Analysis of Saturation Signal Fluctuation in CCD Image Sensors

Abstract

The saturation signal fluctuation that appears on the reproduced images of interline CCD image sensors with a vertical overflow drain is caused by fluctuations in the substrate impurity concentration. Besides such fluctuation, nonuniformity in the saturation level, i.e., bright and dark spots, is generated on CCD chips and wafers. These spots severely degrade image quality. We clarified the origin of the non-uniformity mainly by comparing images obtained at the saturation signal level with those obtained by electron injection from the substrate. We calculated the readout currents for both types of images for photo-diodes having various impurity concentration profiles. From these experimental and calculated results, we concluded that the bright and dark spots are caused by potential profile differences among photo-diodes, brought about by a slight variation in the impurity concentration in the diodes. We also suggest that the origin of the impurity variation is introduced by the dislocations formed during device fabrication.