Development of a Silicon Interposer: Toward an Ultralow Radioactivity Background Photodetector System

It is of great importance to develop a photodetector system with an ultralow radioactivity background in experiments dedicated to rare events’ detection. Silicon photomultipliers (SiPMs) and application-specific integrated circuits (ASICs) are two ideal candidates for low background photosensors and readout electronics, respectively, because they are mainly composed of silicon, which can achieve good radio-purity without considerable extra effort. However, interposers, used to provide mechanical support and signal routes between the photosensor and the electronics, are a bottleneck in building ultralow background photodetectors. Silicon and quartz are two candidates to construct the low background interposer because of their good radio-purity; nevertheless, it is nontrivial to produce through-silicon vias (TSVs) or through-quartz vias (TQVs) on the large area silicon or quartz wafer. In this work, motivated by the requirement of the nEXO project, we developed the first prototype of a silicon interposer with a size of $10\times10$ cm and a thickness of $320~\mu \text{m}$ , based on the double-sided TSV interconnect technology. The electrical properties of the interposer are carefully evaluated at room temperature, and its performance is also examined at $- 110\,\,^{\circ} \text{C}$ with an integrated SiPM on the interposer. The testing results reveal quite promising performance of the prototype, and the single photoelectron signals can be clearly observed from the SiPM. The features of the observed signals are comparable with those from the SiPM mounted on a normal FR4-based printed circuit board (PCB).