Time-dependent $O2$ mass balance change and target surface oxidation during mode transition in $Ti–O2$ reactive sputtering

Time-dependent $O2$ mass balance change among the amounts injected into the chamber, pumped out from the chamber, gettered by sputtered Ti metal, residing in the chamber, and consumed to oxidize the target surface has been investigated as a function of time elapsed after the discharge ignition in $Ti–O2$ reactive sputtering. From the mass balance results obtained, target surface coverage has been estimated. In the period of up to 10 s after discharge ignition, the gettering of $O2$ by sputtered Ti dominated the process change. In this period, the target surface oxidation rate was low. In the period of 20–50 s, the amount of $O2$ consumed to target surface oxidation surpassed the amount of $O2$ gettered by deposited Ti, resulting in a drastic increase in the target coverage. After the target surface oxidation was completed, the process became stable. In this period, the amount of $O2$ pumped out without causing any process changes increased and a very small amount of $O2$ was consumed to oxidize the target surface. The equilibrium in the balance between the formation and sputter etching of the oxide layer on the target surface resulted in the stable condition. Calculated target coverage achieved more than 250 monolayers at 600 s after the glow discharge ignition. This value equated with the oxide thickness of about 100 nm. It is concluded that the nonlinear target surface oxidation process causes a nonlinear process change occurring during the mode transition.