Precipitates Caused in Silicon Wafers by Prolonged High-Temperature Annealing in Nitrogen Atmosphere
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Precipitates Caused in Silicon Wafers by Prolonged High-Temperature Annealing in Nitrogen Atmosphere Haruo Nakazawa1, Masaaki Ogino1, Hideaki Teranishi1, Yoshikazu Takahashi1, and Hitoshi Habuka2 1 Corporate R&D Headquarters, Fuji Electric Co., Ltd., 4-18-1 Tsukama, Matsumoto, Nagano 390-0821, Japan 2 Department of Chemical and Energy Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Kanagawa 240-8501, Japan. ABSTRACT The precipitate behavior in a floating zone silicon crystal produced from a Czochralski single-crystal ingot has been studied. Large precipitates of α-Si3N4 crystal, having a dimension of about 2 μm, were formed at the mid-depth in the wafer by means of annealing at a high temperature in an ambient N2 (70%) + O2 (30%) atmosphere. The precipitate number detected by cross-sectional X-ray topography increased with the increasing annealing time. Interstitial silicon is expected to eliminate the precipitate origins. INTRODUCTION For high-yield electronic devices manufacturing, a sufficient number of wafers having a small variance of resistivity should be prepared from the fixed lot. Conventionally, the floating zone (FZ) treatment of raw poly-Si material has been applied to achieve a small variance of resistivity (The resultant wafers are denoted as FZ wafers). However, since the poly-Si raw material was a short ingot, obtaining a large number of FZ wafers from the same ingot was difficult. An effective solution was to apply FZ treatment to a CZ-grown ingot of sufficient length. Thus, the behavior of an FZ crystal grown from a CZ ingot should be studied, particularly in terms of the crystalline defects which could be easily enhanced and detected by prolonged annealing, necessary in the manufacturing of electronic devices. In this study, for hightemperature long-time annealing, N2 (70%) + O2 (30%) ambient was conducted on FZ wafers grown from CZ single-crystal rods. The precipitates were detected and analyzed by X-ray topography analysis, transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), and electron diffraction analysis [1-5]. The nitrogen in ambient was shown to play an important role in the formation of nitrided silicon precipitates. EXPERIMENT FZ (100) silicon wafers were used in this study. An FZ wafer produced from a CZ singlecrystal ingot is hereafter referred to as an FZ1 wafer. The diameter and thickness of both types of wafer were 150 mm and 500 μm, respectively. Nitrogen was not intentionally doped during the FZ process. There was no considerable surface defect by conventional characterization. Fig. 1 shows the process and evaluation flow in this study. After RCA cleaning, thermal annealing at 1300 oC for 50 h, 75 h, or 100 h was carried out in an ambient N2 (70%) + O2 (30%)
atmosphere. After annealing, the wafers were evaluated by transmission X-ray topography (XRT-150, Rigaku, Tokyo) and cross-sectional X-ray topography (XRT-100, Rigaku, Tokyo), SIMS (IMS-6f, CAMECA, France), electron diffraction and cross-sectional TEM analyses (JEM-2
