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The erosion rate of an aluminum target bombarded with Ar+ and O2+ ions was simulated using TRIDYN. An abrupt change of the erosion rate is noticed at a critical mole fraction of O2+ ions. This target behavior can also be described by a simple analytical model showing that the abrupt change finds its origin in an avalanche induced by reaction ion implantation. Indeed, by the reduction of the average sputter yield by compound formation, more reactive ions become implanted into the target as the number of implanted ions depends inversely on the average sputter yield. As such, an avalanche situation can develop which finally results in a completely oxidized target. It is also shown that the critical mole fraction depends linearly on the sputter yield of the target material and that target poisoning induced by ion implantation can result in a hysteresis behavior for the target condition.

Between 0 and 20%-25%, VD increases with O2. It is explained by a reactive species implantation in the target as proposed by Depla and De Gryse (ref 15). The curves reach a maximum and the VD values decrease dramatically to reach a minimum at 40%. The sharp decrease is attributed to the compound layer formation on the target, which can be correlated to the RD decrease (Fig. 2). Indeed, it is known that the secondary electron emission is higher for an oxide than for the corresponding metal (refs 5 & 6). Consequently, the discharge impedance and the target voltage decrease. Finally, at high %O2 (>40%), the discharge voltage increases due to the change of the gas mixture from a mainly argon atmosphere to a 100% oxygen atmosphere. Indeed, the oxygen molecules can get electrons to produce O2-species in the discharge; consequently the discharge impedance decreases and the target voltage increases.