US3144035A

US3144035A - High vacuum system

Info

Publication number: US3144035A
Application number: US255524A
Authority: US
Inventors: Hablanian Marsbed; Robert J Powers
Original assignee: Nat Res Corp
Current assignee: National Research Corp
Priority date: 1963-02-01
Filing date: 1963-02-01
Publication date: 1964-08-11
Anticipated expiration: 1981-08-11

Description

g- 11, 1964 M. HABLANIAN ETAL 4 HIGH VACUUM SYSTEM Filed Feb. 1, 1963 MECH VACUUM PUMP 1 TO VACUUM SYSTEM O 202 /-o l 138 I30 I34 or r Fig. 4

I10 I02 [ULTRA HIGH VACUUM SYSTEM I V L INVENTORS Fig 3 MARSBED HABLANAN BY ROBERT J. POWERS United States Patent 3,144,035 HIGH VACUUM SYSTEM Marshed Hablanian, Waltham, and Robert J. Powers, Natick, Mass, assignors to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed Feb. 1, 1963, Ser. No. 255,524 12 Claims. (Cl. 137-1) This invention relates primarily to vacuum chambers and more specifically to demountable closures for such chambers. The vacuum systems of interest herein comprise a large chamber with a chamber door which 1s adapted to go through many open and close cycles, a door seal and a pumping system for evacuating the chamber via a port in the chamber wall. Such vacuum systems are used for a variety of tasks in metallurgy, semiconductor manufacture, vapor deposition, materials research and space simulation. The advanced state of these arts requires vacuum systems capable of reaching the ultra high vacuum range, i.e. pressures of l l() mm. Hg abs.(torr) and less. This requirement poses great difficulty for systems having chamber sizes of more than a few cubic feet or large door seals.

A major source of leakage at ultra high vacuum operation is leakage of air into the chamber via the door seal and internal leakage of volatile components of the door seal itself. One approach to circumventing this known problem is the use of soft metal gaskets. However, metal gaskets are permanently deformed by closure since they do not have the resiliency or organic gaskets. Thus, fre-- quent changing of the gaskets is required--an awkward and expensive step in large systems.

Another prior art problem which we have noted is that during preliminary bake-out of vacuum system which use diffusion pumps, a vapor pump oil fills the chamber. Where conventional gaskets, such as butyl rubber, are used for sealing, they must be cooled since they cannot withstand the high bakeout temperatures. This permits condensation of the oil vapor in the region of the door seal and builds up a potential source of vapor which will create difiiculty during the later operation of the vacuum system.

It is therefore a principal object of the invention to provide a vacuum chamber with demountable closure for operation at ultra-high vacuum.

It is a further and more specific object of the invention to provide a door seal which will permit operation at ultra-high vacuum and will have a long operating life.

It is another object of the invention to provide 'a demountable closure for vacuum chambers, pipelines, and the like operating in the ultra-high vacuum range.

It is a further object of the invention to provide a demountable closure which will permit bakeout of the portion of the chamber around the closure seal and of the closure seal itself.

These and other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the combination of elements and the arrangement of parts which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

Patented Aug. 11, 1964 "ice For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed disclosure taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic, partly sectional, view of an ultra-high vacuum system having a demountable closure sealed in accord with the invention;

FIG. 2 is an expanded view of a portion of FIG. 1;

FIG. 3 is a schematic, partially sectional view showing another application of the invention-to demountable closures for feedthroughs; and

FIG. 4 is a schematic, partly sectional view showing still another application of the invention-to demountable closures for pump inlets, pipe couplings and the like.

In accordance with the invention, the gasket material selected is chosen from those rubbery materials having the characteristics of low vapor pressure, 10* mm. Hg (at 20 C.), and less, such as Teflon, Mylar and silicone rubber. The latter material is preferred. Such materials have high permeabilities which would permit air to leak through the gasket itself. This limitation is overcome by the apparatus arrangement described below. Volatile impurities absorbed in the gasket are cleaned out by the technique of double pumping on the gasket in the manner described below. The high permeability of the gasket makes it possible to accomplish this in a reasonable time. The use of such low vapor pressure materials permits bakeout at the seal since these materials can withstand temperatures on the order of 250 C.

Referring now to FIG. 1, there is shown a vacuum system comprising a chamber 10, a pumping system 12 and a door 14 for the chamber. The pumping system comprises an optical baffle cold trap 16, a first diffusion pump 18 backed by a second diffusion pump 20 and a mechanical vacuum pump 22.

Annular mating flanges

24 and 26 are welded to chamber 10 and door 14 respectively to make a tight closure. The leakage path from the atmospheric pressure side of the interface 28 to the vacuum side is blocked by inner and outer gaskets. The inner gasket is silicone rubber O-ring 30 housed in an annular recess 32 in one of the flanges. The outer gasket is another silicone O-ring 34 housed in an annular recess 36 in one of the flanges. An annular recess 38 is provided between the O-rings and means are provided for pumping out recess 38. The means for pumping out recess 38 is a pipe line 40 leading to the mechanical pump. The outer gasket 34 can be a conventional elastomer or silicone rubber or the like.

Referring now to FIG. 2, there is shown an enlarged view of a section of FIG. 1. It is seen in this figure that the interface 28 is actually a narrow passage which is blocked by the O-ring seals. The slight passage that is inevitably formed serves as a pumping passage. Undercuts 29 are provided in flange 24 to facilitate double pumping of seal 30. Volatiles outgassing from the silicone rubber O-ring 30 are rapidly pumped away during the pumpdown cycle via the main chamber 10, recess 38 and line 40; the O-ring is said to be double-pumped. The pumping means acting on the recess 38 should maintain it at a pressure on the order of about 0.1 mm. Hg abs. and below to support ultra-high vacuum operation in the main chamber. However, pressures as high as 10 mm. Hg abs. can be maintained in recess 38 with some beneficial effect from the double pumping.

Example In an experimental run, a system generally comparable to FIG. 1 was built. This was a large 3-foot diameter by 4-foot long chamber. A separate diffusion pump was used to pump out the groove between the O-rings. Silicone rubber was used for both O-rings. The groove 38 was evacuated to less than 10- mm. Hg and 1.5 l was held in the main chamber for over a day. Then the separate pump was shut down and the line 40 was connected to mechanical pump 22. The pressure in the groove rose to mm. Hg and 1.5 X10 was still held in the main chamber for two more days.

The guiding criterion for optimum pumping out of the recess 38 is to reduce the pressure therein to the point where the rate of diffusion through the permeable siliconerubber O-ring 30 is less than the rate at which volatile components evolve from the O-ring itself. Thus, the permeability of the low vapor pressure material becomes an asset rather than a liability since it permits quick removal of the volatile components. Yet, leakage of gas from the recess 38 through the O-ring 30 to the main chamber does not constitute a serious gas load on the ultra-high vacuum system when the pressure in recess 38 is kept at a low value.

After the initial cleaning of the inner O-ring, the groove 38 may bepumped out continuously or intermittently. When the line 40 is valved off, pressure in groove 38 rises slowly and leaks into the main chamber via the inner vO-ring. It is preferred to pump groove 38 continuously.- V v ,Referring now to FIG. 3, there is shown another application of the invention. An ultra-high vacuum system 110 is provided with a port 102. The port is covered by a disc 114 providing a demountable closure therefor. A conventional electricalor mechanical feedthrough 104 passes through the disc. Coaxial inner and outer O-rings 130 andl34 are provided for sealing the leakage path between the disc and wall of chamber 110. An annular double pumping groove 138 is provided between the 0- rings and a line 140 is provided for connecting groove 138 to avacuum pump system which may coincide with the vacuum pumping system for chamber 110 or may be an independent pumping system. The disc issecured to chamber 110 by a peripheral series of welded studs 108. The inner O-ring 130 is selected from the low vapor pressure materials discussed in connection with FIGS. 1 and2 above.

Referring now to FIG. 4, there is shown the application of the invention to a diffusion pump inlet. A diffusion pump 210 is provided with a vapor jet assembly 202, cooling coils 204 and an inlet flange 224. A pipe 214, with exit flange 226, connects the pump to the vacuum system. The inlet flange 224 is provided with an inner O-ring 230, a double pumping groove 238 and an outer 'Q-ring 234. The double pumping groove 238 is connected with a low stage of the diffusion pump via pipeline 240. v This provides a simple way of double pumping the O-ring 230. The material for O-ring 230 is selected from the low ;vapor pressure materials discus'ed in connection with FIGS. 1 and. 2.

There can be many variations within'the scope of the invention in addition to the ones noted above. For instance the gaskets can be water cooled during bakeout and during the operation of the ultrahigh vacuum system. Water supplies are available in connection with the diffusion pumps generally used in such systems. A separate pump can be used for, pumping out gasket 38. However, the main pumping system can usually be relied on to handle this additional. gas load. The groove 38 can be located inflange 26 rather than flange 24, as shown.

It will be appreciated that theplumbing for rough pumping has been omitted from the above description and drawings since such apparatus is Well known -to those skilled in the art.

Since many changes can be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A demountable closure for ultra high vacuum systems, such as a door seal, pipe coupling, or the like, comprising means forming mating metal surfaces to establish a demountable interface extending from an inner low pressure region to an outer high pressure region, spaced inner and outer gaskets blocking said interface, vacuum pumping means connected to the space between the gaskets, the inner gasket being made of a rubbery mate rial having a low characteristic vapor pressure on the order of about 10 mm. Hg abs. and less at 20 C.

2. The demountable closure of claim 1 wherein the inner gasket is made of silicone rubber.

3. The demountable closure of claim 1 wherein the said vacuum pumping means are adapted to reduce the pressure in the space between the gaskets to the point Where the rate of diffusion through the inner gasket is less than the rate at which volatile components evolve from the inner gasket.

4. An ultrahigh vacuum system comprising a chamber connected to a vacuumrpump system and including a demountable closure, the demountable closure comprising abutting metal walls which define an interface extending from an ultrahigh vacuum zone to a zoneof higher pressure, spaced inner and outer gaskets sealing the interface, the inner gasket comprising a rubbery material having a characteristic vapor pressure less than 10- mm. Hg at 20 C. and means for pumping outthe space between the gaskets.

5. The vacuum system of claim 4 wherein the abutting Walls are constituted by annular flanges and the gaskets are O-rings.

6. The vacuum system of claim 4 wherein'one of said walls is grooved to receive both said gaskets.

7. The vacuum system of claim 6 wherein said grooved -wall also comprises a pumping groove intermediate of said gaskets.

8. The vacuum system 'of claim 4 wherein said means for pumping out the space between the gaskets comprises a passage extending into the first vacuum pumping system.

9. An ultra high vacuum system comprising a cham'-. ber, a diffusion pump connected to said chamber, said diffusion pump including a top vapor jet, said system having at least one demountable closure '(such as 'a diffusion pump inlet, chamber door, feedthrough and the like) with an interface extending from a zone of ultra high vacuum to a zone of higher pressure, spaced inner and outer gaskets blocking the interface of each such demountable gasket, the inner gasket being a flexible organic material a'ndhavin'ga characteristic vapor pressure less than 10* mm. Hg abs., at 20 C. and passage means extending from the spaces between the inner and outer gaskets of each demountable closure into a region of the dlifusion pump below the top vapor jet thereof.

10. A method of sealing the leakage path through a demountable closure of an ultra high vacuum system comprising the steps of providing a flexible gasket in said leakage path, said gasket being made of an organic material having a characteristic vapor pressure less than 10" mm. Hg at 20 C., and double pumpingthe gasket.

11. The method of claim 10 wherein the double pumping comprises the exposure of an inner face of the gasket to the ultra high vacuum. 7

12. The method of claim 11 wherein the double pumping further comprises the exposure of an outer surface of the gasket to a zone of the system for producing the ultra high vacuum, in'which zone the pressure is higher than in the main ultra high vacuum zone.

(References on following page) 6 References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS Scientific Foundations of Vacuum Technique, Dush- 2,193,135 Lamm Man 12, 1940 Inan (Lafferty, Edltor), Wiley, New York (second ed1- 3,0ss,232 Farkass 0a. 16, 1962 and 5 Principles of Vacuum Engineering, Pirani and Yarwood, Chapman and Hall, Ltd., London (1961), pages FOREIGN PATENTS 187-196, 202-207 and 319. 564,280 Belgium July 28, 1958 Ultrahigh Vacuum and Its Applications, Roberts and 1,237,219 France June 20, 1960 Vanderslice,Prentice-Hall,Eng1ewood,N.J. (1963), pages

Claims

10. A METHOD OF SEALING THE LEAKAGE PATH THROUGH A DEMOUNTABLE CLOSURE OF AN ULTRA HIGH VACUUM SYSTEM COMPRISING THE STEPS OF PROVIDING A FLEXIBLE GASKET IN SAID LEAKAGE PATH, SAID GASKET BEING MADE OF AN ORGANIC MATERIAL HAVING A CHARACTERISTIC VAPOR PRESSURE LESS THAN 10**-8 MM. HG AT 20*C., AND DOUBLE PUMPING THE GASKET.