Effects of sputtering through small apertures on the refractive index of niobium pentoxide films

Abstract

Niobium pentoxide (Nb2O5) can be reactively rf sputtered to produce high‐index low‐loss amorphous films for use as integrated optical (IO) components. Tight tolerances are required on refractive index, loss, scatter, and geometric form. Of particular interest to IO devices are thin‐film Luneburg lenses which have been successfully employed on lithium niobate (LiNbO3) based acousto‐optic spectrum analyzers. To develop a particular lens configuration it is necessary to sputter through a circular aperture mask in contact with the LiNbO3. Experimental evidence has shown that there is a significant drop in refractive index under fixed deposition conditions as the ratio of aperture diameter to mask thickness A/t is decreased. For an A/t ratio of 7.0 the index is 2.272 while an A/t ratio of 1.0 results in an index of 2.224. Increasing the power to the Nb target and bias sputtering have been successfully applied to raise the index. An increase in power from 70 to 400 W raised lens indices from 2.220 to 2.280. Bias sputtering similarly increases lens indices but distorted the lens profile. The mechanisms influencing refractive index change in amorphous Nb2O5 films sputtered under varying deposition conditions are discussed.