Influence of oxygen addition on the target voltage during reactive sputtering of aluminium

The voltage of an aluminium target changes during magnetron sputtering when oxygen is added to the argon plasma. The variation in target voltage has been ascribed to a target surface modification, which alters the ion-induced secondary electron emission (ISEE) coefficient. As most models assume that the target surface modification was induced by chemisorption of oxygen on the aluminium target, we have measured the influence of chemisorption on the target voltage. At low oxygen exposure an absolute target voltage increase as a function of the exposure was noticed. Extending the oxidation period resulted in an absolute target voltage decrease if the exposure was increased. Comparing these results with the measurements performed regarding reactive sputtering, we came to the conclusion that chemisorption cannot explain the target surface modification during reactive sputtering. Indeed. experiments concerning the stability of the target surface modification induced by reactive sputtering clearly indicated that it is not stable. This shows that the target surface modification during reactive sputtering is not the formation of a stable surface compound by a chemical reaction between oxygen molecules and the aluminium surface, as noticed during chemisorption. The chemical reaction between implanted reactive gas atoms and the target atoms forms the basic idea of the presented approach to describe the target surface changes.