Revisitation of reactive direct current magnetron sputtering discharge: Investigation of Mg-CF₄, Mg-O₂, and Ti-O₂ discharges by probe measurements

The reactive direct current (DC) magnetron sputtering discharges of Mg-CF4, Mg-O-2, and Ti-O-2 were investigated using probe measurements as a function of reactive gas flow ratio. The emission spectroscopy, which was conducted before the probe measurements, demonstrates that all the three DC discharges transit from nonreactive to reactive discharge mode with increasing reactive gas flow ratio. The probe measurements show that the plasma potentials of the Mg-O-2 and Ti-O-2 DC discharges slightly increase or remain almost constant with increasing reactive gas flow ratio, whereas that of the Mg-CF4 DC discharge drastically decreases at the mode transition. For the same change in reactive gas flow ratio, the discharge voltage of the Mg-CF4 DC discharge slightly increases and that of the Mg-O-2 DC discharge drastically increases at the mode transition, whereas that of the Ti-O-2 DC discharge slightly decreases at the mode transition. The changes in the cathode sheath potential difference at the mode transition differ between the Mg-CF4 and Ti-O-2 DC discharges and the Mg-O-2 DC discharge because of the difference in the probability of secondary electron emission at the cathode surface; furthermore, the changes in the anode sheath potential difference at the mode transition differ between the Mg-CF4 DC discharge and the Mg-O-2 and Ti-O-2 DC discharges because of the difference in the probability of negative-ion formation in the plasma bulk. The most informative results obtained in this study were the differences in the potential differences at the cathode and anode sheaths among the Mg-CF4, Mg-O-2, and Ti-O-2 DC discharges. They well demonstrated the effects of the change in secondary-emitted species at the cathode surface and the change in reactive gas concentration in the plasma on the potential configuration.