Charge-coupled area image sensor using three levels of polysilicon

Charge-coupled area image sensors with 220 by 256 cells have been built using a three-phase overlapping electrode structure. Each of the three sets of electrodes is formed in a separate level of polysilicon which are isolated from each other by a thermally grown oxide, This approach relaxes the demands on mask making and photolithography that would otherwise be necessary and reduces the incidents of fatal shorts that render devices inoperable. The overlapping electrode structure results in stable performance and good transfer efficiency. The semitransparent polysilicon electrodes make the device usable with circuit side illumination although the spectral response is not very uniform. Average quantum efficiency in the visible part of the spectrum is 0.25. Measured resolution limits are 110 line pairs horizontally and 100 pairs vertically in accordance with present day PICTUREPHONE® specifications.