GB2153914A

GB2153914A - Improvements in cryogenerator pumps

Info

Publication number: GB2153914A
Application number: GB08500291A
Authority: GB
Inventors: Richard Derek Amos
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1984-01-07
Filing date: 1985-01-07
Publication date: 1985-08-29
Also published as: GB8400349D0; DE3500320A1; FR2557930A1; GB8500291D0; JPH0451669B2; JPS60228779A; GB2153914B

Abstract

A cryogenerator pump has at least two cryopanels (8, 10) maintained within an enclosure (4) which has an open end (6) adapted for attachment to the chamber to be pumped, the cryopanels being effective to operate at relatively higher and lower temperatures respectively with the higher temperature panel (8) being spaced from and adjacent the enclosure and circumscribing the lower temperature panel (10), the lower temperature panel being capable of communicating with the chamber through the opening in the enclosure and also to the space (16) between the enclosure and the high temperature panel (8). Louvres (14) in the base of the higher temperature panel (8) may provide the flow path between the space (16) and the lower temperature panel (10). In operation the pressure within the space (16) is reduced by gas flow through the louvres (14), flow into the space (16) from the chamber to be pumped being restricted e.g. by the close spacing between the enclosure (4) and the panel (8) in the region of the end (6). <IMAGE>

Description

SPECIFICATION Improvements in cryogenerator pumps This invention relates to cryogenerator pumps and is particularly though not exclusively directed to cryogenerators adapted to produce high vacuum in ion sputtering and like equipment. The invention is particularly concerned with such cryogenerators embodying at least two stages of cooling respectively at cryopanels effective to operate at relatively higher and lower temperatures.

Cryogenerator pumps are now well established for the production of a high vacuum in a sealed chamber. Such cryogenerators operate by the controlled reduction of pressure of gas generally supplied by a suitable pump provided independently of and physically seperated from the cryogenerator body. The gas pump is included in a closed gas circuit with the cyrogenerator and is arranged to supply gas, generally helium, to the generator at ambient temperature and at a pressure of typically 20 bar.

The pressure of gas supplied to the cryogenerator is internally reduced in controlled manner, by two stages of expansion respectively within the swept volume of two pistons moving within co-operating cylinders connected in series. Expansion is controlled by indirectly damping the stroke of the cylinders by way of restrictive orifices introduced into gas conduits within the cryogenerator and by gas reservoirs effective to accumulate gas pressure during a part of each gas reduction cycle.

A cryopanel in heat exchange relationship with each gas reduction stage is provided externally on the cryogenerator body and is cooled by the controlled reduction of gas pressure. Typically the cryopanel associated with the first higher pressure reduction stage will operate at a temperature of about 40-1 00 K with the cryopanel associated with the second lower pressure reduction stage operating at a temperature of about 10 K.

The cryopanels are effective as a pump to reduce gas pressure in a chamber by providing condensation of gas in the chamber on the cooled cryopanel surfaces. In general water and contaminants such as volatile hydrocarbons will be condensed upon the higher temperature panel with condensable gases such as nitrogen oxygen and argon being condensed and collected on the lower temperature panel. In a typical pumping arrangement such as is illustrated in Figure 1 of the accompanying drawings, cryopanels secured to the body of a cryogenerator are disposed within an enclosure which is sealed at one end to the cryogenerator body and which has an opening at the other end adapted to interface with the chamber to be pumped. A suitable valved inlet at the cryogenerator end of the enclosure permits the connection of a mechanical or other pump for low pressure roughing.

The configuration of cryopanels in the arrangment of Figure 1 produces condensation of water vapour and volatile contaminants together with for example, carbon dioxide on the radially outer high temperature panel which operates typically at a temperature within the range 40-100 K. Nitrogen, oxygen, argon and other condensable gases are condensed upon and retained on the outside of the lower temperature panel which is nested within the outer panel and which operates typically at a temperature of about 1 5 K.

In addition, non condensable gases such as hydrogen, helium and neon, characterised by a vapour pressure of about 1 torr at 1 5 K., cannot be so condensed and must be adsorbed on to a layer of charcoal suitably bonded to the inner surface of the low temperature panel.

When used to produce and to maintain a high vacuum in a chamber enclosing for example ion sputtering or like equipment which produces a relatively high emmission of gas and other molecules, the cryogenerator must be capable of pumping effectively at pressures of the order of 10-3 to 10 2 torr so produced during operation of such equipment.

At these relatively high pressures, the higher temperature cryopanel will receive heat relatively quickly from the surrounding enclosure by conduction and convection through the intermediate gas space. This heat gain and the increased thermal load which is consequently placed on the cryogenerator will result in- an increase in temperature of both cryopanels and in particular the lower temperature cryopanel which is effective to pump condensable and non condensable gases. Since the ability of this lower temperature panel to pump these non condensable gases is critically temperature dependent, a relatively small pressure induced increase in temperature can lead to desorption of these non condensable gases in particular from the panel and into the vacuum chamber being pumped. This reduces pumping speed for these gases with consequent reduction of vacuum obtainable.

One expedient effective to maintain low cryopanel temperatures independently of increasing pressure is to up-rate the power and thereby the pumping capacity of the cryogenerator; this will however unacceptably increase both the capital as well as the running cost of the pumping system including the cryogenerator.

It is accordingly an object of the present invention to produce a cryogenerator pump capable of maintaining effective pumping at relatively high pressure.

The present invention according to its broadest aspect provides a cryogenerator pump having at least two cryopanels maintained within an enclosure which has an open end adapted for attachment to the chamber to be pumped, the cryopanels being effective to operate at relatively higher and lower temper atures respectively with the higher temperature panel being spaced from and adjacent the enclosure and circumscribing the lower temperature panel, the lower temperature panel being capable of communicating with the chamber through the opening in the enclosure and also to the space between the enclosure and the high temperature panel.

In a preferred embodiment, a constriction to direct gas flow is provided between the low temperature panel and the chamber being pumped, to enable the volume adjacent the lower temperature panel and thereby the space between the high temperature panel and the enclosure to be maintained at a lower pressure than that prevailing in the chamber.

By this means, the effects of heat gain arising from relatively high chamber pressure can be better overcome.

In one embodiment of the invention, the lower temperature panel communicates with the space through a series of apertures provided at the base of the high temperature panel adjacent the cryogenerator body.

Suitably the apertures are in the form of spaced overlapping louvres or panels which are inclined to the direction of pumped gas flow and which are also effective to prevent radiant heat exchange between the enclosure and the low temperature panel.

The conductance of the apertures ideally is so matched to the conductance of the space between the high temperature panel and the enclosure to ensure that the pressure of gas within the space in general and adjacent the apertures in particular is maintained as closely as possible to that adjacent the lower temperature panel. By this means heat transfer to the high temperature panel from the enclosure arising from any increase in gas pressure is reduced and the temperature of both cryopanels can be maintained without the provision of any increase in cooling power of the cryogenerator.

Conveniently the conductance of the space between the high temperature panel and the enclosure is reduced by reducing the spacing therebetween over the whoie or over a part of the length of the panel adjacent the enclosure wall. Alternatively conductance may be reduced by reducing the spacing at discrete points along the lengths of the high temperature panel or by introducing a convoluted gas path at the inlet end of the space.

Embodiments of the invention will now be particularly described by way of example with reference to the accompanying drawings in which: Figure 1 is a sectional side view of a known cryogenerator including cryopanels for pumping a chamber, Figure 2 is a sectional side view of a cryogenerator for pumping a chamber in accordance with the invention and providing a pumping path between the lower temperature cryopanel and the space between the higher temperature panel and the enclosure.

Figure 3 is a sectional side view of part of the cryogenerator shown in Figure 2 and illustrates one alternative means for reducing conductance between the high temperature cryopanel and the pump enclosure.

Figure 4 is an alternative embodiment of the means for reducing conductance.

Figure 5 is a further embodiment of the means for reducing conductance shown in Figure 3; and Figure 6 is a sectional side view of a yet further embodiment of a cryogenerator for pumping a chamber and provided with an alternative arrangement to enable the lower temperature panel to pump the space between the high temperature panel and the enclosure.

Referring again to Figure 1, this illustrates a conventional cryogenerator adapted to produce a low pressure in a chamber, for example a chamber including ion sputtering equipment or the like.

The cryogenerator comprises a body portion 2 having inlets and outlets respectively for receiving and for discharging high pressure helium from a seperate compressor (not shown), provided independently of the cryogenerator.

Secured to the body 2 is an enclosure 4 which forms part of the pump envelope and which has an upper flanged end 6 adapted to be sealingly secured, for example by way of O rings, to the chamber being pumped.

Disposed within the enclosure 4 and secured in heat exchange relationship to the high temperature stage of the cryogenerator is a higher temperature cryopanel 8 in the form of a cylinder having an open end adjacent the flanged opening provided in enclosure 4.

Nested within the high temperature panel and in heat exchange relationship with the low temperature stage of the cryogenerator is a low temperature cryopanel 10 which communicates through one path with the chamber to be pumped by way of a louvred variable throttle 1 2 provided at the open end of cryopanel 8.

Provided also in the enclosure 4 and adjacent the cryogenerator body 2 is an opening (not shown) enabling the space within the enclosure to be connected to a mechanical or other pump for roughing the vacuum system.

In use of the cryogenerator, the cryopanel 8 will operate at a temperature of about 40-100 K and will be effective to condense water vapour together with volatile hyrocarbon and like condensable contaminants together with carbon dioxide if present. Cryopanel 10 will operate at a temperature of about 1 2 K and will be effective to condense nitro gen and oxygen together with other condensable gases on the radially outside surface.

Cryopanel 10 will also adsorb non condensable gases on a charcoal layer provided on its radially inner surface to produce within the pumping chamber a pressure of the order of 10-8, torr with variable throttle 12 open.

At the relatively high pressures of the order of 10-3 torr produced by operation of ion sputtering or like equipment in the chamber being pumped, the high pressure induced heat transfer through the space between cryopanel 8 and the envelope 4, typically at a temperature of 300 K, will tend to increase the operating temperature of both of the cryopanels 8 and 10 and will seriously impair the pumping efficiency of the cryogenerator.

In order to reduce this heat transfer and to maintain the efficiency of the cryopanels at such relatively high pressures the region of the high temperature panel 8 adjacent the body portion 2 is provided with apertures, in this embodiment in the form of overlapping louvres 1 4 inclined to the direction of gas glow. Louvres 1 4 are effective to permit pumping of the space 1 6 between the high temperature panel 8 and the envelope 4 in order to reduce the pressure of gas within this space and consequently to reduce substantially heat transfer from the cryopanel 8 to the enclosure.

Louvres 1 4 also are effective to reduce radiant heat exchange between the enclosure 4 and low temperature panel 1 0.

In order to enable the pumping action through the louvres 1 4 to maintain a low pressure in the space 16, the conductance between the space 1 6 and the chamber being pumped must in general be reduced to ensure that the gas flow through the louvers 1 4 is greater than that into space 1 6 from the chamber.

As shown in Figure 2 this reduced conductance is achieved by a reduced separation between the cryopanel 8 and the envelope 4 at the upper region thereof adjacent the opening 6.

The additional pumping route of the invention. provided by louvres 14, increases the area of cryopanel available for pumping, particularly with the variable throttle closed, reduces heat loss to the pump envelope 4 and accordingly increases the pumping efficiency of the cryogenerator without the need to uprate pump capacity.

An alternative embodiment of the cryogenerator of Figure 2 is illustrated in Figure 3. In this figure the conductance between the cryopanel 8 and the envelope 4 is reduced by way of spaced ribs 30 which are provided in the panel 8 and which are effective to form constrictions to the flow of gas within the space.

In the embodiment of Figure 4 the low conductance is produced by overlaping flanges 40 42 provided respectively at the opening of envelope 4 and the adjacent edge of cryopanel 8 to produce a convoluted gas path.

In the embodiment of Figure 5 an insulating panel 50 partially bridging the gap between the adjacent outer edges of the envelope 4 and the cryopanel 8 is effective to produce the required reduction of conductance.

In an alternative embodiment, (not illustrated) the decrease in conductance of the annulus between the cryopanel 8 and the envelope 4, is achieved by a plurality of spaced heat shields disposed parallel to the surface of panel 8 and envelope 4. The decrease in conductance provided by these heat shields will depend upon the number provided, their separation and their axial length; the shields will in addition to decreasing conductance also provide a reduction of heat loss to the envelope 4 of the cryopump.

In the embodiment of Figure 6 the low temperature panel of Figures 1 to 5 is provided with a frusto-conical extension 60. The inner surface of the extension is in this embodiment effective to pump the space between the enclosure 4 and the high temperature panel 8 to maintain high cryopanel efficiency. A baffle 64 provided conveniently on the high temperature panel is effective to restrict gas flow from the chamber being pumped to the space and thereby to maintain a low pumped pressure in the space, irrespective of any increase in chamber pressure.

An annular space 62, between the frustoconical extension 60 and the baffle 64 is effective to restrict heat gain by conduction from the baffle. The gap 22 may alternatively be bridged by an insulating collar in the manner exemplified by the insulator of Figure 5.

In this embodiment of the invention the pressure differential between the space and the chamber being pumped can be maintained without the need for variable throttle 1 2 of the embodiment of Figures 1 to 5.

It will be appreciated that the relative conductance in each of the embodiments disclosed, of the additional pumping path provided for example by louvres 14, and the space between panel 8 and envelope 4 may be selected to produce the required pressure in the space 1 6. This gas pressure itself can be determined to provide the optimum pumping characteristics of the cryogenerator in accordance with the equipment being pumped.

Claims

1. A cryogenerator pump having at least two cryopanels maintained within an enclosure which has an open end adapted for attachment to the chamber to be pumped, the cryopanels being effective to operate at relatively higher and lower temperatures respectively with the higher temperature panel being spaced from and adjacent the enclosure and circumscribing the lower temperature panel, the lower temperature panel being capable of communicating with the chamber through the opening in the enclosure and also to the space between the enclosure and the higher temperature panel.

2. A cryogenerator pump as claimed in claim 1 wherein the panels and the enclosure are of substantially cylindrical form.

3. A cryogenerator pump as claimed in claim 1 or claim 2 wherein a constriction to direct gas flow is provided between the lower temperature panel and the chamber being pumped.

4. A cryogenerator pump as claimed in claim 3 wherein the constriction is disposed at the enclosure opening.

5. A cryogenerator pump as claimed in claim 4 wherein the constriction is variable.

6. A cryogenerator pump as claimed in any preceding claim wherein the lower temperature panel communicates with the space by way of apertures provided in the higher temperature panel.

7. A cryogenerator pump as claimed in claim 6 wherein the aperatures are provided at the base of the higher temperature panel.

8. A cryogenerator pump as claimed in claim 6 or claim 7 wherein the aperatures are in the form of spaced overlapping louvres of panels inclined to the direction of gas flow.

9. A cryogenerator pump as claimed in claim 8 wherein the louvres or panels are adapted to prevent heat exchange between the enclosure and the lower temperature panel.

1 0. A cryogenerator pump as claimed in any one of claims 6 to 9 wherein the aperatures have a conductance which is matched to the conductance of the space between the higher temperature panel and the enclosure whereby to ensure balanced gas pressure between the space and the region adjacent the lower temperature panel.

11. A cryogenerator pump as claimed in claim 10 including means for reducing the conductance of the space between the higher temperature panel and the enclosure.

1 2. A cryogenerator pump as claimed in claim 11 wherein the means for reducing conductance comprises a region of increased diameter at the region of the higher temperature panel adjacent the opening in the enclosure.

1 3. A cryogenerator pump as claimed in claim 11 wherein the means for reducing conductance comprises a plurality of spaced heat shields disposed in the space between and parallel to the enclosure and the higher temperature panel.

1 4. A cryogenerator pump as claimed in claim 11 wherein the means for reducing conductance comprises spaced ribs provided in the higher temperature panel and extending towards the enclosure.

1 5. A cryogenerator pump as claimed in claim 11 wherein the means for reducing conductance comprises flanges which are respectively provided at the adjacent ends of a higher temperature panel and of the enclosure and which overlap to provide a convoluted gas flow path.

16. A cryogenerator pump as claimed in claim 11 wherein the means for reducing conductance comprise an insulating panel disposed between the enclosure and the higher temperature panel.

1 7. A cryogenerator pump as claimed in any preceding claim wherein the lower temperature panel is provided with an additional panel extending toward the higher temperature panel and effective to increase the pumping rate of the space between the enclosure and the higher temperature panel.

1 8. A cryogenerator pump as claimed in claim 1 7 wherein the additional panel is substantially frustoconical.

1 9. A cryogenerator pump as claimed in claim 1 8 wherein a low heat conductance path is provided in the additional panel.

20. A cryogenerator pump as claimed in claim 1 9 wherein the low heat conductance path comprises a discontinuity in the panel.

21. A cryogenerator pump as claimed in claim 20 wherein the discontinuity comprises a solid insulating material.

22. A cryogenerator pump substantially as herein before described with reference to any one of Figures 2 to 6 of the accompanying drawings.

23. A cryogenerator pump substantially as shown in and adapted to operate substantially as herein before described with reference to any of Figures 2 to 6 of the accompanying drawings.

24. A vacuum pumping system including a cryogenerator pump as claimed in any preceding claim.