Silicon Wafers for CCD Imagers

Abstract

The sensitivity to defects in CCD imagers is about two orders of magnitude higher than in the most sophisticated DRAM circuits. This makes CCD imagers one of the most difficult and challenging VLSI circuits to manufacture. In the present work, we discuss the origin of the material‐related defects that affect CCD imager performance in the various types of silicon wafers. The following silicon starting materials have been evaluated: internally gettered CZ, MCZ, FZ, FZ doped with nitrogen, and epitaxial silicon on internally gettered substrates. The assets and liabilities in the use of these various types of material were assessed. Issues addressed were mechanical strength (susceptibility to defect generation and wafer breakage), presence of residual defects in as‐grown crystals, presence of inhomogeneities, and gettering potential. The best results were obtained using MCZ wafers and epitaxial layers grown on internally gettered substrates. The CCD imagers made on these wafers had the least problems related to materials defects. This can be attributed to the absence of inhomogeneities, the presence of oxygen pinning of dislocations (which provides good mechanical strength, i.e., low wafer breakage, and resistance to dislocation generation), and low grown‐in defect density for MCZ silicon and gettering ability for the epilayers on IG‐CZ.