1. Field of the Invention
The present invention relates to a method for manufacturing a silicon-on-insulator (SOI) substrate, a semiconductor device using the SOI substrate, and a method for manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, a silicon-on-insulator (SOI) substrate has been used for a semiconductor device for a high-performance device. By utilizing features of a thin single crystal silicon layer formed over an insulating layer, transistors formed in the integrated circuit can be electrically separated from each other completely. Further, each transistor can be formed as a fully-depleted transistor, and thus a semiconductor integrated circuit with high added value such as high integration, high speed driving, and low power consumption can be realized.
As a method for manufacturing such an SOI substrate, a so-called hydrogen ion implantation separation method in which hydrogen ion implantation and separation are combined is known. A typical process of a hydrogen ion implantation separation method will be described below.
First, hydrogen ions are implanted into a silicon substrate to form an ion implantation layer at a predetermined depth from a surface of the substrate. Then, another silicon substrate which functions as a base substrate (a supporting substrate) is oxidized to form a silicon oxide layer on its surface. After that, the silicon substrate into which the hydrogen ions are implanted and the silicon oxide layer on the other silicon substrate functioning as the base substrate are disposed in close contact with each other, so that the two silicon substrates are bonded to each other. Then, heat treatment is performed, so that one of the silicon substrates is split and a thin single crystal silicon layer is formed on the base substrate side.
The single crystal silicon layer formed by the above method is very thin and has a thickness of approximately 50 nm to 300 nm in general. Therefore, the single crystal silicon layer formed by the above method is really suitable for a transistor for which high integration, high speed driving, and low power consumption are needed. On the other hand, in the case where use of a power device, a photoelectric conversion device, and the like is considered, the single crystal silicon layer needs to have a certain thickness from the viewpoint of improvement in withstand voltage, improvement in photoelectric conversion efficiency, and the like.
The thickness of a single crystal silicon layer formed by a hydrogen ion implantation separation method mainly depends on accelerating voltage in a step of ion implantation. Since an ion implantation layer is formed in a shallow region when accelerating voltage is reduced, the single crystal silicon layer is formed thin. On the other hand, when accelerating voltage is increased, the single crystal semiconductor layer is formed thick.
This shows that acceleration voltage needs to be increased simply to make the single crystal semiconductor layer thick. However, it is not practically easy to form a thick single crystal semiconductor layer while increasing accelerating voltage. This is because, in the case of using an ion implantation apparatus suitable for mass production (an apparatus capable of supplying a large amount of current), accelerating voltage cannot be more increased than a certain level due to its limit. In the case of using an ion implantation apparatus in which a small amount of current is supplied, acceleration voltage can be increased. However, it takes time to obtain a predetermined injection volume of ions, so that it is not preferable in terms of productivity. Further, in the case where ions are accelerated by high voltage exceeding 100 kV, harmful radiation may be generated; therefore, there is a safety problem.
In order to solve the above-mentioned problems, a method is examined in which a single crystal semiconductor layer is made thick not by acceleration voltage in a step of ion implantation but by epitaxial growth (for example, see Patent Documents 1 and 2).
In Patent Document 1, a silane-based gas is subjected to hydrogen reduction to epitaxially grow on a single crystal semiconductor layer at 1100° C. to 1200° C. by vapor-phase growth (vapor-phase epitaxial growth) such as a chemical vapor deposition (CVD) method. Alternatively, the silane-based gas is made to epitaxially grow at 600° C. to 900° C. by a molecular beam epitaxy method.
In Patent Document 2, an amorphous silicon layer is provided on a surface of a single crystal semiconductor layer by a plasma CVD method or the like. After that, heat treatment is performed at 1100° C. or higher for 60 minutes, so that the amorphous silicon layer is made to solid-phase epitaxially grow with the use of the single crystal semiconductor layer as a nucleus.
In Patent Document 3 that is a patent application, in order that epitaxial growth proceeds on a single crystal silicon layer, laser treatment or the like is performed on the single crystal silicon layer so as to repair crystal defects of the single crystal silicon layer serving as a seed layer. The reason of this is as follows: epitaxial growth does not proceed well due to crystal defects which are generated in the single crystal semiconductor layer by a hydrogen ion implantation step or a separation step in a hydrogen ion implantation separation method.