The analysis of dark signals in the CMOS APS imagers from the characterization of test structures

The characteristics of dark signals have been investigated in the CMOS active pixel sensor (APS) with test structures fabricated using the deep-submicron CMOS technology. It is found that the periphery of the photodiode (PD) is the dominant source of dark currents in our test structure, and this factor is very sensitive to the distance between the sidewall of the shallow trench isolation and the n-type region of the PD. The dark currents from the transfer gate can be effectively reduced by the tail of p⁺ region on the surface of the transfer gate, and those from the floating diffusion (FD) node were estimated to be negligible in the normal operational mode. However, because of the enhanced thermal generation velocity caused by the severe process-induced damages, the FD node was considered as the main source of increased dark currents in the single frame capture mode. The characteristics of quantized dark currents causing the white pixels in the CMOS APS were examined using the dark current spectroscopy method. Three distinct deep-level bulk traps have been identified with the location in the silicon bandgap at |E_t - E_i| ∼ 0.020 (eV), |E_t - E_i| ∼ 0.082 (eV), and |E_t - E_i| ∼ 0.058 (eV), and capture cross sections of 7.80 × 10^-15 cm², 1.83 × 10^-13 cm², and 1.46 × 10^-13 cm² respectively.