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TDI image sensors for advanced reconnaissance cameras preferably have pixels that are both small in size and high in performance. The high performance refers especially to primary responsivity (quantum efficiency convolved over the optical bandwidth of the camera), charge handling capacity and MTF, but other properties such as dark current, readout speed and noise are also very important. These image sensors are typically assembled into focal planes having 6000-15,000 pixels in the electronic scan direction. This paper describes one such array and reports on its performance in strobe-light bench tests. The pixel size is 15um square. In order to store a maximum amount of charge in such a small pixel, we have used 4-phase CCD registers and implanted channel stops to achieve a 40% charge storage area as has been done before in an array with larger pixels. The gate dielectric structure has an equivalent oxide thickness of only 600A providing a pixel charge storage capacity of 1.5X106 el with 10V clocks. (Whereas a television sensor can give a high quality signal for commercial television from pixels that saturate near 1.5X105 el, reconnaissance cameras often require significantly higher capacity.) In order to achieve high responsivity, the optimum thickness of polysilicon, silicon nitride and silicon dioxide were determined by optical modeling for a 5500K blackbody illumination spectrum and for the spectral range of wavelengths greater than 500nm. Then, using a wafer-stepper with very high alignment accuracy, it was possible to achieve a CCD gate structure with overlaps of the order of 0.5um so that 85% of the pixel area has only one layer of polysilicon. With proper control of the thickness and quality of each layer, it was possible to achieve quantum efficiencies that averaged greater than 50% from 500 to 900nm. This high responsivity is very close to the predicted value of the model; this value was reduced from that of older designs because of the new constraint on dielectric thickness. High MTF in the TDI direction was achieved even though most of the signal charge in this device is stored at the silicon-silicon dioxide surface. The Nyquist MTF for wavelengths 4600nm is greater than 57%. This high value is attributed to the good charge transfer efficiency that is possible with a buried channel structure even when charge is stored at the surface.
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