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 gettere-d ~ubstrates. 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.

Proximity Gettering Design of Hydrocarbon–Molecular–Ion–Implanted Silicon Wafers Using Dark Current Spectroscopy for CMOS Image Sensors

“…Metallic impurities form deep-energy-level defects in the silicon band gap. These defects in turn strongly affect electrical device parameters such as dark current, white spot defect, recombination lifetime, and transfer gate oxide breakdown voltage [4,5,6,7,8,9]. Thus, CMOS image sensor manufacture requires metallic impurities to be eliminated from the device active region.…”

A Review of Proximity Gettering Technology for CMOS Image Sensors Using Hydrocarbon Molecular Ion Implantation

“…(5,6) These defects strongly affect device electrical performance parameters such as white spot defect density, recombination lifetime, dark currents, and gate oxide breakdown voltage. (7)(8)(9) Thus, CMOS image sensor manufacturers make much effort to eliminate metallic impurities from the device active region using gettering techniques. Intrinsic gettering (IG) is the most popular gettering technique in the semiconductor device fabrication process.…”

Impact of tungsten contamination on the sensing margin of a CMOS image sensor cell

“…One of the main causes of white spots in a CIS cell is a high leakage current in the photodiode region. 10) This leakage current is originated from the dark current of the cell, decreasing the sensitivity of the photodiode, which is associated with the degradation of the minority-carrier recombination lifetime. 9,11) However, it has not been reported that tungsten contamination degrades the minority-carrier recombination lifetime and there is a correlation between the minority-recombination lifetime degradation and the leakage current in a photodiode.…”