Measurement and modeling of plasma parameters in reactive high-power impulse magnetron sputtering of Ti in Ar/O₂ mixtures

A reactive high-power impulse magnetron sputtering (HiPIMS) process using a titanium target in a mixture of Ar/O₂ has been investigated for different modes of operation including pure argon, metallic, transition, and compound mode. The trends and changes in the plasma density n_e and the effective electron temperature T_eff, have been measured by the time-resolved Langmuir probe technique. The same experimental process conditions have also been studied using a recently developed reactive ionization region model (R-IRM), making it possible to compare the acquired experimental results with the model results. It was found that trends in the plasma density and mean electron energy as measured by the Langmuir probe are in good agreement with the results obtained from the R-IRM model for different pulse discharge current densities. The effective electron temperature generally increases with an increasing oxygen flow rate. It is likely due to a reduction of sputtered Ti, due to compound formation on the target, which forces the discharge to increase the electron energy to increase the ionization rate of the process gas (Ar/O₂) to maintain a high HiPIMS discharge current. Small variations in the plasma density were detected between the middle part of the plasma pulse as compared to the end of the plasma pulse, when transitioning from the metal mode to the poisoned mode. It is found that the time-evolution of the electron density is rather well correlated with the discharge current waveform. On the other hand, the mean electron energy did not change significantly between the middle and the end of the plasma pulse. For the lower pulse discharge current, both the model and experimental data have shown a slight increase in the plasma density with increasing O₂ mass flow rate.