A method for improving the spatial resolution of frontside-illuminated CCD´s

A method is proposed for improving the spatial resolution of frontside-illuminated silicon CCD imagers. The technique involves building the device on an epitaxial layer deposited on a more highly doped substrate of the same conductivity type, creating a high-low junction. A simple theoretical model for the carrier diffusion limited modulation transfer function (MTF) is developed for this structure. Calculations of the MTF using this model are compared to similar calculations for other configurations, e.g., a thinned, backside-illuminated device and a frontside-illuminated imager built on a uniformly doped substrate. The calculations show that the new structure has MTF performance comparable to or better than the backside-illuminated device and has, for λ = 1.06 µm and Nyquist spatial frequency, an MTF which is nearly an order of magnitude higher than that for the nonepitaxial frontside-illuminated device. At the same time, the quantum efficiency of the epitaxial device is reduced by about one order of magnitude. The effects of substrate doping and epitaxial layer thickness are also explored.