Optimization of backside-illuminated single-photon avalanche diodes in 90 nm CIS technology for scintillator-based biomedical applications

Single-photon avalanche diodes (SPADs) fabricated in CMOS technology enable high-sensitivity imaging for applications such as light detection and ranging, positron emission tomography (PET), and x-ray imaging. While backside-illuminated (BSI) SPADs offer high fill factors and compatibility with 3D stacking, their deep junction architecture limits photon detection probability (PDP) in the blue–green spectrum (400–550 nm), which is critical for scintillator-based biomedical applications. This study presents a BSI SPAD optimized for blue–green wavelength sensitivity through three structural optimizations: (1) aggressive backside thinning from 5 to 3.3 μm, (2) active area enlargement from 5 to 10 μm, and (3) backside scattering patterning (BSP). Four SPADs were fabricated in a 90 nm CMOS image sensor process and characterized through electrical and optical measurements. All devices exhibit low dark currents with stable breakdown voltages, while the dark count rate characteristics remain almost identical, showing only minor increases attributed to pixel scaling and BSP-related surface effects. PDP improves dramatically, from 7.69% in the default structure to 53.4% in the fully optimized structure at 500 nm, which is ∼7× enhancement. The results demonstrate that the proposed approach effectively bridges the blue–green sensitivity gap between frontside-illuminated and BSI SPADs, enabling high-performance SPAD arrays for scintillator-based biomedical applications like PET and x-ray imaging.