Buried Oxide Precipitates in a Si Wafer Due to He ION Implantation and High-Temperature Oxidation
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BURIED OXIDE PRECIPITATES IN A Si WAFER DUE TO He ION IMPLANTATION AND HIGH-TEMPERATURE OXIDATION Sadao Nakashima, Jyoji Nakata,1 Junzou Hayashi2 and Kazuo Imai NTT Telecommunications Energy Laboratories, 3-1, Morinosato Wakamiya, Atsugi, 243-0198 Japan 1 Kanagawa University Faculty of Science, 2946, Tsuchiya, Hiratsuka, 259-1293 Japan 2 NTT Basic Research Laboratories, 3-1, Morinosato Wakamiya, Atsugi, 243-0198 Japan
ABSTRACT A new method of producing a buried oxide is proposed. It involves implanting a silicon wafer with helium rather than oxygen ions and then subjecting it to hightemperature oxidation. The voids formed by helium ion implantation and subsequent annealing enhance the diffusion of oxygen atoms into the silicon. The oxygen atoms cause thermal oxide to grow at the void/silicon interface and transform the voids into buried oxide precipitates. Auger electron spectroscopy revealed that the total number of oxygen atoms in the precipitates was 9.3 x 1016 cm-2 and that the peak value of oxygen atom concentration in the wafer was approximately 25%. INTRODUCTION Silicon-on-insulator (SOI) wafers have been studied for nearly the past three decades for the fabrication of low-power high-speed ULSIs [1] and radiation-hardened CMOS [2] and so on. Among SOI wafers, separation by implanted oxygen (SIMOX) [3] is one of the most promising. This is due to the uniformity of the top-Si thickness, which results from oxygen ion implantation. But the crystallinity of the top Si layer tends to deteriorate because a lot of oxygen ions pass through the surface of the wafer. It is necessary to maintain the wafer temperature at around 550°C [4] during the implantation in order to minimize the degradation of the top Si layer. It is known that when hydrogen or helium ions are implanted into a silicon wafer, innumerable small bubbles [5] are generated in the wafer. Subsequent annealing of the wafer causes the bubbles to grow into relatively large voids [6]. The voids have been demonstrated to be a powerful tool for gettering transition metals [7]. Hydrogen and helium ions are so light that they hardly degrade the quality of the top Si layer at all. If an SOI wafer could be formed by light-ion implantation, a high-quality top Si layer may be obtained. We have been investigating the behavior of voids during oxidation with a view to producing an SOI wafer. This paper describes how voids formed by helium ion implantation are transformed into buried oxide during high-temperature oxidation. A mechanism to account for the formation of the oxide is also proposed.
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EXPERIMENTAL
Helium ions were implanted into 150-mm silicon wafers with doses ranging from 3.3 x 1016 to 1.6 x 1017 cm-2 at an acceleration energy of 40 or 55 keV. The implanted wafers were annealed at 1200°C for 30 min in an argon-oxygen gas mixture to form voids. After that, the wafers were oxidized at temperatures between 1200 and 1300°C. The structures of annealed and oxidized wafers were analyzed using cross-sectional transmission electron microscopy (XTEM). The o
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