Fabrication and characterization of PECVD oxynitride rectangular and rib optical waveguides

Silicon oxynitride films with varying oxygen/nitrogen ratios have been grown by PECVD using SiH₄, N₂O, N₂ and NH₃ gases. The refractive index of the film can be smoothly varied by changing the SiH₄/N₂O flow ratio. Rectangular and rib optical channel waveguides are successfully fabricated by growing a sandwich oxynitride structure together with dry etching. The waveguide propagation loss has been characterized and annealing is shown to be an effective means to reduce the propagation loss significantly. A broader waveguide will also have a smaller loss than a narrow waveguide if the side-wall is not sufficiently smooth. A rib waveguide has a smaller propagation loss than a rectangular waveguide of the same width because of its larger waveguide size and shallower etch depth. The coupling loss is certainly smaller in the rib waveguide than in the rectangular waveguide.