Hydrogen/oxygen radio-frequency plasma processing of LiNbO3

Lithium niobate substrates were processed in plasma of radio-frequency glow discharge in hydrogen and subsequently in oxygen. The modified surface layers were examined using the infrared spectroscopy and 6 Li depth profiling. It was found that hydrogen plasma treatment enhances the OH band absorption which is explained as hydrogenation of Li- reduced samples. At the same time the sample surface is modified so that the reflectivity increases and the extinction coefficient of a wafer is substantially lowered. The Li-reduction is not higher than 50 percent, but the depth of the modified surface layer extends to about 20 micrometers. It is attributed to a relatively high sample heating caused by plasma which facilitates the Li mobility. A possibility of this method as an alternative technique to the annealed proton exchange is mentioned. An interesting reversed 6 Li depth profile was observed, i.e. decreasing with depth. The reason of this phenomenon might be a negative surface biasing in plasma, which makes the Li + ions to drift to the surface. It suggests an idea of thus produced 'buried' waveguide.