| Concepts involved | | | | Typically, there is no single vacuum pump that can |
| Advanced Definition | | | | operate all the way from atmospheric pressure to |
| Kinetic theory of gases | | | | ultra high vacuum. Instead, a series of different |
| Gas transport and pumping | | | | pumps is used, according to the appropriate pressure |
| Vacuum pumps and systems | | | | range for each pump. Pumps commonly used to |
| Typical uses for ultra high vacuum | | | | achieve UHV include: |
| Ultra high vacuum is necessary for many surface | | | | Turbomolecular pumps (especially compound and/or |
| analytic techniques such as: | | | | magnetic bearing types) |
| X-ray photoelectron spectroscopy (XPS) | | | | Ion pumps |
| Auger electron spectroscopy (AES) | | | | Titanium sublimation pumps |
| Secondary ion mass spectrometry (SIMS) | | | | Non-evaporable getter (NEG) pumps |
| Thermal desorption spectroscopy (TPD) | | | | Cryopumps |
| Thin film growth and preparation techniques with | | | | UHV pressures are measured with an ion gauge, |
| stringent requirements for purity, such as molecular | | | | either a hot filament or an inverted magnetron type. |
| beam epitaxy (MBE) and UHV chemical vapor | | | | Finally, special seals and gaskets must be used |
| deposition (CVD) | | | | between components in a UHV system to prevent |
| Angle resolved photoemission spectroscopy (ARPES) | | | | even trace leakage. Nearly all such seals are all metal, |
| UHV is necessary for these applications to reduce | | | | with knife edges on both sides cutting into a soft, |
| surface contamination, by reducing the number of | | | | copper gasket. This all-metal seal can maintain |
| molecules reaching the sample over a given time | | | | pressures down to 100 pPa (~1012 Torr). |
| period. At 0.1 mPa (106 Torr), it only takes 1 second | | | | Measuring high vacuum |
| to cover a surface with a contaminant, so much | | | | Main article: Pressure measurement |
| lower pressures are needed for long experiments. | | | | Measurement of high vacuum is done using a |
| UHV is also required for: | | | | nonabsolute gauge that measures a pressure-related |
| Particle accelerators | | | | property of the vacuum, for example, its thermal |
| Atomic physics experiments which use cold atoms, | | | | conductivity. See, for example, Pacey. These gauges |
| such as ion trapping or making Bose-Einstein | | | | must be calibrated. The gauges capable of the |
| Condensates | | | | measuring the lowest pressures are magnetic gauges |
| Achieving ultra high vacuum | | | | based upon the pressure dependence of the current |
| Extraordinary steps are required to reach UHV, | | | | in a spontaneous gas discharge in intersecting electric |
| including the following: | | | | and magnetic fields. |
| High pumping speed possibly multiple vacuum pumps | | | | UHV manipulator |
| in series and/or parallel | | | | A UHV manipulator allows an object which is inside a |
| Minimize surface area in the chamber | | | | vacuum chamber and under vacuum to be |
| High conductance tubing to pumps short and fat, | | | | mechanically positioned. It may provide rotary motion, |
| without obstruction | | | | linear motion, or a combination of both. The most |
| Use low-outgassing materials such as certain stainless | | | | complex devices give motion in three axes and |
| steels | | | | rotations around two of those axes. To generate the |
| Avoid creating pits of trapped gas behind bolts, | | | | mechanical movement inside the chamber, two basic |
| welding voids, etc. | | | | mechanisms are commonly employed: a mechanical |
| Electropolish all metal parts after machining or welding | | | | coupling through the vacuum wall (using a |
| Use low vapor pressure materials (ceramics, glass, | | | | vacuum-tight seal around the coupling), or a magnetic |
| metals, teflon if unbaked) | | | | coupling that transfers motion from air-side to |
| Bake the system (250 C to 400 C) to remove water | | | | vacuum-side. Various forms of motion control are |
| or hydrocarbons adsorbed to the walls | | | | available for manipulators, such as knobs, handwheels, |
| Chill chamber walls to cryogenic temperatures during | | | | motors, stepping motors, piezoelectric drives, and |
| use | | | | pneumatics. |
| Avoid all traces of hydrocarbons, including skin oils in | | | | The manipulator or sample holder may include |
| a fingerprint always use gloves | | | | features which allow additional control and testing of |
| Outgassing is a significant problem for UHV systems. | | | | a sample, such as the ability to apply heat, cooling, |
| Outgassing can occur from two sources: surfaces | | | | voltage, or a magnetic field. Sample heating can be |
| and bulk materials. Outgassing from bulk materials is | | | | accomplished by electron bombardment or thermal |
| minimized by careful selection of materials with low | | | | radiation. For electron bombardment, the sample |
| vapor pressures (such as glass, stainless steel, and | | | | holder is equipped with a filament which emits |
| ceramics) for everything inside the system. Even | | | | electrons when biased at a high negative potential. |
| materials which are not generally considered | | | | The impact of the electrons bombarding the sample |
| absorbent can outgas, including most plastics and | | | | at high energy causes it to heat. For thermal |
| some metals. For example, vessels lined with a highly | | | | radiation, a filament is mounted close to the sample |
| gas-permeable material such as palladium (which is a | | | | and resistively heated to high temperature. The |
| high-capacity hydrogen sponge) create special | | | | infra-red energy from the filament heats the sample. |
| outgassing problems. | | | | References and notes |
| Outgassing from surfaces is a subtler problem. At | | | | ^ DJ Pacey (W. Boyes, editor) (2003). Measurement |
| extremely low pressures, more gas molecules are | | | | of vacuum; Chapter 10 in Instrumentation Reference |
| adsorbed on the walls than are floating in the | | | | Book (Third Edition ed.). Boston: |
| chamber, so the total surface area inside a chamber | | | | Butterworth-Heinemann. p. 144. ISBN 0750671238. |
| is more important than its volume for reaching UHV. | | | | ^ LM Rozanov & Hablanian, MH (2002). Vacuum |
| Water is a significant source of outgassing because a | | | | technique. London; New York: Taylor & Francis. |
| thin layer of water vapor rapidly adsorbs to | | | | p. 112. ISBN 041527351X. |
| everything whenever the chamber is opened to air. | | | | ^ LM Rozanov & Hablanian, MH. p. 95. ISBN |
| Water evaporates from surfaces too slowly to be | | | | 041527351X. |
| fully removed at room temperature, but just fast | | | | See also |
| enough to present a continuous level of background | | | | Vacuum engineering |
| contamination. Removal of water and similar gases | | | | Vacuum flange |
| generally requires baking the UHV system at 200 to | | | | Vacuum gauge |
| 400 C while vacuum pumps are running. During | | | | Journal of Vacuum Science and Technology |
| chamber use, the walls of the chamber may be | | | | Vacuum |
| chilled using liquid nitrogen to reduce outgassing | | | | Vacuum state |
| further. | | | | External links |
| Hydrogen and helium are the most common | | | | American Vacuum Society |
| background gases in a well-designed, well-baked UHV | | | | Three major steps to get Ultra High Vacuum |
| system. Hydrogen diffuses out from the grain | | | | Vacuum Systems and Thin-Film Technology Course |
| boundaries in stainless steel. Helium can diffuse | | | | CERN Ultra high vacuum solar thermal collector. |
| through the steel and glass from the outside air. | | | | |