Conventionally, a method for manufacturing a bonded wafer in which the bonded wafer is manufactured by delaminating a wafer implanted ions after bonding (so-called an ion implantation delamination method) has been known. For example, an SOI (silicon on insulator) wafer and the like are manufactured by using the method for manufacturing a bonded wafer.
The method is a technology of (See also Japanese Patent Application Laid-open (kokai) No. H5-211128), for example in the case of manufacturing a SOI wafer, forming an oxide film on at least one of two silicon wafers, implanting hydrogen ions or rare gas ions from a surface of the silicon wafer to form a micro bubble layer (an enclosed layer) in the wafer, then bringing the surface of the wafer that is implanted ions into close contact with the surface of the other silicon wafer via the oxide film, then performing heat treatment (delaminating heat treatment) to delaminate one of the wafers as a thin film so that the micro bubble layer is a cleavage plane and consequently providing an SOI wafer.
Damage caused by the ion implantation remains in the thin film SOI layer by the conventional ion implantation delamination method as described above. This remaining damage affects device characteristics etc. Thus, in order to remove the damage so as to improve the device characteristics, the SOI layer after delamination is subjected process. For example, the surface of the SOI layer after delamination is subjected to so-called sacrificial oxidation process to remove a damaged layer caused by the ion implantation.
However, the present inventors have examined the bonded wafer in which the thin film (SOI layer of SOI wafer etc.) on a surface of the bonded wafer after delaminating was subjected to conventional process, e.g. sacrificial oxidation process and measured the surface of the thin film by AFM (Atomic Force Microscope). As a result, the present inventors have discovered there were pits (hereinafter referred to as concave defect) having a diameter of 0.5 to 2 μm and a depth of 1 to 4 nm in the surface of the thin film. An existence of the concave defect as described above could affect the currently latest device characteristics.