Collisional processes in preferential sputtering of tantalum oxide

In order to understand relevant mechanisms of preferential sputtering, measurements of sputtering of Ta 2 O 5 with 1.5 keV He ions are compared with computer calculations. The experimental data contain the changes in surface composition, the characteristic fluence to reach steady state, the compositional depth distribution of the altered layer and the steady state sputtering yield with 2 keV He + bombardment. All of these experimental values depend on the angle of incidence. The experiments are simulated using the binary collision code TRIDYN in which the time evolution of the surface layer composition is included by a dynamic rearrangement of the surface atoms after each collision cascade. Furthermore, the contributions of different collisional sputtering processes are modelled and the initial and steady state sputtering yields are calculated. The experimental results can be reproduced quantitatively by the calculations, i.e. the results are explained entirely on the basis of collisional processes. The steady state is found to be governed only by the sputtering process itself and not by diffusion or transport phenomena. Starting from the initial sputtering yields of tantalum and oxygen, we can therefore calculate that steady state surface composition using a simple formula based on particle conservation. The importance of various collision processes for the preferential sputtering effects is analysed in detail.