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Ultra High VacuumThe article on the more general subject of vacuum is located elsewhere.
Ultra High Vacuum is a regime of vacuum characterised by pressures lower than typically about 10-9 torr. Concepts Involved - Kinetic theory of gases
- Gas transport and pumping
- Vacuum pumps and systems
Degrees of vacuum - Rough (low) vacuum = 1 - 10-3 Torr
- Medium vacuum = 10-3 - 10-5 Torr
- High vacuum (HV) = 10-6 - 10-8 Torr
- Ultrahigh vacuum (UHV) = < 10-9 Torr
Why do we need Ultra high vacuum? Ultra high vacuum is necessary for many surface analytic techniques such as X-ray photoelectron spectroscopy, Auger electron spectroscopy, Secondary Ion Mass Spectrometry etc, as well as thin film preparation techniques that have stringent requirements for purity. Types of Pumps - Low vacuum pumps
- Mechanical Pump
- Sorption Pump
- High and Ultrahigh Vacuuum Pumps
There is typically no pump that can take you all the way from atmospheric pressure to the regime of Ultra high vacuum. Thus we typically use a combination of pumps and switch over from one regime to the other. At extremely low pressures, outgassing of the vacuum vessel occurs over time. Even if a high vacuum is generated in a hermetically sealed container, there is no guarantee that an adequately low pressure will continue unless outgassing has been accounted for. Outgassing is generally worse at higher temperatures. Even materials which are not naively considered absorbent will outgas. Water vapor is a primary outgas component, even in hard metal vessels (such as stainless steel or titanium). Outgassing can be reduced by desiccation prior to vacuum pumping. Baking the UHV chamber is a common procedure to get rid of the adsorbed gases on the surface of the chamber. It involves heating the chamber to roughly 250 to 400 deg. C. During chamber use, the walls of the chamber may be chilled using liquid nitrogen to reduce outgassing further. Vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems. To achieve UHV - High pumping speed - possibly multiple pumps in series
- Smaller vacuum chamber
- High conductance
- Special low outgassing stainless steel chamber
- Low vapor pressure materials (ceramics, glass, metals)
- Bake out procedures (250C - 400C)
- Chill chamber walls (77K) during use
- Avoid Organic Materials (Avoid even finger grease - use gloves always)
See also External links
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