Single Event Upsets and Hot Pixels in Digital Imagers

From extensive study of digital imager defects, we found that permanent "Hot Pixels" are the main long term digital camera defects, and are caused by high energy cosmic ray particles. Clearly, as in other microelectronic integrated circuits, most of the particles do not induce permanent damage but instead, inject a short term charge that may cause a transient fault, known as a Single Event Upset (SEU). Unlike standard digital ICs, pixels in a digital imaging sensor can be monitored at almost any desirable frequency. Since an SEU manifests itself as one or more brighter pixels in an otherwise dark image, the rate of SEUs can be measured at a considerably higher accuracy by taking dark-field pictures at different exposure times and different frequencies. In this paper we describe an experimental approach to measuring the occurrence rate and resulting characteristics of SEUs. The SEU rate that we have observed for digital imagers, of about 4 SEUs for every 30 seconds, is considerably higher than was previously reported for ordinary ICs. For the same imager, permanent hot pixels have a rate of 1 every 12.6 days, while SEUs occur 145,000 times more often. Ordinary IC SEU rates have been reported to be about 100x of permanent fault rates. In addition, we found that some SEUs in digital imagers do not impact a single pixel, as do hot pixels, but can create a line of injected charges which appears as a bright line in the dark image.