1. Field of the Invention
The present invention relates to methods for manufacturing SOI substrates, semiconductor devices formed using the SOI substrates, and methods for manufacturing the semiconductor devices. Note that “semiconductor device” in this specification refers to general devices that can function by use of semiconductor characteristics.
2. Description of the Related Art
A variety of research related to silicon on insulator (SOI) technology is being carried out because integrated circuits that are manufactured using SOI substrates formed of a single-crystalline silicon film on an insulating surface have advantages that cannot be obtained with integrated circuits that are manufactured using bulk silicon substrates.
For SOI technology, for example, separation by implanted oxygen (SIMOX) technology by which oxygen ions are implanted to a single-crystalline silicon substrate to form a buried oxide layer; bonding SOI technology by which two silicon substrates are bonded to each other with a silicon oxide film interposed therebetween; ELTRAN (registered trademark) technology in which selective etching of a porous silicon layer is used; and the like have been disclosed.
Of these technologies, bonding SOI technology has been attracting attention because therewith an SOI layer with high crystallinity can easily be formed over substrates other than silicon substrates (quartz substrates, ceramic substrates, and the like, for example). However, in manufacture of a bonding SOI substrate, there are problems in that deterioration in quality due to contamination of the SOI layer, warping of the substrate, and the like occur, which adversely affects device characteristics.
Thus, for solving these kinds of problems, technology for manufacture of bonding SOI substrates, by which two wafers are directly bonded to each other, has been proposed (for example, in Reference 1: Japanese Published Patent Application No. 2004-320050). In Reference 1, a method for manufacturing an SOI substrate has been disclosed, in which a wafer having a structure in which a silicon oxide layer, a silicon nitride layer, and a polycrystalline silicon layer are stacked in order over a single-crystalline silicon layer to be an active layer and a wafer for a supporting substrate, which is provided separately, are washed with a solution so that the surface of each of the wafers is activated; the surfaces of the two wafers are directly bonded to each other; heat treatment is performed to bond the wafers; and an SOI substrate is formed by grinding and polishing of the single-crystalline silicon layer to be an active layer.