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Integrated photonics features applications in high-speed telecommunication, computing, and sensing. These devices are ultimately limited by the optical loss occurring in the waveguide structures. One of its primary sources is surface-roughness- induced scattering and bend-losses. Surface roughness is unavoidably introduced during deposition, mainly during etching and lithographic steps. In photonic integrated circuits, tight bends enable a compact footprint yet increase the mode mismatch loss , radiation loss and scattering loss. Previously, the bend losses were estimated from a parametric model. However, it lacks flexibility w.r.t. the waveguide platform. We apply a recently developed model of the surface-roughness-induced scattering in guided-mode systems to substantiate the dependence of the scattering loss on the bend-radius for waveguides based on a silicon nitride platform. The model incorporates the surface roughness via its autocorrelation. Further, it inherently considers the overlap of the modes with the roughness. As waveguide material, we used both plasma-enhanced CVD silicon nitride as a low-temperature, back-end-compatible process, and low-pressure CVD silicon nitride, as a high-temperature frontend process. As bottom and top cladding, we deposited high-density plasma (HDP) and sputtered silicon oxide, respectively. The latter offers flexibility to adapt the platform for sensing purposes. We evaluate different waveguide widths, bend radii, and wavelengths in the visible and near-infrared ranges. We set the observed propagation losses into context with estimated absorption, scattering, and mode-overlap loss sources and point to their shifting importance at the measured wavelengths. We believe that this model allows to increase our knowledge about the various aspects of loss in guided mode systems and predict the propagation loss based on foregoing absorption and roughness measurements.
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