A Discussion on the Relation between Substrate Impurity Concentration Fluctuation and Saturated Signal Fluctuation in the Charge Coupled Device Image Sensors

Abstract

The saturation signal level of a photodiode with vertical overflow drain, that is generally used in CCD image sensors, depends markedly on a substrate impurity concentration. The relation between them was obtained analytically, for the first time. The analytical result was compared with that of a 3 dimensional numerical analysis to examine the applicability of the analytical solution. Results of the two analyses coincided qualitatively. To examine subsrate impurity fluctuation, the relation between saturation signal and substrate impurity concentration, that was obtained from the 3 dimensional analysis, was applied to a saturated horizontal line signal that was obtained from a CCD image sensor made on a peripheral region of a 5 inch CZ wafer.
The substrate impurity fluctuation of the sensor was estimated to be ±65.7%. Also, an unique photodiode structure was proposed to minimize the effect of the substrate impurity cocentration fluctuation on the saturated signal fluctuation.