Doping profile extraction for predictive modeling of Single Photon Avalanche Diodes

Predictive and reliable models are key tools for the development of the next generations of single photon avalanche diodes (SPADs). Models are indeed crucial to evaluate the performance of prospective detector structures and to down select the most promising solutions before developing and running a dedicated fabrication process. To ensure predictability, models must be extensively validated against experimental data. In particular, the model must be applied to existing detectors and the results obtained from simulations must be compared with the measurements performed on the same detectors. The ability to accurately extract the doping profile along the SPAD active region plays a crucial role in the validation flow, because SPAD properties generally exhibit a strong dependence on the electric field. In this paper we will discuss why widely-adopted doping profile techniques do not allow the calculation of the electric field with an accuracy sufficient for predictive SPAD modeling. Then, we will present a technique we developed to extract accurate doping profiles starting from an approximate device-structure built with process simulations and/or secondary ion mass spectroscopy (SIMS) measurements. The technique combines capacitance-vs-voltage measurements with device simulations to implement an inverse modeling scheme. Finally, we will show how the proposed technique allows us to accurately reproduce the breakdown voltage of a large set of SPADs.