Architecture-Level Optimization on Digital Silicon Photomultipliers for Medical Imaging

Abstract

Silicon photomultipliers (SiPMs) are arrays of single-photon avalanche diodes (SPADs) connected in parallel. Analog silicon photomultipliers are built in custom technologies optimized for detection efficiency. Digital silicon photomultipliers are built in CMOS technology. Although CMOS SPADs are less sensitive, they can incorporate additional functionality at the sensor plane, which is required in some applications for an accurate detection in terms of energy, timestamp, and spatial location. This additional circuitry comprises active quenching and recharge circuits, pulse combining and counting logic, and a time-to-digital converter. This, together with the disconnection of defective SPADs, results in a reduction of the light-sensitive area. In addition, the pile-up of pulses, in space and in time, translates into additional efficiency losses that are inherent to digital SiPMs. The design of digital SiPMs must include some sort of optimization of the pixel architecture in order to maximize sensitivity. In this paper, we identify the most relevant variables that determine the influence of SPAD yield, fill factor loss, and spatial and temporal pile-up in the photon detection efficiency. An optimum of 8% is found for different pixel sizes. The potential benefits of molecular imaging of these optimized and small-sized pixels with independent timestamping capabilities are also analyzed.

Enhancing photodetection efficiency of CMOS SiPMs using virtual guard rings in a standard 0.35 µm process

“…Other techniques try to optimize and the geometry of the APD cells, for example, by replacing physical guarding with virtual guard rings for individual SPADs [6]. The technique of virtual guard rings is employed to enhance the active region of each SPAD and mitigate the effects of edge breakdown [6,7]. Rather than employing physical guard rings, virtual guard rings are created through the overlapping of a thin, lightly doped n-region with a heavily doped n+ region, which is connected to a metal contact that serves as the cathode.…”