Influence of negative ions on the stoichiometry and structure of carbon nitride films deposited by reactive magnetron sputtering

Magnetron-sputtered CNx thin films are primarily employed as hard, low-friction protective and tribological coatings, as solar cells, and for catalytic applications. Lower growth rates and a reduced N content, favoring graphitic sp2 structures, hinder industrial scalability due to prolonged deposition times and produce softer, less dense films with inferior hardness, elasticity, and wear resistance. Carbon nitride films deposited by magnetron sputtering exhibit growth behavior strongly influenced by plasma–surface interactions. However, nitrogen resputtering and reduced film growth rates are commonly attributed to chemical etching by positive ions. We propose an additional, unreported power-dependent mechanism involving negative ions formed at the carbon target. These ions are accelerated through the plasma sheath, reaching the substrate with high kinetic energy and inducing both chemical and physical resputtering. This effect is localized to the geometrical projection of the target, as shown by spatially resolved analysis: ellipsometry reveals thickness reduction, and x-ray photoelectron spectroscopy and Raman spectroscopy indicate nitrogen depletion within this region. Correlation between stoichiometry and structural signatures confirms the decisive role of negative ions in modifying the film composition and microstructure. At the same time, the composition of the gas mixture exerts only a minor effect.