Chromium reactive sputtering in argon-oxygen and argon-water vapor mixtures

Reactive magnetron sputtering is widely used for various protective coatings deposition. In order to control the mechanisms occurring during the sputtering, we have focused our study on the plasma. A chromium target was sputtered in two reactive gas mixtures: argon/oxygen and argon/water vapor. The argon flow and the sputtering power are kept constant. Increasing progressively the reactive gas flow, we have measured the cathode voltage, the total pressure in the chamber and the intensity of a Cr atom optical emission line. Following these parameters, the transition between a metallic and a poisoned target is clearly described. When beyond this transition, the reactive gas flow is decreased, a hysteresis is observed on the response curves of the different signals. The plasma behavior in the argon/oxygen mixture is different than in argon/water. In the first case, the target voltage increases with the oxygen flow and becomes constant when the target is poisoned. With water vapor, a high target voltage fall is observed at the transition and after this a slow decrease is still observed. Furthermore, any hysteresis in the pressure curves has never been observed with water. Determining the plasma ion species by mass spectrometry allows understanding these different behaviors: in Ar-O2, the target is poisoned by CrO; in Ar-H2O, oxide and hydroxide species are observed. The compounds formed on the target surface are different and therefore induce different voltage variations. The gas reactivity variations and the pumping speed influence the pressure behavior.