This invention relates to solid state focal planes which form an image of a scene by detecting the light emanating from the scene.
Advanced imaging systems have recently been the subject of considerable research and development, particularly in the field of infrared detection. These systems employ a focal plane, which is an integrated device incorporating a large array of light-sensitive detectors and appropriate electronic components to process the signals generated by the detectors. Two basic approaches, monolithic and hybrid, have been followed in designing infrared focal planes. A monolithic focal plane is fabricated with the multiplexer forming an integral part of the detector structure, while the photodetector array and the signal multiplexer of a hybrid focal plane are manufactured as separate components, then joined together using an advanced interconnection technology (See, e.g., U.S. Pat. No. 4,067,104).
Whether the monolithic or hybrid configuration is chosen for a particular application, the focal plane must accomplish the complementary functions of photon detection, including prefiltering of the optical signal, and signal multiplexing. Photon detection can be achieved in either intrinsic or extrinsic semiconductors, by using either photovoltaic, photoconductive, or MIS (metal-insulator-semiconductor) detectors. Detection can also be implemented through internal photoemission over a Schottky barrier or with pyroelectrics. The detectors can be either frontside or backside illuminated and, for hybrid arrays, where the sensed charge from the detectors is coupled into the CCD multiplexer for readout, the interface can be either source coupled or gate coupled. Multiplexing can be achieved by x-y addressed MOSFETs, a charge transfer device, or charge injection.
Some of the early focal plane designs utilized a linear array of detectors which was physically scanned over a scene to obtain a two dimensional image. The advantages of a two dimensional staring focal plane, however, have been well established and include a long integration time, leading to a favorable mean resolution temperature, and the elimination of the increased complexity and reduced reliability introduced by a scanning mechanism. The maximum number of pixels which can be included in a staring focal plane, however, has been limited in the past by the number of detectors which may be included on the focal plane. The number of possible detectors has, in turn, been relatively low, approximately 10.sup.3 -10.sup.4 as compar scene which contains approximately 2.times.10.sup.5 pixels. In a hybrid focal plane, this pixel limitation results from a combination of constraints. First, neither a silicon multiplexer input cell nor a hybrid interconnect can be reduced in size below approximately 50.times.50 .mu.m.sup.2. Although further research may ultimately achieve sizes as small as 25.times.25 .mu.m.sup.2 for these elements of the design, the detectors themselves can be made even smaller. Second, the overall size of a hybrid array is limited both by the yield of the silicon multiplexer chip and the differential thermal expansion between silicon and the detector material (InAsSb or HgCdTe, for example, in the case of an infrared detector) to approximately 1.times.1 cm.sup.2.
Because of these constraints on multiplexer cell size, interconnect size, and overall chip size, an array of detectors can be made with a center-to-center spacing much smaller than the spacing of the highest density focal plane which is available. Consequently, a need has developed for a multiplexing scheme which would enable several detectors to be connected to each input cell of the multiplexer. If, for example, the focal plane unit cell were 50.times.50 .mu.m.sup.2 and each detector 25.times.25 .mu.m.sup.2, the unit cells could be multiplexed to receive input for 25% of the viewing cycle from each of four detectors, thereby increasing the number of pixels for the imaging system by a factor of four.