Ultra-high vacuum (often spelled ultrahigh in American English, UHV) is the vacuum regime characterised by pressure lower than about 1×10−9 torrs (1×10−9 mbar; 1×10−7 Pa). UHV conditions are created by pumping the gas out of a UHV chamber, and in some cases by cooling the chamber to cryogenic temperatures. In general, there are two regimes of gas flow, laminar and free molecular, defined by whether the dominant interactions are particle-particle (laminar) or particle-chamber (molecular). At UHV pressures the mean free path of a gas molecule is greater than approximately 40 km, so the gas is in free molecular flow, and gas molecules will collide with the chamber walls many times before colliding with each other, on average. Almost all molecular interactions therefore take place on various surfaces in the chamber. UHV conditions are integral to scientific research. Surface science experiments often require a chemically clean sample surface with the absence of any unwanted adsorbates. Surface analysis tools such as X-ray photoelectron spectroscopy and low energy ion scattering require UHV conditions for the transmission of electron or ion beams. For the same reason, beam pipes in particle accelerators such as the Large Hadron Collider are kept at UHV.
Overview
Maintaining UHV conditions requires the use of unusual materials for equipment. Useful concepts for UHV include:
Sorption of gases Kinetic theory of gases Gas transport and pumping Vacuum pumps and systems Vapour pressure Typically, UHV requires:
High pumping speed — possibly multiple vacuum pumps in series and/or parallel Minimized surface area in the chamber High conductance tubing to pumps — short and fat, without obstruction Use of low-outgassing materials such as titanium and certain stainless steels Avoid creating pits of trapped gas behind bolts, welding voids, etc. Electropolishing of all metal parts after machining or welding Use of low vapor pressure materials (ceramics, glass, metals, teflon if unbaked) Baking of the system to remove water or hydrocarbons adsorbed to surfaces, and at higher temperature/longer bakes, diffuse hydrocarbons, hydrogen, etc trapped in the bulk volume Some chambers have walls chilled to cryogenic temperatures during use Avoiding all traces of hydrocarbons, including skin oils in a fingerprint — gloves must always be used Hydrogen and carbon monoxide are the most common background gases in a well-designed, well-baked UHV system. Both Hydrogen and CO diffuse out from the grain boundaries in stainless steel. Helium could diffuse through the steel and glass from the outside air, but this effect is usually negligible due to the low abundance of He in the atmosphere.
Measurement
Pressure
Measurement of high vacuum is done using a nonabsolute gauge that measures a pressure-related property of the vacuum. See, for example, Pacey. These gauges must be calibrated. The gauges capable of 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 pressures are measured with an ion gauge, either of the hot filament or inverted magnetron type.
Leak rate In any vacuum system, some gas will continue to escape into the chamber over time and slowly increase the pressure if it is not pumped out. This leak rate is usually measured in mbar L/s or torr L/s. While some gas release is inevitable, if the leak rate is too high, it can slow down or even prevent the system from reaching low pressure. There are a variety of possible reasons for an increase in pressure. These include simple air leaks, virtual leaks, and desorption (either from surfaces or volume). A variety of methods for leak detection exist. Large leaks can be found by pressurizing the chamber, and looking for bubbles in soapy water, while tiny leaks can require more sensitive methods, up to using a tracer gas and specialized Helium mass spectrometer.
Outgassing
Outgassing is a problem for UHV systems. Outgassing can occur from two sources: surfaces and bulk materials. Outgassing from bulk materials is minimized by selection of materials with low vapor pressures (such as glass, titanium, certain [304 and 316] stainless steel, and ceramics) for everything inside the system. Although it is cost-prohibitive, titanium is the ideal material for metal chambers, because it has outgassing rates several orders of magnitude lower than any non-heat-treated stainless steel. 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 (392 to 752 °F) while vacuum pumps are running. During chamber use, the walls of the chamber may be chilled using liquid nitrogen to reduce outgassing further.
Bake-out
In order to reach low pressures, it is often useful to heat the entire system above 100 °C (212 °F) for many hours (a process known as bake-out) to remove water and other trace gases which adsorb on the surfaces of the chamber. This may also be required upon "cycling" the equipment to atmosphere. This process significantly speeds up the process of outgassing, allowing low pressures to be reached much faster. After baking, to prevent humidity from getting back into the system after it is exposed to atmospheric pressure, a nitrogen gas flow that creates a small positive pressure can be maintained to keep the system dry. Stainless steel parts can be heat treated at 900 °C for more than 24 hours to achieve much lower outgassing rates.
System design
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