Detection of non-congruent lithium niobate crystals using the non-destructive derivative spectrophotometry

A method called derivative spectrophotometry of higher-order (DSHO) is proposed for quality control to detect stoichiometry variations in lithium niobate crystals. The measured parameter is the wavelength of the absorption edge, which is strongly affected by the composition of the substrate in terms of the Li₂O mole fraction. Thus this measurement can be performed with high accuracy of within ±0.05% and is not influenced by the crystal cut, thickness, and surface finish. It is concluded that this method enables crystal producers and customers of wafers to maintain a high-quality standard of the SAW material with respect to relevant physical properties