Abstract: The oxynitride or oxide layer is formed on a semiconductor substrate by subjecting the substrate to UV radiation while exposed to a gaseous atmosphere of O2 and one or more of N2, N2O, H2 and NH3. Thereafter, a silicon nitride layer is formed according to known 4-step gate stack dielectric processing techniques. Alternatively, a 3-step gate stack process is used, namely following UV-oxidation, a further UV-radiation in NH3 may be applied, followed by a rapid thermal anneal process in an inert ambient. By using UV-oxidation as the first step in either a 4-step or 3-step gate stack process, very thin composite dielectric films with equivalent oxide thickness (EOT) below 16 Å and as low as 14.2 Å can be obtained with significant improvement in leakage current density.