Simulation for estimating smear noise and sensitivity of CCD image sensors having square pixels less than 2.0 μm

Abstract

As a consequence of the reduction in the pixel sizes of charge coupled device (CCD) image sensors, the sensitivity of these sensors has decreased, which means that their signal-to-noise ratio (SNR) has also decreased even though the amount of noise is kept constant. In order to maintain and even increase the SNR, we evaluated a simulation method for estimating the sensitivity and smear noise. Smear noise and sensitivity are defined by the number of electrons in vertical registers and photodiodes, respectively. We used a finite-difference time-domain (FDTD) method to simulate the light energy which is proportional to electrons generated in a Si substrate. Using this simulation, we were able to estimate sensitivity and smear noise accurately and optimize the structure of on-chip lenses (OCLs) with respect to these parameters. When we optimized an OCL structure for an interline transfer (IT)-CCD having 1.86-&mgr;m-square pixels, we found that the optimal thickness of the OCL in regards to the smear noise was 0.25 &mgr;m thinner than the optimal thickness for the sensitivity. This result demonstrates that when designing the structures of image sensors, including the OCL shape, it is not only necessary to consider the sensitivity, but it is also important to take the smear noise into consideration.