Dependence of electrical properties on nitrogen profile in ultrathin oxynitride gate dielectrics formed by using oxygen and nitrogen radicals

We studied nitrogen incorporation in ultrathin oxynitride films by using oxygen and nitrogen radicals, and investigated the dependence of the electrical properties on the nitrogen profile. We found that the nitrogen position in the films could be controlled by using different processing sequences, and that the N concentration could be controlled at values up to 16%. In this process, the interface roughness depends on nitrogen position and nitrogen concentration: the interface roughness tends to increase as the N position close to the SiO2–Si interface and increase with N concentration. The results of an analysis of the electrical properties of these oxynitride films indicated that the best way to form the film was by radical nitridation after radical oxidation. These results show that the nitrogen position should be kept away from the SiO2–Si interface and nitrogen amount should be localized at the surface. Using this process, we have successfully achieved a low-leakage 1.5 nm oxynitride (equivalent oxide thickness) and maintained good device performance. This 1.5-nm-thick oxynitride has a leakage current two orders of magnitude less than that of 1.5-nm-thick SiO2 without decreasing the drain current. Radical oxynitridation should thus be very useful in making high-quality ultrathin gate-insulator films.