Ultra-smooth silicon nitride waveguides based on the Damascene reflow process: fabrication and loss origins

On-chip optical waveguides with low propagation losses and precisely engineered group velocity dispersion are important to nonlinear photonic devices such as soliton microcombs, and likewise can be employed for on-chip gyroscopes, delay lines, or Brillouin lasers. Yet, despite intensive research efforts, nonlinear integrated photonic platforms still feature propagation losses orders of magnitude higher than in standard optical fiber. The tight confinement and high index contrast of integrated waveguides make them highly susceptible to fabrication-induced surface roughness, causing dominant scattering losses. Therefore, microresonators with ultra-high-$Q$-factors are, to date, attainable only in polished bulk crystalline or chemically etched silica-based devices, which pose, however, challenges for full photonic integration. Here, we demonstrate the fabrication of silicon nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) waveguides with unprecedentedly smooth sidewalls and tight confinement with record-low propagation losses. This is achieved by combining the photonic Damascene process with a novel reflow process, which reduces etching roughness, while sufficiently preserving dimensional accuracy. This leads to previously unattainable mean scattering $Q$-factors of $12\times {10}^6$ for tightly confining waveguides with anomalous dispersion. Via systematic process step variation and two independent characterization techniques, we differentiate the scattering and absorption loss contributions and reveal metal-impurity-related absorption to be an important loss origin. Although such impurities are known to limit optical fibers, this is the first time, to the best of our knowledge, they are identified—and play a tangible role—in absorption of integrated microresonators. Taken together, our work provides new insights into the origins of propagation losses in ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguides and provides the technological basis for integrated nonlinear photonics in the ultra-high-$Q$ regime.