A silicon-on-insulator (SOI) wafer is a semiconductor substrate used for high-performance devices. An ultrathin and extremely uniform SOI layer is required with the downsizing of devices. In this paper, numerically controlled sacrificial oxidation using localized atmospheric-pressure plasma was applied to the ultraprecision finishing of an SOI wafer. In a basic experiment, the relationship between the oxidation thickness profile and the dwell time was described. Surface roughness was also measured using an atomic force microscope. As a result of applying this finishing technique to a commercially available phi 300 mm SOI wafer, the standard deviation of the thickness of the top silicon layer was, successfully improved from 0.415 to 0.127 nm. (C) 2007 Elsevier B.V. All rights reserved.