FTIR analysis of a-SiCN:H films deposited by PECVD

Abstract

Hydrogenated amorphous silicon carbonitride films (a-SiCN:H) have been deposited at low temperature by mid-frequency pulsed plasma enhanced chemical vapour deposition (PECVD) from systematically varied mixtures of trimethylsilane (SiH(CH3)3), nitrogen, hydrogen and argon. The elemental composition of the films was measured by elastic recoil detection analysis (ERDA). The film stoichiometry was varied in a wide range, i.e. between nitrogen-free and nitrogen rich films, but typically carbon was the most abundant element.The main effort was aimed at getting information about the various chemical bonds in the films by Fourier transform infrared spectrometry (FTIR) in the range 400–7000 cm−1. For the analysis of the transmission spectra optical modelling of the layer stack “vacuum/a-SiCN:H/monocrystalline silicon/vacuum” was used. The applied dielectric function model describing the amorphous a-SiCN:H material comprises Sellmeier dispersion, Urbach absorption and a superposition of Kim oscillator susceptibilities, representing the different infrared active modes of network bonds (e.g. Si–C, Si–N) and terminal hydrogen bonds (e.g. Si–Hx, Si–CHx, C–Hx). The FTIR analysis showed systematic changes in the optical film properties (refractive index, absorption coefficient) as a function of the deposition conditions. The oscillator strengths are well correlated with the a-SiCN:H film composition. Furthermore, the wave number positions of absorption peaks also shift in a systematic manner. The FTIR results were interpreted in terms of the chemical environment of the bonds.