Investigation of thick, low-temperature plasma deposited silica films for waveguide fabrication

Abstract

Optical devices, such as directional couplers made from silica waveguides, are used to route optical signals at points where optoelectronic conversions are necessary. We report the fabrication of silica waveguides using low-temperature plasma deposition and etching processes. The devices are buried-ridge, single-mode waveguide structures with a refractive index difference (Δn) of 0.03 between the guiding and cladding layers. Two different plasma deposition processes with different reactant chemistries (SiH4/N2/N2O) and 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS)/O2 were used to fabricate the waveguides. The deposition rates, uniformities, film stress, and refractive index control obtained using the two techniques are discussed. The plasma etching process (CHF3/CF4/Ar chemistry) defining the guiding layer varied from a vertical profile (90°) down to 75°, to minimize void formation in the coupling regions of the devices. The step coverage obtained using the TMCTS process showed marked improvement when compared to silica films deposited using plasma-enhanced chemical-vapor deposition with SiH4/N2/N2O chemistry. The TMCTS chemistry produced plasma deposited films with increased deposition rates and the required precision for both the thickness and refractive index parameters necessary for waveguide device fabrication.