Fabrication of low-loss channel waveguides in Al₂O₃ and Y₂O₃ layers by inductively coupled plasma reactive ion etching

Etching of amorphous $Al_2O_3$ and polycrystalline $Y_2O_3$ films has been investigated using an inductively coupled reactive ion etch system. The etch behaviour has been studied by applying various common process gases and combinations of these gases, including $CF_4/O_2$, $BCl_3$, $BCl_3/HBr$, $Cl_2$, $Cl_2/Ar$ and $Ar$. The observed etch rates of $Al_2O_3$ films were much higher than $Y_2O_3$ for all process gases except for Ar, indicating a much stronger chemical etching component for the $Al_2O_3$ layers. Based on analysis of the film etch rates and an investigation of the selectivity and patterning feasibility of possible mask materials, optimized optical channel-waveguide structures were fabricated in both materials. In $Al_2O_3$, channel waveguides were fabricated with $BCl_3/HBr$ plasma and using a standard resist mask, while in $Y_2O_3$, channel waveguides were fabricated with Ar and using either a resist or a sputter deposited $Al_2O_3$ mask layer. The etched structures in both materials exhibit straight sidewalls with minimal roughness and sufficient etch depths (up to 530 nm for $Al_2O_3$ and 250 nm for $Y_2O_3$) for defining waveguides with strong optical confinement. Using the developed etch processes, low additional optical propagation losses (on the order of 0.1 dB/cm) were demonstrated in single-mode ridge waveguides in both $Al_2O_3$ and $Y_2O_3$ layers at 1550 nm.