## Vacuum system In high resolution microscopes, FIB, atom probe, or other characterization instruments, ultra high vacuum level (UHV) may be quite common. This is also true for the study of specific phenomenon, like surface behavior or quantum process, when we wanna avoid scattering/attaching of gas molecules disturbing our systems of interest. To obtain and maintain such vacuum level, pumps are applied. ### Obtaining UHV - construction geometry: large diameter tubing for connection; avoid bends. This is because when vacuum level is high, mean free path will become very large, bending/small radius would make molecules difficult to get out from the system. - proper pump selection at different level. ### Pumps for UHV #### [Rotary vane pump](https://en.wikipedia.org/wiki/Rotary_vane_pump) Backing pump, for $10^3-10^{-3}$ mbar, oil in older pumps can lead to contamination of the vacuum chamber. #### [Sorption pump](https://en.wikipedia.org/wiki/Sorption_pump) Backing pump, for $10^3-10^{-5}$ mbar, molecular sieve (particles with high specific surface area)![[Drawing 2023-09-30 21.00.21.excalidraw.svg]] #### [Turbomolecular pump](https://en.wikipedia.org/wiki/Turbomolecular_pump) UHV pump, for $10^{-2}-10^{-11}$ mbar. Gas molecules in the pump are given a desired orientation by impacts with a rotating turbine. Rotor speed should match thermal speed of molecules. The pump is noisy and could introduce vibration, which is very unfavorable for scanning microscopes. #### [Diffusion pump](https://en.wikipedia.org/wiki/Diffusion_pump) UHV pump, for $10^{-2}-10^{-9}$ mbar. Gas molecules are given desired orientation by impacts with a supersonic vapor jet of oil molecules. This type of pump is cheap and effective for all gases, but energy consuming and requires additional cooling system. #### [Titanium sublimation pump](https://en.wikipedia.org/wiki/Titanium_sublimation_pump) UHV pump, for $10^{-1}-10^{-11}$ mbar. Must be used together with other UHV pump. Gas molecules get absorbed on the thermally vapor-deposited titanium. #### [Ion pump](https://en.wikipedia.org/wiki/Ion_pump_(physics)) UHV pump, for $10^{-5}-10^{-11}$ mbar. Gas molecules get ionized, the accelerated under applied field, the get attached on a solid titanium electrode. This process will further sputter Ti away from the electron, and these Ti will react with other gas molecules to maintain this process. Due to the requirement for acceleration in electric field, noble gases are not very suitable for this pump. #### [Cryopump](https://en.wikipedia.org/wiki/Cryopump) UHV pump, for $10^{-3}-10^{-11}$ mbar. Inside the pump a surface is cooled by liquid nitrogen/helium to have other gases condensed onto it. It has high pumping rate and universally applicable. The same instrument installed in TEM to ensure UHV is called a [cold trap](https://en.wikipedia.org/wiki/Cold_trap). ### Measuring UHV For low vacuum, $P>10^{-3}$, the pressure can be measured by thermal conductivity, using a hot wire in a gas. This is because both pressure and thermal conductivity are linked to mean free path of gas molecules. For $P<10^{-3}$, this method no longer works well due to the increasing mean free path. The pressure can then be measured with [ion gauges](https://en.wikipedia.org/wiki/Hot-filament_ionization_gauge). Such measurement is based on the ionization of gases. >[!Info] >The contents in this page are partially adapted from [327-0505-00L  Surfaces, Interfaces and their Applications I](https://www.vvz.ethz.ch/Vorlesungsverzeichnis/lerneinheit.view?lerneinheitId=162186&semkez=2022W&lang=en). Find more information in the lecture slides. >