Low-temperature Ar/N2 remote plasma nitridation of SiO2 thin films

Abstract

Low-temperature nitridation of SiO2 thin films by Ar/N2 remote plasma processing was investigated using on-line Auger electron spectroscopy, angle-resolved x-ray photoelectron spectroscopy (ARXPS), and optical emission spectroscopy (OES). Nitridation experiments were performed at 300 °C using 30 W Ar/N2 remote plasmas at 0.1 and 0.3 Torr. Ar/N2 remote plasma exposure of 5 nm SiO2 films for 30 min results in nitrogen incorporation throughout the films, independent of process pressure and plasma reactor configuration (i.e., upstream versus downstream N2 injection). ARXPS indicates a N–Si3 local bonding configuration with second nearest neighbor oxygen atoms. Ar/N2 remote plasma exposure at 0.1 Torr results in higher nitrogen concentrations (8–10 at. %). Reactor configuration has a negligible effect at 0.1 Torr; conversely, downstream N2 injection results in higher nitrogen concentrations (5–6 at. %) than upstream injection (3–4 at. %) at 0.3 Torr. OES indicates that the Ar/N2 remote plasmas contain N2 triplet excited states and ground-state N atoms. The Ar emission intensities and the saturation N concentrations in the resultant films follow similar trends with processing pressure and reactor configuration; the N2 first positive emission intensities run counter to these trends. We infer that low-temperature SiO2 nitridation by Ar/N2 remote plasmas is a two-step process: O removal by Ar+ ion bombardment and N insertion by plasma-generated active N species. Moreover, the first step appears to be rate limiting under the conditions employed in this study. Annealing the oxynitride films in N2 at 900 °C decreases the N concentration and results in a more uniform nitrogen distribution.