We present the world's first backside-illuminated (BSI) single-photon avalanche diode (SPAD) based on standard silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) technology. This SPAD achieves a good dark count rate (DCR) after backside etching, comparable to DCRs of BSI SPADs fabricated on bulk wafers. Unlike bulk-wafer-based BSI SPADs, which typically suffer from poor violet and blue sensitivity, the proposed BSI SPAD features increased near-ultraviolet sensitivity as well as significant sensitivity in the violet and blue spectral ranges, thanks to the ultrathin-body SOI. To the best of our knowledge, this is the best result ever reported for any BSI SPAD in the standard CMOS technology. In addition, it also shows high sensitivity at long wavelengths thanks to the interface between silicon and silicon-dioxide layers. Therefore, it achieves a photon detection probability over 26% at 500 nm and 10% in the 400-875 nm wavelength range at 3 V excess bias voltage. The timing jitter is 119 ps full width at half maximum at the same operation condition at 637 nm wavelength. For the proposed BSI SPAD, the buried oxide layer in SOI wafers is used as an etching stop during the wafer backside-etching process, and therefore it ensures the excellent performance uniformity in large arrays. ... Read more

Photon-counting imaging technology has applications in many fields such as fluorescence lifetime imaging microscopy (FLIM), time-resolved Raman spectroscopy, 3D imaging, and even space communications. The requirement to detect single photons with picosecond temporal resolution makes single-photon avalanche photodiode (SPAD) a popular choice. Advanced biomedical imaging applications such as pill cameras, retinal prosthesis, and implantable biocompatible monitoring sensors require a compact image system, which can be implanted into a living body. To meet these requirements, novel single-photon image sensor solution needs to be developed, in which new substrate post-processing and backside illumination or even dual-side illumination are core technologies, with inherent CMOS compatibility as a prerequisite. This chapter proposed and demonstrated the world’s first flexible CMOS single-photon avalanche diode image sensor, providing a suitable solution for implantable biomedical imaging or monitoring applications, and wherever a curved imaging plane is essential. ... Read more