Simulation-based study of afterpulsing dependence on STI guard ring separation in 180 nm CMOS SPADs subjected to neutron irradiation.

This work quantifies the effect of shallow trench isolation (STI) guard ring separation (9, 8, 7, 6 and 5 μm) on afterpulsing in 180 nm CMOS SPADs subjected to neutron irradiation at a fluence of 4.29 × 1010 n/cm2. The afterpulsing probability is calculated with an analytical model using TCAD-extracted junction capacitance Cj and trapping/de-trapping parameters at an excess bias of Vex = 3.5 V. Decreasing the guard ring separation lowers Cj and the avalanche charge, giving a monotonic reduction in the pre-irradiation afterpulsing probability from 2.355% at 9 μm to 0.661% at 5 μm, evaluated over the 50 ns to 100 μs observation window. Adding the radiation-induced trap densities predicted by NIEL scaling raises the afterpulsing probability by 30.8% in the unannealed state, by an identical factor for every geometry within the present uniform-bulk-damage model. The results identify the guard ring separation as a geometric lever for capacitance-driven afterpulsing mitigation, show the trade-off between afterpulsing probability, fill factor and photon collection area, and provide layout guidance for radiation-tolerant SPADs. All results are obtained from the TCAD and analytical simulation in this work.