Effect of chemical bonds on the properties of SiN in thin film transistor liquid crystal display

Abstract

The hydrogenated amorphous silicon nitride (a-SiNx:H) thin films were produced in a radio-frequency plasma-enhanced chemical vapor deposition system using NH3∕SiH4∕N2 mixture source gases at 330°C. In this study the authors determine the structural properties of a-SiNx:H thin films with Fourier transform infrared measurement and relate these to both the processing parameters and their physical/optical properties. The Si–H and N–H bond densities are affected by the processing parameters. They find that the plasma power density and chamber pressure play minor roles compared to the gas flow rate of SiH4, and the role of the electrode spacing is indistinctive. The Si–H, N–H, and Si–N bond densities are important parameters affecting the physical/optical properties of low deposition rate thin film (the thin films in the proximity of a-SiNx:H∕a-Si interface) and high deposition rate (HDR) thin film (bottom gate insulator thin films). The optical band gap (E04) could be tuned by the N radical density. The N radical density increases with increasing N–H and Si–N bond densities and decreasing Si–H bond density. The Si–N bond density and the Si and N dangling bonds enhance the dielectric constant, but the Si–H and N–H bond densities have different effects. The dielectric constant increases with increasing Si–H bond density for the HDR thin films, because the high plasma power density (>38.5W∕cm2) could break more Si–N bonds. Finally, process parameters are obtained for optimized performance of the thin film transistors.