Gas Release from Solids, Materials for Vacuum and Leak Detector

VACUUM AND PLASMA SCIENCE AND TECHNOLOGY W9 Gas Release from Solids, Materials for Vacuum and Leak Detector Ming-Show Wong Nov. 2015

Gas Release from Solids Vaporization Thermal Desorption First Order Desorption Second Order Desorption Real Surfaces Diffusion Vacuum Baking Permeation Stimulated Desorption Electron-stimulated Desorption Ion-stimulated Desorption Stimulated Chemical Reactions Photodesorption Pressure Limits

Gas Release from Solids the mechanisms of gas evolution from solid surfaces explains how they affect the pumping rate and ultimate pressure in vacuum chambers. Gas is dissolved in and adsorbed on solids. This gas release, collectively referred to as outgassing, is actually a result of several processes. Figure 4.1 shows all the possible sources of gas in addition to the gas located in the volume of the chamber. Gases and vapors release from surface are a result of.

Vaporization (I) A vapor is a gas near its condensation temperature Vaporization is the thermally stimulated entry of molecules into the vapor phase. In dynamic equilibrium the rate at which molecules leave the surface of solid or liquid equals the rate at which they arrive at the surface. The pressure of the vapor over the surface in dynamic equilibrium is the vapor pressure of the solid or liquid, provided the solid or liquid and the vapor are at the same temperature

Vaporization (II) In Chapter 2 we stated the molecular flux of vapor crossing a plane was nv/4. In equilibrium this is therefore the rate of molecular release from the surface. For the case of free evaporation of a solid from a heated source, (2.9) given here in a different form may be used to calculate the maximum rate of evaporation of a solid from its temperature, vapor pressure, surface area, and molecular weight: (molecules/s) = 2.63x10 24 PA/(MT) 1/2 (4.1)

4.2 THERMAL DESORPTION Thermal desorption is the heat-stimulated release of gases or vapors previously adsorbed on the interior walls of the system. They may have been adsorbed on the chamber surface while it was exposed to the atmospheric environment and then slowly released as the pump removed gas from the chamber. Desorption is also the final step in the processes of diffusion and permeation. The rate of desorption is a function of the molecular binding energy, the temperature of the surface, and the number of monolayers of surface coverage Physisorption: van der Waal s force (< 40 MJ/(kg-mole)) Chemisoption: van der Waal s force (> 40 MJ/(kg-mole))

Materials in Vacuum

Selection Criteria Materials for Use in Vacuum Materials commonly used in vacuum metals, glasses, ceramics and polymers gases release from solids at low pressures A very important part of the design and should be considered in consultation with the user Not only must the material be capable of being fabricated into the required components but it must stand up to the environmental conditions of temperature, pressure, and chemical attack etc. imposed on it by the vacuum processes,without limiting the attainable pressure that is required

Material Property Requirements Vaporization, Permeability and Outgassing rate Mechanical Properties must be capable of being machined and fabricated. must have adequate strength at max. and min. temp. to be encountered, and must retain it's elastic, plastic, and/or fluid properties over the expected temp. range Thermal Properties The material's vapor pressure must remain low Thermal expansion coef. match Gas Loading Materials must not be pourous. Materials must be free of cracks and crevices Surface and bulk desorption rates must be acceptable

Commonly Used Materials In Vacuum metals, glasses, ceramics and polymers

Metals for the Vacuum Envelope Austenitic Stainless Steel the dominant materials for construction of vacuum system Aluminum and Aluminum Alloys the second common material used for vacuum construction Mild Steel may be used down to about 10E-3 mbar Oxygen Free High Conductivity (OFHC)Copper Brass has good corrosion resistance and may be suitable Glass to Metal & Glass to Ceramic Seals: None of the Above are suitable for glass to metal seals, which are usually made of Inconel or Kovar.

Stainless Steel Chamber

Stainless Steel Chamber

Metal Bell Jar Baseplate Feedthrough Collar

Pyrex Glass Bell Jars, Metal Guards, and "L" Gaskets

Metals Used in Demountable Seals Copper Rings are commonly used for high and ultrahigh vacuum applications. Aluminum Wire Rings are cheap and bakeable to 200 C Indium Wire can be used between flat flanges It is very soft and continues to flow after initial tightening Gold wire is often used for Ultra-High Vacuum seals between flat surfaces,and can be baked to 450 C Swagelok,Tylok,and other Proprietary Brand Seals are now widely used

Glasses and Ceramics A glass is an inorganic material that solidifies without crystallization formulated from silicon oxide base soft glasses: addition of sodium and calcium oxides (soda-lime) hard glasses: addition of boric oxide (Borosilicate, Pyrex ) A ceramic is a polycrystalline, non-metallic inorganic material may be formed by heat treatment may contain entrapped gas pore and are not so dense Fully vitrified electrical porcelain and vitrified alumina

Polymers, Plastics, Elastomers, Epoxy Should be kept to a minimum due to their high gas permeability and high desorption rates PTFE has self-lubricating properties Nylon has self lubricating properties Acrylics has undesirable vacuum properties as nylon. Polycarbonates and Polystyrene PVC has a high outgassing rate Polyethylene may be usable if well outgassed. Nalgene t.m. bell jars from Fisher Scientific Vespel Polyimide is ultra-high vacuum compatible, Elastomers Nitrile Rubber used in demountable seals,i.e. "O" rings. Viton is bakeable to 200 Degrees C. O Ring Chart

Vacuum Compatible Epoxies and Leak Sealers Glue is no substitute for having done the job right in the first place Varian Torr Seal is a solvent free epoxy resin Kurt J. Lesker KL-325K, a solvent free epoxy Vacseal Vacseal is a low vapour pressure silicon resin Vacuum Compatible Tapes 3M 850 Polyester Film Tape has a low outgassing rate

Pumping Fluids Rotary Pumps High Quality,Low Vapour Pressure Mineral Oils Fomblin (t.m. Montedison Sp.A.), is used for pumping oxygen and other aggressive or corrosive gases or vapours. Diffusion Pumps Mineral Oils prepared by molecular distillation Silicon Fluids produce ultimate pressures down to 10E-09 mbarr. Polyphenyl Ethers are thermally stable Perfluoro Polyether (a.k.a. Fomblin t.m.)is chemically stable

High Vacuum Compatible Lubricants Krytox A perfluorinated grease Torrlube Polyphenyl Ethers

Materials That Should Not Be Used In Vacuum Cadmium Plating, often used for small screws, must be avoided for vacuum applications due to some really nasty outgassing problems Small screws used in the chamber can be nickel plated brass or plain copper, and should be drilled or relieved (file off one side of the threaded area) to prevent virtual leaks PVC Insulated wire has a high outgassing rate should be replaced with teflon insulated wire Paint is a source of outgassing and virtual leaks Yellow Transformer Tape outgasses like mad can be replaced with ordinary (plumbers) teflon tape

Residual Gas Analyzers & Mass Spectrometers

RGA & MASS SPECTROMETER A residual gas analyzer (acronymed by the term RGA) is a small and usually rugged mass spectrometer used as sensitive in-situ, helium leak detectors. With vacuum systems pumped down to lower than 10 5 Torr, checking of the integrity of the vacuum seals and the quality of the vacuum; air leaks, virtual leaks and other contaminants at low levels my be detected before a process in initiated. An instrument which produces a beam of ions from a sample, separates the resulting mixture of ions according to their mass-to-charge ratios, and provides output signals which are measurements of the relative abundance of the ionic species present. A mass spectrometer usually consists of an ion source, a detector and a mass analyzer.

RESIDUAL GAS ANALYZER A device for measuring the amounts and species of various gases present in a vacuum chamber Also referred to as a partial pressure analyzer or partial pressure gauge.

Partial Pressure Analysis (1) Ion Sources Electron-Impact lonization Process Open Ion Source Closed Ion Source Ion Detection Faraday Cup Ion Detection Secondary Electron Multiplier Detection MicroChannel Plate Detector Mass Analysis Quadrupole Mass Analyzer Magnetic Sector Analyzer Time-of-Flight Mass Analyzer Trochoidal (Cycloid) Mass Analyzer Omegatron

Partial Pressure Analysis (2) Optical Measurement of Partial Pressures Photoionization Measurement of Partial Pressure Infrared Absorption Measurement of Partial Pressure Computer Control, Data Acquisition, and Presentation Residual Gas Analysis Pressure Reduction Sampling Methods for Vacuum Process Analysis Calibration of Partial Pressure Analyzers

Residual Gas Analyzers & Mass Spectrometers Instrument description Installation and operation Instrument selection Interpretation of RGA data

Instrument Description Residual Gas Analyzers and mass spec. are used to measure the ratio of mass to electric charge of a molecule or atom. the molecules or atoms are ionized then directed through a mass separator and finally detected Various methods have been developed for each of three stages of particles identification Ionization sources - electron impact ionization Mass separation - magnetic sector and RF quadrupole Detection - Faraday cup-electron multiplier detector

Magnetic deflection mass spectrometer mv r r 2 1 B Bev 2mV e 1/ 2

Quadrupole Mass Spectrometer

Quadrupole MS

Interpretation of RGA Data Cracking Patterns Dissociative ionization Isotopes Multiple ionization Combined effects Qualitative Analysis Quantitative Analysis Isolated spectra Overlapping spectra

Cracking Patterns Ionization create fragments of several mass-to-charge ratios unique for each gas species peak amplitudes depend on the gas and instrumental conditions cracking pattern forms a fingerprint - used for identification of a gas Dissociation ionization: Electron dissociation of molecule into lighter fragments Isotopes: Several neighboring peaks in the cracking pattern may due to isotropes Multiple Ionization: Higher degree of ionization, doubly ionized peaks and even triply ionized peaks Combined Effects: cracking patterns may show the combined effects of dissociation, isotopes ad multiple ionization

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Leak Detection and Leak Detectors (1) Principles of Vacuum Leak Detection Types of Leaks and Leak Rate Units Total Pressure Measurements Partial Pressure Measurements Measurement of Leakage Rates with Helium Leak Detectors Helium Leak Detection of Vacuum Components Helium Leak Detection of Vacuum Systems Special Methods and Other Tracer Gases

Leak Detection and Leak Detectors (2) Mass Spectrometer Leak Detectors Mass Spectrometer System for Helium Leak Detection Direct-Flow Helium Leak Detectors Simple Counterflow Helium Leak Detectors Advanced Counterflow Helium Leak Detectors Oil-free and Dry Helium Leak Detectors Specifications of Mass Spectrometer Leak Detectors Quantitative Leakage Rate Measurements Mass Spectrometer Leak Detectors for Other Tracer Gases and Future Developments in Leak Detection