A High-Resolution Direct-Conversion CMOS X-Ray Imager with Code-Division-Multiplexed Readout

Abstract

This thesis presents the design and characterization of a high-spatial-resolution direct-conversion X-ray imager built using complementary metal-oxide-semiconductor (CMOS) technology integrated with an amorphous-selenium photoconductor. Our imager targets applications such as dark field X-ray microscopy and aims to improve the performance of existing imagers by reducing pixel size, and increasing conversion efficiency and frame rate. We achieve high spatial resolution by reducing the pixel size of the imager. We present the design of a 2.252um x 2.363um two-transistor passive-pixel-sensor (PPS) array using analog code-division-multiplexed (CDM) readout. The PPS allows us to reduce the number of components in the pixel and reduce its size. The major drawback of using the PPS is low signal-to-noise ratio (SNR) due to the large column capacitance. To overcome the low SNR, we implement analog CDM readout which enables a longer effective integration time. Our design also contains a column-parallel dual integrator and switched-capacitor difference amplifier (SCDA) to implement the CDM readout. We also show how the pixel size, frame rate, and readout resolution design specifications are met. We implement our imager in a 180-nm CMOS process and also discuss the design of external hardware to operate the imager. Our preliminary experimental results focus on the characterization of the imager readout circuit. Specifically, we present the measured response of the integrator and SCDA and compare these to simulation results. We also suggest improvements for future iterations of the imager.