JPH0549826B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to cryogenic pumps, and more particularly to cryogenic pumps cooled by a two-stage closed cycle cooler. The invention also relates to a method for preventing crossover hang-up in a cryogenic pump.

Whether cooled by an open cryogenic cycle or a closed cryogenic cycle, currently available cryogenic pumps generally rely on the same design concept.
A cryogenic second stage array, typically operating in the 4 to 25 K range, constitutes the primary pumping surface. This surface is surrounded by a hot cylinder that typically operates in the temperature range of 70 to 130 K and provides radiation shielding for the cold array. The radiation shield typically includes a housing that is closed except for the portion of the frontal array located between the main pumping surface and the chamber to be evacuated. At these elevated temperatures, the first stage front array acts as a pumping point for high boiling gases, such as water vapor.

In operation, high boiling gases, such as water vapor, condense on the front array. The low boiling point gas passes through the array and into a chamber within the radiation shield.
Condense on stage array. Additionally, a surface coated with an absorbent, such as charcoal, or a molecular sieve operating at or below the temperature of the second stage array may be provided within the chamber to remove very low boiling point gases. By condensing or absorbing the gas onto the pumping surface, the working chamber becomes vacuum-only.

In systems cooled by closed cycle coolers, the cooler is typically a two-stage refrigerator with cooling fingers extending through the rear of the radiation shield. The cooling end of the second, coldest stage of the cryocooler is at the tip of the cooling finger. The main pumping surface, i.e. the cold panel, is the second part of the cooling finger.
Connected to a heat sink at the coldest end of the stage.
The cold panel may be a simple metal plate or an array of metal baffles disposed around and connected to the second stage heat sink. The second stage cryogenic panel also supports a cryogenic absorbent.

Radiation shielding is provided at the coldest end of the first stage of the refrigerator.
Connected to a heat sink or heat station. The shield surrounds the first stage cryogenic panel to protect it from radiant heat. The frontal array is cooled by a first stage heat sink via side shields or thermal struts as shown in US Pat. No. 4,356,701.

Experienced by a user of a cryogenic pump system1
One problem is known as a crossover "hangup." This problem is particularly relevant to systems such as sputtering systems where the process proceeds in an argon, oxygen or nitrogen atmosphere.
Crossover is a process step in which a valve between the working chamber and the cryopump opens to expose the very high vacuum of the cryopump to the low vacuum of the working chamber. In that case, the pressure in the working chamber is reduced by a cryo-pump. To bring the working chamber pressure to, say, 10 ^-7 Torr, in the case of argon, the gas needs to be condensed in a cold 22nd stage array at a temperature of 28.6 K. Condensing rougon at high temperatures increases the partial pressure of argon and therefore the pressure in the working chamber.

During normal operation of the system, where the first stage array is held at a temperature of e.g. 77K, argon does not condense in the first stage array, but passes directly to the second stage array with moderate condensation in the second stage array. do. However, under low heat load conditions, the temperature of the front array can drop to around 40K. At that temperature, argon does not condense in the front array, and at that temperature the partial pressure resulting from the equilibrium between the evaporation of solid argon and the condensation of argon molecules reduces the partial pressure to only 10 ^-3
to 10 ^-4 torr. As long as the argon remains in this sublimated state in the front array, the working chamber pressure cannot be reduced to the desired 10 ^-7 Torr.

As the argon gas evaporates during sublimation, it eventually moves to the cooler second stage and
It is captured there. However, the rate of sublimation is slow and the pressure in the system "hangs up" at high temperatures by the time sublimation is complete.

As a possible solution to "hang-up", it has been proposed to warm up the first stage array by introducing an electrical thermal load to prevent the temperature of the first stage array from cooling down. However, adding a heat load to the first stage array typically increases the cooling time of the refrigerator. Minimizing cooling time is a significant concern in cryogenic pump design, and furthermore, electrical elements can be dangerous in areas with high hydrogen concentrations.

Another problem associated with cryogenic pump systems is that the pulsating heat load does not provide constant pressure within the working chamber. For example, as a low emissivity valve door is opened to expose the front array to a high emissivity radiating surface, heat loads may increase and pressure instability may occur.

In accordance with the principles of the present invention, cross-over hang-up in cryogenic pumps is eliminated by providing a passive heat load to the first stage such that the first stage is maintained at a temperature above about 50K. During the initial stage of cooling, the cooling time is not significantly affected since the passive heat load is significantly less than in the final cooling temperature state.

Preferably, the heat load is due to radiant heating of the radiation shield. The radiant heat load on the first stage is increased by increasing the effective emissivity between at least a portion of the radiation shield and the vacuum vessel. When the first stage is cold, the radiant heat load on the first stage is large due to the fact that the heat flux is a function of the temperature difference to the fourth power. As a result, when the first stage cools down to a temperature close to 50K, the heat load becomes significant and prevents the first stage from dropping below a temperature of 50K. It has been found that as long as the temperature is kept above 50K, crossover hang-up is eliminated. At high temperatures,
Due to the fact that the temperature difference between the radiation shield and the vacuum vessel is low and the radiation heat flux is a function of the temperature difference,
The heat load is significantly lower. If the system is initially at ambient temperature, the heat load is negligible. Therefore, by providing a radiant heat load to the first stage, the heat load is minimized at cooling temperatures;
At very low temperatures it is significantly high enough to prevent the first stage from dropping below 50K. Cooling time is not significantly compromised and crossover hang-ups are eliminated.

Preferably, the effective emissivity between the radiation shield and the vacuum vessel is obtained by painting the outer surface of the radiation shield black. Coating the interior surfaces of the vacuum vessel will also increase the effective emissivity, but at elevated temperatures may result in outgassing from the coating on the vacuum vessel.

The problem with the crossover "Hang Up" is
It can be caused by gas condensing on the sides of the second stage refrigerator cylinder. This problem is particularly evident when using an open second stage array to provide maximum flow to the absorbent material on the back side of the array. At normal operating temperatures, approximately
There is a temperature gradient across the length of the refrigerator cylinder from the first stage heat sink at 77 K to the second stage heat sink at 15 K. Argon and other gases can condense along zones of the refrigerator cylinder at temperatures below 50K. The temperature in the zone is determined by the system pressure. When a heat load is applied to the first stage, for example by opening a valve in the system, the temperature of the first stage increases, moving a zone of 50 K over the length of the refrigerator cylinder. As the zone moves, the gas frozen on the cylinder is rapidly released. This rapid evaporation causes the pressure in the working chamber to increase rapidly. moreover,
Even when the heat load of the first stage is constant, the displacement device within the refrigerator cylinder reciprocates, causing continuous movement of the critical zone. Said movement of the critical zone causes frequent fluctuations in the pressure within the working chamber.

To eliminate problems caused by condensation of argon and other gases in the second stage refrigerator, a sleeve of a sealing device surrounds the refrigerator cylinder. The sleeve is in thermal contact with the second stage heat sink but not with the refrigerator cylinder. Most of the gas passing through the second stage array condenses on the shield before reaching the cylinder. A narrow air space of approximately 0.1 inch or less between the shield and the cylinder ensures that even gas passing below the cylinder is rapidly condensed and trapped on the cold shield. . By keeping the radiation shield at the low temperature of the second stage heat sink, gases that condense on the shield will be retained on the shield and evaporated even as mobile equipment or higher heat loads move to the first stage. Never.

The foregoing and other objects, features and advantages of the invention will become apparent from the following detailed description of a preferred embodiment of the invention, illustrated in the accompanying drawings in which like reference numerals refer to like parts throughout the various drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

The low pressure pump shown in FIGS. 1 and 2 includes a vacuum vessel 12 attached to the wall of the working chamber along a flange 14. The front opening 16 of the container 12 communicates with the circular opening of the working chamber.

As shown in FIG. 2, a removable cover 17 is provided over the opening for transportation. Alternatively, the cryopump assembly may project into the chamber and include a vacuum seal on the rear flange. A second stage cooling finger 18 of the refrigerator projects through the opening 20 and into the container 12 . In this case, the refrigerator is a U.S. patent to Chellis et al.
3218815, a Gifford-MacMahon refrigerator. A second stage moving device for cooling finger 18 is driven by motor 22 . During each cycle, helium gas introduced under pressure into the cooling finger via line 24 is expanded, cooled and discharged via line 26. A first stage heat sink or station 28 is attached to the cold end of the first stage section 29 of the refrigerator. Similarly, a heat sink 30 is attached to the cooled end of the second stage 32. A suitable temperature sensitive element 34 is mounted at the rear of the heat sink 30.

The primary pumping surface is an array attached to the heat sink 30. The array includes a disk 38;
It consists of a set of circular chevron-shaped diaphragms 40 arranged vertically and attached to the disk 38 by thermal struts 41. The strut 41 extends through the diaphragm 40 and a cylindrical spacer 43 provided between the diaphragms, and a nut at the end of the strut compresses the chevron-shaped diaphragm and the spacer. to form a tight stack. A cold absorbing material, such as charcoal powder, is adhered to the lower back surface portion of the diaphragm. The low boiling point gas approaches the absorbent material through the open chevron-shaped diaphragm. By arranging the chevron-shaped diaphragm member in the open manner as described above by the struts, assembly is simplified and gas can easily flow through the front side of the diaphragm member 40 to the absorbent material. Alternatively, the chevron-shaped diaphragm may be supported by an internal cylinder to which the absorbent material can be adhered.

Sleeve 52 is located over second stage refrigerator cylinder 32 for reasons discussed below. The sleeve 52 has two semi-cylindrical members 5 attached to and extending downwardly from the second stage heat sink 30.
It is formed from 4,56. A small cavity 55 is provided between the sleeve and the cylinder 32.

A pot-shaped radiant heat sink 44 is attached to the first stage high temperature heat sink 28. A second stage cooling finger extends through the radiation shield opening 45. The radiation shield 44 surrounds the second stage array to the rear and sides to minimize heating of the array by radiation. Preferably, the temperature of the radiation shield is about 120K or less.

The front cryo panel array 46 acts as both a radiation shield for the main cryo panels and a cryo-pumping surface for high boiling point gases such as water vapor. Said array is connected by a rim 50 of the armor plates 4
Contains 8. The front array 46 is attached to a radiation shield 44 that supports the front array and acts as a thermal path from the heat sink 28 to the array. The shape of the array need not be limited to the arrangement shown, and may be designed to act as a radiant heat shield and as a cold pumping panel at high temperatures, while providing a path for low boiling point gases to the second stage array. There should be a deflector array in place.

As previously mentioned, the crossover hangup problem arises from argon and other gases freezing on the front array of the first stage rather than passing directly to the second stage. Experiments have shown that argon hang-up can be eliminated by maintaining the front array temperature above 50K degrees. This can be achieved by providing a heat load to the cold first stage. On the other hand, it is preferred to minimize the heat load of the first stage at high temperatures in order to increase the cooling rate. To this end, the outside of the radiation shield 44 is coated with a matte black paint to provide a radiant heat load to the first stage. This increases the emissivity of the radiation shield, increasing the flow of radiant heat from the vacuum vessel to the shield. The radiant heat flow is a heat load on the first stage refrigerator.

The heat load on the first stage is due to the radiant heat flow Q to the radiation shield 44.

Q=Aδe _eff (T _H ⁴ −T _L ⁴ ) (1) A is surface area, δ is constant, e _eff is effective emissivity, T _H
is the temperature of the vacuum vessel and T _L is the temperature of the radiation shield.

The effective emissivity is a function of the emissivity e ₀ of the outer surface of the radiation shield and the emissivity e _i of the inner surface of the vacuum vessel.

e _eff 1/1/c ₀ +1/e _i −1 (2) In the past, the surface has been polished to obtain a very low emissivity of about 0.1 or less, for an effective emissivity of about 0.05 or less. The low effective emissivity minimizes the flow of radiant heat and the heat load on the first stage. To provide a reasonable heat load to the first stage according to the present invention, the effective emissivity should be at least about 0.10. This effective emissivity is 1 of the outer surface of the radiation shield 44.
and an emissivity of about 0.1 for the inner surface of the vacuum vessel 12.

It is important that high emissivity is provided on the radiation shield 44 and not on the front array 46. By providing the front array 46 with a high degree of emissivity, the effective emissivity can be greatly varied. As the valve door to the working chamber opens, the emissivity on the front array changes from 0.1 to near 1. As the emissivity on the array approaches 1, the effective emissivity varies from about 0.1 to about 1. This results in a change in heat load of several watts.

With the current arrangement, the emissivity of the front array is about 0.1, and as the valve opens, the emissivity of the front array changes from about 0.05 to about 0.1.
Regardless of the valve position, the effective emissivity between the radiation shield and the vacuum vessel remains approximately 0.1. Thus, the heat load on the first stage remains much more constant at about 1 or 2 watts.

It can be seen that the radiant heat flow is a function of the fourth power of the temperature difference. Thus, as the temperature difference increases, the heat flow increases. It has been found that painting the radiation shield 44 black provides an emissivity of about 0.9, and in the cold conditions of the first stage, the flow of radiant heat provides a significant heat load on the first stage. The above heat load increases the temperature of the first stage including the front array 46.
Enough to hold over 50K. However, at high temperatures the radiant heat load is much lower and does not significantly impede system cooling.

Another device for obtaining the desired heat load only at low temperatures is shown in FIG. In this arrangement, the thermal switch provides a thermally conductive thermal path between the vacuum vessel 12 and the radiation shield 44 at low temperatures. Said switch is formed from bimetallic elements 57,58. At low temperatures close to 50K, the bimetallic elements bond and provide a heat flow path for the radiation shield, allowing the front array to reach a temperature of 50K.
Avoid dropping below. However, at high temperatures, the elements are separated;
The vacuum between the elements 57 and 58 provides good insulation.

Radiant heat loads are preferred over conductive heat loads because they provide a more uniform load on the second stage and do not introduce any structural changes to the system.
Both radiant and conductive heat loads eliminate the need for electrical heating elements in the system, and both provide a greater heat load as the first stage temperature decreases.

The heat load provided by increasing radiation to the radiation shield 44 prevents argon and other gases with low freezing temperatures from condensing in the front array, but prevents argon from condensing in the second stage refrigerator cylinder 32. It turns out that there are still problems. 1st stage heat sink 28 is 77K, 2nd stage heat sink 30
Even at normal operating temperatures of 15K, there is a temperature gradient between the heat sinks over the length of cylinder 32. For example, if the chamber pressure is 10 ^-4 Torr,
Defines a limited temperature range below 50K in which argon gas condenses and evaporates in an equilibrium state. Thus, at any given point along the length of the cylinder, there is a critical zone on the cylinder 32 at which temperature the argon gas condenses and evaporates at equilibrium. As the moving device within the cylinder 32 reciprocates up and down, the critical zone also moves up and down along the cylinder. As the zone moves upward, the area that was supporting the condensed argon is warmed to a high temperature where the argon evaporates. The argon evaporates fairly quickly, increasing the pressure in the system. As the transfer device reciprocates, oscillations in the critical region are seen as oscillations in the chamber pressure.

Another consequence of argon condensation on cylinder 32 is pressure instability as the heat load on the first stage changes. For example, when the valve is opened to the actuation chamber, a large heat load is placed on the first stage, increasing the temperature of the first stage heat sink 28. Therefore, the critical zone moves rapidly and the pressure inside the chamber becomes unstable.

It has been found that argon condensation on the cylinder can be virtually eliminated by placing a tightly fitted shield 52 over the cylinder and maintaining the shield at a stable low temperature. Most of the gas that passes through the second stage array and would otherwise come into contact with the second stage cylinders 32 is cut off by the shield.
Furthermore, any gas that may pass from below the shield into the area between the shield and the cylinder is immediately trapped on the inner surface of the shield. Once the argon condenses on the 15K screen, evaporation is extremely limited. On the other hand, any gas that condenses on the cylinder evaporates relatively quickly. Then, upon equilibrium, gas entering the air space between the shield and the cylinder is rapidly captured by the cylinder, and condensation on the cylinder is virtually eliminated. 2.16 mm (0.085 inch) for this purpose
It turned out that the sky was suitable.

While the invention has been illustrated and described in detail with respect to preferred embodiments, those skilled in the art will recognize that various changes can be made in form and detail without departing from the spirit and scope of the claims. That is understood. For example, although a closed-cycle, two-stage refrigerator is shown, a cryo-pump cooled by an open-cycle refrigerator, such as liquid nitrogen, hydrogen, or helium, could also be used. A combination of single stage and two stage closed cycle refrigerators can also be used to provide cooling.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a cryo-pump embodying the present invention, FIG. 2 is a cross-sectional side view of the cryo-pump shown in FIG. 1, and FIG. 3 is a diagram showing an alternative embodiment of the thermal switch. In the figure, 12... vacuum container, 16, 20... opening, 18... cooling finger, 28, 30... heat sink,
29...First stage, 32...Second stage, 34...Temperature sensing element, 38...Disc, 40...Chevron shaped member, 4
1... Heat strut, 43... Spacer, 44... Heat shield, 4
6...Front array, 52...Sleeve, 54, 56...
Semi-cylindrical member, 48... armor plate.

Claims (1)

[Claims] 1. A cryogenic pump having at least two stages 29, 32 of a refrigerator, with a passive heat load on the first stage 29, the first stage 29 being heated to a temperature of about 50K or more. The heat load is lower at the temperature of the first stage 29 , and the passive heat load is based on the radiant heat flow through the radiation shield 44 . A cryogenic pump characterized in that the outer surface of the radiation shield 44 is black. 2. The cryo-pump according to claim 1, wherein the outer surface of the radiation shield 44 is painted black. 3. The cryogenic pump according to claim 1 or 2, wherein the radiation efficiency of the radiation shielding 44 is about 0.1 or more. 4. A vacuum container 12 whose outer surface of the radiation shield 44 has an emissivity of approximately 1 and which surrounds the radiation shield 44.
4. The cryogenic pump according to claim 3, wherein the inner surface of the cryogenic pump has an emissivity of about 0.1. 5. A cryopump according to any one of claims 1 to 4, wherein the argon, nitrogen and oxygen gases in the system are prevented from condensing on the cryo-pumping surface of the first stage. 6 the radiation shielding 44 surrounds the second stage cold pumping surfaces 38, 40, 41, 43;
3 is in thermal contact with at least the heat sink 30 disposed on the second stage 32 of the refrigerator, and is configured to condense low-temperature condensed gas. Cryogenic pump as described in . 7. A method of preventing crossover hang-up in a cryogenic pump having at least two stages 29, 31 of refrigerators, the first stage 29 providing a passive heat load to the first stage 29, the first stage 29 having a temperature of approximately 50K. in which the heat load is lower at the temperature of the first stage 29 and the passive heat load is based on radiant heat flow through a radiation shield 44; Shielding 44
A method characterized by making the outer surface of a black color. 8. The method of claim 7, wherein the radiation efficiency of the radiation shield 44 is greater than or equal to about 0.1.