Wide range investigation of duty cycle and frequency effects on bipolar magnetron sputtering of chromium nitride

Among the different techniques of reactive sputtering, the bipolar and high power impulse magnetron sputtering are growing in interest for the thin films research community. However, the combination of both processes in presence of a reactive atmosphere is extremely complex and the role of the sputtering parameters are key points to control the deposited material properties. In this study, we have investigated the effect of the duty cycle and the pulse frequency on the reactive bipolar sputtering efficiency of chromium in presence of nitrogen. The study has been performed on a wide range of parameters: from 12.5 to 87.5% for the duty cycle, and from 62.5 to 5000 Hz for the frequency. In situ measurements of the magnetron discharge characteristics have been performed (excitation temperature, peak target current and voltage, energy influx at substrate position) in addition to ex situ characterizations of the deposited thin films (structure, microstructure, density, composition, optical and mechanical properties). It appears that the modulation of the duty cycle allows a better control of the mechanical properties due to higher ionization level at the target, while the frequency is better adapted to tune the optical properties that are attributed to a change of texturation and density of the deposited film (confirmed by simulation). All films present a similar microstructure due to the absence of bias applied to the substrate during the deposition process, which leads to a similar energy per atom of deposited species.