Discharge voltage measurements during reactive sputtering of oxides

During reactive sputtering, the plasma impedance changes on addition of the reactive gas to the plasma. At constant current, this change in plasma impedance is manifested by a discharge voltage change. Depending on the target material, the discharge voltage decreases or increases when the target becomes fully positioned. To understand this discharge voltage behaviour, 13 different target materials were sputtered in pure oxygen, to fully oxidize the target. To compare with the discharge voltage measured in metallic mode, i.e. in pure argon, the discharge was interrupted after target stabilization in pure oxygen, and oxygen was substituted by argon. After exchanging the gases, the discharge was ignited again and the discharge voltage was measured. In agreement with published results, we distinguish between two groups of target materials: for Al, Mg, Ce and Y the discharge voltage of the fully oxidized target is lower than the discharge voltage measured in pure argon. The opposite behaviour is noticed for the second group of target materials (Ag, Au, Cr, Cu, Nb, Pt, Re, Ta, Ti).