This paper proposes a 10 µm thick oxide layer structure, which can be used as a substrate for RF circuits. The structure has been fabricated by anodic reaction and complex oxidation, which is a combined process of low temperature thermal oxidation (500, for 1 hr at H2O/O2) and a rapid thermal oxidation (RTO) process (1050, for 1 min). The electrical characteristics of oxidized porous silicon layer (OPSL) were almost the same as those of standard thermal silicon dioxide. The leakage current through the OPSL of 10 µm was about 100-500 pA in the range of 0 V to 50 V. The average value of breakdown field was about 3.9 MV/cm. From the X-ray photo-electron spectroscopy (XPS) analysis, surface and internal oxide films of OPSL, prepared by complex process, were confirmed to be completely oxidized. Also the role of RTO was important for the densification of the porous silicon layer (PSL), oxidized at a lower temperature. For the RF test of Si substrate, with thick silicon dioxide layer, we have fabricated high performance passive devices such as coplanar waveguide (CPW) on OPSL substrate. The insertion loss of CPW on OPSL prepared by complex oxidation process was -0.39 dB at 4 GHz and similar to that of CPW on OPSL prepared at a temperature of 1050 (1 hr at H2O/O2). Also the return loss of CPW on OPSL prepared by complex oxidation process was -23 dB at 10 GHz which is similar to that of CPW on OPSL prepared by high temperature oxidation.

Microcrystalline silicon embedded in silicon oxide has been prepared by means of a wet oxidation of porous silicon (PS) anodically produced from degenerate Si wafers in a HF solution. As the oxidation proceeded, optical absorptions of the PS specimen in the visible light region shifted obviously to the higher energy side. Visible light emission from the oxidized specimen was observed at room temperature with photoexcitation by a He-Cd laser while the as-prepared specimen emitted no visible lights. These results are discussed in relation to the quantum size effect of the microcrystalline silicon confined in the oxide matrix as well as visible emissions from as-prepared specimens produced from non-degenerate Si wafers.