Integrated circuits using an SOI (silicon on insulator) substrate where a thin single-crystal silicon layer is formed over an insulating layer, instead of using a bulk silicon wafer, have been developed. By taking advantage of features of a thin single-crystal silicon layer, transistors in the integrated circuit can be formed in such a way that the transistors are electrically isolated for each element completely. Further, since the transistors can be formed as fully depleted transistors, a semiconductor integrated circuit can be manufactured to have high added value such as high integration, high-speed driving, and low power consumption.
As one method of manufacturing an SOI substrate, there is a known method of manufacturing an SOI substrate in accordance with a bonding technique in which a hydrogen ion implantation step and a separation step are combined. In this method, an SOI substrate is manufactured mainly by the following process. Hydrogen ions are implanted into a silicon wafer to form a damaged region at a predetermined depth from the surface. A silicon oxide film is formed by oxidizing another silicon wafer which serves as a base substrate. The silicon wafer with the hydrogen ions implanted therein is bonded to the silicon wafer with the silicon oxide film formed therein, so that the two silicon wafers are attached to each other. Heat treatment is performed thereon so that the wafers are cleaved from each other at the damaged region. Another heat treatment is performed in order to improve bonding force of a silicon layer attached to the base substrate.
Moreover, there is another known method of manufacturing an SOI substrate, in which a silicon layer separated from a silicon wafer is attached to a glass substrate (see Patent Document 1: Japanese Published Patent Application No. 2004-087606 and Patent Document 2: Japanese Published Patent Application No. H11-163363).