MicroRaman investigation of optical quality lithium niobate wafers

MicroRaman spectroscopy has been used for the surface characterization of lithium niobate (LiNbO3) crystals. 3" wafers with different Li/Nb ratio, i.e., conventional congruent (CLN) and quasi-stoichiometric LiNbO3 have been analyzed. A correlation between the width of the 150cm-1 and 870cm-1 line and the crystal composition has been found. A narrowing of the linewidths for quasi-stoichiometric crystals has been observed, showing an ordered structure, if compared with CLN. The 870cm-1 line has been used to study the surface quality of 3" Z-cut CLN crystals after the wafering process. The presence of a surface structural disorder up to 30micron has been found. Various etching methods have been employed in order to minimize both the thickness of the damaged layer and the degree of damage during the wafer slicing and polishing processes. A reliable surface stress release method has been found for optical surface finishing of LiNbO3 substrates