Oxygen '96

Early Stages of Oxygen Precipitation in Silicon

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ANOMALOUS DISTRIBUTION OF OXYGEN PRECIPITATES IN A SILICON WAFER AFTER ANNEAL PROCESSES

H. Ono, T. Ikarashi, S. Kimura, A. Tanikawa and K. Terashima

Microelectronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305, Japan.

Keywords: oxygen, silicon, X-ray topography, infrared absorption, distribution

Oxygen precipitation after thermal annealing is generally controlled by interstitial oxygen atoms and other unknown defects. It is known that anomalous ring-shaped distribution of oxygen precipitates appears in a wafer which is cut from an ingot grown at a specific growth rate. This exhibits a typical example of a wafer in which the precipitation process is different in regions of the wafer due to the heterogeneous distribution of the unknown defects.

Samples were cut from a 5 inch p-type <100> Si crystal grown at a constant growth rate of 0.8 mm/min. Mirror-polished wafers were annealed at 450-1150 degrees C for up to 64h in N₂ or O₂ atmosphere, preceding a post-anneal at 1000 degrees C for 16h, if needed. The distribution of interstitial oxygen and oxygen precipitates was studied by infrared absorption spectroscopy. Room-temperature infrared absorption profiles were obtained using a conventional micro-FTIR system. The distribution was also obtained at liquid-He temperature, in order to separate an oxygen precipitate band at 1100 cm^-1 from an interstitial oxygen peak. Lang X-ray topographs were taken to show the distribution of strains due to precipitates and/or oxidation-induced stacking faults (OSFs).

``Ring area'' is defined as a region where OSFs are distributed with ring-shaped and anomalously rich in density. We found that the density of the precipitation nuclei formed by a preanneal is lower in the ring area than in the other area. This might imply that the OSF nuclei (or micro precipitates) already exist in an as-grown wafer, which were generated during cooling process after crystal growth. However, the FTIR measurements result in homogeneous distribution of oxygen precipitates, interstitial oxygen and thermal donors. It is concluded that, in the precipitation nucleus formation process during an anneal, the renewal nucleation of oxygen precipitates should be strongly affected by already-existing precipitates with large size and very small amount, instead of point native defects, impurities of their clusters.

Last modified: Mon Feb 19 17:18:26 GMT 1996 JG