Abstract
We present and discuss parameters of a high dynamic range (HDR) image sensor with LED flicker mitigation (LFM) operating in automotive temperature range. The total SNR (SNR including dark fixed pattern noise), of the sensor is degraded by floating diffusion (FD) dark current (DC) and dark signal non-uniformity (DSNU). We present results of FD DC and DSNU reduction, to provide required SNR versus signal level at temperatures up to 120 °C. Additionally we discuss temperature dependencies of quantum efficiency (QE), sensitivity, color effects, and other pixel parameters for backside illuminated image sensors. Comparing +120 °C junction vs. room temperature, in visual range we measured a few relative percent increase, while in 940 nm band range we measured 1.46x increase in sensitivity. Measured change of sensitivity for visual bands—such as blue, green, and red colors—reflected some impact to captured image color accuracy that created slight image color tint at high temperature. The tint is, however, hard to detect visually and may be removed by auto white balancing and temperature adjusted color correction matrixes.
Highlights
Image sensors with unsurpassed high dynamic range (HDR) and light and LED flicker mitigation (LFM) performance [1,2] are powering the latest advancements in autonomous and assisting driving.These sensors employ CMOS back side illumination (BSI) technology to consistently outperform sensors of the previous generations
We have found that floating diffusion (FD) dark signal non-uniformity (DSNU) fixed pattern noise (FPN) is a dominant noise source for lateral overflow (LO) HDR pixels at high temperatures
dark current (DC) and DSNU compared to the PD DC and DSNU can be reduced by decreasing the FD junction electric field (Efld)
Summary
Image sensors with unsurpassed high dynamic range (HDR) and light and LED flicker mitigation (LFM) performance [1,2] are powering the latest advancements in autonomous and assisting driving. In order to meet these HDR LFM requirements, attenuating the number of integrated photoelectrons by temporal operation has been proposed [1] It is efficient for high speed operation with the extension of the effective full-well capacity (FWC) by programming the duty cycle of photoelectron integration, but has a poor noise floor due to the uncorrelated double sampling (UDS) readout. This paper presents a BSI lateral overflow (LO) 3.0 μm 2.6 M pixels HDR LFM CMOS image sensor with single exposure 97 dB dynamic range. The effects investigated included quantum efficiency (QE), sensitivity, source follower (SF) gain, pixel transaction factor (PTF) gain, color ratios, and color image quality
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