argon glow discharge[19,20] with a current of 1 A at about 300 Vac for 20 min under a pressure of B40 Pa, during which the temperature of the pipe and the temperatures of the pump and the dome were kept at 150 C and 200 C, respectively. Relatively accurate pressure measurements are very important to calculate the gas-flow rates by the 3PP method. Relative calibrations among the gauges were carried out for nitrogen at several pressure levels. Nitrogen was intermittently introduced into the closed system of Fig. 2 before the ion pump was switched on. The pressure readings of G1, G2, and G3 were recorded to correct their relative sensitivities at a long elapsed time after introducing nitrogen. The sensitivity of the gauge G2 was 12% higher than those of the gauges G1 and G3. On the other hand the dependence of relative sensitivities of the gauges upon pressure levels was negligibly small. So, the pressure readings of G2 were divided by 1.12 regardless of pressure levels. After the system was evacuated to a pressure less than 1 3 1026 Pa, nitrogen was leaked into the pipe through a variable leak valve to make the pressure P1B1 3 1025 Pa. In the course of reaching equilibrium pressures, P1, P2, and P3 were read at elapsed times of 5, 15, and 25 min. The leak rate was then increased to obtain higher pressures. The pressures P1, P2, and P3 were read at elapsed times of 5, 15, and 25 min. The process was repeated at various pressures up to B3 3 1023 Pa. The pressure distributions in the pipe at 25 min at two different pressure levels are shown in Fig. 3, where the solid curve shows the pressure distribution in the 1025 Pa range and the broken curve in the 1023 Pa range.

Pressure (Pa)

1×10–4

P3

P2

5×10–5

1×10–2

5×10–3

P1 1×10–5

1×10–3 0.1

0.4 Fraction k

0.9

FIG. 3 Pressure distributions in the pipe at an elapsed time of 25 min for nitrogen. K in the 1025 Pa range, - - x- - in the 1023 Pa range.

The convex curve in the 1025 Pa range shows that the outgassing rate of the pipe wall is appreciable compared with the leak rate, while the straight line in the 1023 Pa range shows that the effect of the outgassing is negligibly small. The values QL , QW , Qð 5 QL 1 QW Þ calculated by Eqs. (9)(11), and the ratio QW =QL are given in Table 1, together with the value Q0123 calculated by the 2PP method.

Methods for measuring outgassing rates Chapter | 3

85

The table shows the following evidence: 1. After an elapsed time of 15 min the pressures reach an equilibrium where the inner surface of the pipe wall must be in a saturated state of adsorbed gases. 2. The leak rates QL are almost independent of the elapsed time. On the other hand the outgassing rates QW depend on the elapsed time with the exception of the rates QW for P1 5 3:72 3 1025 Pa. The QW =QL ratios decrease with an increase of pressure and finally the ratios become negative values in the 1023 Pa range. The negative ratios mean that the pipe wall sorbs some of the introduced leak gases effectively. The large positive ratios in the 1025 Pa range indicate that the outgassing rate of the pipe wall is comparable to the leak rate. 3. The Q0123 values are smaller than the corresponding Q values with the exception of Q0123 in the 1023 Pa range. The Q0123 values in the 1025 Pa range are 20%30% smaller than the corresponding Q values, seen in the last column in Table 1.

TABLE 1 Gas-flow rates QL ,QW ,Qð 5 QL 1 QW Þ, and the ratios QW =QL by the 3PP method, compared with the rates Q0123 by the conventional 2PP method. Elapsed times (min)

5

Gas-flow rates (Pa s21)

Measured pressure (Pa) P1 (k 5 0.1)

P2 (k 5 0.4)

P3 (k 5 0.9)

By the new 3PP method

By conventional 2PP methoda

QL

QW

QW/QL

Q

Q0 1-3

0.80

4.1 3 1024

3.2 3 1024

0.78

24

24

1.23 3 1025

2.13 3 1025

3.33 3 1025

2.3 3 1024

1.8 3 1024

25

25

25

24

24

Ratios Q0 1-3/Q

15

1.18 3 10

2.07 3 10

3.19 3 10

1.9 3 10

2.2 3 10

1.13

4.1 3 10

3.0 3 10

0.73

25

1.15 3 1025

2.07 3 1025

3.19 3 1025

1.8 3 1024

2.5 3 1024

1.36

4.3 3 1024

3.1 3 1024

0.71

25

25

24

24

24

23

23

5

3.72 3 10

6.87 3 10

1.10 3 10

7.6 3 10

6.7 3 10

0.89

1.4 3 10

1.1 3 10

0.76

15

3.72 3 1025

6.87 3 1025

1.10 3 1024

7.6 3 1024

6.7 3 1024

0.89

1.4 3 1023

1.1 3 1023

0.76

25

25

25

24

24

24

0.89

23

23

0.76

22

3.72 3 10

24

6.87 3 10

24

1.10 3 10

23

7.6 3 10

22

6.7 3 10

23

1.4 3 10

22

1.1 3 10

5

3.46 3 10

6.39 3 10

1.09 3 10

1.0 3 10

2.2 3 10

0.22

1.2 3 10

1.1 3 10

0.91

15

3.46 3 1024

6.45 3 1024

1.11 3 1023

1.0 3 1022

2.0 3 1023

0.19

1.2 3 1022

1.1 3 1022

0.92

25

24

3.46 3 10

24

6.45 3 10

23

1.12 3 10

22

1.1 3 10

23

1.4 3 10

0.13

22

1.2 3 10

22

1.2 3 10

0.94

5

3.46 3 1023

6.04 3 1023

1.05 3 1022

1.1 3 1021

29.6 3 1023

0.09

1.0 3 1021

1.1 3 1021

1.05

23

23

22

21

23

0.03

21

21

1.02

21

1.02

15 25

3.46 3 10

23

3.46 3 10

6.04 3 10

23

6.04 3 10

1.04 3 10

22

1.04 3 10

1.1 3 10

21

1.1 3 10

23.6 3 10

23

23.6 3 10

0.03

1.0 3 10

21

1.0 3 10

1.0 3 10 1.0 3 10

2PP, Two-point-pressure; 3PP, three-point-pressure. a Calculated by the conventional 2PP method.

The outgassing rate varies widely depending on the pressure level. Under such situations the effect of the outgassing from the pipe wall must be reflected on the equations for calculating gas-flow rates. The 3PP method, taking into account the outgassing, gives real gas-flow rates.

3

Discussion

As previously seen in Section 2, the pressure distributions in the conducting pipe are convex to the k axis at lower pressure ranges due to the appreciable outgassing from the pipe wall. In this case the gas-flow rates calculated by the conventional 2PP method depend strongly on the gauge positions in the pipe, as discussed later. The pressures P1 and P2, or P2 and P3 can also be used to calculate gas-flow rates by the conventional 2PP method, although the pressures P1 and P3 were used in the previous section in order to make a large pressure difference between two gauges.

86

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

The gas-flow rates Q0122 and Q0223 were calculated by the following equations: Q0122 5 40ðP2 2 P1Þ; Q0223 5 24ðP3 2 P2Þ; where 40 and 24 are the conductance (given in L s21) between the two gauges G1 and G2, and G2 and G3, respectively. The rates Q0123 , Q0122 , and Q0223 calculated by the conventional 2PP method at an elapsed time of 25 min are given in Table 2, together with the flow rate Q calculated by the 3PP method.

TABLE 2 Gas-flow rates Q0123 , Q0122 , and Q0223 by the conventional 2PP method, compared with the rates Q by the 3PP method, all at a time of 25 min and various pressures. Gas-flow rates [(Pa L) s21]

Measured pressures (Pa) P1 (k 5 0.1)

P2 (k 5 0.4)

P3 (k 5 0.9)

By the 3PP method

By the conventional 2PP method

Q

Q0123

Q0122

Q0223

1.15 3 1025

2.07 3 1025

3.19 3 1025

4.3 3 1024

3.1 3 1024

3.7 3 1024

2.7 3 1024

3.72 3 1025

6.87 3 1025

1.10 3 1024

1.4 3 1023

1.1 3 1023

1.3 3 1023

9.9 3 1024

3.46 3 1024

6.45 3 1024

1.12 3 1023

1.2 3 1022

1.2 3 1022

1.2 3 1022

1.1 3 1022

3.46 3 1023

6.04 3 1023

1.04 3 1022

1.0 3 1021

1.0 3 1021

1.0 3 1021

1.0 3 1021

2PP, Two-point-pressure; 3PP, three-point-pressure.

As is clearly shown in Table 2, the differences among the Q0123 , Q0122 , and Q0223 rates become larger as the pressure level is lowered, among which the Q0122 rate is the largest and close to the Q rate calculated by the 3PP method. The gas-flow rate at the upper stream is lower than that at the lower stream for an outgassing tube. Therefore when using the conventional 2PP method, the pressures in the pipe should be measured at two positions which are located at the smallest possible distance from each other just before the entrance to the dome. However, the gauges must be located at a distance from the entrance in order not to be influenced by the gas beaming effect. In the 3PP method, on the other hand, the positions of three gauges can be optionally best selected for pressure measurements. In addition, the pumping speed of the ion pump used can be calculated using the flow rate Qð 5 QL 1 QW Þ and the dome pressure P4 measured by the gauge G4 (shown in Fig. 2) under the following assumptions: (1) The outgassing rate of the test dome is negligibly small compared with the rate of gases flowing into the test dome. (2) The ultimate pressure of the pump is negligibly low compared with the test pressure. The pressure P0 and the pumping speed S0 at the pump mouth for nitrogen were calculated as P0 5 P4 2 S0 5

Q ; 1200

Q ; P0

where 1200 is the conductance (given in L s21) between the pump mouth and the gauge G4 and Q is the gas-flow rate given by Eq. (11). The calculated values S0 and P0 are given in Table 3, together with the values S00 and P00 calculated using Q0123 instead of Q. The pumping speeds S0 are appreciably higher than the speeds S00 , especially in lower pressure regions.

TABLE 3 Values of Q,P0, and S0 by the 3PP method and Q0123 , P00 , and S00 by the 2PP method at elapsed times of 5, 15, and 25 min at various pressure levels. The ratios S00 =S0 are also presented. Elapsed times

Measured pressures

Values calculated by the 3PP method

Values calculated by the conventional 2PP method

t (min)

P4 (Pa)

Q [(Pa L) s21]

P0 (Pa)

S0 (L s21)

Q0123 [(Pa L) s21]

P00 (Pa)

5

8.24 3 1026

4.1 3 1024

7.9 3 1026

51.2

3.2 3 1024

26

24

26

53.5

24

15 25

8.01 3 10

26

7.78 3 10

25

4.1 3 10

24

4.3 3 10

23

7.7 3 10

26

7.4 3 10

25

57.9

3.0 3 10

24

3.1 3 10

23

Ratios S00 (L s21)

S00 =S0

8.0 3 1026

39.5

0.77

26

38.0

0.73

26

40.7

0.70

25

7.8 3 10 7.5 3 10

5

2.40 3 10

1.4 3 10

2.3 3 10

62.6

1.1 3 10

2.3 3 10

47.3

0.76

15

2.40 3 1025

1.4 3 1023

2.3 3 1025

62.6

1.1 3 1023

2.3 3 1025

47.3

0.76

25

25

23

25

62.6

23

25

47.3

0.76

24

64.2

0.90

24

62.1

0.92

24

63.0

0.94

23

57.0

1.05

23

5 15 25 5

2.40 3 10

24

1.83 3 10

24

1.94 3 10

24

1.94 3 10

23

1.94 3 10

23

1.4 3 10

22

1.2 3 10

22

1.2 3 10

22

1.2 3 10

21

1.0 3 10

21

2.3 3 10

24

1.7 3 10

24

1.8 3 10

24

1.8 3 10

23

1.9 3 10

23

71.1 67.9 67.0 54.3

1.1 3 10

22

1.1 3 10

22

1.1 3 10

22

1.2 3 10

21

1.1 3 10

21

2.3 3 10 1.7 3 10 1.8 3 10 1.8 3 10 1.9 3 10

15

2.06 3 10

1.0 3 10

2.0 3 10

51.8

1.0 3 10

2.0 3 10

52.8

1.02

25

2.06 3 1023

1.0 3 1021

2.0 3 1023

51.8

1.0 3 1021

2.0 3 1023

52.8

1.02

2PP, Two-point-pressure; 3PP, three-point-pressure.

88

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

Pumping speed (L s–1)

Pumping speeds for nitrogen as a function of pressure at an elapsed time of 25 min are presented in Fig. 4, where the solid curve shows the speed S0 derived using the value Q calculated by the 3PP method and the broken curve S00 derived using the value Q0123 by the conventional 2PP method. The speed S00 gradually deviated from the speed S0 and becomes lower with a decrease of the pressure level. This tendency of the calculated pumping speeds is expected from the relatively large ratios of QW =QL in lower pressure ranges. FIG. 4 Pumping speeds for nitrogen as a function of pressure at an elapsed time of 25 min.K S0 by the 3PP method, - - x- - S0 0 by the conventional pipe method.

100

50

0

10–5

10–4 Inlet pressure (Pa)

10–3

The gradual decrease of speed S0 with the decrease of inlet pressure P0 in the lower pressure region may be due to the pressure dependence of the discharge intensity of the test pump,[21,22] and the gradual decrease of S0 with the increase of P0 in the higher pressure region may be partly due to the decrease in the gauge sensitivity at pressures above 1023 Pa.[23] For instance, as seen in the last row of Table 1, the pressure P3, 1.04 3 1022 Pa, is approximately three times higher than P1, 3.46 3 1023 Pa, so the gauge G3 is presumably more influenced by the decrease of the gauge sensitivity at the high pressure levels than the gauge G1 or G2 is.

4

Conclusion

The 3PP method can give real gas-flow rate when three pressures are measured accurately. Moreover, the outgassing rate of the pipe wall can be separately measured from the introduced leak rate. The flow rates measured by the conventional 2PP method were always lower than those by the 3PP method under the pressures lower than 1023 Pa. This means that the effect of the outgassing from the pipe wall cannot be neglected to obtain gas-flow rates. Appendix The following equation in matrix form is obtained for three pressures Pk1 , Pk2 , and Pk3 in the pipe from Eq. (2): 0

1 0 Pk1 1 @ Pk2 A 5 @ 1 1 Pk3

k1 k2 k3

10 1 k1 ð1 2 k1 =2Þ P0 k2 ð1 2 k2 =2Þ A@ QL =C A: QW =C k3 ð1 2 k3 =2Þ

Let jαj, jβj, and jγj represent the following determinates:    1 Pk1 k1 ð1 2 k1 =2Þ    jαj 5  1 Pk2 k2 ð1 2 k2 =2Þ ;    2 k3 =2Þ  1 Pk3 k3 ð1  1 k1 Pk1    jβj 5  1 k2 Pk2 ;     1 k3 Pk3  1 k1 k1 ð1 2 k1 =2Þ    jγj 5  1 k2 k2 ð1 2 k2 =2Þ :  1 k3 k3 ð1 2 k3 =2Þ  Then, the values QL and QW are derived using Cramer’s formula, as QL 5 C 3 jαj=jγj and QW 5 C 3 jβj=jγj, which are the same as Eqs. (3) and (4), respectively.

Methods for measuring outgassing rates Chapter | 3

89

References in the paper[1,2] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

Dayton BB. Ind Eng Chem 1948;40:795. Buhl R, Trendelenburg EA. Vacuum 1965;15:231. Fischer E, Mommsen H. Vacuum 1967;17:309. Denison DR, Mckee ES. J Vac Sci Technol 1974;11:337. Sharma JKN, Sharma DR. Vacuum 1982;32:253. Landfors AA, Hablanian MH. Transactions of the 5th AVS national vacuum symposium; 1958;22. Munro DF, Tom T. Transactions of the 3rd international vacuum congress, vol. 2; 1965;377. Blears J, Hill RW. Rev Sci Instrum 1948;19:847. Apparatus of AVS tentative standard 4.1, Suppl. III(b). Milleron N, Reinath FS. Transactions of the 9th AVS national vacuum symposium; 1962;356. Steckelmacher W. Vacuum 1966;16:561. Dayton BB. Transactions of the 3rd AVS national vacuum symposium; 1956;5. Fu¨sto¨ss L. Vacuum 1983;33:13. Levenson LL, Milleron N, Davis DH. Transactions of the 7th AVS national vacuum symposium; 1960;372. Hamacher H. Vacuum 1982;32:729. Teubner W. Vak Tech 1967;16:69. Welch KM. Vacuum 1973;23:271. Denison DR. J Vac Sci Technol 1975;12:548. Govier RP, McCracken GM. J Vac Sci Technol 1970;7:552. Calder R, Grillot A, Le Normand F, Mathewson A. Proceedings of the 7th international vacuum congress and third international conference solid surfaces, vol. 2; 1977;231. [21] Hartwig H, Kouptsidis JS. J Vac Sci Technol 1974;11:1154. [22] Pierini M. J Vac Sci Technol A 1984;2:195. [23] Redhead PA. Transactions of the 7th AVS national vacuum symposium; 1960;108.

Reviewed paper [1-2] Hirano H, Yoshimura N. A three-point-pressure method for measuring the gas-flow rate through a conducting pipe. J Vac Sci Technol A 1986;4(6):252630.

[1-3] “Two-point pressure method for measuring the outgassing rate” (Yoshimura and Hirano, 1989) Yoshimura and Hirano (1989) presented the paper,[1-3] “Two-point pressure method for measuring the outgassing rate,” in which the new “2PP method” and the new “one-point-pressure (1PP) method” are presented. In this book the paper is fully introduced. Abstract The 2PP method for measuring the outgassing rate of a solid material has been introduced, in which the pressures at two points in a pipe are measured. This method was applied to measuring the outgassing rates of two kinds of SUS304 plates, belt-polished plates, and buff-polished plates. The outgassing rates were measured under a pressure as low as that expected in the actual high-vacuum system. In addition, the 1PP method has been introduced, whose validity was ascertained using measured pressures in the experimental setup. The outgassing rates for the same kinds of SUS304 plates were again measured by the conventional orifice method for comparison.

1

Introduction

Data on outgassing rates of the constituent materials are essential to calculating the pressures in a high-vacuum system.[1,2] Two methods for measuring the outgassing rate have been conventionally used; one is the orifice method and the other is the build-up method. In the former the pressure in the chamber evacuated through a small

90

PART | 1 Adsorption, desorption, diffusion, and outgassing/pumping

orifice is comparatively high, which is undesirable because the outgassing rate of the sample depends on its history under vacuum and on the pressure itself.[3] In the latter method, on the other hand, it is difficult to measure the pressure-rise rate accurately at the minimum base pressure, because a large number of gas molecules are evolved from the valve seat when the metal valve is closed tightly. The density of molecules adsorbed on the surfaces of the sample and the chamber wall is increased with the pressure rise in the build-up period. The surfaces thus covered with the higher density of molecules show a higher outgassing rate for an appreciable period after the valve is again opened. The present report introduces a new method for measuring the outgassing rate under a sufficiently low pressure, in which the pressures at two points in a pipe are measured.

2

Two-point-pressure method

The 3PP method for measuring the gas-flow rate through an outgassing pipe has been recently introduced,[4] in which the pressures at three different points in the pipe are measured. The pumping speed of an ion pump was successfully measured by the 3PP method.[4] A similar method, simplified from the 3PP method, was applied to measuring the outgassing rate of the two kinds of SUS304 plates.

A Principle First, let us review the principle of the 3PP method.[4] Consider an outgassing pipe of conductance C with an unknown outgassing rate QW , accompanied with an unknown introduced leak rate QL . The pipe is evacuated by an unknown pumping speed, as shown in Fig. 1A, and, we assume the same outgassing rate per unit length of the pipe regardless of the position. Then, the pressure Pk at a position indicated by the fraction k is given by Pk 5 P0 1

k½QL 1 ð1 2 k=2ÞQW  : C

(1)

Eq. (1) contains three unknown factors, QL , QW , and P0 . These three factors can be calculated from Eq. (1) using three pressures Pk1 , Pk2 , and Pk3 measured at three different points in the pipe indicated by ki . Next, consider a simplified system of an outgassing pipe without a leak rate QL , as shown in Fig. 1B. When the outgassing rate of the end plate is negligibly low, which is applicable in general, the pressure Pk at k is given by Pk 5 P0 1

kð1 2 k=2ÞQW : C

(2)

Eq. (2) contains two unknown factors P0 and QW . And so, the value QW can be calculated from Eq. (2) using two pressures Pk1 and Pk2 measured at two different positions k1 and k2 , respectively, as QW 5

CðPk2 2 Pk1 Þ : ðk2 2 k1 Þ½1 2 ðk1 1 k2 Þ=2

(3)

We call this measuring method based on Eq. (3), the “2PP method.” Finally, let us consider the case that the pipe is evacuated by an effective pumping speed much higher than the pipe conductance C, as shown in Fig. 1C. In this case, one could treat the pressure P0 at the pipe port as zero. And consequently, 1PP Pk at k can derive the outgassing rate QW simply as QW 5

CPk : kð1 2 k=2Þ

We call this method based on Eq. (4), the “1PP method.” Comment by Yoshimura: When one sets k to 0.8, Eq. (4) reduces to CP0:8 QW 5 0:8ð1 2 0:4Þ 5 2:08CP0:8 D2CP0:8 :

(4)

Methods for measuring outgassing rates Chapter | 3

FIG. 1 Outgassing pipe systems: (A) Pipe of a conductance C with a leak rate QL , (B) pipe with an end plate whose outgassing rate is negligibly small, and (C) pipe evacuated by a high pumping speed.

(A) Ql

Qw+Ql 0 P0

(k)

1

91

(B) Qw l P0

(k) (C) Qw P1

B

l P0