Contents I. Introduction
IT HAS been showed that silicon oxynitride is a promising material for replacing the existing silica-based passive optical devices such as grating, interferometers, ring-resonator filters, finite impulse response (FIR) filters, Y-junctions and splitters [1] – [5]. Silicon oxynitride has readily tunable optical properties such as refractive index as well as the reduction of absorption loss in spectral regions of interest. The foremost constraint on waveguide design is the selection of the refractive index of core and cladding layer materials. The larger the index difference, the lower the minimum allowable bending radius for waveguide design. Hence the high index-contrast silicon oxynitride permits the design and fabrication of more compact waveguiding devices. This is particular important for on-chip optical interconnect in microelectronic technology. It was reported recently that the radii of curvature in a ring-resonator filter could be as small as a few hundred micrometers by using the silicon oxynitride material [3]. However, the loss of this material, including the scattering losses and fiber-to-waveguide coupling losses, is still a major concern. The optical absorption loss at 1.55 um wavelength in plasma enhanced chemical vapor deposition (PECVD) or low-pressure chemical vapor deposition (LPCVD) oxynitride is mainly due to the vibration band of N-H bond stretching [2], [6]. A number of methods for solving this drawback have been proposed. The hydrogen, incorporated into the silicon oxynitride film was found to decrease by more than 40% by thermal oxidation of LPCVD silicon-rich silicon nitride film [7] – [8]. In PECVD samples, the hydrogen content can also be reduced significantly with a post-deposition thermal oxidation or annealing [9]. Despite several tryouts for fabricating oxynitride based waveguide devices were presented [10] – [13], compositional study, especially the hydrogen bonds within the oxynitride layer is lacking. In this work, we report on the optical and compositional properties of silicon oxynitride films prepared by PECVD from nitric oxide, ammonia, and silane gas mixtures.
