The influence of substrate biasing on the crystallinity of Al$_2$O$_3$ films

Substrate biasing is an established technique to control and adjust material properties during thin film deposition from a plasma. Film properties of Al$_2$O$_3$ like adhesion, wear resistance, or film hardness are related to the degree of crystallinity. The external bias voltage manipulates the energy distribution function of the ions impinging on the substrate (IEDF). In our contribution, we report the influence of sinus RF biasing at \mbox{1 MHz} on the crystallinity of aluminium oxide films. The films are prepared in a magnetically enhanced, capacitively coupled argon/oxygen discharge, which is excited by \mbox{13.56 MHz} and \mbox{71 MHz} frequencies and used for reactive sputtering of an aluminium target. The target is mounted on the powered electrode and the silicon substrate is placed on an additionally biased electrode maintained at a temperature of \mbox{600$^\circ$ C}. A feedback loop is used to control the partial pressure of O$_2$ in the plasma. The IEDF is measured with a retarding field analyser. The results show that the energy distribution with which the ions hit the surface is important for the control of crystallinity. The diffraction pattern of films biased by a rectangular signal indicates a defined crystal orientation. Additionally, stoichiometry is identified by XPS measurements. The results of the study will be used in the future to get a deeper insight into the nucleation of the film.