We have developed separation by bonding Si islands (SBSI) process for advanced CMOS LSI applications. In this process, the Si islands that become the SOI regions are formed by selective etching of the SiGe layer in the Si/SiGe stacked layers, and those are bonded to the Si substrate with the thermal oxide layers by furnace annealing. The etching selectivity for SiGe/Si and surface roughness after the SiGe etching were found to be improved by decreasing the HNO3 concentration in the etching solution. The thicknesses of the fabricated Si island and the buried oxide layer also became uniform by decreasing the HNO3 concentration. In addition, it was found that the space formed by SiGe etching in the Si/SiGe stacked layers was able to be filled with the thermal oxide layer without furnace annealing.