Abstract: The present invention provides a method for processing a semiconductor device wherein a dielectric layer is partially converted into a silicon-oxy-nitride by incorporation of nitrogen atoms into the dielectric layer, which comprises a silicon oxide. Before the introduction of the nitrogen atoms into the dielectric layer, the dielectric layer is provided as a silicon oxide in which the atomic silicon to oxygen ration is greater than ½. In this way, MOS transistors are obtained with a high quality interface between the dielectric region and semiconductor substrate, and a dielectric region which is impermeable to impurity atoms from the gate region and which has a thickness which is substantially equal to the dielectric layer as deposited.