A method for reducing roughness of an exposed surface of an insulator layer on a substrate, by depositing an insulator layer on a substrate wherein the insulator layer includes an exposed rough surface opposite the substrate; treating the first substrate to form a zone of weakness beneath the insulator layer; and smoothing the exposed rough surface of the insulator layer by exposure to a gas plasma in a chamber. The chamber contains therein a gas at a pressure of greater than 0.25 Pa but less than 30 Pa, and the gas plasma is created using a radio frequency generator applying to the insulator layer a power density greater than 0.6 W/cm.sup.2 but less than 10 W/cm.sup.2 for at least 10 seconds to less than 200 seconds. Substrate bonding and layer transfer may be carried out subsequently to transfer the thin layer of substrate to the insulator layer and to a second substrate.