When interline CCD image sensors increase in size beyond 4 million pixels, CCD dark current begins to degrade the signal. Some scientific and photographic applications use very slow readout rates (less than 1 MHz) to reduce the noise level. At a 1-MHz readout rate, a 4-megapixel imager will take at least 4 s to read out. This extended time period allows a significant amount of dark current, to build up and frustrate efforts to reduce noise. Often this situation leads to the additional expense of a low-temperature operation. The accumulation-mode readout method for interline CCD image sensors is being developed at Eastman Kodak Company. Previously, accumulation mode could only be applied to the full-frame architecture because the p-type substrate acted as a source for holes. Interline CCD image sensors with n-type substrates have no ready source of holes to accumulate the surface of the CCD under all phases. This problem has been overcome, allowing room-temperature operation without significant dark current generation.
