Measuring the degree of ionization of the metal flux in reactive ionized physical vapor deposition

The standard method for determining the ionization degree of the metal flux at the substrate position in ionized physical vapor deposition involves charge-selective quartz crystal microbalance (QCM) measurements in both ion-admitting and ion-repelling states. This paper outlines the theoretical basis and assumptions of this approach and extends it to reactive deposition processes, where variations in film composition between the two measurements may affect accuracy. The extended method is applied to three high-power impulse magnetron sputtering discharges of titanium in argon–nitrogen gas mixtures. In all cases, a higher nitrogen content is observed in films grown when the charge-selective QCM is in the ion-repelling state. For two of the discharges, this compositional difference leads to significant corrections in the measured ionization degree. Possible causes for this variation and its impact on the measurement are discussed. Additionally, a formula is derived to estimate the upper bound of the compositional correction, providing a criterion for when the extended measurement procedure is necessary.