1.  Field of the Invention
The present invention relates to a method of manufacturing a thin film transistor (hereinafter referred to as TFT) using a single crystal silicon thin film formed on a substrate having an insulating surface, and to a method of manufacturing a semiconductor device including a semiconductor circuit constituted by TFTs. 
Incidentally, in the present specification, the semiconductor device indicates any device capable of functioning by using semiconductor characteristics, and the category thereof includes an electro-optical device typified by a liquid crystal display device, a semiconductor circuit in which TFTs are integrated, and an electronic apparatus including such an electro-optical device or semiconductor circuit as a part. 
2.  Description of the Related Art
In recent years, VLSI techniques have been remarkably developed, and an attention has been paid to an SOI (Silicon on Insulator) structure for realizing low power consumption.  This technique is such a technique that an active region (a channel formation region) of an FET, which has been conventionally formed of bulk single crystal silicon, is formed of a single crystal silicon thin film. 
In an SOI substrate, a buried oxide film made of silicon oxide exists on single crystal silicon, and a single crystal silicon thin film is formed thereon.  Although various methods are known as methods of manufacturing such an SOI substrate, an attention has been recently paid to a bonded SOI substrate.  The bonded SOI substrate realizes the SOI structure by bonding two silicon substrates as suggested by its name.  This technique has a possibility that a single crystal silicon thin film can be formed in future also on a glass substrate or the like. 
Among the bonded SOI substrates, in recent years, an attention has been especially paid to a technique called Smart-Cut (registered trademark of SOITEC Co.).  Smart-Cut method is a technique developed by SOITEC Co. in France in 1996, and is a method of manufacturing a bonded SOI substrate using hydrogen embrittlement.  The particular technique of the Smart-Cut method is disclosed in “Industrial Research Society (Kogyo Chosa Kai); Electronic Material, August, pp.  83-87, 1977” in detail. 
As another method, there is known a technique called ELTRAN (trademark of Canon K.K.).  This technique is a method of manufacturing an SOI substrate using selective etching of a porous silicon layer.  The particular technique of ELTRAN method is disclosed in “T. Yonehara, K. Sakaguchi and T. Hamaguchi: Appl.  Phys.  Lett.  43[3], 253 (1983)” in detail. 
Even if either one of the methods is used, a single crystal silicon thin film having a desired thickness can be formed on a substrate.  However, in both methods, since a high temperature heat treatment is carried out in a step of bonding two substrates, there arises a problem in which intense stress is generated and remains in the formed single crystal silicon film. 
If the stress at this time remains in an active layer of a TFT formed of the single crystal silicon thin film, it may function as trap levels for carriers or may become a factor to cause change in TFT characteristics with time elapses.  This problem is a very important problem when Smart-Cut method or ELTRAN method is used, and a fundamental solution thereof has been required.