CHEMICAL AND THERMODYNAMIC INFLUENCES IN ION-BEAM-ASSISTED THIN-FILM SYNTHESIS

The main parameter of ion beam assisted deposition of compound films i s the ratio gamma 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 the ir partial pressure in the vacuum system. The corresponding process pa rameter is the ratio delta of impinging gas molecules to condensing at oms. In this case, the final phase and elemental composition of the fi lm is strongly affected by chemical and thermodynamic influences. This is shown by a comparison of ion beam assisted deposition of TiN, VN a nd CrN. The according metal was evaporated and the growing film irradi ated with energetic nitrogen ions. The physical process parameters suc h as evaporation rate, ion current density and ion energy were the sam e for all three elements. X-ray phase analysis shows that, whereas it is easily possible to synthesize TiN over a wide range of gamma-values , CrN requires a very high irradiation intensity or high gamma-value t o be deposited as a monophase. VN is intermediate in its behaviour bet ween 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 t he process chemical driving forces also play an important role.