GB2220449A - Cryopump - Google Patents

Abstract

A cryopump includes a two-stage cryogenerator in which a cryopanel 8 is mounted on and in thermal contact with the second stage 6 and a radiation shield 10 is mounted on but is thermally separated from the first stage 4 by means of a gasket 20 of insulating material. <IMAGE>

Description

IMPROVED CRYOPUMP The present invention relates to cryopumps.

A typical commercial cryopump is illustrated in Figure 1 of the accompanying drawings.

The cryopump 1 includes a two stage cryogenerator 2 having a first stage cooling station 4 and a second stage cooling station 6. Mounted on the second stage cooling station 6 is a cryopanel 8 which can be cooled to temperatures in the region of 10 to 20K. The inside surface of the cryopanel 8 is coated with charcoal for the absorption of so called "non-condensible" gases namely helium, hydrogen and neon which cannot be condensed on the main cryopanel.

Mounted on the first stage cooling station 4 and surrounding the cryopanel 8 is a radiation shield 10 which can be cooled to temperatures in the region of 30 to 70K. It is normal practice to use an indium metal gasket or a gasket of similar high thermal conductivity material between the radiation shield 10 and the first stage cooling station 4. In order to permit the passage of gas to the cryopanel 8 while preventing the entry of room temperature radiation the top face of the radiation shield is in the form of a baffle 12.

In United States Patent 4546613 reference is made to a problem that has been experienced by certain users of cryopump systems known as cross over "hang up". As explained in United States Patent 4546613 cross over is the processing step in which a valve between the work chamber and cryopump is open to expose the very high vacuum cryopump to a lower vacuum work chamber.

The e pressure in the work chamber is then reduced by the cryopump to bring the work chamber pressure to a vacuum of, for example, 10 7 torr. It is necessary in the case of argon that the argon be condensed on the cold second stage cooling station 6 and the cryopanel 8. Condensation of argon at higher temperatures results in a higher partial pressure of the argon and thus a higher pressure in the work chamber.

There are occasions when under low thermal load conditions the radiation shield 10 can drop to as low as 30k. At that temperature argon condenses on the radiation shield 10, and at that temperature the partial pressure resulting from the balanced evaporation of solid argon and condensation of argon molecules results in a partial pressure of only 10 5 to 10-6 torr in the work chamber. So long as any argon is in this state of sublimation on the radiation shield 10, the pressure in the work chamber cannot be taken down to the desired 10 7 torr.

In order to overcome this problem US Patent 4546613 teaches the painting of the outer surface of the radiation shield black thus providing a high emissivity (low reflective) surface.

This, in effect, provides a passive radiant heat load to the first stage cooling station to ensure that the radiation shield is held at a temperature above 50k thereby preventing the condensation of argon on the radiation shield for pressures up to 10'1 torr.

It is an aim of the present invention to provide a cryopump in which the radiation shield is maintained above a predetermined temperature by means of a thermal barrier placed between the radiation shield and the first stage cooling station.

According to the present invention, a cryopump comprises a cryogenerator having first and second stage cooling stations, a cryopanel mounted on and in thermal contact with the second stage cooling station and a radiation shield mounted on the first stage cooling station, the radiation shield surrounding the cryopanel, in which a thermal barrier is placed between the radiation shield and the first stage cooling station.

Preferably, the thermal barrier is in the form of a gasket made from polytetrafluoroethelyne (PTFE).

An embodiment of the invention will now be described by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which: Figure 1 is a diagrammatic cross section through a known cryopump; and Figure 2 is a diagrammatic cross section through a cryopump embodying the invention.

The e construction of, and operation of a cryopump of the kind.

with which the present invention is concerned is described generally with reference to Figure 1. The e cryopump 1 includes a two stage cryogenerator 2 having a cold head drive unit 12 connected to a source of working fluid, for example, helium under pressure and a two stage cold finger 14.

As previously explained, mounted on the second stage cooling station 6 of the cold finger 14 is a cryopanel 8 and mounted on the first stage cooling station 4 is a radiation shield 10.

In normal use, it is not unusual for the temperature of the cryopanel 8 to reach between 10 and 20k and the temperature of the radiation shield 10 between 30 and 50k.

In some applications, it is necessary to raise the temperature of the radiation shield 10 to prevent the condensation of argon on the shield.

Referring now to Figure 2 where like parts are given like reference numerals, the radiation shield 10 is mounted on the first stage cooling station 4 but is thermally separated therefrom by means of a thermal barrier 20.

The thermal barrier 20 is in the form of a PTFE gasket, or other similar insulating material, which provides a significant temperature differential between the first stage cooling station 4 and the radiation shield 10. This differential is created by the heat flow of conduction and radiant heat, collected by the radiation shield 10 across the thermal barrier to the first stage cooling station, and is set by the thickness of the gasket and the thermal conductivity of the material used. The temperature of the radiation shield 10 can now be arranged such that the condensation of argon can be prevented for all pressures within the normal operating range of the cryopump.As the temperature of the radiation shield 10 can now be set by selection of the gasket dimensions and/or material there is no longer any need to provide a high emissivity exterior surface to the radiation shield as contemplated by United States Patent 4546613.

The return to a low emissivity that is, polished highly reflective surface will then result in a lower radiant load being imposed on the cold finger 14: The e embodiment described with reference to Figure 2 has the following advantages: 1. With a highly reflective radiation shield outside surface radiant loading to the cold finger 14 can be kept to a minimum.

2. Using a PTFE gasket, the temperature of the radiation shield 10 can be controlled by simply selecting the thickness and type of the thermal barrier material.

3. Without the radiant heat exchange between the pump body and the radiation shield 10 imposed by the use of a high emissivity radiation shield exterior surface there is a reduced risk of atmospheric moisture condensing onto the pump body during normal operation of the cryopump.

Claims (4)

1. A cryopump comprising a cryogenerator having first and second stage cooling stations, a cryopanel mounted on and in thermal contact with the second stage cooling station and a radiation shield mounted on the first stage cooling station, the radiation shield surrounding the cryopanel, in which a thermal barrier is placed between the radiation shield and the first stage cooling station.

2 A cryopump as claimed in claim 1, in which the thermal barrier is in the form of a gasket of heat insulating material.

3. A cryopump as claimed in claim 2, in which the gasket is made from polytetrafluoroethelyne.

4. A cryopump constructed and arranged substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.