On the mechanism of crystal growth orientation of ion beam assisted deposited thin films

The high number of degrees of freedom of the process parameters of ion beam assisted thin film deposition allows the formation of thin films with special features. Among these process parameters are ion energy, ion irradiation intensity, atom condensation rate and ion impact angle. By contrast to plasma deposition methods, these parameters are independent of each other and can be selected over a wide range. By carefully selecting the ratio I/A of condensing atoms to impacting ions, the ion energy and the angle of ion incidence, films with a high degree of crystallographic orientation can be formed. To-date several mechanisms for the development of texture are discussed. Among them is the sputter theory which explains the growth of particular planes by sputtering and channeling. In a comparative study, this theory is applied to transition metal nitrides of group 4, 5 and 6 of the periodic system of elements. Based on results of structural measurements, the mechanisms which are involved in texture formation are discussed as a function of the process parameters arrival ratio I/A, ion energy and ionic species. It turns out that it is not the ion energy deposited per atom but the momentum transfer per unit volume which correlates with the observed changes in preferred crystal orientation.