1. Field of the Invention
The present invention relates to a method for producing an SOI substrate, the method in which an epitaxial layer is laid on an SOI layer of an SOI substrate obtained by using an ion-implantation delamination method.
2. Description of the Related Art
As a method for producing a bonded SOI substrate, a method in which an SOI layer is formed into a thin film by grinding and polishing and an ion-implantation delamination method (also called Smart Cut method™) are generally known. Of these methods, the method using grinding and polishing is performed as follows. Two silicon single crystal wafers are bonded together via an oxide film without using an adhesive and, after the bonding strength is enhanced by heat treatment (1000 to 1200° C.), one of the wafers is formed into a thin film. The advantage of this method is that the crystallinity of the SOI layer and the reliability of the buried oxide film are equal to those of a normal silicon single crystal wafer, and the disadvantage thereof is that the SOI layer has limited film thickness uniformity (of the order of ±0.3 μm) and material costs are high because two silicon wafers are used to produce one SOI substrate.
On the other hand, the ion-implantation delamination method is performed as follows. An oxide film is formed on at least one of two silicon single crystal wafers. After at least one kind of ion of hydrogen ion and a rare gas ion is implanted into one main surface of one wafer (a bond wafer) such that an ion-implanted layer (a delaminated layer) is formed inside the wafer, the ion-implanted surface and one main surface of the other silicon single crystal wafer (a base wafer) are brought into intimate contact with each other via the oxide film. Then, the product thus obtained is subjected to heat treatment at 500° C. or higher, whereby separation is performed along the delaminated layer. This method has the advantage that a thin-film SOI substrate having an SOI layer with film thickness uniformity of ±10 nm or less can be produced with ease and material costs can be reduced by reusing the delaminated bond wafer more than once.
Incidentally, a thick-film SOI substrate with an SOI layer having a film thickness of several to several tens of micrometers is a highly useful wafer for use in bipolar devices and power devices. However, it is known that producing a high-quality SOI wafer at low costs is relatively difficult even when the above-described method using grinding and polishing or the ion-implantation delamination method is used.
The reason is as follows. In the case of the method in which an SOI layer is formed into a thin film by grinding and polishing, it is necessary to bond a wafer with an oxide film and a bare wafer together, perform heat treatment at 1100° C. or higher to join them together, and perform grinding and polishing until an intended SOI film thickness is obtained. This requires a complicated process, and it is extremely difficult to improve the SOI film thickness uniformity. On the other hand, in the case of the ion-implantation delamination method, the thickness of an SOI layer is determined by the depth to which ions can be implanted (that is, the accelerating voltage of an ion implanter). Thus, with a commonly-used implanter, since the maximum accelerating voltage thereof is of the order of 200 keV, only an SOI layer having a thickness of the order, at most, of 2 μm can be obtained. Therefore, with this method alone, it is impossible to obtain a thick-film SOI layer thicker than 2 μm.
As a method for obtaining such an SOI substrate having a thick SOI layer, an epitaxial layer is formed on the SOI layer of the SOI substrate obtained by the above-described ion-implantation delamination method so as to make the SOI layer thicker. However, the surface of the just-delaminated SOI layer obtained by the ion-implantation delamination method has a high degree of roughness (P-V value: 50 nm or less). If epitaxial growth is carried out on such a surface, an epitaxial layer of inferior quality with a high degree of roughness or high particle level is undesirably obtained. Therefore, some measures have to be taken against this problem.
To address this problem, JP-A-2001-85649 describes that polishing is performed by CMP after delamination and then epitaxial growth is carried out on the surface thus obtained. However, the above measures have the following drawbacks. When a rough and uneven surface after delamination is polished by the CMP equipment, it takes considerable time to flatten the surface, leading to reduced productivity. In addition, this results in poor film thickness uniformity due to high stock removal. Furthermore, due to poor bonding strength, film exfoliation occurs during CMP polishing, or an SOI substrate having insufficient bonding strength is obtained after epitaxial growth.