Intrinsic Gettering of Iron in Czochralski Silicon Crystals
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INTRINSIC GETTERING OF IRON IN CZOCHRALSKI SILICON CRYSTALS M. AOKI, A. HARA, AND A. OHSAWA Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi
243-01,
Japan
ABSTRACT We present a new experimental approach to studying the mechanism of intrinsic gettering of Fe in Czochralski silicon crystals. We present our experimental method and results for as-grown and intrinsic gettered wafers with high and low-level Fe surface contamination. We found that when annealing at the Fe supersaturation temperature, Fe concentration decreases faster in intrinsic gettered wafers than in as-grown wafers. Concentration saturated with annealing time for each sample and the saturated Fe concentration followed a simple Arrhenius relationship. Re-emission of Fe from the bulk defect region occurred above the gettering temperature. We conclude that in intrinsic gettering, Fe precipitates preferentially in the bulk defect region when the Fe impurities supersaturate as temperature drops. INTRODUCTION Gilles et al.[1] and Weber and Gilles[2] proposed the kinetics of intrinsic gettering(IG) based upon the difference between the precipitation kinetics of metallic impurities in the denuded zone and in the bulk near the oxygen precipitate. They asserted that intrinsic gettering requires supersaturation of heavy-metal impurities and occurs during wafer cooling or low-temperature processing. We recently reported that supersaturation of Fe impurities is necessary for intrinsic gettering of Fe. [3,4] Here, we examine how the contamination level of Fe introduced into the wafer affects the temperature at which gettering occurs and discuss the low-temperature solid solubility of Fe. We also examine re-emission of Fe from the bulk defect region. EXPERIMENT Silicon wafers were Czochralski (CZ) and float zone (FZ) grown, (100), boron-doped, and 10 ohm-cm. We measured an oxygen concentration in CZ wafers of about 1.7 x 1018 cm-3 using an infrared absorption intensity with a conversion coefficient of 4.81 x 101 7cm-2 . [5) We annealed some wafers using a three-step gettering process (high, low, and hightemperature) to from a denuded zone and an oxygen-precipitated region.[6] We used a high temperature of 1100 0C and low temperature of 650 0C. We also prepared long-term gettered wafers by increasing the low temperature annealing time. The denuded zone was about 20 gim, irrespective of annealing time in the three-step gettering process. The defect density in the gettered wafer was about 5.0 x 105 cm-2 and in the long-term gettered wafer was 5.0 x 106 cm- 2. The wafer surfaces were intentionally contaminated by immersion in an NH4OH, H20 2, and H20 mixture containing 5 ppb and 100 ppb of Fe. The Fe surface concentration was measured by atomic absorption spectrophotometry with HF vapor-phase decomposition [7] as around 4.2 x 1011 cm-2 for 5 ppb Fe and 3.2 x 1013 cm- 2 for 100 ppb Fe. The sample wafers were cut into 25 by 15 mm pieces. We annealed the samples as shown Fig. 1. After Fe dissolution annealing at 1150 0 C for 30 min, we annealed to evaluate th
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