WATER PLASMA §IS OF BURIED OXIDE BY PLASMA IMPLANTATION WITH OXYGEN AND

SPIMOX wafers were fabricated using an oxygen-plasma from an ECR plasma source operating at 2.45 GHz. The implantation was carried out with an implant voltage of 70 kV for 3 minutes to achieve a dose of 2x1017 cm-2. The wafer temperature was held around 600 C during the implantation. The secondary ion mass spectroscopy (SIMS) spectrum of the as-implanted sample is shown in figure 3, which reveals a peaked oxygen profile below the surface. The implanted wafer was annealed at 1250 C for 2 hours. A cross-sectional transmission electron microscopy (XTEM) micrograph of the annealed sample is shown in figure 4. It shows an SO1 structure with a silicon over-layer of 500 8, and buried oxide of 250 8, thickness. There is no mass selection in the SPIMOX process and hence, both 0' and 0; ions present in the oxygen-plasma are implanted. The difference in the mass of these ions causes a spread in the oxygen implant profile giving rise to non-uniformity in the oxide formation and alternative modes of SPIMOX formation [2]. A water-plasma may be used as a source of oxygen for achieving a smaller spread in the oxygen implant profile. The ions present in the water plasma are H20+, OH' and 0' which have comparable masses. This helps in reducing the straggle in the implant-profile of oxygen in the silicon (figure 5), leading to a more uniform buried oxide layer. An implantation with water-plasma was carried out for 20 minutes at an implant energy of 60 keV to obtain an oxygen dose of 6.8~10'~ cm-2. The wafer was annealed at 1250 C for 2 hours. The XTEM micrograph of the annealed sample is shown in figure 6. The silicon over-layer thickness was 200 A and the oxide thickness was 700 W. The silicon over-layer obtained was polycrystalline because the wafer temperature during implantation was below 450 OC. Buried oxide has been successfully formed by SPIMOX using both oxygen plasma and water plasma. The implant time could be further reduced with improved wafer temperature control. The buriedoxide structure could be improved by annealing at higher temperatures and for longer time intervals. The crystallinity of the top over-layer can be preserved by implanting pre-heated wafers. The success of this SPIMOX technique can lead to low-cost and high-volume production of SO1 wafers.