The Influence of Thermal Treatment on Defect Characteristics in CZ-Silicon Wafers Investigated by Positron Annihilation
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THE INFLUENCE OF THERMAL TREATMENT ON DEFECT CHARACTERISTICS IN CZSILICON WAFERS INVESTIGATED BY POSITRON ANNIHILATION SPECTROSCOPY 4 4 3 2 P. Mascherl, W.Puff , S. Hahn , K.H. Cho , and B.Y. Lee 1. Centre for Electrophotonic Materials and Devices, Department of Engineering Physics, McMaster University, Hamilton, Ontario, Canada L8S 4L7 2. Institut fOr Kemphysik, Technische Universit~t Graz, Graz, Austria 3. Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 4. Siltron Inc., #283 Imsoodong, Gumi Kyungsangbukdo, Korea
ABSTRACT
Positron lifetime and Doppler-broadening experiments as well as Fourier-transform infrared spectroscopy (FTIR) were performed on a variety of six-inch Czochralski (CZ) silicon wafers. Measurements were done at 14 equidistant locations across the wafers which were cut from the seed-, middle-, and tail-sections of two boules grown at different pull-speeds. In the as-grown wafers, the positron response consisted of components from small oxygen-related clusters and "perfect" bulk silicon only. Possible contributions from vacancytype defects were at or just below the detection limit. After a two-step heat treatment (750°C/ 4 hrs + 1050°C/6 hrs in N2 ) FTIR showed that significant amounts of oxygen (4-8 ppma) had precipitated in wafers taken from the seed-sections of the boules but not in any of the other wafers. The positron data did not reflect this distinctive difference, however, both lifetime and Doppler-broadening results strongly indicate the creation of vacancy-type defects at concentrations in the 1016 cm-3-range. INTRODUCTION
Oxygen and carbon are two dominant impurities in CZ silicon. Oxygen is typically present in concentrations from 10 to 20 ppma and has a dramatic impact on device performance and the yield of circuits. The high concentration of oxygen in CZ silicon arises from the incorporation of oxygen into the growing crystal from the slow dissolution of the crucible by the molten silicon. Although the equilibrium oxygen solubility decreases rapidly with decreasing temperature, the excess oxygen can remain in a metastable solid solution during the cooling of the as-grown silicon crystal. However, oxygen complexing and precipitation typically occur during subsequent thermal treatments of the crystals. It is generally accepted that point defects play an important role in the precipitation process as evidenced by a multitude of observations of oxygen-defect complexes after heat treatment [11. In this contribution, we report results of a systematic investigation by both positron lifetime and Doppler-broadening spectroscopy in an effort to monitor the involvement of vacancies in the oxygen precipitation process. During the recent years this technique has been demonstrated to be a valuable tool for the characterization of defects in crystalline silicon [2]. The results shown and discussed here are directly related to a first series of measurements presented at the latest Conference on Defects in Semiconductors [3]. EXPERIMENTAL
All the wafers were
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