Indirect time-of-flight pixel study: 3D Monte Carlo simulation approach

The use of 3D Monte Carlo simulations for the study of an indirect time of flight (iToF) pixel revealed underlying information compared to conventional simulation tools. Experimental tendencies and results are systematically compared with results obtained numerically. During the sensor operation, iToF pixels reconstruct the depth information using an optical signal modulated in intensity at high operation frequencies of hundreds of MHz. A demodulation operation samples the photogenerated charges at different times in a single pixel. An efficient transfer is dependent of the charge carrier path in the pixel volume. Through the coupling of 3D Monte Carlo with a commercial Poisson solver and optical simulation tools, a complete and accurate simulation methodology was developed allowing the estimation of iToF main figures of merit such as demodulation contrast, parasitic light sensitivity and quantum efficiency. The method consists of generating a light impulse and studying the distribution and collection of each unitary charge in time through MC simulations. Detailed information can be obtained in the 3D volume of a pixel for the photogenerated carriers. The efficiency of charges transfer from the pixel volume to sensing nodes is given at the operation frequency by the demodulation contrast. The electrostatic potential barriers reducing the transfer efficiency can be easily identified and lost photogenerated carriers can be estimated. The prediction accuracy of Monte Carlo simulation is further improved through the coupling of photogeneration and electron mobility profiles extracted from optical simulation and drift-diffusion-based-technology computed-aided design tools respectively. A non optimized small pitch pixel was optimized thanks to these advanced multi-physics simulations.