Strain and surface damage induced by proton-exchange in y-cut LiNbO3

When Y-cut LiNbO] substrates are proton exchanged in pure benzoic acid to fabricate optical waveguides, they suffer surface damage, and a consequent degradation in optical properties. This effect is mainly produced by a remarkably large strain in the exchanged layer in a direction normal to the surface. This strain leads to a large number of cracks and to the peeling off of the exchanged layer itself. This paper presents a probable explanation of the mechanism involved.