Ultra-high vacuum
Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about 10−7pascal or 100 nanopascals (10−9mbar, ~10−9torr). UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is approximately 40 km, so gas molecules will collide with the chamber walls many times before colliding with each other. 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.
Maintaining UHV conditions requires the use of unusual materials for equipment. Heating of the entire system above 100 °C for many hours ("baking") to remove water and other trace gases which adsorb on the surfaces of the chamber is required upon "cycling" the equipment to atmosphere. To save time, energy, and integrity of the UHV volume an "interlock" is often used. The interlock volume has one door or valve facing the UHV side of the volume, and another door against atmospheric pressure through which samples or workpieces are initially introduced. After sample introduction and assuring that the door against atmosphere is closed, the interlock volume is typically pumped down to a medium-high vacuum. In some cases the workpiece itself is baked out or otherwise pre-cleaned under this medium-high vacuum. The gateway to the UHV chamber is then opened, the workpiece transferred to the UHV by robotic means or by other contrivance if necessary, and the UHV valve re-closed. While the initial workpiece is being processed under UHV, a subsequent sample can be introduced into the interlock volume, pre-cleaned, and so-on and so-forth, saving much time. Although a "puff" of gas is generally released into the UHV system when the valve to the interlock volume is opened, the UHV system pumps can generally snatch this gas away before it has time to adsorb onto the UHV surfaces. In a system well designed with suitable interlocks, the UHV components seldom need bakeout and the UHV may improve over time even as workpieces are introduced and removed.
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