Development and evaluation of a high resolution CMOS Image Sensor with 17 μm × 17 μm pixel size for X-ray imaging

In this research, we designed and fabricated a CIS (CMOS Image Sensor) with 17 μm × 17 μm pixel size and 190 × 190 pixels using 0.25 μm standard CMOS process as a test version sample for developing high resolution X-ray image sensors. Active pixel sensors, area efficient sample and hold circuits, and a switched capacitor amplifier are integrated in a single chip. A unit pixel of the sensor consists of a photodiode and a 3-transistor active pixel structure. A sample and hold circuit is designed to reduce silicon area by including only one capacitor. Finally, a current mirrored operational transconductance amplifier is used to construct a switched capacitor amplifier. Also, in order to analyze the characteristics of the CIS and obtain images, we developed a data acquisition system. The system is responsible for communicating with a personal computer as well as controlling the CIS and an external ADC. The evaluation procedure of the CIS is divided into two categories: one is to investigate the performance of the CIS itself, and the other is to evaluate the quality of the obtained image. We measured not only the linearity, sensitivity and charge-to-voltage conversion gain of the CIS, but also the spatial resolution of the X-ray image acquired by the CIS coupled with a CsI(Tl) scintillator.