Color-reproduction-driven CMOS image sensor design

Abstract

Reducing crosstalk in small pixels has become one of the major efforts in CMOS image sensor design. In this paper, instead of focusing on lowering crosstalk, we explore the problem at color imaging system level. First we study the components that affect the spectral response of CMOS image sensor, including micro-lens, color filter, sensor quantum efficiency (QE), and spectral response shifts caused by crosstalk inside the sensor. This is performed using a commercial tool. Based on the results, a super-linear cross-talk function is constructed. A novel model for color reproduction of image sensor system under crosstalk is then proposed. Hypothetical spectral sensitivity method using smooth cubic spline function model with controllable peak positions and widths is applied to simulate the spectral sensitivity of color filters. The peaks and widths are optimized based on the μ-factor of spectral responses of image sensor system under different crosstalk level. It is found that, with crosstalk, the imaging system can still provide color production as good as, if not better than, the situation without crosstalk. The spectral sensitivity of color filters has to be slightly modified to compensate the effect of crosstalk. The overlaps of spectral responses of color filters should be slightly extended, which means in certain circumstances higher overlap of spectral sensitivities of color filters may be desirable to compensate the effect of crosstalk. Keywords: CMOS image sensor, QE, crosstalk, color reproduction, color filter, color filter array, μ-factor, spectral sensitivity