Measurement of Substrate Impurity Fluctuation on CCD Image Sensor

Abstract

It is well known that the saturation signal level of a CCD image sensor including photo diode with vertical overflow drain (VOD) varies strongly with substrate impurity concentration. However, analytical results between the saturation signal level and substrate impurity concentration have not been obtained yet. In this paper, the relation between the saturation signal level and substrate impurity concentration was calculated using the drift-diffusion model as a function of photo diode structures and operation conditions.By comparing analytical and experimental results obtained from CCD image sensor chips, we found that the impurity fluctuations at the peripherals of CZ, MCZ, and epitaxial wafers were +11%, ±16%, and ±6%, respectively. Those at the center were ±7%, ±9%, and ±6%, respectively. The distinctive features of the impurity fluctuation pattern for these wafers were therefore clarified. The minimum impurity fluctuation level that can be detected is almost ±0.1%.