The role of active species in the N2 and N2-H2 RF afterglows on selective surface nitriding of ALD-grown TiO2 films

We find that surface modification characteristics of TiO2 using N2 RF plasma are strongly dependent on the detailed composition of active species in the plasma and the afterglows. The surface nitriding of ALD-grown TiO2 films in pure N2 RF afterglows at room temperature (RT) is found to be more effective in the late afterglows than in the early afterglows. Adding a small fraction of H2 in N2 results in suppression of surface nitriding, suggesting that the change in the composition of the active species in the afterglows by H2 is the origin to the suppressed nitriding performance. Here, we present our analysis on the surface chemical composition after the plasma modification as well as the densities of excited species such as N atoms, N2(A) and N2(X, v) metastable molecules and N2+ ions in the afterglows of RF N2 and N2-H2 (< 5%) at different positions along the downstream by emission spectroscopy. The early afterglow of N2 changes from a pink to a late afterglow where the N + N recombination is the dominant process with the introduction of H2. The roles of active species such as N-atoms and N2+ ions on TiO2 surface nitriding are found to oppose to each other. We find that N atoms enhance the surface nitriding, while N2+ ions are likely to deplete the surface-bound N species.