Monitoring of magnetron target poisoning by measurement of higher harmonics of discharge voltages

Reactive magnetron sputtering suffers from processing stability problems and the optimal experimental conditions for thin film deposition usually lie very close to an abrupt transition from the metallic to the compound mode. Therefore, a fast feedback method is needed to automatically control either the flow of the reactive gas, or the discharge power to keep the process at the desired operating point. A promising method for this process of monitoring, based on the measurement of amplitudes of higher harmonic frequencies of discharge voltages, is proposed. The measurement of the amplitudes can be performed either by an uncompensated probe in the plasma or on the cable between the RF generator and the sputtered target. The sensitivity of the proposed method is significantly better than measurement of other electrical quantities conventionally used for process control. Physical reasons for the change in amplitudes of higher harmonic frequencies during the mode transition are found. The paper shows that the observed changes in amplitudes of higher harmonics by the transition are not caused either by the pressure change, or by the changes in the composition of the gas in the reactor volume. It is found that the amplitudes of higher harmonics reflect primarily the state of the magnetron target poisoning. The amplitudes thus belong to a group of process parameters representing the target state, which are considered to be the best for reliable process control.