Process for producing semiconductor device substrate using polishing guard

Claim 1

A process for producing an SOI-structured semiconductor device substrate having the steps of: bonding polished surfaces of two Si-monocrystal wafers at least one of which has an oxide film formed on the bonded polished surface; and uniformly eliminating part of the thickness of one of the two Si-monocrystal wafers in parallel to the polished surface so that a thin film is resulted from the one Si-monocrystal wafer in addition, the process for producing the SOI-structured semiconductor device substrate comprising the steps of: reducing the diameter of the one Si-monocrystal wafer to be slightly smaller than that of the other Si-monocrystal wafer at least prior to the thin film making step so that the width of the unbonded annular peripheral margin exposed on the surface of the other Si-monocrystal wafer is uniform along all the periphery; forming an annular polishing guard on the cylindrical surface of the one Si-monocrystal wafer and said margin prior to the thin film making step, the guard having a predetermined thickness and being made of a material of a lower polishing rate relative to the polishing rate of the one Si-monocrystal wafer; and then polishing the one Si-monocrystal wafer so as to make the one Si-monocrystal wafer thinned into a thin film, said polishing guard acting to control the polishing rate.