Abstract:
The pulse tubes ( 165, 175 ) and regenerators ( 160, 170 ) contained within a cryopump housing ( 210 ) are arranged in a way that facilitates the fabrication and installation of the cryopanels ( 265 ). The pulse tubes and regenerators are located in a common plane in the center of the cryopump housing and the cold (second stage) panels ( 265 ) that are pitched parallel to the plane with the tubes.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/346,674, filed Jan. 8, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The Gifford-McMahon (G-M) type pulse tube refrigerator is a cryocooler, similar to G-M refrigerators, that derives cooling from the compression and expansion of gas. However, unlike the G-M systems, in which the gas expansion work is transferred out of the expansion space by a solid expansion piston or displacer, pulse tube refrigerators have no moving parts in their cold end, but rather an oscillating gas column within the pulse tube that functions as a compressible displacer. The elimination of moving parts in the cold end of pulse tube refrigerators allows a significant reduction of vibration, as well as greater reliability and lifetime, and is thus potentially very useful in cooling cryopumps, which are often used to purge gases from semiconductor fabrication vacuum chambers.  
         [0003]     G-M type pulse tube refrigerators are characterized by having a compressor that is connected to a remote expander by high and low pressure gas lines. The pulse tube expander has a valve mechanism that alternately pressurizes and depressurizes the regenerators and pulse tubes to produce refrigeration at cryogenic temperatures.  
         [0004]     Two stage G-M refrigerators, which are presently being used to cool cryopumps, cool a first stage cryopanel at about 60 K and a second stage cryopanel at about 15 K. The expander is usually configured as a stepped cylinder with a valve assembly at the first stage warm end, a first stage cold station (60 K) at the transition from the larger diameter first stage to the smaller diameter second stage, and a second stage cold station (15 K) at the far end. The cryopanels are typically axi-symetric around the cold finger. The cryopump operates equally well in all orientations.  
         [0005]     Longsworth, U.S. Pat. No. 4,150,549, dated Apr. 24, 1979 and entitled “Cryopumping Method and Apparatus”, describes a typical cryopump that uses a two stage G-M refrigerator to cool two axi-symetric cryopanels. The first stage cools an inlet (warm) panel that pumps group I gases, e.g. H 2 O, and blocks a significant amount of radiation from reaching the second stage (cold) panel but allows group II, e.g. N 2 , and m, e.g. H 2 , gases to pass through it. The Group II gases freeze on the front side of the cold panel(s) and Group III gases are adsorbed in an adsorbent on the backside of the cold panel(s).  
         [0006]     Unlike a typical GM expander that has a single stepped cylinder that lends itself to attaching axi-symetric cryopanels, the two stage pulse tube expander has two pulse tubes and two or more tubes to house the regenerators. The pulse tubes themselves tend to be as long as the most common size cryopump which has a diameter of 200 mm. G-M type pulse tube refrigerators that operate below 20 K have the disadvantage of requiring that the hot end of the pulse tube be above the cold end in order to avoid the thermal losses associated with convective circulation within the pulse tube. Conventional two-stage GM type pulse tube refrigerators typically have the valve mechanism and the hot end of the pulse tube on top. This enables the heat that is rejected at the hot end of the pulse tube to be easily transferred to the low-pressure gas and returned to the compressor where it is rejected.  
         [0007]     Most cryopumps are mounted below the vacuum chamber where space above the cryopump housing is very limited. Having the valve mechanism above the cryopump housing limits the applications of the cryopump. Thus, any options to orient the pulse tube refrigerator with the valve behind or below a cryopump housing that has a side inlet are highly desirable. The first and second stage pulse tubes are two separate tubes that have two or three regenerator tubes with them. The arrangement of the pulse tubes and regenerators in the cryopump housing makes it very difficult to make conventional axi-symetric cryopanels because they have so many cut outs to fit around the tubes. This problem has not been recognized or solved in the prior art.  
         [0008]     It is an object of the present invention to provide an arrangement of the tubes within the cryopump housing that facilitates the fabrication and installation of the cryopanels.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention has two essential features. First, the pulse tubes and regenerators are located in a common plane in the center of the cryopump housing, and second, the cold (second stage) panel(s) are in planes that are pitched parallel to the plane with the tubes, (a line can be drawn on a cryopanel surface that is parallel to the line where the plane of the tubes intersects the inlet plane). This arrangement simplifies the construction of the cryopanels and enables the cryopanels to be mounted more easily.  
         [0010]     It is preferred that the cryopump housing be generally cylindrical with a horizontal centerline and an inlet on one end. The pulse tube valve assembly is either below the housing or mounted on the end plate opposite the inlet. The pulse tubes used to illustrate this invention have two separate pulse tube and regenerator assemblies, and operate with passive interphase control and a buffer volume. The hot ends of the pulse tubes are an integral part of the cryopump housing and the buffer volume is external to the housing.  
         [0011]     If the inlet to the cryopump is parallel to the pulse tubes then the first stage pulse tube may be in front of or behind the second stage pulse tube. It is preferred that the first stage is behind the second stage so that the cold panel shields the temperature gradients in both pulse tubes from freezing gases at intermediate temperatures where they can cause undesirable pressure transients when the gas load changes.  
         [0012]     The first stage panel, or shield, is generally cup shaped with a small gap between the panel and cryopump housing. It may have some cut outs to fit around the pulse tubes when being installed. It serves to shield the second stage panel from radiant heat and may also serve to transport heat from the inlet louver. It is connected to the first stage cold station by a thermal bus. When the first stage pulse tube is behind the second stage pulse tube the thermal bus is parallel to the second stage pulse tube. When the first stage pulse tube is in front of the second stage pulse tube the orientation of the thermal bus is optional. In this case the thermal bus also cools the inlet louvers.  
         [0013]     Inlet louvers are conventionally pitched at about 45° and are circular (truncated cones), but they may be straight, or transverse to the inlet in the form of a grid.  
         [0014]     In the preferred embodiment with the second stage pulse tube in front of the first stage pulse tube the second stage cryopanels can be a group of simple flat plates folded over with different pitches, and attached to, the cold station. They are oriented parallel to both pulse tubes. When the first stage is in front of the second stage the second stage cryopanel can consist of two separate but identical assemblies that each has a plate that attaches to the second stage cold station and extends along side the first stage pulse tube. Flat fins extend from the base plate to provide the cryopumping surface.  
         [0015]     An example is also given of a conventional two-stage pulse tube that has the valve assembly on top, with the pulse tubes and regenerators oriented hot ends up, cold ends down. The cryopump inlet is on the bottom. The design is unconventional in having all of the tubes in a common plane so the second stage cryopanel can consist of flat plates that are pitched parallel to the same centerline. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1A  is a cross section of a first embodiment of a cryopump with a two stage pulse tube that has the pulse tubes in a common plane and the second stage pulse tube between the inlet louver on one side and the first stage pulse tube on the other. The valve assembly is on the bottom.  
         [0017]      FIG. 1B  is a view of the cryopump of  FIG. 1A  from the front, or inlet side.  
         [0018]      FIG. 1C  is a view of the cryopump of  FIG. 1A  from the bottom, showing a cross section through the regenerators, cryopanels, and housing.  
         [0019]      FIG. 2  is a cross section of a second embodiment of a cryopump with a two stage pulse tube that has the pulse tubes in a common plane and the second stage pulse tube between the inlet louver on one side and the first stage pulse tube on the other. The valve assembly is on the backside, opposite the inlet.  
         [0020]      FIG. 3A  is a cross section of a third embodiment of a cryopump with a two stage pulse tube that has the pulse tubes in a common plane and the first stage pulse tube between the inlet louver on one side and the second stage pulse tube on the other. The valve assembly is on the bottom.  
         [0021]      FIG. 3B  is a view of the cryopump of  FIG. 3A  from the front, or inlet side, with the inlet louver removed.  
         [0022]      FIG. 3C  is a view of the cryopump of  FIG. 3A  from the bottom, showing a cross section through the regenerators, cryopanels, and housing.  
         [0023]      FIG. 4  is a cross section of a fourth embodiment of a cryopump with a two stage pulse tube that has the pulse tubes in a common plane and the first stage pulse tube between the inlet louver on one side and the second stage pulse tube on the other. The valve assembly is on the backside, opposite the inlet.  
         [0024]      FIG. 5  is a cross section of a fifth embodiment of a cryopump with a two-stage pulse tube that has the pulse tubes and regenerators in a common plane. The valve assembly is on the top, opposite the inlet, which is on the bottom. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 1A  is a cross section of a first embodiment of a cryopump, cryopump  200 , with cryopanels cooled by a two-stage pulse tube refrigerator. The pulse tubes are oriented vertically; pulse tube hot ends up, in line with their respective regenerators, and mounted in a common vertical plane that includes the horizontal axis of the housing. Cryopump  200  includes housing  210 , inlet flange  215 , first stage regenerator  160 , first stage cold station  115 , first stage pulse tube  165 , first stage hot station  117 , restrictor  145 , second stage regenerator  170 , second stage cold station  116 , second stage Pulse tube  175 , second stage hot station  119 , restrictor  150 , valve assembly  118 , gas inlet  110 , gas outlet  111 , cryopump inlet grid  245 , radiation shield  220 , thermal bus  215 , and second stage cryopanel  265 . The pressure in the two pulse tubes cycles 180° out of phase, with gas being exchanged at the hot ends through flow restrictors  145  and  150 , with buffer tank  180  in between. The hot ends of the pulse tubes are an integral part of the top of the cryopump housing and extend through the wall to facilitate the rejection of heat and the connection of control piping.  FIG. 1A  shows the preferred embodiment in which the second stage pulse tube is located between the inlet grid on one side and the first stage pulse tube on the other. The valve assembly is on the bottom.  
         [0026]      FIG. 1B  is a view of cryopump  200  from the front, or inlet side. Component callouts are the same as  FIG. 1A . The inlet grid  245  consists of a long ribbon, about 1.5 cm wide made of a material with a high thermal conductivity, such as Cu, which may be formed into the shape shown. The ribbon is mechanically and thermally connected to a strip of Cu in the middle, thermal bus  216 , and to shield  220  on the circumference. Grid  245  will freeze out most of the water vapor in the air entering the pump. The top of cryopanel  265 , which is attached to the second stage cold station  116 , is seen through the inlet grid. The second stage pulse tube assembly is directly behind cryopanel  265  and the first stage pulse tube assembly is behind that.  
         [0027]      FIG. 1C  is a cross section view of cryopump  200  from the bottom, showing the regenerators, cryopanels, and housing. This is the best view to illustrate the essential principles of this invention and the preferred embodiment. The second stage pulse tube assembly, as represented by cold station  116  and regenerator  170 , and first stage pulse tube assembly, as represented by cold station  115  and regenerator  160 , are in a common vertical plane that includes the axis of the housing. The second stage cryopanel  265  is a series of flat plates pitched at different angles that extend parallel to, and on the inlet side of, the second stage pulse tube. Shield  220  is split into two halves, each half being attached to cold station  115  by thermal bus  215 , which extends from one side of the shield to the other. Grid  245  and thermal bus  216  are attached at the inlet end of shield  220 .  
         [0028]      FIG. 2  shows a second embodiment of a cryopump, cryopump  300 , with a two stage pulse tube that has the pulse tubes in a common plane and the second stage pulse tube between the inlet louver on one side and the first stage pulse tube on the other. The valve assembly is on the backside, opposite the inlet. The component designations are the same as  FIG. 1A . The cryopanel arrangements are the same as  FIGS. 1B and 1C . Cryopump  300  differs from cryopump  200  in that regenerator  160  and regenerator  170  are parallel to the centerline of the cryopump housing  210 , perpendicular to the inlet grid  245 , and are mounted at the warm end to valve assembly  118 . This arrangement minimizes the top to bottom height of the cryopump.  
         [0029]      FIG. 3A  is a cross section of a third embodiment of a cryopump, cryopump  400 , with cryopanels cooled by a two-stage pulse tube refrigerator. The pulse tubes are in line with their respective regenerators, and mounted in a common plane that includes the horizontal axis of the housing. Cryopump  400  differs from cryopump  200  in that the location of the pulse tube assemblies is interchanged. This embodiment has the first stage pulse tube located between the inlet grid on one side and the second stage pulse tube on the other. Cryopump  400  includes housing  210 , inlet flange  215 , first stage regenerator  160 , first stage cold station  115 , first stage pulse tube  165 , first stage hot station  117 , restrictor  145 , second stage regenerator  170 , second stage cold station  116 , second stage pulse tube  175 , second stage hot station  119 , restrictor  150 , valve assembly  118 , gas inlet  110 , gas outlet  111 , cryopump inlet louver  240 , radiation shield  220 , thermal bus  216 , and second stage cryopanel  267 . The pressure in the two pulse tubes cycles 180° out of phase with gas being exchanged at the hot ends through flow restrictors  145  and  150 , with buffer tank  180  in between. The hot ends of the pulse tubes are an integral part of the top of the cryopump housing and extend through the wall to facilitate the rejection of heat and the connection of control piping. The valve assembly is on the bottom.  
         [0030]      FIG. 3B  is a view of cryopump  400  from the front, or inlet side, with inlet louver  240  and thermal bus  216  removed. The component callouts are the same as  FIG. 3A .  
         [0031]      FIG. 3C  is a view of cryopump  400  from the bottom, showing a cross section through the regenerators, cryopanels, and housing. This view illustrates the essential principals of this invention. The second stage pulse tube assembly, as represented by cold station  116  and regenerator  170 , is in line with the first stage pulse tube assembly, as represented by cold station  115  and regenerator  160 , and the second stage cryopanel  265  is a series of flat plates pitched at different angles that extend parallel to the second stage pulse tube. Shield  220  is slotted at both sides so it can be fitted over the pulse tubes and regenerators when it is installed from the inlet side. Another panel, that is not shown, might be installed to cover the slot. Thermal bus  216  extends from one side of shield  220  to the other. It is attached to cold station  115 , shield  220 , and louver  240 . A second shield  222 , which is also cooled by cold station  115 , extends over the cold sections of pulse tube  165  and regenerator  160  to prevent gases from freezing out at intermediate temperatures. Cold stations  115  and  116 , cryopanel  267 , shields  220  and  222 , thermal bus  216 , and louver  240 , are all made of a metal with high thermal conductivity, such as Cu.  
         [0032]     Cryopanel  267  consists of two halves that are attached to either side of second stage cold station  116 . The individual louvers of louver  240  are shown as being tapered. Looking at Louver  240  straight on would show the louvers to be overlapped in the center, and to have gaps of increasing width as the outer edge is approached. This provides essentially the same gas flow pattern as the typical louvers that are presently being used, which are quite open in the outer region. Straight louvers of constant width and circular louvers can also be used.  
         [0033]      FIG. 4  is a cross section of cryopump  500  which is a fourth embodiment of a cryopump with a two stage pulse tube that has the pulse tubes in a common plane and the first stage pulse tube between the inlet louver on one side and the second stage pulse tube on the other. The valve assembly is on the backside, opposite the inlet. The component designations are the same as  FIG. 3A . The cryopanel arrangements are the same as  FIGS. 3B and 3C . Cryopump  500  differs from cryopump  400  in that regenerator  160  and regenerator  170  are parallel to the centerline of the cryopump housing  210 , perpendicular to the inlet louver  240 , and are mounted at the warm end to valve assembly  118 . This arrangement minimizes the top to bottom height of the cryopump.  
         [0034]     The most common configuration of a two stage GM type pulse tube is a warm end base with the valve assembly mounted above it and the pulse tubes and regenerators mounted below it. The cold ends are at the bottom, and the hot ends are connected to the base and valve assembly.  FIG. 5  shows cryopump  600 , which incorporates the basic features of a conventional two stage GM type pulse tube, but the pulse tubes and regenerators are in a common plane, in accordance with the present invention. The second stage cryopanels are flat, pitched, surfaces that are essentially the same as those shown in  FIGS. 3B and 3C  but the orientation is parallel to the plane of the pulse tubes and regenerators rather than parallel to the second stage pulse tube. The inlet to cryopump  600  is on the bottom. The configuration shown in  FIG. 5  has the first stage cold station  115  between inlet louver  240  and cold station  116 , thus the similarity of the cryopanels to cryopump  400 . The component designations are the same as cryopump  400 .  
         [0035]     It is equally possible to have cold station  116  between inlet louver  240  and cold station  115 . This would result in a cryopanel geometry that is essentially the same as shown in  FIGS. 1A and 1B . The use of an inlet louver or a grid is a designers&#39; choice.  
         [0036]     It is understood that it is within the scope of this description to allow for the pulse tubes and regenerators to be generally in a plane and for the cryopanels to be generally flat.