Tradeoffs in imager design parameters for sensor reliability

Abstract

Image sensors are continuously subject to the development of in-field permanent defects in the form of hot pixels. Based on measurements of defect rates in 23 DSLRs, 4 point and shoot cameras, and 11 cell phone cameras, we show in this paper that the rate of these defects depends on the technology (APS or CCD) and on design parameters the like of imager area, pixel size, and gain (ISO). Increasing the image sensitivity (ISO) (from 400 up to 25,600 ISO range) causes the defects to be more noticeable, with some going into saturation, and at the same time increases the defect rate. Partially stuck hot pixels, which have an offset independent of exposure time, make up more than 40% of the defects and are particularly affected by ISO changes. Comparing different sensor sizes has shown that if the pixel size is nearly constant, the defect rate scales with sensor area. Plotting imager defect/year/sq mm with different pixel sizes (from 7.5 to 1.5 microns) and fitting the result shows that defect rates grow rapidly as pixel size shrinks, with an empirical power law of the pixel size to the -2.5. These defect rate trends result in interesting tradeoffs in imager design.