Chemical and thermodynamic influences in ion beam assisted thin film synthesis

Abstract

The main parameter of ion beam assisted deposition of compound films is the ratio γ of impacting ions to condensing atoms. It determines the energy input into the growing film and influences the composition of the film. When highly reactive elements such as transition metals are involved in the process, the elemental composition of the film may also be influenced by the presence of reactive gas molecules, and their partial pressure in the vacuum system. The corresponding process parameter is the ratio δ of impinging gas molecules to condensing atoms. In this case, the final phase and elemental composition of the film is strongly affected by chemical and thermodynamic influences. This is shown by a comparison of ion beam assisted deposition of TiN, VN and CrN. The according metal was evaporated and the growing film irradiated with energetic nitrogen ions. The physical process parameters such as evaporation rate, ion current density and ion energy were the same for all three elements. X-ray phase analysis shows that, whereas it is easily possible to synthesize TiN over a wide range of γ-values, CrN requires a very high irradiation intensity or high γ-value to be deposited as a monophase. VN is intermediate in its behaviour between TiN and CrN. This result is in accordance with the reactivity of the metals towards nitrogen gas and the thermodynamic stability of the resulting nitride. It shows that besides the physical parameters of the process chemical driving forces also play an important role.