460:
medium-high vacuum. In some cases the workpiece itself is baked out or otherwise pre-cleaned under this medium-high vacuum. The gateway to the UHV chamber is then opened, the workpiece transferred to the UHV by robotic means or by other contrivance if necessary, and the UHV valve re-closed. While the initial workpiece is being processed under UHV, a subsequent sample can be introduced into the airlock volume, pre-cleaned, and so-on and so-forth, saving much time. Although a "puff" of gas is generally released into the UHV system when the valve to the airlock volume is opened, the UHV system pumps can generally snatch this gas away before it has time to adsorb onto the UHV surfaces. In a system well designed with suitable airlocks, the UHV components seldom need bakeout and the UHV may improve over time even as workpieces are introduced and removed.
526:) as gasket materials can be considered if metal gaskets are inconvenient, though these polymers can be expensive. Although through-gassing of elastomerics can not be avoided, experiments have shown that slow out-gassing of water vapor is, initially at least, the more important limitation. This effect can be minimized by pre-baking under medium vacuum. When selecting O-rings, permeation rate and permeation coefficients need to be considered. For example the penetration rate of nitrogen in Viton seals is 100 times lower than the penetration of nitrogen in silicon seals, which impacts the ultimate vacuum that can be achieved.
654:: 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, which is commonly known as a "virtual leak". This can be mitigated by designing components to include through-holes for all threaded connections, or by using vented screws (which have a hole drilled through their central axis or a notch along the threads). Vented Screws allow trapped gases to flow freely from the base of the screw, eliminating virtual leaks and speeding up the pump-down process.
706:
mechanical movement inside the chamber, three basic mechanisms are commonly employed: a mechanical coupling through the vacuum wall (using a vacuum-tight seal around the coupling: a welded metal bellows for example), a magnetic coupling that transfers motion from air-side to vacuum-side: or a sliding seal using special greases of very low vapor pressure or ferromagnetic fluid. Such special greases can exceed USD $ 400 per kilogram. Various forms of motion control are available for manipulators, such as knobs, handwheels, motors,
609:: 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. In a more esoteric application, the low melting point of Indium is taken advantage of as a renewable seal in high vacuum valves. These valves are used several times, generally with the aid of a torque wrench set to increasing torque with each iteration. When the indium seal is exhausted, it is melted and reforms itself and thus is ready for another round of uses.
343:
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 (392 to 752 °F) while vacuum pumps are running. During chamber use, the walls of the chamber may be chilled using
103:
722:
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.
623:
consistent layer of oxide, it has become increasingly accepted that aluminum is a suitable UHV material without special preparation. Paradoxically, aluminum oxide, especially when embedded as particles in stainless steel as for example from sanding in an attempt to reduce the surface area of the steel, is considered a problematic contaminant.
721:
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
478:
10 Torr). Although generally considered single use, the skilled operator can obtain several uses through the use of feeler gauges of decreasing size with each iteration, as long as the knife edges are in perfect condition. For SRF cavities, indium seals are more commonly used in sealing two flat
622:
Aluminum: Although aluminum itself has a vapor pressure which makes it unsuitable for use in UHV systems, the same oxides which protect aluminum against corrosion improve its characteristics under UHV. Although initial experiments with aluminum suggested milling under mineral oil to maintain a thin,
342:
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
285:
In any vacuum system, some gas will continue to escape into the chamber over time and slowly increase the pressure if it is not pumped out. This leak rate is usually measured in mbar L/s or torr L/s. While some gas release is inevitable, if the leak rate is too high, it can slow down or even prevent
269:
that measures a pressure-related property of the vacuum. See, for example, Pacey. These gauges must be calibrated. 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
705:
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
369:
on the surfaces of the chamber. This may also be required upon "cycling" the equipment to atmosphere. This process significantly speeds up the process of outgassing, allowing low pressures to be reached much faster. After baking, to prevent humidity from getting back into the system after it is
459:
or UHV angle valve, facing the UHV side of the volume, and another door against atmospheric pressure through which samples or workpieces are initially introduced. After sample introduction and assuring that the door against atmosphere is closed, the airlock volume is typically pumped down to a
532:: special glues for high vacuum must be used, generally epoxies with a high mineral filler content. Among the most popular of these include asbestos in the formulation. This allows for an epoxy with good initial properties and able to retain reasonable performance across multiple bake-outs.
816:
The Large Hadron
Collider (LHC) has three UH vacuum systems. The lowest pressure is found in the pipes the proton beam speeds through near the interaction (collision) points. Here helium cooling pipes also act as cryopumps. The maximum allowable pressure is
487:
Many common materials are used sparingly if at all due to high vapor pressure, high adsorptivity or absorptivity resulting in subsequent troublesome outgassing, or high permeability in the face of differential pressure (i.e.: "through-gassing"):
297:(either from surfaces or volume). A variety of methods for leak detection exist. Large leaks can be found by pressurizing the chamber, and looking for bubbles in soapy water, while tiny leaks can require more sensitive methods, up to using a
718:. The use of motors in a vacuum environment often requires special design or other special considerations, as the convective cooling taken for granted under atmospheric conditions is not available in a UHV environment.
331:) for everything inside the system. 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
322:
is a problem for UHV systems. Outgassing can occur from two sources: surfaces and bulk materials. Outgassing from bulk materials is minimized by selection of materials with low vapor pressures (such as glass,
391:
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. In the first stage, a
638:
is often used to reduce the surface area from which adsorbed gases can be emitted. Etching of stainless steel using hydrofluoric and nitric acid forms a chromium rich surface, followed by a nitric acid
66:, and gas molecules will collide with the chamber walls many times before colliding with each other. Almost all molecular interactions therefore take place on various surfaces in the chamber.
696:) are obviously not. To avoid trapping gas or high vapor pressure molecules, welds must fully penetrate the joint or be made from the interior surface, otherwise a virtual leak might appear.
396:
clears most of the gas from the chamber. This is followed by one or more vacuum pumps that operate at low pressures. Pumps commonly used in this second stage to achieve UHV include:
802:
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 millipascals (7.5
246:
in stainless steel. Helium could diffuse through the steel and glass from the outside air, but this effect is usually negligible due to the low abundance of He in the atmosphere.
581:(which provides no corrosion resistance). Common designations include 316L (low carbon), and 316LN (low carbon with nitrogen), which can boast a significantly lower
474:
Metal seals, with knife edges on both sides cutting into a soft, copper gasket are employed. This metal-to-metal seal can maintain pressures down to 100 pPa (7.5
479:
surfaces together using clamps to bring the surfaces together. The clamps need to be tightened slowly to ensure the indium seals compress uniformly all around.
361:
In order to reach low pressures, it is often useful to heat the entire system above 100 °C (212 °F) for many hours (a process known as
85:
require UHV conditions for the transmission of electron or ion beams. For the same reason, beam pipes in particle accelerators such as the
561:
are preferred. These steels include at least 18% chromium and 8% nickel. Variants of stainless steel include low-carbon grades (such as
437:
Turbo pumps and diffusion pumps rely on supersonic attack upon system molecules by the blades and high speed vapor stream, respectively.
806:
10 Torr), it only takes 1 second to cover a surface with a contaminant, so much lower pressures are needed for long experiments.
912:. High vacuum reduces oxidation and contamination, hence enables imaging and the achievement of atomic resolution on clean metal and
780:
940:
1181:
Kumar, Abhay; Ganesh, P; Manekar, Meghmahlar; Gupta, Ram; Singh, Rashmi; Singh, Mk; Mundra, Garvit; Kaul, Rakesh (October 2021).
1018:
1080:
1049:
859:
10 mbar) in order to eliminate temperature fluctuations and sound waves which would jostle the mirrors far too much for
242:
are the most common background gases in a well-designed, well-baked UHV system. Both
Hydrogen and CO diffuse out from the
676:(with an appropriate heat profile and properly selected filler material) is necessary. Other clean processes, such as
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1090:
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1032:
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78:
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step, which forms a chromium oxide rich surface. This surface retards the diffusion of hydrogen into the chamber.
603:. Occasionally pure lead is used as a gasket material between flat surfaces in lieu of a copper/knife edge system.
752:
746:
148:
Maintaining UHV conditions requires the use of unusual materials for equipment. Useful concepts for UHV include:
946:
909:
433:, especially when used with a cryogenic trap designed to minimize backstreaming of pump oil into the systems.
231:
Avoiding all traces of hydrocarbons, including skin oils in a fingerprint — gloves must always be used
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experimental apparatus is housed in a 10,000 cubic metres (350,000 cu ft) vacuum chamber at 1
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There are a variety of possible reasons for an increase in pressure. These include simple air leaks,
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Cleaning is very important for UHV. Common cleaning procedures include degreasing with detergents,
673:
631:
558:
302:
1107:
58:). UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the
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or load-lock vacuum system is often used. The airlock volume has one door or valve, such as a
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experiments often require a chemically clean sample surface with the absence of any unwanted
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growth and preparation techniques with stringent requirements for purity, such as
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Use of low vapor pressure materials (ceramics, glass, metals, teflon if unbaked)
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Ultra-high vacuum is necessary for many surface analytic techniques such as:
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403:(especially compound pumps which incorporate a molecular drag section and/or
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170:
62:
of a gas molecule is greater than approximately 40 km, so the gas is in
39:
1158:"VAT 54.1 Ultra High Vacuum All-Metal Angle Valve - Easy Close - VAT Valves"
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956:
619:: High vapor pressures during system bake-out virtually preclude their use.
542:
290:
195:
High conductance tubing to pumps — short and fat, without obstruction
128:
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exposed to atmospheric pressure, a nitrogen gas flow that creates a small
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186:
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1183:"Development of Low-Magnetic-Permeability Welds of 316L Stainless Steel"
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While not compulsory, it can prove beneficial in applications such as:
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Avoid creating pits of trapped gas behind bolts, welding voids, etc.
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with special welding techniques making them preferable for
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can render a stainless steel less resistant to oxidation.
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To save time, energy, and integrity of the UHV volume an
277:, either of the hot filament or inverted magnetron type.
1180:
1078:
1047:
217:
Baking of the system to remove water or hydrocarbons
589:
applications. Chromium carbide precipitation at the
69:
UHV conditions are integral to scientific research.
549:is used. Particularly, non-leaded and low-sulfur
672:and potential introduction of voids or porosity.
1246:
545:, which greatly increases adsorption area, only
1079:LM Rozanov & Hablanian, MH (4 April 2002).
365:) to remove water and other trace gases which
22:(often spelled ultrahigh in American English,
1105:
211:of all metal parts after machining or welding
185:High pumping speed — possibly multiple
510:: plastics in other uses are replaced with
339:sponge) create special outgassing problems.
265:Measurement of high vacuum is done using a
869:experiments which use cold atoms, such as
374:can be maintained to keep the system dry.
202:materials such as certain stainless steels
1198:
781:Angle resolved photoemission spectroscopy
668:cannot be used, due to the deposition of
16:Artificial vacuum with very low pressure
1106:Walter Umrath (1998). "Leak Detection".
1016:
941:Journal of Vacuum Science and Technology
1072:
1048:LM Rozanov & Hablanian, MH (2002).
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286:the system from reaching low pressure.
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569:), and grades with additives such as
192:Minimized surface area in the chamber
1010:
979:
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96:
1134:"Load-lock vacuum system explained"
522:) and perfluoroelastomers (such as
273:UHV pressures are measured with an
13:
998:Communication Group. February 2009
700:
14:
1271:
1233:
1109:Fundamentals of Vacuum Technology
1017:DJ Pacey (2003). W. Boyes (ed.).
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421:Non-evaporable getter (NEG) pumps
77:. Surface analysis tools such as
896:and other deposition techniques.
735:X-ray photoelectron spectroscopy
377:
101:
79:X-ray photoelectron spectroscopy
753:Thermal desorption spectroscopy
747:Secondary ion mass spectrometry
725:
599:: Soldering is performed using
1215:
1174:
1150:
1126:
1099:
947:Orders of magnitude (pressure)
906:on the cantilever oscillation.
347:to reduce outgassing further.
249:
116:format but may read better as
1:
1240:Online Surface Science Course
967:
910:Scanning tunneling microscopy
308:
224:Chilling of chamber walls to
829:Gravitational wave detectors
280:
7:
1223:"Vented Screws - AccuGroup"
933:
916:surfaces, e.g. imaging the
902:. High vacuum enables high
741:Auger electron spectroscopy
577:to reduce the formation of
514:or metals. Limited use of
440:
350:
254:
92:
10:
1278:
1211:– via Research Gate.
1023:(Third ed.). Boston:
985:"CERN FAQ: LHC: The guide"
809:UHV is also required for:
666:shielded metal arc welding
467:
444:
416:Titanium sublimation pumps
382:
354:
335:(which is a high-capacity
312:
258:
1085:. CRC Press. p. 95.
928:Electron-beam lithography
875:Bose–Einstein condensates
787:Field emission microscopy
767:chemical vapor deposition
189:in series and/or parallel
181:Typically, UHV requires:
163:Gas transport and pumping
83:low energy ion scattering
674:Gas tungsten arc welding
647:Technical limitations:
632:chlorinated hydrocarbons
541:: due to oxidization of
463:
303:Helium mass spectrometer
30:regime characterised by
900:Atomic force microscopy
775:pulsed laser deposition
771:atomic layer deposition
228:temperatures during use
159:Kinetic theory of gases
125:converting this section
918:surface reconstruction
886:Molecular beam epitaxy
763:molecular beam epitaxy
694:flux-cored arc welding
1200:10.29391/2021.100.029
1025:Butterworth-Heinemann
814:Particle accelerators
796:Atom Probe Tomography
690:submerged arc welding
678:electron beam welding
662:gas metal arc welding
583:magnetic permeability
87:Large Hadron Collider
1056:Taylor & Francis
1054:. London; New York:
992:CERN Document Server
791:Field ion microscopy
712:piezoelectric motors
688:inclusions (such as
660:: Processes such as
587:particle accelerator
483:Material limitations
401:Turbomolecular pumps
261:Pressure measurement
861:gravitational waves
387:There is no single
64:free molecular flow
1115:. pp. 110–124
952:Vacuum engineering
920:of the unoxidized
890:E-beam evaporation
682:laser beam welding
127:, if appropriate.
34:lower than about 1
494:organic compounds
372:positive pressure
270:magnetic fields.
267:nonabsolute gauge
146:
145:
89:are kept at UHV.
20:Ultra-high vacuum
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636:Electropolishing
628:organic solvents
601:lead-free solder
591:grain boundaries
579:chromium carbide
516:fluoroelastomers
496:cannot be used:
492:The majority of
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405:magnetic bearing
301:and specialized
244:grain boundaries
209:Electropolishing
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123:You can help by
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1193:(10): 323–337.
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708:stepping motors
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701:UHV manipulator
670:impure material
553:grades such as
547:stainless steel
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431:Diffusion pumps
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176:Vapour pressure
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1234:External links
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962:Vacuum state
957:Vacuum gauge
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871:ion trapping
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726:Typical uses
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221:to the walls
187:vacuum pumps
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167:Vacuum pumps
147:
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129:Editing help
111:
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23:
19:
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765:(MBE), UHV
641:passivation
389:vacuum pump
250:Measurement
198:Use of low-
1249:Categories
1167:2022-06-01
1143:2022-06-01
1119:2020-03-22
968:References
894:sputtering
873:or making
716:pneumatics
575:molybdenum
551:austenitic
468:See also:
457:gate valve
445:See also:
320:Outgassing
315:Outgassing
309:Outgassing
299:tracer gas
295:desorption
200:outgassing
75:adsorbates
1209:238754443
1162:VAT Valve
1138:sens4.com
904:Q factors
759:Thin film
518:(such as
426:Cryopumps
411:Ion pumps
333:palladium
281:Leak rate
275:ion gauge
226:cryogenic
137:June 2019
32:pressures
26:) is the
1002:June 19,
934:See also
924:surface.
845:TAMA 300
831:such as
512:ceramics
500:Plastics
441:Airlocks
363:bake-out
357:Bake-out
351:Bake-out
337:hydrogen
329:ceramics
255:Pressure
236:Hydrogen
219:adsorbed
155:of gases
153:Sorption
93:Overview
54:10
46:10
922:silicon
841:GEO 600
783:(ARPES)
769:(CVD),
658:Welding
617:cadmium
571:niobium
453:airlock
447:Airlock
383:Pumping
171:systems
40:pascals
1260:Vacuum
1207:
1089:
1062:
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847:. The
843:, and
749:(SIMS)
714:, and
652:Screws
607:Indium
539:steels
524:Kalrez
407:types)
367:adsorb
327:, and
293:, and
112:is in
28:vacuum
1205:S2CID
1113:(PDF)
988:(PDF)
837:VIRGO
798:(APT)
777:(PLD)
755:(TPD)
743:(AES)
737:(XPS)
630:, or
537:Some
530:Glues
520:Viton
464:Seals
118:prose
50:; 7.5
1087:ISBN
1060:ISBN
1029:ISBN
1004:2016
996:CERN
849:LIGO
833:LIGO
789:and
692:and
686:slag
664:and
613:Zinc
597:Lead
573:and
567:316L
565:and
563:304L
557:and
508:PEEK
506:and
504:PTFE
238:and
169:and
114:list
81:and
56:Torr
48:mbar
42:(1.0
1195:doi
1191:100
680:or
559:316
555:304
38:10
24:UHV
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