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Ultra-High Vacuum and Precision in Measurement

Ultra-high vacuum replicates conditions somewhere between the stratosphere and the most immediate reaches of outer space

Brandoch Cook, PhD

Brandoch Cook, PhD, is a freelance scientific writer. He can be reached at:

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Ultra-high vacuum (UHV) systems are essential tools for researchers and engineers to achieve extremely low pressures in contained environments. They are powerful enough to accurately measure and alter the smallest semiconductors, or to unravel some of the mysteries of the vastness of space-time. Units of pressure can be as confusing as units of British currency. If you live at sea level, you can reasonably assume a pressure of roughly one atmosphere, unless you decide to climb a mountain, or the weather turns foul. One atmosphere corresponds to 760 millimeters of mercury (mmHg), a unit your doctor uses when recording blood pressure. Scientific and engineering literature regularly report pressures in Torr, which are roughly equivalent to mmHg, although the international standard (SI) unit is the Pascal (Pa). For every Torr there are 133.3 Pa, so that one atmosphere equals about 101,300 Pa, which is also 1013 millibar (mbar). For Americans filling the tires of your minivans, this is also about 14.7 pounds per square inch, or a little less than half of what you need. Do you follow?

If you dive in the ocean, the pressure increases one additional atmosphere for about every 10 meters you descend. Conversely, in the long line to reach Mount Everest’s death zone, the atmospheric pressure drops to less than 20 percent of what it was at sea level, a precipitous decline related to a molecular gradient in which the majority of mass in Earth’s atmosphere occupies the bottom few kilometers of it. Pressure trends more slowly toward an asymptotic lower limit within the stratosphere. Gas molecules become much more widely dispersed, and hence less likely to collide.

Ultra-high vacuum replicates conditions somewhere between the stratosphere and the most immediate reaches of outer space, achieving pressures in the range of 10-7 to 10-12 Pa. Reaching pressures this low is necessarily a stepwise process. First, the receiving chamber has to be baked at high temperature, and so is most often made of high quality stainless steel, or of alloys that promote the exclusion of gases. A rough vacuum obtains 10-3 Pa, then a turbo molecular pump reaches about 10-8 Pa. In the final step, a specialized pump with metallic seals and gaskets and a dedicated gauge brings it to its target. Like dust in a seemingly empty room, under ordinary high vacuum, residual gases will settle on chamber surfaces in a monolayer in milliseconds. Under UHV, monolayer deposition takes days, with the hypothetical remaining molecules needing to travel about 50 kilometers to find each other. This becomes very important for multimillion-dollar particle physics experiments that might best be described as quantum existentialism, which depend greatly on the predictability of the path of a proton or ion beam through a long, sealed chamber. A lot is riding on ensuring the beam is not scattered by remnant gases.

In the comparatively humble laboratory space, UHV is broadly applicable to many fields of measurement and materials science. Notable among these are various iterations of electron and atomic force microscopy, and the production and manipulation of nanoscale constructions, for instance in the development of thin film-based semiconductors for high-fidelity electronics. The combination of these fields enables the alteration of graphene- or transition metal-based two-dimensional nanomaterials, while characterizing those changes at the atomic level using transmission electron microscopy. Microscope companies such as Zeiss and Nikon offer transmission electron microscope platforms, however the upgrade to UHV requires customization with specialized pumps and gauges. Gamma Vacuum is an industry leader, providing a wide array of ion, titanium sublimation, and non-evaporable getter pumps, as well as controllers and other accessories. Additionally, there is a wealth of contractors that can customize UHV builds to the ideal specifications of a given research goal.

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