Amorphous silicon image sensor for x-ray applications

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Abstract
An amorphous silicon photodiode array was fabricated using a thin-film PECVD process. The 256 by 256 array contains 65,536 pixels on 200 micrometers centers and measures 5 cm on a side. A pixel configuration containing switching diodes instead of the more common TFT switch approach is used to address pixels. The resulting optical fill factor for each pixel is 66%. This device geometry can be scaled both to smaller pixels and large image sensor panels. Custom support electronics mounted on the sensor substrate using a chip-on-glass technique provide the row addressing and pixel charge readout. A scintillator film attaches to the array to convert x-rays to visible light. The basic sensor performance is evaluated under visible light without the scintillator. The device exhibits a dynamic range of 60 - 80 dB, low image lag and good uniformity. The low dark current allows integration times of up to one minute at room temperature without saturating the device. With the scintillator screen attached the device MTF is measured under x-ray illumination. Large changes in MTF are observed for two different types of commercial scintillator.

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