Abstract: A method of producing a semiconductor device comprising a silicon body in which there is located an oxide layer that is sunk locally from a surface of said body and over at least part of the thickness of said body, comprising the steps of providing said silicon body, locally providing on said silicon body surface a first masking layer capable of masking the underlying silicon against oxidation, forming a recess in a part of said silicon body at the area at which said sunken oxide layer is to be provided, covering the side walls of said recess with a second masking layer to protect the underlying silicon against oxidation and oxidizing said silicon at the unmasked area at the bottom of said recess to form said sunken oxide layer.