Effects of total gas flow rate and sputtering power on the critical condition for target mode transition in Al–O2 reactive sputtering

Reactive sputtering is one of the most commonly used techniques for the fabrication of compound thin films, and the critical condition for target mode transition from metal mode to oxide mode is very important. We investigated the effects of total gas flow rate and sputtering power on the critical condition in Al–O2 reactive sputtering. It was found that the ratio of the number of sputtered Al atoms (NAl) to the number of supplied O atoms (NO) at the critical condition was almost constant, and the ratio of NAl to NO was close to the stoichiometric ratio of Al2O3 (2 to 3). It is thought that the introduced oxygen is gettered by Al atoms almost completely and the target remains in the metal mode below the critical condition. By increasing the amount of supplied O atoms above the stoichiometric ratio of Al2O3, the oxygen supply overcomes the gettering effect. Then, oxygen concentration in the plasma increases abruptly and the target mode changes from metal mode to oxide mode.