Investigation of Breakdown Behaviour and Improvement of Spatial Resolution for Silicon Pixel Detectors

Abstract

In high-energy physics, the demand for silicon pixel detectors is increasing due to the elevated particle collision rates in collider experiments, since silicon detectors can handle high particle fluxes and provide precise determination of particle tracks. Silicon pixel sensors fabricated using commercial CMOS technologies (passive CMOS sensors) offer an attractive alternative to conventional planar pixel sensors. Commercial CMOS technologies provide a well-tuned set of fabrication parameters and special features, such as multiple metal layers, ensuring the reliability of the final products. However, this advantage comes at the cost of limited customisability for doping profiles, making it difficult to transfer previous knowledge from planar sensors directly. The studies presented in this work focus on two crucial features of passive CMOS silicon pixel sensors: breakdown performance and spatial resolution. The breakdown voltage determines the upper limit of the operational voltage of silicon pixel detectors. It is influenced by the design of the implant structures in the area between the pixel matrix and the chip’s edge, where a large voltage drop occurs. The goal of optimising the sensor design is to provide a smooth potential drop to suppress unexpected high electric fields. N-on-p passive-CMOS test structures were fabricated, measured, and simulated using TCAD to study the relationship between guard ring design and breakdown performance. In the second part of the thesis, a concept for improving spatial resolution using directional charge sharing between pixels is proposed and validated through dedicated simulations. Directional charge sharing can be achieved via subdivision of pixels and capacitive cross-couplings, which can be realised using commercial CMOS technologies. Results show an improvement in spatial resolution of approximately 30% compared to conventional pixel sensors with the same pitch size.