Cosmetic Defects in CCD Imagers

Abstract

The requirement to fabricate CCD imagers free of defects brings about the need to identify the origin of the defects in an effort to improve the imager technology. The following defects in the video display were commonly observed: vertical and horizontal lines, localized spots, and striations. Their origin was analyzed using a combination of optical microscopy, chemical etching, and EBIC. It was establishd that they are related to defects present in the oxide and polysilicon, defects introduced into the bulk silicon by processing, and defects present in the starting material. A detailed discussion of their relation to the starting material, device processing steps, and their appearance in the displayed output as a function of the device operating conditions is given.

Deep Levels Study in Float Zone Si Used for Fabrication of CCD Imagers

“…7). It has been established,that this pattern reflects nonuniform collection efficiency, caused by inhomogeneous distribution of recombination centers in the silicon substrate (6). The typical fluctuations in the recombination center concentration observed at the edge of the wafer (measured by EBIC under plasma generation conditions) are shown in Fig.…”

“…Two types of unacceptable cosmetic defects, i.e., localized spots or lines and striation bands, are usually observed in CCD imagers. Most of the lines and spots are caused by electrically active crystallographic defects in the silicon substrate (1)(2)(3)(4)(5), while "striations" in the image are generated by inhomogeneities of resistivity and concentration of recombination centers (6). It should be pointed out that the ratio of crystallographic defects, which are electrically active, to the total amount of crystallographic defects as well as striation contrast in the displayed image depends on the heat-treatment involved in CCD imager fabrication (7).…”

Silicon Wafers for CCD Imagers

“…2. The diagnostic proc e d u r e s used to identify the origins of the defects have been described previously (5), and the origin of various c o s m e t i c defects is s u m m a r i z e d in Table I. The formation of defects in CCD imagers is affected by the follow-ing material factors: (i) the presence of striations, (ii) susceptibility to formation of crystallographic defects during processing, (iii) crystallographic defects present in as-grown wafers, (iv) m e c h a n i c a l strength of silicon, (v) presence of recombination centers, and (vi) gettering ability of silicon wafers.…”

“…Studies on solid-state and semiconductor electrolyt e junctions are now in progress in view of their utilization either as ion selective electrodes (1)(2)(3)(4), as photoanodes or photocathodes in electrochemical solar cells (5), and as detectors for x-ray, nuclear, and IR spectrometers (6)(7)(8)(9). The study of the behavior of the semiconductor electrode in various chemical media is growing, promising further development in instrumental measuring parameters.…”

“…Some defects observed in a television display of an output signal from these devices involve malfunction of one or more pixels. These failures often can be eliminated by improved control over processes that are used in the fabrication of CCD structures such as mask fabrication, photolithography, etching, and ion implantation (5). Other defects arise from inhomogeneities and defects in the silicon wafers, which are present in virgin wafers (7) or which can be generated during processing (6).…”