Defect Annihilation in Czochralski-Grown Silicon During Out-Diffusion Process Probed with Variable-Energy Positron Beam
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DEFECT ANNIHILATION IN CZOCHRALSKI-GROWN SILICON DURING OUTDIFFUSION PROCESS PROBED WITH VARIABLE-ENERGY POSITRON BEAM T. KITANO', L. WEI-, Y. TABUKI", S. TANIGAWA" AND H. MIKOSHIBA"
"*VLSI
Development Division, NEC Corporation, 1120 Shimokuzawa, Sagamihara, Kanagawa, 229 Japan "Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki, 305 Japan ABSTRACT The defect annihilation in CZ-crystal was first detected during the process of oxygen out-diffusion, by the positron measurements with a variable-energy beam. The defects, which were related to oxygen atoms such as oxygen cluster, were mainly annihilated at the high temperature, ex., 1150°C. The defect concentration was decreasing down to one tenth, compared with that for an as-grown crystal. INTRODUCTION Oxygen is inevitably introduced into Czochralski-grown silicon from a quartz crucible during crystal growth. In its precipitated form, oxygen plays an important role in the creation of internal gettering centers, used for removing harmful metallic impurities from an active layer of silicon devices [1]. Suitable heat treatment cycles are carried out to form the denuded-zone near the silicon surface and intentionally to generate the oxygen precipitation inside silicon crystal. The first process in the heat treatment cycles is outdiffusion of oxygen from the silicon surface at high temperature (>1 100"C). One aim of this process is to reduce the concentration of supersaturated oxygen below its solubility [2,3]. Other aim of the first process is to eliminate grown-in defects from the denudedzone (solution treatment). The crystal quality in the denuded-zone is strongly dependent on the existence of grown-in defects such as vacancy, silicon self-interstitial, oxygen clusters and vacancy-oxygen complex. However, the direct observation concerning with the annihilation of grown-in defects has not been reported yet. In the present work, the annihilation of grown-in defects was first detected during the process of oxygen out-diffusion, by the positron measurements with a variable-energy beam. The positron annihilation technique is very effective for studying vacancy-type defects and oxygen clusters in silicon, because a positron has a tendency to be trapped by neutral and negatively charged defects [4,5]. Especially, the usage of a variable-energy positron beam makes it possible to characterize defects near the surface and/or with a depth distribution [6,7]. EXPERIMENTAL The starting materials used in this work were 6 inch p-type (001) Czochralski (CZ)grown silicon wafers cut from the same place in an ingot (as-grown crystals). The concentration of oxygen in the as-grown crystals was about 1.5x10'3 atoms/cm3 , and their resistivity was about 10 Clcm. Two series of samples were made from the as-grown crystals. One series of samples were isothermally annealed at 1150 0C in nitrogen ambient in order to investigate the annihilation behavior of grown-in defects during the process of oxygen out-diffusion. The annealing times were 0, 0.25, 0.5, 1, 1.5, 2, 4, 6 and
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