Development of a 3D silicon coincidence avalanche detector for charged particle tracking in medical applications

This work describes the development of a novel position sensitive charged particle detector suitable for nuclear physics applications, in the field of High Energy Physics experiments and emerging medical applications such as hadrontherapy and Proton Computed Tomography (pCT). The “3D Silicon Coincidence Avalanche Detector” (3D-SiCAD) pixel consists of a pair of 3D vertically aligned Geiger-mode avalanche diodes (SPAD) working in time-coincidence mode. This novel detector features single charged particle detection capability, compatibility with both standard CMOS technology and commercially available 3D integration techniques, and very low noise. In order to demonstrate this latter point, a first 3D-SiCAD prototype has been designed and fabricated using a standard CMOS technology and a 3D assembly technique based on gold micro-bumps. In a first phase, preliminary measurements over two adjacent “in-plane” avalanche diodes operated in coincidence-mode have demonstrated a very high noise rejection capability, up to 3-4 orders of magnitude lower than the intrinsic dark count rate of each SPAD cell. Afterwards, measurements on a real 3D pixel have been carried out, obtaining however noise rejection capabilities lower than expected. Optical cross-talk occurring between the two sensing level of the 3D-SiCAD pixel has been found to be the phenomenon responsible for this underperformance. Nevertheless this limitation can be easily overcome by interposing a fully absorbing/reflecting material between the two sensing levels. These first results are very encouraging for the realization of a fully CMOS integrated 3D-SiCAD detector, targeting application requiring high-precision tracking, ultra-fast response times and very low noise.