Diagnostics of phase composition of lithium niobate films on sapphire by principal component analysis using Raman spectra

Thin films of lithium niobate were deposited by RF diode sputtering of a LiNbO3 single-crystal target on a heated sapphire substrate (R-cut, 650 degrees C). Radial distributions of the phase composition of the films along the substrate were obtained by principal component analysis (PCA) using Raman spectra. It was found that at small targetsubstrate distances (12 mm is the distance between the target and the substrate, 76 mm is the diameters of the target and the substrate holder, 30 mm is the size of the film deposition area in axial geometry), the effects of resputtering of the deposited film do not lead to a considerable decrease in the lithium content: no phases other than LiNbO3 are formed in the extended central part of the film, the edge of the film contains no more than 2 % of the undesirable LiNb3O8 phase, and the difference in the measured values of deviation of LiNbO3 phase composition from stoichiometry in the film compared to the target is 0.7 mol% Li2O. These results indicate that the technology for depositing of lithium niobate films can be improved since there is no need to produce a target additionally enriched with Li2O.