High resolution, high fill factor a-Si:H sensor arrays for medical imaging

We describe new amorphous silicon (a-Si:H) image sensor arrays which are the highest resolution imagers so far reported. The pixel sizes of 64 mu m (resolution 8 lp/mm) and 75 mu m (6.7 lp/mm) are made possible using a photodiode technology that enables high sensor fill factor even in very small pixels. This approach allows the a-Si:H imagers to satisfy high resolution requirements of digital mammography. Each array contains 512x512 pixels with matrix addressing provided by a-Si:H thin film transistors (TFT). The high fill factor structure contains a continuous a-Si:H photodiode layer grown on top of the TFT array, with contacts to each pixel through a patterned metal/n(+) layer. X-ray detection is accomplished by use of a phosphor layer superimposed on the array. The continuous photodiode layer maximizes light absorption from the phosphor and provides high x-ray conversion efficiency. Since the photodiode forms a continuous layer, crosstalk between adjacent pixels due to the lack of isolation is a particular concern, and has been extensively studied. We find that the high fill factor structure can be made such that the lateral charge leakage is minimal in the dark or under moderate illumination, although small amount of charge spreading is observed under conditions of sensor saturation. The measured MTF for optical illumination exceeds 60% at the Nyquist frequency, even for long integration times.