A 140-dB Single-Exposure Wide-Dynamic-Range CMOS Image Sensor Combining LOFIC and Selective Overflow Technology

This article presents a wide-dynamic-range (WDR) CMOS image sensor (CIS) integrating lateral overflow integration capacitor (LOFIC) technology and selective overflow architecture, achieving a 140-dB dynamic range (DR) in a single exposure. The proposed pixel architecture modulates the barriers of overflow paths to enable saturated charges overflow through two distinct pathways: effective overflow (captured by LOFIC) and ineffective overflow (discharged via ${V} _{\mathbf {DD}}$ ). This unique selective overflow mechanism equivalently enhances the charge storage capacity of the LOFIC capacitor, strengthening high-light detection capabilities and thereby extending DR. In addition, we propose a count-range-selectable single-slope (SS) analog-to-digital converter (ADC) circuit to resolve the dual-channel readout issue caused by inconsistent readout sequences between high-gain and low-gain signals in LOFIC architectures. This design achieves single-channel readout while maintaining digital correlated double sampling (DCDS) functionality. The prototype chip is fabricated using an 110-nm backside illumination (BSI) CIS process, featuring a $5\times 5~\mu $ m pinned photodiode (PPD) pixel and a 14.32-fF LOFIC capacitor. Compared with conventional LOFIC architectures, the proposed selective overflow mechanism extends DR from 101 to 140 dB, with a switching-point signal-to-noise ratio (SNR) of 20.5 dB. By selecting appropriate overflow gate parameters, the photo-response non-uniformity (PRNU) at the half-full code for the high-conversion-gain (HCG) output is measured at 0.50%, while PRNU for the low-conversion-gain (LCG) output is 0.89%. At an ambient temperature of $60~^{\circ }$ C, the dark current of the HCG signal is 181 e ${}^{-}$ /s, and that of the LCG signal is 621 e ${}^{-}$ /s.