Abstract:
Faster cool down time is achieved in a low profile cryopump by having heat transferred directly from the inlet louver to the first stage heat station through one or more tapered thermal busses, and by obviating the need of a thermal shield over the second stage cylinder of the expander by having second stage cryopanels that form a nested tent like structure, at least one of which, extends over the cylinder.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to cryopump used in limited space applications. 
     2. Discussion of the Related Art 
     Cryopumps are typically made with the inlet in a plane perpendicular to the axis of the expander cylinder, “in line”, or parallel to the axis of the cylinder, “low-profile”. Low profile cryopumps are preferred over in line cryopumps in applications where space is limited and are usually mounted under or on the side of the vacuum chamber. 
     Two stage G-M (“Gifford-Mchon”) refrigerators are used to cool cryopumps. These cool a first stage cryopanel at 50 to 100 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 warm end of the first stage, a first stage cold station (at 50 to 100 K) at the transition from the larger diameter first stage to the smaller diameter second stage, and a second stage cold station (at about 15 K) at the far end. An example of an expander that is used in cryopumps as described in this application is found in U.S. Pat. No. 6,256,997. 
     U.S. Pat. No. 4,150,549 describes a typical in line 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 CO 2 , and blocks a significant amount of radiation from reaching the second stage (cold) panel but allows Group II gases, e.g. Ar and N 2 , and Group III gases, e.g. H 2  and He, to pass through it. The Group II gases freeze on the front side of a cup shaped cold panel and Group III gases are adsorbed in an adsorbent on the backside of the cold panel. U.S. Pat. No. 4,530,213 describes a cold panel design that consists of a series of concentric rings of increasing diameter from the inlet region to the back of the housing. This design has more room for large amounts of Ar to collect than the cup design, as is the case with sputtering, and there is more surface area on which the Ar is distributed. 
     U.S. Pat. No. 4,530,213 shows an arrangement that is typical for in line cryopumps of having heat from the inlet louver, typically consisting of segments of circular cones, transported radially to the inlet end of the warm panel then conducted to the first stage heat station through the warm panel. A similar arrangement is shown for a low profile cryopump in U.S. Pat. No. 5,974,809. U.S. Pat. No. 6,155,059 shows an in line cryopump with straight inlet louvers and two straight bars, thermal busses, that carry heat to opposite sides of the inlet end of the warm panel. 
     U.S. Pat. No. 4,691,534 describes straight louvers that are individually attached to the warm panel at the inlet end. U.S. Pat. No. 5,542,257 shows in  FIGS. 1 and 2  a low profile cryopump that has straight inlet louvers and a single cross bar that transfers heat radially to opposite sides of the warm panel inlet. 
     U.S. Pat. No. 5,056,319 describes a low profile cryopump with a vibration isolation mechanism. The drawings show straight inlet louvers with a cross rod taking heat to the warm panel but the physical description is lacking. 
     U.S. Pat. No. 4,356,701 describes an in line cryopump that has conventional conical inlet louvers with radial bars to conduct heat but these bars are not connected to the inlet end of the warm panel, rather they are connected to rods that conduct heat through the second stage cryopanels to the bottom of the warm panel. 
     U.S. Pat. No. 5,156,007 and U.S. Pat. No. 5,974,809 show a shield that has to be added over the second stage cylinder to avoid having Ar or N 2  freeze at some temperature above the cryopanel temperature. The phenomena of “Ar hang up” that results from Ar freezing on the second stage cylinder is described in U.S. Pat. No. 5,156,007. 
     The cryopanels for in line cryopumps are typically axi-symetric around the expander cylinder. This panel design is frequently adapted to low profile cryopumps by having cutouts in the cold panel for the expander cylinder, e.g. U.S. Pat. No. 5,156,007. This invention improves upon previous designs by having second stage cryopanels that are comprised of flat surfaces that form a nested tent like structure that is attached to the second stage heat station and extends over the second stage cylinder, between the cylinder and inlet, to shield the cylinder from having Group II gases freeze on it. The second stage heat station does not have to be in the middle of the housing because the second stage cryopanels, which consist of folded flat sheets of copper, can be attached any place along their length. 
     SUMMARY OF THE INVENTION 
     The present invention applies to low profile cryopumps having two-stage G-M type refrigerators in which the inlet port to the vacuum chamber is in a plane that is parallel to the axis of the expander cylinder. It generally applies to cryopumps having inlet ports in the size range from 150 mm to 600 mm. The invention has two essential features. First, the inlet louver has straight strips that are oriented transverse to the axis of the expander cylinder and are attached to one or more thermal busses that are attached directly to the first stage heat station. The thermal busses are tapered so the temperature gradient is fairly uniform. Second, the cold (second stage) cryopanel(s) are in planes that are pitched parallel to the axis of the expander cylinder, (a line can be drawn on a cryopanel surface that is parallel to the axis of the expander cylinder) and extend as a shield over most of the second stage expander cylinder. These features reduce the weight of the material to be cooled, resulting in faster cool down times, and simplify the construction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section of a side view of a cryopump showing the main features of the present invention for an inlet diameter of 300 mm. The expander drive mechanism is not shown in  FIG. 1  but can be seen in U.S. Pat. No. 6,256,997. 
         FIG. 2  is a top view of  FIG. 1  showing the inlet louver of the cryopump but not the cold panel assembly. 
         FIG. 3  is a cross sectional view along plane A-A of  FIG. 1 . 
         FIG. 4  is a top view of  FIG. 1  showing the second stage cryopanels but not the inlet louver and thermal bus. 
         FIG. 5  has plots of the temperature pattern in a straight thermal bus and a tapered thermal bus for the cryopump shown in  FIGS. 1-4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The side view cross section of cryopump assembly  7  shown in  FIG. 1  shows the main components including expander cylinder assembly  10 , vacuum housing assembly  20 , first stage cryopanel assembly  30 , and second stage cryopanel assembly  40 . Expander cylinder assembly  10  consists of warm flange  11 , first stage cylinder  12 , first stage heat station  13 , second stage cylinder  14  having a cold end  14   a , and second stage heat station  15 . Vacuum housing assembly  20  consists of inlet mounting flange  21 , cryopanel housing  22 , expander cylinder housing  23 , expander mounting flange  24 , and vent/port  27 . Not shown are mounting ports on cylinder housing  23  that are generally standard for cryopumps to mount a pressure gauge, temperature sensors, purge gas input, and possibly heaters. The first stage cryopanel assembly  30  consists of radiation shield  31  (frequently referred to as the warm panel), inlet louver  32 , thermal bus  35 , louver bracket  36 , and bracket  37 . The second stage cryopanel assembly  40  (cold panel) consists of cryopanels  41 ,  42 ,  43 , etc. which are shown in  FIG. 3 . All of the materials used in the construction of first stage cryopanel assembly  30  and second stage cryopanel assembly  40  have a high thermal conductivity, typically ETP or OFHC copper. 
       FIG. 2  is a top view showing inlet louver  32  which consists of a series of essentially flat strips that are attached to thermal buses  35 . Two busses are a preferred embodiment but one bus may suffice for a 150 mm cryopump. The near end of the thermal busses attached to louver bracket  36  which in turn is attached to first stage heat station  13 . The far ends of thermal busses  35  may be attached to brackets  37  which in turn are attached to warm panel  31 . These provide structural support and do not transfer a significant amount of heat. 
       FIG. 3  shows the flat and folded nature of second stage cryopanels  41 ,  42 ,  43 , etc. Second stage heat station  15  has a flat on one side to provide a large surface for attaching second stage cryopanel assembly  40 . Inlet louver  32  runs straight across the pump inlet port in line with second stage cryopanel assembly  40 . It generally shields the central part of assembly  40  from radiation. The design helps to distribute the Ar so it freezes uniformly on the surfaces of the second stage cryopanels. A lot of space is available for solid Ar to accumulate. The backsides of the second stage cryopanels are coated with charcoal to adsorb H 2 . 
       FIG. 4  shows second stage cryopanel assembly  40  looking into the inlet of the cryopump with the first stage louver  32  removed. Clearance is left between radiation shield  31  and cryopanels  41 ,  42 ,  43 , etc. so that H 2  can flow around the panels to get to the charcoal. This view also shows liquid dam  33  that prevents liquid from flowing out of the inlet when the pump is mounted vertically. First stage heat station  13  is curved so that liquid can flow around second stage cylinder  14  when the pump is oriented vertically. Radiation shield  31  is also mounted to heat station  13  so that liquid cannot flow through openings into the region between first stage cylinder  12  and cylinder housing  23  when the pump is mounted vertically. 
       FIG. 5  illustrates the difference in the temperature pattern of a tapered thermal bus,  FIG. 5   a , and a straight thermal bus,  FIG. 5   b . The comparison is made assuming the same maximum temperature difference between the warmest point on the louver and end of the thermal bus at the near end of the louver. The design is shown in  FIGS. 1 and 2  and is based on a heat load of 50 W on the louvers. The five louvers attached at the far end of the thermal bus are assumed to be thicker than the ones at the near end because a greater temperature rise from the thermal bus to the end of the louver is allowable at the colder end of the thermal bus. The copper in the louvers and the thermal bus behind the louvers are calculated to weigh 1,340 g for the tapered thermal bus and 1,710 g for the straight thermal bus, a difference of 370 g or a reduction in weight of more than 20%. 
     While the low-profile cryopump described in this invention is focused on a 300 mm ID size, the basic concepts of flat panels folded over the second stage cylinder of a low profile cryopump which also shield the cylinder, having the first stage heat station end at the cryopanel vacuum housing, and having heat from a louver conducted directly to the first stage heat station through a thermal bus, can be applied to other size housings but generally in the size range from 150 mm to 600 mm.