Surface morphology evolution in argon ion-beam polishing of barium titanate thin films

Barium titanate (BaTiO3, BTO) is a promising material for integrated photonics, but surface roughness introduced during thin-film growth degrades optical performance. An argon (Ar) ion-beam polishing process is developed for BTO thin films. The effects of ion energy, beam current, incidence angle, and removed thickness on surface morphology evolution are systematically examined. An incidence-angle response function is used to describe the angular dependence of the morphology evolution. At near-normal incidence (0° to −30°), grain-induced surface undulations are suppressed by ion-induced surface-parallel atomic redistribution, which promotes surface smoothing. An effective polishing window is identified at 300 eV, 100 mA, and 0° to −30°. Under these conditions, the root mean square roughness is reduced from 5.69 to 1.34 nm, corresponding to a 76.5% reduction. In addition, post-polishing annealing at 600 °C in oxygen helps relieve Ar-induced compressive stress and facilitates recrystallization in the near-surface damaged region. These results provide a practical polishing–annealing route toward the integration of high-quality BTO thin films into low-loss photonic devices.