Ultra-high vacuum

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Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals (10−9 mbar, ~10−9 torr). UHV requires the use of unusual materials for equipment, and 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. 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 interactions therefore take place on various surfaces in the chamber.

Concepts involved[edit]

Material limitations[edit]

Materials which are not allowed due to high vapor pressure:

  • Majority of organic compounds cannot be used:
    • Plastics other than PTFE and PEEK: gaskets are made of copper, and are single-use; plastics in other uses are replaced with ceramics or metals. Limited use of fluoroelastomers such as Viton and perfluoroelastomers such as Kalrez as gasket materials can be considered if metal gaskets are inconvenient, though these polymers can be expensive.
    • Glues: special glues for high vacuum must be used
  • Common steel: due to oxidizing, which greatly increases adsorption area, only stainless steel is used. Particularly non-lead and non-sulfur containing Austenitic stainless steel such as 304- and 316-based alloys are preferred. These steels include at least 18% chromium and 8% nickel. Variants of stainless steel include low carbon stainless and steels with additives such as niobium (also known as columbium, symbol Cb) and molybdenum that reduce the formation of chromium carbide, and nitrogen to improve strength. Common designations include 316L (low carbon), and 316LN (low carbon with nitrogen). Chromium carbide precipitation at the grain boundaries can render a stainless steel less resistant to oxidation.
  • Lead: Soldering is performed using lead-free solder
  • Indium: Indium is sometimes used as a deformable gasket material for vacuum seals, especially in cryogenic apparatus, but its low melting point prevents use in baked systems.
  • Zinc, cadmium: High vapor pressures during system bake-out.
  • Cleaning is very important for UHV. Common cleaning procedures include degreasing with detergents, organic solvents, or chlorinated hydrocarbons. Electropolishing is often used to reduce the surface area from which adsorbed gases can be emitted. Etching of stainless steel using hyrdrofluoric and nitric acid forms a chromium rich surface, followed by a nitric acid passivation step, which forms a chromium oxide rich surface. This surface retards the diffusion of hydrogen into the chamber.

Technical limitations:

  • Screws: Threads have a high surface area and tend to "trap" gases, and therefore, are avoided. Blind holes are especially avoided due to the trapped gas at the base of the screw and slow venting through the threads. This can be mitigated with either holes or vented screws, which have a hole drilled along the long central axis of the screw or a notch along the threads.
  • Welding: Standard welding cannot be used due to high surface area and introduction of gas chambers, which would collect gas and water vapor at atmospheric pressure, and release it slowly during evacuation (removal of gas). Careful TIG (tungsten inert gas) welding with a proper heat profile and filler material, usually the same alloy as the base material, is the preferred welding method. Welds must be internal or full penetration to reduce the ability of the weld to trap gas or high vapor pressure molecules.

Typical uses for ultra-high vacuum[edit]

Ultra-high vacuum is necessary for many surface analytic techniques such as:

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:

and, while not compulsory, can prove beneficial in applications such as:

Achieving ultra-high vacuum[edit]

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 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 carbon monoxide are the most common background gases in a well-designed, well-baked UHV system. Both Hydrogen and CO diffuse out from the grain boundaries in stainless steel. Helium could diffuse through the steel and glass from the outside air, but the abundance of He is usually negligible in the atmosphere.

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:

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 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[edit]

Main article: Pressure measurement

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.[1] These gauges must be calibrated.[2] The gauges capable of 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.[3]

UHV manipulator[edit]

A UHV manipulator allows an object which is inside a vacuum chamber and under vacuum to be mechanically positioned. It may provide rotary motion, linear motion, or a combination of both. The most complex devices give motion in three axes and rotations around two of those axes. To generate the mechanical movement inside the chamber, two basic mechanisms are commonly employed: a mechanical coupling through the vacuum wall (using a vacuum-tight seal around the coupling), or a magnetic coupling that transfers motion from air-side to vacuum-side. Various forms of motion control are available for manipulators, such as knobs, handwheels, motors, stepping motors, piezoelectric motors, and pneumatics.

The manipulator or sample holder may include features that allow additional control and testing of a sample, such as the ability to apply heat, cooling, voltage, or a magnetic field. Sample heating can be accomplished by electron bombardment or thermal radiation. For electron bombardment, the sample holder is equipped with a filament which emits electrons when biased at a high negative potential. The impact of the electrons bombarding the sample at high energy causes it to heat. For thermal radiation, a filament is mounted close to the sample and resistively heated to high temperature. The infrared energy from the filament heats the sample.

See also[edit]

References[edit]

  1. ^ DJ Pacey (W. Boyes, editor) (2003). Measurement of vacuum; Chapter 10 in Instrumentation Reference Book (Third Edition ed.). Boston: Butterworth-Heinemann. p. 144. ISBN 0-7506-7123-8. 
  2. ^ LM Rozanov & Hablanian, MH (2002). Vacuum technique. London; New York: Taylor & Francis. p. 112. ISBN 0-415-27351-X. 
  3. ^ LM Rozanov & Hablanian, MH. p. 95. ISBN 0-415-27351-X. 

External links[edit]