Differential etch rates in z-cut LiNbO3 for variable HF/HNO3 concentrations

We report the experimental measurements for etch rates of the +z and -z faces of single crystal lithium niobate immersed in HF and HNO3 acid mixtures of varying ratios. We find that pure HF produces an etch rate that is a factor of 2 higher than the rate obtained for the more frequently used mixture of HF/HNO3 in a 1 : 2 ratio. We further observe that the quality of etching is improved for either pure HF or HF/HNO3 in a 1 : 4 ratio, again by comparison with use of a 1 : 2 ratio. These results lead to a discussion of the etch chemistry involved, and an explanation for the observed high degree of differential etching between the +z and -z crystal faces.