Modeling Charge Loss in CMOS MAPS Exposed to Non-Ionizing Radiation

A model, approximating minority carrier diffusion with a discrete random walk and accounting for radiation induced reduction of minority carrier lifetime, is proposed to predict the effects of neutron irradiation on the charge collection properties of monolithic active pixel sensors (MAPS) in CMOS technology. The model has been implemented in a Monte Carlo code to simulate MAPS operation in minimum ionizing particle detection systems. For the purpose of validating it, the results from the characterization of monolithic sensors irradiated up to an integrated fluence of 10¹⁴ 1- MeV-neutron equivalent/cm² have been compared with the outcomes of the Monte Carlo simulations. The monolithic sensors taken into consideration for the model validation are based on two different CMOS processes, one featuring a triple well option, the other one featuring a quadruple well structure and a standard (10 Ω·cm) or high (1 kΩ·cm) resistivity epitaxial layer. Simulation results are shown to be in good agreement with experimental data. The consistency between the model and the measurement results seems to confirm that radiation induced increase in the recombination rate is the main source of charge collection degradation in neutron-irradiated MAPS.