Ultra high vacuum
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Ultra high vacuum (UHV) is the regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals (~10−9 torr). UHV requires the use of special materials, extreme cleanliness, and baking the entire system to remove water and other trace gases. See Vacuum for descriptions of other levels of vacuum. Pressure is also measured by a barometer in the weather form of the "air pressure" of geography. At these low pressures, gas molecules rarely run into each other. The mean free path of a gas molecule at 10−7 pascal or 100 nanopascals (~10−9 torr) is approximately 40 km, so gas molecules will collide with the chamber walls many times before colliding with each other. Almost all interactions therefore take place at various surfaces in the chamber.
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[edit] Concepts involved
- Kinetic theory of gases
- Gas transport and pumping
- Vacuum pumps and systems
[edit] 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)
- Thin film growth and preparation techniques with stringent requirements for purity, such as molecular beam epitaxy (MBE) and UHV CVD
- Particle accelerators
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 (10−6 Torr), it only takes 1 second to cover a surface with a contaminant, so much lower pressures are needed for long experiments.
[edit] 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.
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 (~10−12 Torr).
