Precipitates formed in silicon wafers by prolonged high-temperature annealing in nitrogen atmosphere

A floating zone (FZ) silicon wafer produced from a Czochralski (CZ) single-crystal ingot was subjected to prolonged annealing at a high temperature in a N₂ (70%) + O₂ (30%) ambient. Precipitates were formed and their regions manifested themselves as dark concentric rings in an X-ray topograph. According to the results of cross-sectional transmission electron microscopy and energy-dispersive X-ray spectroscopy elemental analysis, nitrogen was distributed throughout the precipitate regions, while oxygen concentrated in the periphery of the regions. Fourier transform infrared spectroscopy analysis of the precipitate regions revealed that the concentration of nitrogen was high, while that of oxygen was low. A high nitrogen concentration was also directly detected by secondary ion mass spectrometry in the mid-depth of the wafer in the precipitate regions. Electron diffraction analysis of the precipitates showed that their phase was α-Si₃N₄. The number of precipitates detected by cross-sectional X-ray topography increased with increasing annealing time. The precipitates were considered to originate from vacancies that can be eliminated by interstitial silicon.