Nonclassical impedance control of the high rate midfrequency reactive magnetron sputter process using harmonic analysis

A method for the impedance control of midfrequency (MF) reactive sputter process for materials which are inapplicable for classical impedance control (e. g., TiO2 and Nb2O5) has been developed. Analysis of target voltage and sputter current at multiples of the fundamental frequency offers a variety of new deduced quantities which are suitable particularly for process control, because of the monotonic dependence on the oxygen partial pressure. For the TiO2 MF process, the principle ability for nonclassical impedance control was shown. For the Nb2O5 MF process, a simple control using the second harmonic of the target voltage was implemented. This tool enabled overcoming the typical hysteresis and allowed stabilizing the deposition processes in any operating point on the s curve of the corresponding hysteresis loop. The growth rates and optical properties were examined by means of multiangle ex situ spectroscopic ellipsometry.