In recent years, a substrate with an SOI (Silicon-On-Insulator) structure having an Si layer on an insulator has received attention. For example, Japanese Patent Nos. 2,608,351, 2,877,800, and 3,048,201 each disclose a method of manufacturing such an SOI substrate.
There has recently been proposed a semiconductor device with a GOI (Germanium-On-Insulator) structure having a Ge layer on an insulator. A semiconductor device with a GOI structure is expected to be superior to a semiconductor device with an SOI structure particularly in performance speed. For example, U.S. Pat. No. 6,501,135 discloses a semiconductor device with a GOI structure and a method of manufacturing a GOI substrate. The manufacturing method disclosed in U.S. Pat. No. 6,501,135 bonds an Si substrate to an insulating film formed on a Ge substrate, thereby obtaining a GOI substrate. The GOI substrate is used to obtain a semiconductor device which employs a thin Ge film having a thickness of 500 Å as an active layer.
To obtain a semiconductor device which employs a thin Ge film as an active layer using a GOI substrate formed by bonding together a Ge substrate and an Si substrate, as disclosed in U.S. Pat. No. 6,501,135, most of the Ge substrate having a thickness of about several hundred μm needs to be removed by polishing or etching, prior to the step of forming the semiconductor device. It is difficult to control the thickness of the thin Ge film to be uniform in removing the Ge substrate by polishing or the like. Accordingly, the semiconductor device to be obtained cannot sufficiently exhibit its superiority as a device with a GOI structure. The semiconductor device is hard to manufacture at high yield, and a process of removing almost the entire part of the Ge substrate increases the manufacturing cost.