The effect of d.c. substrate bias on the properties of nitrogen-rich CNx films

The influence of d.c. substrate bias on the properties of nitrogen-rich CNx films deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD) utilizing transport reactions has been investigated. The film forming species are CN and/or (CN)2 generated by the interaction of the atomic nitrogen from the ICP with a solid pure carbon mesh; they are deposited on the substrate in the presence of nitrogen species from the plasma. A study of the surface topography of the coatings by atomic force microscopy (AFM) shows that the average roughness slightly increases from below 1 nm without bias to 1.6 nm at −300 V. The deposition rate decreases by a factor of 1.3–1.5 (depending on the working pressure) with increasing the bias up to −300 V, mainly as a result of desorption of CN species from the substrate enhanced by the ion bombardment. The CNx films deposited with bias exhibit nitrogen atomic fraction N/(C+N) in the range of 50–60%, as revealed by surface and bulk techniques. The chemical bonding structure of the layers investigated by Fourier transform infrared (FTIR) spectroscopy showed only a marginal influence of the d.c. substrate bias. The increase of the refractive index n from 1.6 to 1.8 is probably due to slight densification of the films deposited with substrate biasing as a result of reduction of voids.