Development of high-sensitivity CMOS active pixel sensor with wide dynamic range for low-dose X-ray imaging

Abstract

Recently, a large-area CMOS image sensor (CIS) has been in the spotlight as a powerful candidate detector for digital X-ray imaging. The CMOS X-ray detector benefits from the inherited features of well-developed and standardized CMOS technology such as low power consumption, high speed, low production cost, and above all the full integration of signal processing circuitry in the same sensor chip. While a few digital X-ray detectors fabricated in CMOS fabrication processes were already commercialized, the need remains to be able to develop a high-sensitivity detector with low noise to compete with industrial and scientific CCDs. The primary goal of this study is to develop a high-sensitivity CMOS active pixel sensor for low-dose X-ray imaging. First of all, a CIS with 17 um x 17 um pixel size and 190 x 190 pixels was designed and fabricated using 0.25 xm 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 an n+/p-well photodiode and a 3-transitor active pixel structure. A sample and hold circuit is designed to reduce silicon area by including only one capacitor. In addition, a current mirrored operational trans-conductance amplifier is used to construct a switched capacitor amplifier. In order to analyze the characteristics of the CIS and obtain images, a data acquisition system was also developed. From the developed system, 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. The measured sensitivity was 0.16 V/lux.sec and the conversion gain was 0.97 uV/e-. Moreover, the spatial resolution higher than 10 lp/mm was obtained. This means that the fabricated CIS is expected to be applied to intra-oral...