CMOS electronics for back side illuminated SPAD imagers

Single-Photon Avalanche Diode (SPAD) is the viable detector that can fulfill the requirements of single-photon sensitivity and short integration time, which are specific constrains of many applications in the Safety and Security scenario. Front-side illuminated SPAD (FrontSPAD) array relies on a high-voltage fully-standard 0.35µm CMOS technology suitable to monolithically integrate side-by-side SPAD detectors and pixel and global electronics. The trade-off of this approach is low fill-factor, since SPAD occupies a large area and also electronics and the two must be laid out in the same chip. Therefore in order to improve SPAD quality and overall imager’s performance, a “back-side illuminated” assembly is proposed, where the electronics be fabricated on a separated wafer to be bonded together with the wafer containing just the SPAD detectors. Back-side illuminated SPAD (BackSPAD) imagers, object of this thesis, will be based on state-of-the-art processing technologies exploiting a combination of a wafer processed in a silicon-on-insulator (SOI) CMOS technology, containing the SPADs, flipped and bonded onto a standard CMOS wafer, containing the electronics. SPADs will be operated with back-side illumination and the two-wafers mounting will attain two major improvements: • much higher pixel density since SPAD detectors are placed on top of the corresponding smart-pixel electronics, thanks to the wafer-bonded vertical structure, instead of being placed to its side (as in the planar structure of the FrontSPAD); • enhanced spectral sensitivity in the near-infrared, up to 1µm-wavelength, thanks to the thicker active volume of BackSPAD detectors, to the internal steered reflections of photons within the SOI detector wafer (thus increasing the effective absorption photon path), and to the back-side illumination of the active area, as compared to the FrontSPAD. Besides, the detector will be paired with sophisticated in-pixel intelligence able to process at the pixel-level intensity-data and depth-ranging information, with both pulsed-light iTOF and continuous-wave modulation iTOF techniques enabling 3D mapping of rapidly changing scenes in light starved environments.