Direct-Conversion CMOS X-Ray Imager With $5.6 ~\mu \text{m} \times 6.25~\mu \text{m}$ Pixels

We report a monolithic direct-conversion X-ray imager capable of detecting diagnostic level X-rays. The imager is constructed by combining a custom 32×32 CMOS four-transistor active pixel sensor (4T APS) array with an amorphous selenium photoconductive layer deposited on top of the array via a post processing step. A 4T APS with an explicit per-pixel integration capacitor is employed to increase the pixel dynamic range. Under dark conditions, an input-referred electronic noise of <;90 electrons (rms) is estimated based on measured noise data for a 40-ms integration time. The very first X-ray images of copper and stainless-steel objects are included to demonstrate the performance of what is, to the best of our knowledge, a direct-conversion X-ray imager with the smallest pixel pitch reported to date.