Achieving and Maintaining Extreme High Vacuum
 

Marxen Larin, "Vacuum and Cryogenic System" Co Ltd.,
P.O. Box 36 St.- Petersburg, 195256, Russia

Alexey Stepanov, "Applied Materials"
P.O. Box 4030, M/S 5800 Nes-Ziona 70400, Israel

Achieving and maintaining extreme high vacuum with pressures up to 10-12 - 10-13Torr and lower depends on a few factors :

  1. The ultimate pressure of the vacuum pump should be in few times lower than the pressure required in the vacuum chamber .
  2. Selected pump should have enough pumping speeds for all gases, especially for hydrogen and helium with the partial pressure of 10-12 - 10-13Torr and lower .
  3. Speed of desorption, permeation, and other gas sources inside of vacuum closures should not be higher than the pumping speed of a pump at required vacuum level.
It is known that the ultimate pressure for most types vacuum pumps such as turbomolecular, sublimation, cryogenerators, getter (evaporable and non-evaporable) achieve a vacuum of approximately 10-10 – 10-11 Torr.

This connects to the physical limit these pumps by hydrogen pumping speed. The spectrum of gases at ultimate pressure at these pumps shows 95-99% atoms and molecules of hydrogen . Only one type of pump which have an ultimate pressure which is lower than 10-13Torr is condensation-sorption cooled by liquid helium (He) cryopump with pumping surface temperature of 4.2K and lower. More effective and economic then those pumps are liquid helium storage-type cryopump [ 1 ]. These pumps use for a pumping surface, part of an inner pump volume, filled with liquid helium . After filling pump with liquid helium, a pumping surface was condensed with a layer of argon (also possible nitrogen) with 103 up to 106 molecular monolayers thickness [2].

This layer condensed while maintaining a pressure (1-4)x10-4 Torr. The cryopanel formed optimal small porous size structures, of condensed gases with pore size from less then one and up to few tens of nm.

If ratio between adsorbed helium or hydrogen and condensed gas layer will be not less than 1:10 –1:100 and 1:100 – 1:1000 accordingly [3], this cryosorbent absorb effectively molecules of hydrogen and helium and partial pressures of adsorbed gases can be lower than 10-12 – 10-13 Torr. By calculation partial pressure of this sorbent when T=4.2K lower then 10-70Torr.

At this point of time we developed and produced few tens of liquid helium storage-type cryopumps, with pumping speed of 0.2 – 10 m3/s and working pressure from 10-7 up to 10-13 Torr. Main benefits

of these pumps are: very high reliability, economically better, long life time and long maintenance free time. Cost of ownership of those pumps lower than turbo pump. These pumps have been operating for a quite long time in R&D Institutes and Universities of Japan, Germany, Sweden and in production companies in Russia. Cryopumps can to work 2-4 month continuously after filling with liquid helium. A few modifications were developed especially for particle accelerators, thermonuclear synthesis equipment, electron spectrometers, producing electronic equipment and etc..In order to reduce desorption rate of gases, adsorbed on vacuum surfaces and dissolved in bulk of extra high vacuum chamber ( usually made from stainless steel ), special coating was developed. The coating reduced gas extraction by a factor of 104 and it was possible to attain an ultra low pressure of 10-11 – 10-13 Torr even without bakeout. Best properties show Al film 1-3m m thick deposited by thermal evaporation in a flow of very pure gaseous helium produced by liquid helium evaporation at a helium flow pressure of ~2x 10-2 Torr. [4] Formed Al film has relatively big crystalline structure with low numbers of defects (absorption centers ).

Moreover, layer of Al2O3 with thickness up to few nm, which was formed on Al film essentially prevents permeation of atomic hydrogen which dissolved and chemi-sorbed in bulk of stainless steel into ultra-high vacuum . Because of using both factors – cryopumps and coating possible to reach and maintain with match lower consuming of time and many extreme high, super clean and absolutely hydrocarbon free vacuum up to 10-14 Torr.
 
 
 
 

 Important to mention additional benefit of the special Al coating – very low emissivity factor especially with very low temperatures. Emissivity factor of such Al coating with T=4.2K, e al4.2 = 0.002 and for copper film e cu4.2 = 0.0006. Therefore, it is possible to make cryopumps and helium cryostats with very low helium evaporation rate ,that gives to cryosystem continuously working 5-10 times longer, ( 2–6 month ). Pressure measurement of pressures lower than 10-12 Torr done by ( inversion-magnetron gauge with cold cathode PMM – 46 ) and a current measure device with sensitivity limit of 10-15 A and lower .

References

[1] M.P. Larin, Second European Vacuum Conference, 21-26, May, 1990, Trieste, Italy, J. Vuoto, vol. XX, N. 2 – Aprile – Guigno 1990, p. 246 – 251.

[2] M.P. Larin, Soviet Physics : " J. Technical Physics" 58, ( 1988 ), p. 744.

[3] V.B. Juferov, Doctor thesis ( Habilitation ), Charkov, 1974 ( in Russian ).

[4] M.P.Larin, "Electronnaya Tehnika", ser.:Materialy, 5 (1980), p. 14(in Russian) .