Ultra high vacuum

Ultra high vacuum (UHV) is the vacuum regime characterised by pressures lower than about 10⁻⁷ pascal or 100 nanopascals (~10⁻⁹ torr). UHV requires the use of special materials in creating the vacuum system, extreme cleanliness to maintain the vacuum system, and baking the entire system to remove water and other trace gases that are accumulated due to the difficulties of maintaining a UHV. At these low pressures the mean free path of a gas molecule at 10⁻⁷ pascal or 100 nanopascals (~10⁻⁹ torr) is approximately 40 km, so gas molecules will collide with the chamber walls many times before colliding with each other. Almost all interactions therefore take place on various surfaces in the chamber.

Concepts involved

* Kinetic theory of gases
* Gas transport and pumping
* Vacuum pumps and systems

Typical uses for ultra high vacuum

Ultra high vacuum is necessary for many surface analytic techniques such as:
* X-ray photoelectron spectroscopy (XPS)
* Auger electron spectroscopy (AES)
* Secondary ion mass spectrometry (SIMS)
* Thermal desorption spectroscopy (TPD)
* Thin film growth and preparation techniques with stringent requirements for purity, such as molecular beam epitaxy (MBE) and UHV chemical vapor deposition (CVD)
* Angle resolved photoemission spectroscopy (ARPES)

UHV is necessary for these applications to reduce surface contamination, by reducing the number of molecules reaching the sample over a given time period. At 0.1 mPa (10^{−6 Torr), it only takes 1 second to cover a surface with a contaminant, so much lower pressures are needed for long experiments.}

UHV is also required for:
* Particle accelerators
* Atomic physics experiments which use cold atoms, such as ion trapping or making Bose-Einstein Condensates

Achieving ultra high vacuum

Extraordinary steps are required to reach UHV, including the following:
*High pumping speed — possibly multiple vacuum pumps in series and/or parallel
*Minimize surface area in the chamber
*High conductance tubing to pumps — short and fat, without obstruction
*Use low-outgassing materials such as certain stainless steels
*Avoid creating pits of trapped gas behind bolts, welding voids, etc.
*Electropolish all metal parts after machining or welding
*Use low vapor pressure materials (ceramics, glass, metals, teflon if unbaked)
*Bake the system (250 °C to 400 °C) to remove water or hydrocarbons adsorbed to the walls
*Chill chamber walls to cryogenic temperatures during use
*Avoid all traces of hydrocarbons, including skin oils in a fingerprint — always use gloves

Outgassing is a significant problem for UHV systems. Outgassing can occur from two sources: surfaces and bulk materials. Outgassing from bulk materials is minimized by careful selection of materials with low vapor pressures (such as glass, stainless steel, and ceramics) for everything inside the system. Even 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 while vacuum pumps are running. During chamber use, the walls of the chamber may be chilled using liquid nitrogen to reduce outgassing further.

Hydrogen and helium are the most common background gases in a well-designed, well-baked UHV system. Hydrogen diffuses out from the grain boundaries in stainless steel. Helium can diffuse through the steel and glass from the outside air.

Typically, there is no single vacuum pump that can operate all the way from atmospheric pressure to ultra high vacuum. Instead, a series of different pumps is used, according to the appropriate pressure range for each pump. Pumps commonly used to achieve UHV include:
*Turbomolecular pumps (especially compound and/or magnetic bearing types)
*Ion pumps
*Titanium sublimation pumps
*Non-evaporable getter (NEG) pumps
*Cryopumps

UHV pressures are measured with an ion gauge, either a hot filament or an inverted magnetron type.

Finally, special seals and gaskets must be used between components in a UHV system to prevent even trace leakage. Nearly all such seals are all metal, with knife edges on both sides cutting into a soft, copper gasket. This all-metal seal can maintain pressures down to 100 pPa (~10⁻¹² Torr).

Measuring high vacuum

Measurement of high vacuum is done using a "nonabsolute gauge" that measures a pressure-related property of the vacuum, for example, its thermal conductivity. See, for example, Pacey.cite book
author=DJ Pacey (W. Boyes, editor)
title=Measurement of vacuum; Chapter 10 in Instrumentation Reference Book
url=http://books.google.com/books?id=sarHIbCVOUAC&pg=PA144&lpg=PA144&dq=%22measurement+of+vacuum%22&source=web&ots=aWbFsOCsvf&sig=NcVTpfYXN5X-8STfeOIIKeCG0EU#PPA144,M1 |publisher=Butterworth-Heinemann
edition=Third Edition
location=Boston
year=2003
page=p. 144
isbn=0750671238] These gauges must be calibrated.cite book
author=LM Rozanov & Hablanian, MH
title=Vacuum technique
url=http://books.google.com/books?id=8yEGJCtS2XgC&pg=PA79&lpg=PA79&dq=%22measurement+of+vacuum%22&source=web&ots=RmEYhDyVOY&sig=Muaslx8-GZW6zWBCa403si5cuRM#PPA112,M1
publisher=Taylor & Francis
location=London; New York
year=2002
page=p. 112
isbn=041527351X] The gauges capable of the 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.cite book
author=LM Rozanov & Hablanian, MH
title=p. 95
url=http://books.google.com/books?id=8yEGJCtS2XgC&pg=PA79&lpg=PA79&dq=%22measurement+of+vacuum%22&source=web&ots=RmEYhDyVOY&sig=Muaslx8-GZW6zWBCa403si5cuRM#PPA95,M1
isbn=041527351X]

UHV manipulator

A UHV manipulator allows to position, rotate, heat, and cool a sample. It allows to measure the temperature and to apply a specific voltage and magnetic field. Due to the UHV requirements some movements are transferred by bellows into vacuum. Bellows do not allow rotation, but then a typical manipulator is a pole supported outside vacuum to position the sample and has a U-joint at the end to rotate the sample. The U-joint itself gets quite hot and is driven remotely by small bellows, by magnets reaching through thin aluminum, by an electric motor, or by a piezoelectric drive. Any cables and tubes running to the sample run in helix or spiral fashion to bend, when the sample is rotated. In this way nearly 360° rotation is possible. Due to the U-joint consisting of three rotation joints three helices are needed. One of them goes up the whole pole so has lot of space to distribute the bend across, but may need regular support to stay close to the axis. The cooling tubes do not bend quite as well as conductive copper cooling wire, so along the pole the tube is used and a wire is used for the two other joints. Each joint is made of two ball bearings with hard sapphire balls running in steel grooves so that they do not need a lubricant. Cables are isolated by means of glass fibers and/or are held clear by means of ceramic spacers. The sample is electrically connected to thermocouple wire, which also allows to send current through the sample and apply a voltage. The tubes for the cooling liquid can theoretically feed out of the vacuum through some electrical feed through, so that they can also be used to apply a potential or current to the sample. Behind the sample - with nothing obstructing the view – is a thoriated wolfram coil. A current allows it to glow and radiatively heat the sample. At higher temperatures it additionally emits hot electrons. When a voltage is applied to the sample it can be heated effectively by impact of these electrons. Two coaxial coils can compensate the magnetic field of each other or enhance it.

References and notes

See also

* Vacuum engineering
* Vacuum flange
* Vacuum gauge
* Journal of Vacuum Science and Technology
* Vacuum
* Vacuum state

External links

* [http://www.avs.org/ American Vacuum Society]
* [http://www.cae2k.com/ Three major steps to get Ultra High Vacuum]
* [http://www.technology.niagarac.on.ca/courses/phtn1432/ Vacuum Systems and Thin-Film Technology Course]
* [http://www.renewableenergymagazine.com/paginas/Contenidosecciones.asp?ID=1656&Tipo=&Nombre=Solar+Thermal CERN Ultra high vacuum solar thermal collector] .