Particle-in-cell/Monte Carlo collisions treatment of an Ar/O₂ magnetron discharge used for the reactive sputter deposition of TiO_x films

The physical processes in an Ar/O₂ magnetron discharge used for the reactive sputter deposition of TiO_x thin films were simulated with a 2d3v particle-in-cell/Monte Carlo collisions (PIC/MCC) model. The plasma species taken into account are electrons, Ar⁺ ions, fast Ar_f atoms, metastable Ar_m^* atoms, Ti⁺ ions, Ti atoms, O⁺ ions, O₂⁺ ions, O⁻ ions and O atoms. This model accounts for plasma–target interactions, such as secondary electron emission and target sputtering, and the effects of target poisoning. Furthermore, the deposition process is described by an analytical surface model. The influence of the O₂/Ar gas ratio on the plasma potential and on the species densities and fluxes is investigated. Among others, it is shown that a higher O₂ pressure causes the region of positive plasma potential and the O⁻ density to be more spread, and the latter to decrease. On the other hand, the deposition rates of Ti and O are not much affected by the O₂/Ar proportion. Indeed, the predicted stoichiometry of the deposited TiO_x film approaches x=2 for nearly all the investigated O₂/Ar proportions.