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Ultra high vacuum

Concepts involved

Advanced Definition

Kinetic theory of gases

Gas transport and pumping

Vacuum pumps and systems

Typical uses for ultra high vacuum

Ultra high vacuum is necessary for many surface analytic techniques such as:

X-ray photoelectron spectroscopy (XPS)

Auger electron spectroscopy (AES)

Secondary ion mass spectrometry (SIMS)

Thermal desorption spectroscopy (TPD)

Thin film growth and preparation techniques with stringent requirements for purity, such as molecular beam epitaxy (MBE) and UHV chemical vapor deposition (CVD)

Angle resolved photoemission spectroscopy (ARPES)

UHV is necessary for these applications to reduce surface contamination, by reducing the number of molecules reaching the sample over a given time period. At 0.1 mPa (106 Torr), it only takes 1 second to cover a surface with a contaminant, so much lower pressures are needed for long experiments.

UHV is also required for:

Particle accelerators

Atomic physics experiments which use cold atoms, such as ion trapping or making Bose-Einstein Condensates

Achieving ultra high vacuum

Extraordinary steps are required to reach UHV, including the following:

High pumping speed possibly multiple vacuum pumps in series and/or parallel

Minimize surface area in the chamber

High conductance tubing to pumps short and fat, without obstruction

Use low-outgassing materials such as certain stainless steels

Avoid creating pits of trapped gas behind bolts, welding voids, etc.

Electropolish all metal parts after machining or welding

Use low vapor pressure materials (ceramics, glass, metals, teflon if unbaked)

Bake the system (250 C to 400 C) to remove water or hydrocarbons adsorbed to the walls

Chill chamber walls to cryogenic temperatures during use

Avoid all traces of hydrocarbons, including skin oils in a fingerprint always use gloves

Outgassing is a significant problem for UHV systems. Outgassing can occur from two sources: surfaces and bulk materials. Outgassing from bulk materials is minimized by careful selection of materials with low vapor pressures (such as glass, stainless steel, and ceramics) for everything inside the system. Even materials which are not generally considered absorbent can outgas, including most plastics and some metals. For example, vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems.

Outgassing from surfaces is a subtler problem. At extremely low pressures, more gas molecules are adsorbed on the walls than are floating in the chamber, so the total surface area inside a chamber is more important than its volume for reaching UHV. Water is a significant source of outgassing because a thin layer of water vapor rapidly adsorbs to everything whenever the chamber is opened to air. Water evaporates from surfaces too slowly to be fully removed at room temperature, but just fast enough to present a continuous level of background contamination. Removal of water and similar gases generally requires baking the UHV system at 200 to 400 C while vacuum pumps are running. During chamber use, the walls of the chamber may be chilled using liquid nitrogen to reduce outgassing further.

Hydrogen and helium are the most common background gases in a well-designed, well-baked UHV system. Hydrogen diffuses out from the grain boundaries in stainless steel. Helium can diffuse through the steel and glass from the outside air.

Typically, there is no single vacuum pump that can operate all the way from atmospheric pressure to ultra high vacuum. Instead, a series of different pumps is used, according to the appropriate pressure range for each pump. Pumps commonly used to achieve UHV include:

Turbomolecular pumps (especially compound and/or magnetic bearing types)

Ion pumps

Titanium sublimation pumps

Non-evaporable getter (NEG) pumps

Cryopumps

UHV pressures are measured with an ion gauge, either a hot filament or an inverted magnetron type.

Finally, special seals and gaskets must be used between components in a UHV system to prevent even trace leakage. Nearly all such seals are all metal, with knife edges on both sides cutting into a soft, copper gasket. This all-metal seal can maintain pressures down to 100 pPa (~1012 Torr).

Measuring high vacuum

Main article: Pressure measurement

Measurement of high vacuum is done using a nonabsolute gauge that measures a pressure-related property of the vacuum, for example, its thermal conductivity. See, for example, Pacey. These gauges must be calibrated. The gauges capable of the measuring the lowest pressures are magnetic gauges based upon the pressure dependence of the current in a spontaneous gas discharge in intersecting electric and magnetic fields.

UHV manipulator

A UHV manipulator allows an object which is inside a vacuum chamber and under vacuum to be mechanically positioned. It may provide rotary motion, linear motion, or a combination of both. The most complex devices give motion in three axes and rotations around two of those axes. To generate the mechanical movement inside the chamber, two basic mechanisms are commonly employed: a mechanical coupling through the vacuum wall (using a vacuum-tight seal around the coupling), or a magnetic coupling that transfers motion from air-side to vacuum-side. Various forms of motion control are available for manipulators, such as knobs, handwheels, motors, stepping motors, piezoelectric drives, and pneumatics.

The manipulator or sample holder may include features which allow additional control and testing of a sample, such as the ability to apply heat, cooling, voltage, or a magnetic field. Sample heating can be accomplished by electron bombardment or thermal radiation. For electron bombardment, the sample holder is equipped with a filament which emits electrons when biased at a high negative potential. The impact of the electrons bombarding the sample at high energy causes it to heat. For thermal radiation, a filament is mounted close to the sample and resistively heated to high temperature. The infra-red energy from the filament heats the sample.

References and notes

^ DJ Pacey (W. Boyes, editor) (2003). Measurement of vacuum; Chapter 10 in Instrumentation Reference Book (Third Edition ed.). Boston: Butterworth-Heinemann. p. 144. ISBN 0750671238. http://books.google.com/books?id=sarHIbCVOUAC&pg=PA144&lpg=PA144&dq=”measurement+of+vacuum”&source=web&ots=aWbFsOCsvf&sig=NcVTpfYXN5X-8STfeOIIKeCG0EU#PPA144,M1. 

^ LM Rozanov & Hablanian, MH (2002). Vacuum technique. London; New York: Taylor & Francis. p. 112. ISBN 041527351X. http://books.google.com/books?id=8yEGJCtS2XgC&pg=PA79&lpg=PA79&dq=”measurement+of+vacuum”&source=web&ots=RmEYhDyVOY&sig=Muaslx8-GZW6zWBCa403si5cuRM#PPA112,M1. 

^ LM Rozanov & Hablanian, MH. p. 95. ISBN 041527351X. http://books.google.com/books?id=8yEGJCtS2XgC&pg=PA79&lpg=PA79&dq=”measurement+of+vacuum”&source=web&ots=RmEYhDyVOY&sig=Muaslx8-GZW6zWBCa403si5cuRM#PPA95,M1. 

See also

Vacuum engineering

Vacuum flange

Vacuum gauge

Journal of Vacuum Science and Technology

Vacuum

Vacuum state

External links

American Vacuum Society

Three major steps to get Ultra High Vacuum

Vacuum Systems and Thin-Film Technology Course

CERN Ultra high vacuum solar thermal collector.

Categories: Vacuum

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