Effects of substrate position and angle of incidence on the refractive index of sputtered, birefringent Ta2O5Nx films

The refractive indexes of reactively sputtered Ta2O5Nx waveguides can be controlled by adjustment of the fraction (r) of O2 in the N2/O2 gas mixture during deposition. We have extended this work by studying the effect of varying the distance (z) and the angle (ϑ) of the substrate from the usual position. At r<1, the indexes for TE and TM modes vary with z and ϑ and they bear the same relationship to each other as for varying r. However, the changes in geometry also affect the indices at r=1 and birefringence (Δn) is observed for the first time in this case. Variations in nE from 2.07 to 1.97 were observed; Δn increased from 0 to 0.03 over this range. The indexes do not depend on film thickness or deposition rate and the results are not consistent with a stress‐induced birefringence. It is proposed that the changes are due to the effect of the electric field near the substrate, variations in the field being caused both by the substrate position and the fraction of nitrogen in the discharge. In the latter c...