Sputtered Carbon Nitride Thin Films

Sammanfattning: The relation between the growth conditions and the film structure and properties of reactively magnetron sputtered carbon nitride CNx (0 ≤ x ≤ 0.6) thin films has been studied. The growth of CNx films has been studied when varying the process parameters, such as substrate temperature, N2 (partial) pressure and ion flux. A wide variety of analytical techniques have been employed for analyzing the films, with respect to composition, microstructure, chemical structure, as well as mechanical and tribological properties.Three characteristic structures have been identified depending on the deposition parameters; for growth temperatures below ∼200 °C, the films are homogeneously amorphous, independent of nitrogen concentration and degree of ion bombardment during growth. For temperatures above ∼200 °C, the film structure is predominantly turbostratic or graphite-like, with the basal planes preferentially oriented with their c-axis parallel to the film surface. For low nitrogen concentrations and/or low ion bombardment, the films are porous and the three-dimensional strength of the material is rather poor. If the ion bombardment and the nitrogen incorporation are sufficiently high, the structure evolves into a dense fullerene-like structure, with frequently curved and cross-linked basal planes. Due to the three-dimensionally bonded structure, this phase exhibits high hardness values combined with an very high elasticity. The structural evolution depending on nitrogen concentration, ion bombardment and substrate temperature can to a large extent be explained by chemical sputtering processes, resulting in desorption of volatile CN-compounds from the growth surface.Using scanning tunneling microscopy and spectroscopy, the local electronic structure could be measured and correlated to the microstructure at a specific position. In the fullerene-like films, regions with locally higher band gaps, assumably due to a higher sp3 content, could be observed. This supports the assumption that the high hardness of these films can be correlated to cross-links between graphitic basal planes through sp3-coordinated carbon.In the transition region between the fullerene-like material and the porous graphite-like structure, a material containing densely packed well-oriented nanotubular features is observed. These nanotubes typically have a diameter of 10-50 nm, and protrude by 20-50 nm from the surrounding film. High-resolution transmission electron microscopy revealed that the tubular features consists of concentric layers of CN basal planes, comparable to the case of multi-wall carbon nanotubes.The role of hydrogen on the growth and properties of carbon nitride films has also been investigated. When H2 is added to the discharge gas, the growth rate decreases considerably due to chemical sputtering of, e.g., hydrocarbons, NH3 and HCN molecules from the growth surface. The hydrogen incorporation into the films reduces the mechanical strength of the material, due to hydrogen terminating the strong covalently bonded networks. The hard and elastic fullerene-like phase, however, seems to be the least sensitive to the presence of H2 during growth.