Abstract:
A bolometer type focal plane is made up of a plurality of silicon sensors. Within each sensor, interconnection between co-planar stages is provided by elongated “I” beam type bridge members having a generally rectangular cross-section including unequal wider (height) and narrower (width) dimensions, and wherein the bridge members are oriented such that the narrower width dimension is in the direction of the common plane and the wider height dimension is perpendicular to the common plane. A sensor with these bridges accommodates stress/strain by rotation while preventing out-of-plane deflection and deformation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to Non-provisional application Ser. No. 11/239,275, (Northrop Grumman Ref. No. 000775-078) entitled “Focal Plane Antenna to Sensor Interface For An Ultra-Sensitive Silicon Sensor” filed on Sep. 30, 2005; Non-provisional application Ser. No. 11/239,297, (Northrop Grumman Ref.: No. 000776-078), entitled “Ultra-Sensitive Silicon Sensor Readout Circuitry” filed on Sep. 30, 2005; and Non-provisional application Ser. No. 11/240,772 (Northrop Grumman Ref.: No. 000800-078), entitled “Sensitive Silicon Sensor and Test Structure for an Ultra-Sensitive Silicon Sensor”, filed on Oct. 3, 2005.  
       CROSS REFERENCE TO RELATED ART  
       [0002]     This application is also related to U.S. Pat. No. 6,489,615 entitled “Ultra-Sensitive Silicon Sensor”, granted to Nathan Bluzer, one of the present inventors, on Dec. 3, 2002, and assigned to the assignee of this invention. U.S. Pat. No. 6,489,615 is intended to be incorporated herein by reference for any and all purposes. 
     
    
     FIELD OF THE INVENTION  
       [0003]     This invention relates generally to a bolometer type focal plane having radiation sensors for detecting thermal radiation and more particularly to the interconnecting bridge members in an ultra-sensitive silicon sensor having stages interconnected in a common plane for improving sensitivity.  
       DESCRIPTION OF RELATED ART  
       [0004]     Bolometers are well known in the art and comprise devices which generate a voltage output when thermal radiation is absorbed. These devices, moreover, have been successfully used for infra-red (IR) imaging in the long wave infra-red (LWIR) band of the electromagnetic spectrum. Extending these devices to other spectral bands has proven relatively difficult in the past. However, efforts are currently under way to extend this capability to the millimeter wave (mm) and terahertz (THz) spectral bands and thus there is a need for imagers operating in the mm and THz spectral bands. Applications for such devices include, for example, multi-spectral imaging for improved navigation, target recognition and detection as well as homeland defense applications. Such applications would all greatly benefit from the use of bolometers. Therefore, realizing bolometers with acceptable performance with mm-THZ-LWIR cameras requires the formulation of new approaches for overcoming conventional limitations such as the requirement for faster response time and improved sensitivity.  
         [0005]     In U.S. Pat. No. 6,489,615, there is disclosed, inter alia, the structure of a three tiered silicon sensor including a detector stage, an intermediate stage and a heat bath stage with the intermediate stage being located between the detector stage and the heat bath stage. The intermediate stage is also part of an electro-thermal feedback loop including an amplifier which generates heat proportional to the temperature difference between the detected temperatures provided by a pair of back-to-back temperature sensing silicon diodes respectively located in the intermediate stage and detector stage. The heat provided by the amplifier acts to actively zero the temperature difference between the detector stage and the intermediate stage so as to eliminate any net heat flow between the detector stage and the intermediate stage.  
         [0006]     In related application Ser. No. 11/239,275 (Northrop Grumman Ref. No. 000775-078) entitled, “Focal Plane Antenna To Sensor Interface For An Ultra-Sensitive Silicon Sensor”, there is disclosed both a three tiered semiconductor and a two tiered semiconductor sensor structure including three temperature stages, namely a detector stage, an intermediate stage, and a heat bath stage. In the two tiered silicon sensor, the detector stage and the intermediate stage are mutually coplanar with the upper section of the heat bath stage.  
       SUMMARY  
       [0007]     It is an object of the present invention to provide an improvement in a bolometer type focal plane including a plurality of sensors, each including a detector stage, an intermediate stage and a heat bath stage. The detector stage, the intermediate stage and portion of the heat bath stage comprise stages which are generally co-planar and are interconnected so as to permit mutual co-planar rotation while preventing out of plane deflection and deformation. Interconnection between the three sensor stages is provided by elongated “I” beam type bridge members having a generally rectangular cross section including unequal relatively wider height and relatively narrower width dimensions, and wherein the bridge members are oriented such that the relatively narrower width dimension is in the direction of the common plane of the co-planar stages while the relatively wider height dimension is perpendicular thereto.  
         [0008]     Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific example, while indicating the preferred embodiment of the invention, is provided by way of illustration only. Accordingly, various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the following detailed description of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The present invention will become more fully understood from the detailed description provided hereinafter and the accompanying drawings which are provided by way of illustration, and thus are not meant to be considered in a limiting sense, and wherein:  
         [0010]      FIG. 1  is a cross section of a three tiered semiconductor ultra-sensitive silicon sensor in accordance with related art;  
         [0011]      FIG. 2  is a cross section of a two tiered semiconductor ultra-sensitive silicon sensor in accordance with the related art;  
         [0012]      FIG. 3  is a top plan view generally illustrative of a two tiered semiconductor ultra-sensitive silicon sensor (with the microantenna left out for clarity) where the detector stage, intermediate stage, and heat bath stage are interconnected by bridge elements in accordance with the subject invention; and  
         [0013]      FIG. 4  is illustrative of the prior art and desired orientation of the interconnecting bridge elements shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     Insufficient thermal isolation in conventional bolometer type sensors is generally known and presents an ongoing problem. In order to overcome the limitation of passive thermal isolation between stages, an active thermal isolation scheme was developed by the present assignee as shown and taught, for example, in the above referenced U.S. Pat. No. 6,489,615. Overcoming these and other limitations associated with the known prior art would also result in a further improvement in responsivity. For example, increased detector responsivity would provide a large improvement in sensitivity. Also, the impact of electronic readout noise would also be reduced.  
         [0015]     Referring now to the subject invention and more particularly to  FIG. 1 , shown thereat is an embodiment of an ultra-sensitive silicon sensor for a bolometer type focal plane including active thermal isolation as disclosed in the above-referenced related U.S. Pat. No. 6,489,615. In  FIG. 1 , reference numeral  10   1  denotes a three tiered semiconductor “ultra-sensitive” bolometer pixel receiving thermal radiation directly or by means of an antenna  12  and including three major stages, a detector stage  14 , an intermediate stage  16  and a heat bath stage  18 . Two temperature sensors  20  and  22  are respectively located in the detector stage  14  and the intermediate stage  16  and comprise semiconductor diodes connected in back-to-back relationship to an amplifier  24 , also located in the intermediate stage  16 . The amplifier  24  generates heat in an electro-thermal feedback loop to zero the difference in temperature between the temperature T D  of the sensor  20  in detector stage  14  and the temperature T IN  of the sensor  22  in the intermediate stage  16  and is achieved by means of the bridge members G 1A  and G 1B  linking the detector stage  14  and the intermediate stage  16 . Thus when the detector stage temperature T D  changes, electro-thermal feedback causes the intermediate stage temperature T IN  to change by the same amount. The back-to-back connection of the temperature sensors  20  and  22  produces a positive (negative) voltage signal if the detector stage  14  is at a higher (lower) temperature than the intermediate stage  16  and the temperature difference signal is amplified by the amplifier which generates heat. The thermal conductivity of these links are reduced proportionally to the reduction in the temperature difference between the detector stage  14  and the intermediate stage  16 .  
         [0016]     The intermediate stage  16  is also shown coupled to the heat bath stage via a pair of bridge members G 2A  and G 2B . Thus, the combination of the adjustable heat power with constant cooling provided by the heat bath stage  18  via the bridge members G 2A  and G 2B  provides for bi-polar temperature tracking of the detector stage  14  by the intermediate stage  16 .  
         [0017]     The implementation of a sensor shown in  FIG. 1  consisting of a three tiered semiconductor device exhibits certain fabrication problems, since each one of the three tiered semiconductor sensor stages  14 ,  16  and  18 , require the use of special wafer bonding techniques. This problem can be alleviated by resorting to a simplified two tier sensor structure  10   2  shown in  FIG. 2  where one tier includes the active components, namely, the detector stage  14  and the intermediate stage  16  and an upper section  17  of the heat bath stage  18  arranged in a common plane as shown, for example, in  FIG. 3 . There the solid circular detector stage  14  is surrounded by an annular intermediate stage  16 . The detector stage  14  and the intermediate stage  16  are located above a planar lower section  19  of the heat bath stage  18 . The upper section  17  of the heat bath stage  18  includes a generally circular cavity  28  in which is located the co-planar detector stage  14  and the intermediate stage  16 . The top flat surface  21  of the heat bath section  17  is used for the placement of a generally annular antenna  12 , which consists of a passive element and is readily integratable with the active sensor stages  14  and  16 . When desirable, the antenna can also be placed on the intermediate stage  16 .  
         [0018]     The three stages  14 ,  16  and  18  of the sensor  10   2  are typically fabricated in silicon and are interconnected by connecting bridge members made from sandwiched layers of oxide and nichrome. Given the fabrication temperature and the different thermal expansion coefficients of these materials, provisions must be made to accommodate these differences.  
         [0019]     This now leads to a consideration of  FIGS. 3 and 4  which are directed to the preferred embodiment of the invention which comprises the interconnect bridge members for the three stages  14 ,  16  and  18  of a sensor  10   2  shown in  FIG. 2  included in a bolometer type focal plane. As shown in  FIG. 3 , two sets of elongated curvilinear bridges are utilized. The first set includes two interconnecting bridges G 1A  and G 1B  for connecting the detector stage  14  with the intermediate stage  16  while the second set includes four interconnecting bridges G 2A , G 2B , G 2C  and G 2D  connecting the intermediate stage  16  with the heat bath stage  18 .  
         [0020]     The detector stage  14  in a typical embodiment of the sensor  10   2  as shown in  FIG. 3  is about 8 μm in diameter and the annular intermediate stage  16  is approximately 10 μm wide. The two gaps between these stages are typically between 3 and 4 μm wide with the interconnecting bridges G 1A , G 1B  and G 2A , G 2B , G 2C  and G 2D  being respectively located in the gaps identified by reference numerals  32  and  30 .  
         [0021]     Accommodating thermal induced stress in the interconnecting bridges G 1A , G 1B  and G 2A , G 2B , G 2C  and G 2D  is required to prevent physical distortion of the sensor structure shown in  FIG. 2 . Strain or stress induced distortion will normally cause out of plane deformation or canting of the detector stage  14  and/or intermediate stage  16  relative to the microantenna  12 , thereby reducing the signal sensed from an external scene, not shown. Also, distortion can also cause the antenna to come in mechanical contact with stages from which it is supposed to be thermally isolated.  
         [0022]     Thus the detector stage  14 , the intermediate stage  16 , and the heat bath stage  18  as shown in  FIG. 2  need to be interconnected by bridge elements that accommodate strain or stress without canting or out of plane distortion. What is desired is to accommodate stress or strain in the interconnecting bridges is by in-plane rotation as opposed to out-of-plane canting. Small in-plane rotation less than about 2 μm, for example, would maintain proper alignment between the detector stage  14 , and/or the intermediate stage  16  and the microantenna  12 .  
         [0023]     With respect to the two sets of elongated curvilinear interconnects G 1A , G 1B  and G 2A  . . . G 2D  of the subject invention, they have a rectangular cross section measuring about 2 μm high and 0.2 μm wide overlaid by a thin Nicrome layer of about 0.03 μm thick.  
         [0024]     Furthermore, as shown in  FIGS. 4A and 4B , all of the bridges G 1A , G 1B  and G 2A  . . . G 2D  have the same rectangular cross section with the same moments of inertia. The moment of inertia about the axis I Y1  is parallel to the wider dimension  34  as shown in  FIG. 4A , and one moment of inertia about the axis I Y2  is perpendicular to the wider dimension  34 , as shown in  FIG. 4B . For a given area, the moment of inertia about I Y1  is much less than that about I Y2 , since the moment of inertia varies with the distance squared from the center of mass. Specifically, for a 10:1 ratio between the wider and narrower dimensions, the ratio I Y2 /I Y1 =100. Accordingly, the “I” beam bridge shown in  FIG. 4B  will be much stiffer than that shown in  FIG. 4A  against vertical bending. Conversely, the “I” beam bridge shown in  FIG. 4A  will be much stiffer than the bridge shown in  FIG. 4B  against lateral or horizontal bending.  
         [0025]     Heretofore, bridges as shown in  FIG. 4A  have been utilized. In such a configuration, any residual strain or stress in the bridge arms will be accommodated by a vertical out of plane bending since stiffness is least in the out-of-plane direction, i.e., perpendicular to the axis I Y1 .  
         [0026]     By rotating the bridge by 90 degrees as shown in  FIG. 4B , it will stiffen the bridge to vertical or out-of-plane (perpendicular to I Y2 ) direction by 100 times relative to the configuration shown in  FIG. 4A . Utilizing such bridge elements stiffens the structures shown in  FIGS. 2 and 3  to out-of-plane movements. Thus, any contraction (stress) or expansion (strain) in the bridge lengths will be accommodated by in-plane length changes manifesting themselves by relative rotation between the detected stage  14 , the intermediate stage  16 , and the heat bath stages  18  and  26  shown in  FIG. 3 . Rotation does not produce out-of-plane distortion thereby maintaining electrical coupling and mechanical isolation between the antenna and detector stage, and intermediate stage (if the microantenna is not placed there). Providing means for alleviating stress and/or strain by rotation (and not by out-of-plane distortion) is very important since fabrication of these small structures and temperature changes always include stress and/or strain. Controlling stress/strain by fabrication techniques is very difficult and it is much more practical to provide means for mechanical relief by rotation.  
         [0027]     Accordingly, the “I” beam approach shown in  FIG. 4B  for the interconnecting bridges between the detector, intermediate and heat bath stages  14 ,  16  and  18  provides one with the flexibility of optimizing the structural features of the silicon sensor  10   2  without worry of vertical out-of-plane distortion due to mechanical stresses or strains in the film because the geometry of the bridges G 1A , G 1B  and G 2A  . . . G 2D  is used to stiffen the bridge elements in the vertical direction while accommodating the stress and strain by in-plane rotation.  
         [0028]     This same type of bridge member structure can be used in connection with MEMS devices.  
         [0029]     Having thus described the preferred embodiment of the invention, any variations therefrom are not to be regarded as a departure from the spirit and scope of the invention nor for applications including other sensors where MEMS like structures are required that are tolerant to strain and stress. Thus all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.