Design and characterization of radiation-tolerant CMOS 4T Active Pixel Sensors

The design and radiation characterization of pinned photodiode 4T Active Pixel Sensors (APS) is presented in this paper. Four types of pixel layout are characterized and irradiated by 60Co γ-rays up to 170krad(Si). A variety of radiation hardening techniques are applied in these pixels. The variations between the four pixels allow us to optimize the design parameters for radiation hardness. Our experimental results exhibit increase in dark current and its non-uniformity after radiation. It is also found that Enclosed Layout Transistor (ELT) transfer gate (TX) becomes the major cause of the dark current increase. The dark current increase attributed to the shallow trench isolation (STI) around the photodiode can be mitigated by recessed-STI photodiode structure when the total dose is low. Large floating diffusion (FD) shows more immunity to the radiation-induced leakage current and random noise.