Evaluation of a Direct Detection Selenium-CMOS 8×8 Passive Pixel Sensor Array for Digital X-Ray Imaging Applications
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Digital imaging systems for medical applications use amorphous silicon thin-film transistor (TFT) technology due to its ability to be manufactured over large areas, making it useful for X-ray imaging, which requires imagers to be the size of the subject, unlike optical imaging. TFT technology is used to make imaging arrays coated with an X-ray detector called amorphous selenium (a-Se), which can be grown easily over large areas by being evaporated on a substrate. However, TFT technology is far inferior to crystalline silicon CMOS technology in terms of the speed, stability, noise susceptibility, and feature size. Where CMOS technology falls short is its inability to be manufactured in large wafers at a competitive cost, allowing TFT technology to continue to be dominant in the medical imaging field, unlike the optical imaging industry. This work investigates the feasibility of integrating an imaging array fabricated in CMOS technology with an a-Se detector. The design of a CMOS passive pixel sensor (PPS) array is presented, in addition to how it is integrated with the amorphous selenium detector. Results show that the integrated Selenium-CMOS PPS array has good responsivity to optical light and X-rays, leaving the door open for further research on implementing CMOS imaging architectures going forward. Demonstrating that the PPS chips using CMOS technology can use a-Se as a detector is thus the first step in a promising path of research which should yield substantial and exciting results for the field. Though area may still prove challenging, larger CMOS wafers can be manufactured and tiled to allow for a large enough size for certain diagnostic imaging applications and potentially even large area applications like digital mammography.