Polycrystalline Silicon Grown on Porous Silicon-on-Insulator Substrates
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ABSTRACT Inexpensive full-wafer SOI substrates are appealing for various applications such as ULSI. As an attempt to achieve this goal, low-temperature deposition of silicon on novel porous Si-on-insulator (PSOI) substrates was performed in this work. The bottom insulator was obtained by anodically oxidizing a pre-formed porous silicon film in HCI solution. The thickness, uniformity and quality of the resulted bottom oxide layer as well as the residual porous silicon layer above were well-controlled. Low-temperature PECVD growth of silicon on the PSOI wafer was conducted by using the residual porous silicon as the seed. Crosssectional TEM pictures and electron diffraction patterns showed that poly-Si films were formed on PSOI substrates under the conditions of 98% hydrogen dilution ratio, 20 Watts RF power, and 300 0 C substrate temperature. Further thermal annealing at 1050'C for 30 minutes significantly enhanced the crystallinity of the deposited films. Combined with the excellent insulation ability of the bottom oxide, the technique is suitable for future inexpensive full-wafer SOI fabrication. INTRODUCTION In the area of advanced Si-related materials, silicon-on-insulator (SOI) and porous Si (PS) are two important subjects which contain both scientific interest and the potential for
industrial application. The former presents many advantages over conventional Si substrates in the performance of ULSI. The latter creates the possibility for the birth of Si optoelectronic devices. In recent years, intensive studies in various aspects were conducted on the two materials and the control of materials' quality has been significantly improved. Historically, PS had been used in the fabrication of SOI [1-3]. In these approaches, crystalline Si islands surrounded both at the four sides and at the bottom by PS were first formed on Si wafers. Because PS can be oxidized at a much faster rate than crystal Si, SOI structure was then formed at the Si island regions by applying thermal oxidation to the wafers. For VLSI design and fabrication, separate SOI islands are not practical. The demand for fullwafer scale SOI is obvious. Separation by ion implantation of oxygen (SIMOX) as well as wafer bonding and etch-back are now two commercialized methods to produce full-wafer SOI substrates. However, both of them involve sophisticated and expensive techniques. Therefore the price of these SOI substrates is high. Doubtless to say, inexpensive full-wafer SOI substrates will be appealing for various applications such as ULSI. If full-wafer scale SOI can be produced out of PS, it may play an important role in the field of SOI because PS is easy to form and the production cost is very low. There are two possible ways to fabricate full-wafer SOI by using PS. The first is to oxidize the lower half of a PS layer to form a so-called porous Si-on-insulator (PSOI) structure and then thermally melt the upper PS region to let the Si recrystallize. The second method is to 1007
Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
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