JP2597696B2 - Cryopumps staged optimally - Google Patents

Description

BACKGROUND OF THE INVENTION Cryopumps are commonly used in equipment for the manufacture of integrated circuits and other electronic components and for deposition of thin films in various consumer and production goods. The cryopump is used to freeze or pump out gases in the working environment to create a vacuum. Refrigerators used in cryopumps for pumping out gas are:
Any open or closed cycle cryogenic refrigerator may be used.
The most common refrigerator used is a two-stage gold-finger closed cycle refrigerator.

Generally, the cold end of the second stage, the lowest stage of the two-stage refrigerator, is connected to the main pumping surface. This primary pumping surface operates in the temperature range of 4-25K. The first stage of the two-stage refrigerator is connected to a radiation shield surrounding the main pumping surface. The spacing between the primary pumping surface and the radiation shield must be sufficient to allow unobstructed flow of low boiling point gas from the vacuum chamber created by the shield to the primary pumping surface. Radiation shields generally operate in the range 70-140 ° K. The full array separates the evacuation chamber and the radiation shield, which also serves as a radiation shield on the main pumping surface. The full array is cooled, typically to 110-130 K, by thermally coupling it to the radiation shield.

In operation, high boiling gases, such as water vapor, are condensed on the full array. The low-boiling gas flows via the arrangement into the volume in the radiation shield, where it condenses on the main pumping surface. An absorbent, such as charcoal, is generally placed near the main pumping surface and is operated at the temperature of the surface to absorb gases that have a very low boiling point and are not condensed on the main surface.

Multi-stage refrigerators use a temperature measurement controller in the first and second stages to prevent cross hang-ups to the entire or partial regeneration means during cooling. Such a temperature control system is described in U.S. Pat. No. 4,679,401, in which the refrigerated gas is diverted to exchange heat with a first and / or second stage heat sink of a cryopump. By controlling the flow rate and temperature of the diverted refrigerated gas, the problem of cross solder-up and / or more efficient regeneration can be addressed.

DISCLOSURE OF THE INVENTION The present invention is used to refer to various temperature steps (where step is not meant to be "process" but rather "means") for efficient pumping of gas. The same applies hereinafter).

The present invention relates to a conventional cryopump comprising a temperature stage having an absorbent cooled as low as possible to pump gas that has not been pumped in a first temperature stage, which is generally used to pump water. Are different. In conventional cryopumps, when the absorbent is cooled to a temperature that absorbs a gas with a low critical mobility temperature, the gas with a high critical mobility temperature becomes immobile at the entrance of the hole or recess. In addition, the surface of the absorbent is not efficiently utilized for pumping the gas. As a result, less surface area is not available for absorbing gas. Thus, the advantage of the present invention over conventional cryopumps is that the inner surfaces of holes or recesses are not blocked at their inlets,
That's what it means.

It is generally recognized that in order to achieve optimal pumping of hydrogen in the second stage, the temperature in the second stage must be no greater than 14 ° K. However, it has been found that there is a certain temperature at which optimal pumping occurs under a given load condition, and below this temperature the rate of absorption is significantly reduced. Thus, according to the present invention, the second stage temperature is maintained at an optimum temperature for existing gas and load conditions.

In one embodiment, the cryopump pumps three different temperature stages, a first temperature stage that pumps a gas having a high boiling point such as water, and a gas that is not pumped in the first stage. A second temperature stage having a very low boiling point and a third temperature stage, ie, the coldest, for pumping gas which has not been pumped in the first two temperature stages. In the second and third temperature stages, an absorbent having holes or recesses for efficiently absorbing gases at various critical mobility temperatures is placed. In this embodiment, the third temperature stage is surrounded and isolated from the third temperature stage by the second temperature stage surrounded and isolated from the first temperature stage. The spacing between the temperature stages allows an unobstructed flow of low boiling gas from the first temperature stage to the third temperature stage.

A second embodiment of the present invention utilizes a second stage temperature control system during pump operation to achieve optimal cryogenic adsorption of gas pumped in the second stage. Accordingly, the second stage temperature can be adjusted to maintain the second stage temperature at an optimal level. For hydrogen, the temperature at which the optimum condition is occurring is depending on the particular H ₂ loading pump,
Generally, it is 10-14 ゜ K. This optimum temperature of hydrogen must be maintained so that the pumped molecules can move around the absorbent surface without blocking the pores.

The preferred embodiment of this temperature control system incorporates a temperature sensor in contact with the second stage heatsink and an electrical resistance heater in heat conductive contact with the second stage. The wires used to direct power to the heating filament are hermetically sealed to avoid their exposure to volatile gases in the pumping chamber.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and features and advantages of the present invention are as follows:
A more detailed description of the preferred embodiment of the invention, as illustrated in the accompanying drawings, in which like parts are designated with like reference numerals throughout the various views, will be apparent. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIG. 1 shows an enlarged partial cross section of charcoal.

FIG. 2 is a cryopump embodying the present invention having three temperature stages.

FIG. 3 is a cryopump embodying the present invention with a heater system installed in the first and second stages.

FIG. 4 is an optional illustration of the dependence of effective pumping rate on temperature for hydrogen under certain loading conditions.

DETAILED DESCRIPTION OF THE INVENTION It is well known that the number of molecules absorbed per unit area is equal to the unit area of the surface multiplied by the rate at which the gas strikes the gas, times the average time the molecules spend on the surface. Thus, by increasing the unit of surface area, more molecules can be absorbed by the absorbent. Among the available cryogenic absorbents, charcoal and zeolites are the most commonly used absorbents because they have a large number of pores and depressions along their surface. The large number of pores or recesses in these absorbents provide a large effective surface area for absorbing molecules, which is large relative to the amount of absorbent. Other requirements, such as the temperature and time required for activation, the amount of dust generated by the absorbent, thermal conductivity, etc., make charcoal and zeolites the best choice.

As an example, FIG. 1 shows an enlarged view of a surface portion of charcoal. As absorbed, the gas molecules M migrate along the surface 11 of the charcoal and fall into the latent recess 10 until such time as they receive enough thermal energy to desorb. Gas molecule M
Migrate along surface 11, possibly because they may receive a small amount of energy from the absorbent during a period of time called the residence time that remains on the surface 11 of the absorbent. If the temperature of the absorbent is sufficiently low, the probability that the molecule M will have sufficient energy to escape or migrate along the surface of the absorbent is reduced. The molecule M is therefore less mobile.
Thus, according to conventional theory, the amount of gas absorbed must increase with decreasing temperature.

In the present invention, tests have shown that non-condensables such as helium, neon, and hydrogen have a critical mobility temperature when absorbed by charcoal. In particular, helium has been found to have a critical mobility temperature of less than 5 K, neon has been found to have a critical mobility temperature of about 10 K, and hydrogen has been found to have a critical mobility temperature of about 13 K.゜ K
It has been found to have a critical mobility temperature of Similarly, other non-condensables have a critical mobility temperature. It is believed that below these critical temperatures, the absorbed non-condensate can become immobile on the surface of the absorbent.
As a result, the inlets of the absorbent cavities and holes can become blocked by immobile molecules, due to their immobility being insufficient to penetrate the more inaccessible interior. The above situation is shown in FIG. As a result, less effective surface portion of the absorbent is utilized to absorb gas having a lower critical mobility temperature.

It has also been found that the rate of cryogenic adsorption of hydrogen reaches a maximum in the temperature range of 10-14 ° K and decreases considerably below that level. The exact temperature depends on the level of loading of H _2.

FIG. 4 is an optional illustration that, under a given H ₂ loading condition, the rate at which hydrogen is pumped reaches an optimum at temperature (T ₀ ). As shown above, the conventional theory is:
It teaches that the amount of gas absorbed should increase with decreasing temperature. FIG. 4 shows that the rate of absorption decreases rapidly at temperatures below (T ₀ ).

As a result, in a first embodiment, the present invention provides three temperature stages, namely, a first stage for pumping a gas that readily freezes at a temperature of about 100 ° K, such as water, and a nitrogen or argon phase. Effectively pumps gas that easily freezes at temperatures of about 15 ° K, and also pumps non-condensate with higher critical mobility such as hydrogen and neon with an absorbent An optimal cryopump can be constructed having a second stage, and a small third stage that is kept as low as possible to effectively pump gas with a very low critical mobility temperature, such as helium. . Preferably, the first stage temperature is cooled to 70-140 ° K, the second stage temperature is cooled to 10-14 ° K, and the third stage temperature is cooled to about 5 ° K.

The three temperature stage cryopump can be configured in various ways. For example, in FIG. 2, the cold fingers of the two-stage closed cycle refrigerator R extend through openings 16 into the housing 14 of a conventional cryopump. In this case, the refrigerator is a Gifford McMaon (Giff
ord-MacMahon) A refrigerator, but other refrigerators may be used. In this refrigerator, the displacement unit in the cold finger is driven by the electric motor 12. For each cycle, the helium gas pressurized through the feed line 13 and introduced into the cold finger expands, is thus cooled, and is then discharged through the return line 15. This type of refrigerator is disclosed in U.S. Pat. No. 3,218,815.

The first cold finger stage 18 is attached to a radiation shield 20 that is coupled to a full array 22. Generally, the temperature difference across the heat path from the front array 22 to the first stage 18 of the cold finger is between 30K and 50K. Therefore, the front array 22 is cooled to a temperature low enough to condense the water vapor.
To maintain the cold-finger first stage 90
It must be operated at ゜ ~ 110 ゜ K. Radiation shield
20 and the front array serve as a first temperature stage.

The cold finger cold end 24 and the second stage 26 are attached to a heat sink 28. The heat sink 28 includes a disk 30 and a series of circular chevron 32 mounted to the disk 30 in a vertical array. The heat sink 28 and the vertical array of round chevron 32 form the main pumping surface of the cryopump.
There is a low temperature absorber 34 along the cylindrical surface between the circular members of the main pumping surface. Preferably, the main pumping surface forms a second temperature stage and is cooled to 10-14 ° K. The temperature of the primary pumping surface is reduced by cooling the second stage of the cold finger to about 5K and using a low conductivity material 30 so that the temperature difference across the heat sink 28 is about 9K. This can be maintained by designing vessel 28. The third temperature stage can be accomplished by placing the absorbent 36 in thermal contact with the cold end of the second stage 26 of the cold finger. Thus, the coldest stage of the cold finger, ie both the second stage and the second and third temperature stages, can be obtained. Alternatively, a three-stage closed cycle refrigerator can be used to maintain the three temperature stages.

In operation, gas from the working chamber (not shown) enters front array 22 via cryopump opening 37 where high boiling point gas condenses on the surface of front array 22. The lower-boiling gas passes through the above-mentioned arrangement and the volume inside the radiation shield 20
The gas flows into 38 where the gas condenses on the chevron surface 32 and is absorbed by the absorbent 34 located on the surface between the chevron 32.
Gases having a very low melting point, such as helium, which are not pumped by the main pumping surface, flow to the third temperature stage absorbent 36 for absorption.

In the case of a conventional cryopump, the design of the cryopump conforms to the conventional theory that the colder the absorbent surface, the more gas the absorbent will absorb. In the present case, the absorbent along both the second and third temperature surfaces are operated at different temperatures. Both of these heaters, the second temperature stage sorbent, effectively absorb the gas that would otherwise be immobile at the third temperature stage along the entire surface area, including the absorbent recesses. Let it be. As a result, more gas is absorbed at the surface portion of the second temperature stage than in a conventional cryopump, because pores and recesses in the absorbent are not blocked by immobilized gas molecules. Instead, the gas with the very low critical mobility temperature is pumped in the third temperature stage.

Another preferred embodiment of the present invention is shown in FIG. This embodiment utilizes a heater 40 that extends through the housing 14 and the shield 20. The first heating element 42 contacts the first stage and the second heating element 44 contacts the second stage heatsink 28. The heating element 44 is used to regulate the temperature of the main pumping surface such that there is optimal cryogenic absorption of the pumped gas. This embodiment uses a highly conductive material such as copper for the member 30. The present invention maintains the primary pumping surface to the optimum temperature T ₀ using a heating element 44 which contacts the second stage heat sink.

A temperature measuring device 46, such as a thermistor or thermocouple, is provided at the cold end 24 to monitor the temperature of the primary pumping surface. The temperature measured by the monitor 46 is
The automatically adjusts used to maintain the predetermined temperature T ₀ in. The control circuit 50 obtains a signal to the heater 40 based on the detected temperature.

The heating system 40 may also be of the type described in U.S. Pat. No. 4,679,401, in which the refrigerated gas of the refrigerator R is diverted to a heat exchanger coupled to the first and second stages of the cryopump. it can. The heating system is also used to prevent cross hang-ups and to obtain a more efficient regeneration procedure. Alternatively, a third embodiment of the present invention utilizes three temperature stages with a temperature control system attached to the second stage. In this embodiment, a second stage temperature active controller is used instead of a low conductivity material for the second stage temperature control member 30.

Although the present invention has been illustrated and described in detail in terms of preferred embodiments thereof, it should be understood that various changes in form and detail may be made without departing from the spirit and scope of the invention as defined by the appended claims. It will be understood by traders. For example, a three-stage closed cycle refrigerator may be used to maintain three temperature stages. Also, separate refrigerators may be used to maintain the various temperature stages, as disclosed in U.S. Patent Application No. 007,019, filed January 27, 1987.

Claims (9)

(57) [Claims] 1. A first temperature stage having a first pumping surface for condensing a gas and at least an absorbent cooled therein to a temperature lower than the first temperature stage and pumped by the first pumping surface. A second temperature stage having a second pumping surface for pumping the missing gas to pump gas to be pumped by the second temperature stage absorbent. A cryopump wherein the temperature of the second temperature stage is maintained above a minimum temperature at which the capacity of the absorbent significantly decreases. 2. The cryopump according to claim 1, wherein said cryopump is cooled to a temperature lower than said second temperature stage to pump gas not pumped at said second pumping surface. A cryopump having a third temperature stage with a third pumping surface. 3. A cryopump according to claim 1, wherein the temperature detector detects a temperature in a second temperature stage.
A temperature controller for maintaining the temperature of the second temperature stage at a predetermined level during the pumping operation by the second temperature stage in response to the detected temperature. 4. A method for absorbing gas in a cryopump, comprising: cooling a first temperature stage of the cryopump having a first condensing surface for condensing the gas; and cooling a second temperature stage of the cryopump. Maintaining the temperature of the second temperature stage above a minimum temperature below which the ability of the absorbent to absorb gas not condensed in the first temperature stage is significantly reduced. A method comprising: 5. A method for effectively absorbing gas in a cryopump according to claim 4, comprising a second absorption surface for absorbing gas not absorbed by the second temperature step. Cooling the third temperature stage of the cryopump. 6. A method for effectively absorbing gas in a cryopump according to claim 5, wherein the first temperature step is cooled to about 9-140 ° K. and the second temperature step is about 10 ° C. ~14
Cooling to ゜ K, wherein the third temperature stage is cooled to about 5 ゜ K. 7. The method for effectively absorbing gas in a cryopump according to claim 4, further comprising the steps of: detecting a temperature in a second temperature stage; And maintaining the temperature of the second temperature stage at a predetermined level during the pumping operation by the second temperature stage. 8. The method according to claim 4, wherein:
The minimum temperature at which the second temperature stage is kept higher is about 10
A method characterized in that 方法 K. 9. A cryopump having different temperature stages for pumping a gas, the cryopump being coupled to a two-stage cold finger closed cycle refrigerator and a radiation shield cooled by a first stage of the refrigerator. A first temperature stage comprising a front array, the first pumping surface being surrounded by the radiation shield and separated from the radiation shield, wherein the first pumping surface is the second most refrigerated temperature of the refrigerator. A second temperature stage having an absorbent and a condensing surface cooled by the stage; and a second temperature stage surrounded by and separated from the second temperature stage for absorbing gas having a very low boiling point temperature. A third temperature stage including an absorbent in thermal communication with the second temperature stage of the refrigerator.