Optimization of Pinned Photodiode Pixels for High-Speed Time of Flight Applications

We discuss optimizations of pinned photodiode (PPD) pixels for indirect time of flight sensors. We focus on the transfer-gate and dumping gate regions optimization, on the PPD dimension and shape to assure fast lateral charge transfer and on the epitaxial layer thickness for a good tradeoff between fast vertical charge transfer and high quantum efficiency at near infrared region. The overall performance of the pixel is quantified by the demodulation contrast of the pixel at specific frequencies. The operation frequency of the device is determined by the required ambiguity range of the application and the required distance noise. In order to reach a reasonable distance noise, the pixel needs to allow modulation frequencies up to 100 MHz. In this paper, we present TCAD simulation and experimental data on demodulation contrast, impulse response time, and quantum efficiency of 10×10μm pixels. We introduce a setup for impulse response measurement and we compare this to the demodulation contrast. We also discuss the optimization of the dump gate and dump diffusion. With the best pixel we measured a quantum efficiency of about 45% at 850 nm, a demodulation contrast of 47% at 80 MHz, and an impulse response time < 5 ns.