In manufacturing an SOI (Semiconductor On Insulator) by bonding two substrates, the following representative methods can be used.
In the first method, two substrates are bonded while inserting an oxide film between them. Grinding and polishing are performed from the lower surface of one of the two substrates to leave a semiconductor layer (SOI layer) having a desired thickness on the oxide film.
The second method uses porous Si (see Japanese Patent Laid-Open No. 5-21338). In the representative process of this technique, a porous Si layer, epitaxial Si layer, and oxide film are in turn formed on a seed substrate. After that, the resultant substrate is bonded to a handle substrate to manufacture a bonded substrate, and the bonded substrate is then divided using a porous layer.
The third method uses hydrogen ion implantation (see Japanese Patent Laid-Open No. 5-211128). In the representative process of this technique, hydrogen ions or rare gas ions are implanted into an Si substrate having an oxide film to form an ion-implanted layer. The resultant substrate is bonded to a handle substrate to manufacture a bonded substrate. The ion-implanted layer is then changed into a micro-cavity layer by annealing, thereby dividing the bonded substrate.
SOI substrates manufactured by the above manufacturing methods have a structure in which an insulating film (SiO2) is exposed to the side surface of the SOI substrate. As a result, the insulating film (SiO2) exposed to the side surface of the SOI substrate is selectively etched in forming devices such as a transistor. The SOI layer on the insulating layer overhangs like a terrace, and the strength becomes low. In this state, the SOI layer flakes off and produced Si fragments may cause damage to the substrate surface, resulting in a low yield.
Japanese Patent Laid-Open No. 8-195483 discloses a technique to solve the above problem. In this technique, the central portion of the Si substrate is oxidized while the peripheral portion of an Si substrate is masked by an Si3N4 film. After that, the surface of the Si substrate is polished, thereby manufacturing a first substrate having an oxide film whose peripheral portion is surrounded by Si. The first substrate is bonded to a second substrate to manufacture an SOI substrate.
The method described in Japanese Patent Laid-Open No. 8-195483 can prevent the SOI layer from flaking off by etching the oxide film in manufacturing devices since the oxide film is not exposed to the side surface of the SOI substrate.
However, the method described in Japanese Patent Laid-Open No. 8-195483 complicates the SOI substrate manufacturing process because masking of the peripheral portion of the Si substrate, and polishing after oxidation must be executed.