Transfer-Gate Region Optimization and Pinned-Photodiode Shaping for High-Speed TOF Applications

In this paper, we discuss potential optimizations of the structure and about the shape of pinned photodiodes (PPD) pixels for indirect-ToF sensors. We focus mainly on the transfer-gate (TG) region optimization (particularly pwell mask distance and the threshold adjustment implant) and on the PPD shape. TG region optimization is important to extract as fast as possible the charge from the PPD region. PPD shaping, instead, is important to quickly move the generated charge close to the TG, to be then collected by the floating diffusion. We also present experimental data on the produced 10μmx10μm-pitch pixels, introducing a setup for the direct measurement of the impulse-response-time, which takes into account both the TG transfer time and the photogenerated charge collection time. With the best pixel achieved during this study, we measured a quantum efficiency of about 45% at 850nm and a demodulation contrast of about 70% at 40MHz.