The magnetron discharge voltage, measured in pure argon (or another noble gas) is characteristic for each kind of metallic targets. This discharge voltage is related to the Ion induced Secondary Electron Emission (ISEE) coefficient of the target material. In reactive sputtering a reaction gas is added to the sputter gas so that compounds (oxides, nitrides, carbides…) can be grown on a substrate. However, this reaction gas is also interacting with the target material forming also oxides, nitrides etc. In this paper we will limit ourselves to the situation in which the reaction gas is oxygen yielding oxides as well on the substrate as on the target surface. These oxides, formed on the target have a profound influence on the behaviour of the discharge voltage. Indeed, the higher the amount of oxygen added to the sputter gas (in this case only argon was used) the more oxide will be formed on the target surface and in the target subsurface region. In the limit, the target is fully plasma oxidised. This phenomenon gives rise to a change of the discharge voltage as a function of the oxygen flow, which is generally illustrated by the well known hysteresis experiments published in literature. As it is normally assumed that the ISEE of oxides is higher than the ISEE of the corresponding pure metallic targets, one expects a decrease of the discharge voltage upon the addition of oxygen to the sputter gas. The discharge voltage was measured for 15 different metallic target materials at constant current before and after plasma oxidation. Plasma oxidation of the target surface was achieved by sputtering the target in pure oxygen. It turned out that only 5 out of 15 target materials behaved "normal" i.e. displaying a lower target voltage when sputtering reactively as compared to metallic mode sputtering without oxygen. Taking the Thornton relation into account, it is possible to establish a relation between the measured discharge voltage and published ISEE values for pure metals. Based on this relation a value for the ISEE coefficient of the oxidized target surface can be calculated. Two distinct groups can be discerned: for one group the ISEE coefficient of the oxidized target surface is larger than the ISEE coefficient of the metal, while the opposite behaviour is noticed for the second group. This difference seems to find its origin in the reduction behaviour of the oxides under ion bombardment, since the ISEE coefficient of the oxide can be related to the simulated degree of reduction of the oxide. It is shown that the ISEE coefficient of the reduced oxides decreases with increasing oxygen content in the target. This is confirmed experimentally by sputtering in pure argon reduced titanium oxide targets with a known composition.
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