Characterization of remote inductively coupled CH4–N2 plasma for carbon nitride thin-film deposition

Abstract

We investigated reaction characteristics in a CH4∕N2 plasma for deposition of amorphous CNx thin films (a-CNx) by evaluating the change in electron density using the wave cutoff method, and the behavior of ions and radicals with an optical emission spectroscopy (OES). An inductively coupled plasma source that was 30cm away from the substrate stage was used for the discharge. The change in electron density in the substrate region and OES spectra in the plasma-source region were evaluated to investigate both the reaction mechanism and the remote effect while varying process conditions such as rf power, pressure, and gas-mixing ratio. We found that the electron density in the remote CH4∕N2 plasma was closely related to recombination reactions of major ions such as N2+, CH4+, CH3+, and H2+ during diffusion from the plasma source to the substrate. The electron density and optical emission of major ions and radicals in the CH4∕N2 plasma increase at higher rf power. The ratio [N]∕([N]+[C]) in a-CNx films, as measured by auger electron spectroscopy, also increases with rf power since more excited N and C species are generated. For increasing pressure, the change in electron density and emission spectra showed different behavior, which arose from recombination of ions that generated more CH4, Nx (x=1,2), and CN radicals. The majority of positive ions generated from N2 species are greatly affected by the remote effect, while the majority of positive ions generated from CH4 species are not significantly influenced, since each species has different losses dependent on the pressure. A higher N2 gas fraction in the gas mixture generated more CN radicals, which resulted not only in more N incorporated into a-CNx films but also to a reduction of H passivation that retards formation of hybrid bonding between C and N in the films. These results suggest that efficient H abstraction is required to achieve more NC triple bonding in CH4∕N2 plasma deposition.