A deep-depletion CCD imager for soft X-ray, visible, and near-infrared sensing

A high-resolution 512 × 96 element buried-channel charge-coupled (CCD) imager was designed and fabricated on 1-5 kΩ . cm p-type 〈100〉 uncompensated float-zone silicon substrates for soft X-ray (1-10 keV), and near-infrared (0.75-1 µm) sensing. The large depletion layer width (>50 µm) resulting from the high substrate resistivity minimizes minority-carrier generation in the neutral bulk region and thus modulation transfer function (MTF) degradation due to lateral minority-carrier diffusion effects. A unique design approach was used to integrate both high- and low-resistivity regions on the same chip for deep-depletion CCD´s and MOS transistor peripheral circuitry, respectively. The low-resistivity region prevents transistor punchthrough caused by the very low substrate doping. Present fabrication technology eliminates previous problems associated with high-resistivity silicon processing, namely, resistivity degradation, saturating dark signal at room temperatures, reduction in full-well capacity, and wafer breakage. Typical dark signal and transfer efficiency of the imager at 25°C are 4 nA/cm²and 0.99997, respectively. Experimental results demonstrate MTF improvements over conventional CCD imagers in the near-infrared and X-ray regions.