Single photon counting 센서 적용을 위한 32 x 32 array 디지털 Si-PIN x-ray detector의 제작 및 평가

Abstract

Recently, radiation detection devices and sensor systems have been becoming wide-spread in medical imaging applications due to the large increase of radiation system usages such as nucleation radiation and x-ray radiation systems. This study deals with fabrication and characterization of 32 x 32 array Si-PIN x-ray detector for single photon counting imaging sensor. 4 x 4 mm² double-sided PIN silicon photodiodes have been fabricated with the standard IC fabrication planar technology. Floating-zone (FZ) refined, n-type, <111>-orientation and high resistivity (6 ~ 11 kΩ·cm) silicon wafer (1000 ㎛ thickness) was used as substrate for Si-PIN x-ray detector fabrication. Different gettering processes, ion-implantation and doped poly-Si, had been implemented to remove the impurities and defects from the x-ray detector’s silicon substrate. Multi guard-rings (MGRs) and metal filed plates (MFPs) techniques were proposed to reduce the leakage current and to improve the breakdown performance of x-ray detector at high peak electric field in the vicinity of junction termination. Its electrical properties such as the leakage current and the breakdown voltage were characterized. The structure of 32 x 32 array Si-PIN x-ray detectors were designed by simulations for planar x-ray radiation detector with high voltage and low leakage current operation. To optimize the 32 x 32 array Si-PIN x-ray detectors, which have MGRs and MFPs of various physical and geometrical parameters, were simulated with various spacing between the outermost active area and MGRs and MFPs. The optimal design of low leakage current and high breakdown voltage depends on MGRs spacing and MFPs extension width. The 32 x 32 array Si-PIN x-ray detectors for single photon counting imaging sensor were fabricated based on optimized simulation results. The size of pixel was 4 x 4 mm². The size of 32 x 32 active pixel array was 80 x 80 ㎛². Four p^(+) floating MGRs and MPFs were formed around the external active areas not only to reduce the leakage current but also to improve the breakdown voltage performance. The 32 x 32 p^(+) active pixels and MGRs were formed by the high dose ion-implantation process. The extrinsic getterings on the wafer backside was performed to reduce the leakage current by n^(+) high dose ion-implantation and doped poly-Si deposition process. Dielectric, SiO₂, layer was deposited as an IMD (intermetal dielectric) layer on front side and then, metallizations were conducted on both sides of wafers. Finally, alloy anneal, in hydrogen ambient, process was done to anneal out interface traps at the Si-SiO₂ interface. The breakdown voltage of x-ray detector was improved with MGRs and the leakage current of x-ray detectors was significantly reduced with a MFPs. The extrinsic gettering process and alloy process show significant effect on reduction of leakage current. It was possible to achieve the leakage current lower than 40 pA/cm² at 100 V_(dc).