Abstract:
An improved detector dewar assembly comprised primarily of metal subassemblies, which is easily assembled, and leaves the included detector assembly more readily serviceable.

Description:
BACKGROUND OF THE INVENTION 
     The device of the present invention relates to thermal imaging systems, and, more particularly, to detector dewar assemblies used in such systems. 
     In most thermal imaging systems using semiconductor detection devices which must be cryogenically cooled, it is necessary to provide a housing (or &#34;dewar&#34;) for the detector device. The dewar must be evacuated and provide electrical connections between the detector device and the housing between the detector array and the housing exterior so that the device may be interfaced to other signal processing or imaging subassemblies. 
     Typically, these dewars include several concentric cylindrical tubes or housings. The inner tube is generally comprised of a long cylindrical glass bore which opens to the base of the dewar. This bore, often referred to as the &#34;coldwell&#34;, is multifunctional. First, the detector device or array is mounted at the top of the coldwell (the &#34;endwell&#34;). A &#34;cold finger&#34; or cryogenic cooling device is generally inserted into the bore of the coldwell to effect cooling of the detector array. Often the exterior surfaces of the coldwell are metalized and then etched or delineated in order to define conductive leads which run the length of the bore. 
     Past dewars have used a gold plated glass bore to provide a thermally reflective shield and reduce heat load. In addition, glass has been used because it has the advantage that it is a good electrical and thermal insulator, it is not ductile and once fixed in the system, alignment will remain true unless the bore breaks. However, the glass coldwell has the disadvantage that it is extremely fragile and requires more complex and expensive assembly techniques. 
     In prior art dewars, the base portion of the coldwell supports a flange to which a cap is attached. This cap is typically made of Kovar or some other metal which has appropriate structural and thermal properties and can maintain a hard vacuum. The top part of the cap directly above the detector array holds a small, transmissive window. 
     In one type of dewar, this flange comprises a ceramic material. The horizontal ceramic substrate receives wire connectors from the vertical glass coldwell (jumper wires). The ceramic conductor pattern (usually radial) connects through the vacuum wall to conventional pin connectors outside the vacuum portion of the dewar. Thus, the detector leads can be connected to external imaging or signal processing subassemblies. 
     In a second dewar design, a metal flange, bonded appropriately to the glass coldwell, serves primarily as a supporting member for the vacuum chamber cap and window. The base portion of the glass coldwell is thickened so that it may include buried conductive wires which surface above and below the flange providing an electrical exit from the vacuum chamber. Gold jumper wires are used to connect the axial leads on the coldwell to the detector array and to surfaced conductors on the coldwell above the flange. The conductors surfacing below the flange (outside the vacuum) may be connected to a conventional tape cable or other wiring device. 
     In either design a second larger flange is connected to the coldwell at its base. This flange supports an outer protective housing which encloses the vacuum chamber, as well as supporting the mounting flange for the dewar. The housing has openings for the end of the vacuum cap with transmissive window, and for electrical connectors. In either design, a tape cable or other wiring device connects from the vacuum exit conductors to standard electrical interconnects. 
     These prior designs have several disadvantages. First, to repair the detector array or other connections within the vacuum chamber the entire device must be disassembled; the outer housing removed, wire cable detached, and the vacuum cap removed. Second, since the coldwell is made of glass, the device is particularly susceptible to breakage during assembly, disassembly, and normal use. Third, the gold jumper wire connectors used at the array and flange or the buried lead connections are susceptible to breakage. Fourth, the volume of the vacuum chamber is usually a small percentage of that of the finished assembly, such that when impurities from the materials outgas into the vacuum chamber, the vacuum degrades at a high rate. Finally, because the design uses many parts and requires use of more complex assembly techniques, the cost per unit is usually high. 
     Accordingly, it is a primary object of the present invention to provide an improved detector dewar assembly. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention are achieved by providing a dewar vessel which may have a metal coldwell, a single vessel/vacuum chamber, internal tape cable(s) and hermetically sealed pin connector(s) to affect connection between the detector array and external signal processing devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects of the present invention are achieved in the illustrative embodiment as described with respect to the Figures in which: 
     FIGS. 1A and 1B show a top view and a cross-section through the device of the present invention, respectively; 
     FIG. 2 shows the tape cable apparatus of the present invention; and 
     FIG. 3 illustrates an electrical feed through subassembly of the apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1A and 1B, the apparatus of the present invention includes a metal coldwell 10 with endwell 12 which supports detector assembly 14; a vacuum vessel 15 having a base 22, flange/wall member 16, protective shield ring 27, lower cap 18 and upper cap 20 with transmissive window 24. A flexible tape cable 36 provides electrical connection between detector assembly 14 and external pin connector assembly 38. Connector holder assembly or plate 46 provides a strain free mounting configuration for pin connector 38. A conventional pinch-off tube 28 (used for evacuating the dewar) and &#34;getter&#34; 30 (which absorbs outgassed materials within the evacuated chamber) are mounted through flange/wall member 16 in a conventional manner. 
     Base 22 and other vacuum chamber members 10, 12, 16, 18, 20 and 24 described above, may be made of any material which is mechanically rigid, has appropriate thermal properties, can hold a vacuum and does not outgas impurities at an unacceptable rate, once evacuated. Typically, these members will be made of stainless steel, Kovar or some other metal which may be easily assembled by, for example, soldering, welding or brazing techniques. Similarly, shield ring 27 might be made of any mechanically durable and formable material, such as plastic or stainless steel. Upper cap 20 is attached to lower cap 18 by soldering. Upper cap 20 may be easily removed to provide direct access to detector assembly 14 as well as elements 12, 10 and 36. 
     Coldwell 10 may be made of any material which is easily assembled to base 22 and which has low thermal conductivity, and high mechanical strength. As an example, metal such as a cobalt/nickel/iron alloy might be used. Coldwell 10 wall thickness may be reduced to minimize thermal conductivity. In addition, the external surface of coldwell 10 may be gold plated to provide a thermally reflective surface to lower the overall heat load of the dewar. 
     The flexible tape cable 36 provides electrical connection between detector assembly 14 and external pin connector assembly 38. As shown in FIG. 2, the tape cable 36 of the present invention comprises a flexible nonconductive base 48 upon which conductive tracks 56 have been electrodeposited. A second flexible nonconductive sheath 52 covers tracks 56. The external surface of the cable 36 may be coated with gold to form a new outer surface 54. This provides a further reduction of the heat load of the apparatus. Base 48 and sheath 52 may be made of Kapton, mylar or any other suitable nonconductive material which will not outgas into the evacuated dewar. Tracks 56 might be made of copper, gold or some other suitable conductive material which may be electrodeposited to base 48. Such materials should not be glued because of impurities therein. 
     Referring to FIGS. 1A, 1B and 3, pin connector saver assembly 38 comprises feed through assembly 40, bias resistor board 42 and connector saver 44. Feed through assembly 40 is made of a ceramic or other insulating material capable of holding a hard vacuum. Assembly 40 is attached to ring 60 which is integral with lower cap 18. Feed through assembly 40 comprises multiple layers 68, 70, 72 of material which has been attached as shown in FIG. 3. Electrical connection is made through assembly 40 from pins 62 to pins 64 by a combination of vias 66 and conductor tracks 67 made through and across the surfaces of layers 68, 70 and 72 as shown. It is noted that only one representative pin connection of the several connector pins of connectors 44 and 45 is shown. 
     This configuration provides several advantages. First, it assures the vacuum integrity of the assembly by eliminating straight through pin connection. Second, if necessary, a cross-over assembly may be provided by appropriate arrangement of tracks 67 and vias 66 which allow reordering of the configuration of pin connectors 62 from the tape cable 36 to a desired pattern of pin connectors 64 on the feed through assembly 40. This allows mating with external device connectors via connector saver 44 in a predetermined pattern of electrical connections. 
     A bias resistor board 42 may be inserted between feed through assembly 40 and connector saver 44 to provide a tailored electrical resistance in each detecting element&#39;s circuit, together with grounds as required. 
     To prevent stress on the feed through assembly 40 which might result in breakage or vacuum leaks, a connector saver plate 46 is used to isolate stresses applied during attachment or detachment of any external connectors to connector saver 44. This bracket 46 includes a single shaped disk having recessed wells 80 and 81 whose location is matched to the location of feed through assemblies 40 on lower cap 18. Connector saver 44 is mounted into well 80 via fasteners 78. Bracket 46 is mounted via fasteners 74 to bosses, i.e., stand-offs, 76 which provide a mounting surface. Bosses 76 may be brazed to lower cap 18. The height of bosses 76 is such that when the bracket 46 is in position with connector saver 44, pins 64 mate to receptacles 63 in connector saver 44 without contact between connector saver 44 and feed through assembly 40. In addition, connector saver bracket 46 is designed to provide a shielding cylindrical external surface 47, which protects the connector assembly 38 and the top of the vessel 15 from damage. 
     It should also be noted that pinch-off tube 28 and getter 30 might be positioned elsewhere on vessel 15 to increase accessibility, for example on lower cap 18. In that embodiment, any necessary electrical connection of getter 30 might be made via electrical connection to the fasteners 78 which are a part of connector saver 44.