Effects of Combined Ionic and Molecular Oxygen Flow on the Optical and Structural Characteristics of Sputtered TiO2 Thin Films

Titanium dioxide (TiO2) is widely used in high-power laser coatings; however, its optical performance is strongly limited by oxygen-related defects and the insufficiently understood microstructural evolution that occurs during ion-beam sputtering (IBS). While IBS allows ionic and molecular oxygen to be independently tuned, no studies have elucidated how these oxygen species collectively regulate TiO2 stoichiometry and defect formation, nor have they reported the resulting subsurface pinhole structures and their distinct morphology transitions revealed in this work. In this study, TiO2 films were deposited under systematically varying ionic/molecular oxygen compositions, and their optical constants, surface morphologies, chemical states, and subsurface pinhole structures were examined with post-deposition annealing. The results show that the film’s ability to undergo adequate oxidization and remain compact during early growth was determined by ionic oxygen, whereas molecular oxygen mainly affected the defect-related absorption and development of pinhole precursors. Coordinated adjustment of the two oxygen components governed the formation of substoichiometric TiOx regions and oxygen-deficient sites, and lead to distinct transitions in the pinhole morphology—from shallow inverted-cone voids to deeper multicavity structures—after annealing. This study establishes the correlation among oxygen-flow composition, defect formation, and pinhole evolution, and demonstrates that simultaneous oxygen-flow control and thermal reconstruction is an effective route toward achieving dense, low-defect, and low-absorption TiO2 coatings for high-power laser applications.