A Method for Estimating Quantum Efficiency for CMOS Image Sensors

The standard method for measuring QE for a CCD sensor is not adequate for CMOS APS since it does not take into consideration the random offset, gain variations, and nonlinearity introduced by the APS readout circuits. The paper presents a new method to accurately estimate QE of an APS. Instead of varying illumination as in the CCD method, illumination is kept constant and the pixel output is continuously observed — sampling at regular intervals. This makes it possible to eliminate random offset. The experiment is repeated multiple times to obtain good estimates of the pixel output mean and variance at each sample time. The sensor response is approximated by a piecewise linear function and using the Poisson statistics of shot noise (which are also used in the CCD method) gain, charge and read noise are estimated for each line segment. This procedure is repeated at no illumination so that dark charge may be estimated and subtracted from the total charge estimates. The method can also be used to estimate readout noise and gain FPN. Results from 64x64 pixel APS test structures implemented in a 0.35 um CMOS process are reported. Using 6 different chips and 16 pixels per chip QE=O.37, gain FPN=2%, dark charge=832e-, and readout noise= 40e-, are estimated.