Contents I. Introduction
SILICON-ON-INSULATOR (SOI) wafer is of prime importance for integrated optoelectronic circuits as it offers potentiality for monolithic integration of optical and electronic functions on a single substrate. As silicon is transparent at wavelengths larger than 1.1 , including optical communication bands, the silicon film of SOI substrates can be used to fabricate low-loss optical waveguides [1], [2]. Silicon/silicon dioxide waveguides benefit from a large refractive index difference, inducing high electromagnetic field confinement in the silicon guiding layer that in turn allows to reduce the waveguide size to sub-micrometer values [3]–[6]. Nevertheless, in order to use SOI waveguides for optical communications, both polarization insensitivity and single mode propagation have to be simultaneously fulfilled. It has been shown that these conditions can be achieved using deeply etched rib SOI waveguides with dimensions of the order of 1 [7]. At , compact devices are obtained with square strip waveguides provided a square size smaller than 320 nm is used to insure single-mode condition. Due to the etching process [reactive ion etching (RIE)], those devices generally suffer from sidewall roughness. Such a random phenomenon constitutes the dominant source of propagation loss [8]. The use of an oxidation step or of an anisotropic etching added to an RIE etching process to reduce sidewall roughness has led to demonstrate the feasibility of low-loss SOI submicron waveguides [3]. In order to predict the impact of sidewall roughness on propagation loss, a model based on a planar optical waveguide has been developed [8]. Taking into account two-dimensional (2-D)-confinement, this model has then been extended to the case of 2-D structures [9]. Based on the model described in [8] and [9], a numerical investigation of scattering loss induced by sidewall roughness as a function of the size of SOI square strip waveguides with cross-sections ranging from 500 nm × 500 nm to 150 nm × 150 nm has recently been published [10]. It has been demonstrated that scattering loss is not only linked to sidewall roughness but also strongly depends on the waveguide cross-section and decreases when the size is reduced below a given value, due to a lower optical confinement. As a result, scattering loss is strongly correlated to field confinement and the smallest structures can be useful for three-dimensional (3-D) tapers designed for low-loss coupling between polarization-insensitive microwaveguides and single-mode optical fibers [11].
