Integrated heaters are essential in the photonics toolbox, particularly for microresonator frequency tuning through the thermo-refractive effect. Resonators that are fully embedded in a solid cladding (typically SiO2) allow for straightforward lossless integration of heater elements. However, air-clad resonators, which are of great interest for short wavelength dispersion engineering and direct interfacing with atomic/molecular systems, do not usually have similar low loss and efficient heater integration through standard fabrication. Here, we develop a new approach in which the integrated heater is embedded in SiO2 below the waveguiding layer, enabling more efficient heating and more arbitrary routing of the heater traces than possible in a lateral configuration. We incorporate these buried heaters within a stoichiometric Si3N4 process flow that includes high-temperature (\textgreater1000 ○C) annealing. Microring resonators with a 1 THz free spectral range and quality factors near 10^6 are demonstrated, and the resonant modes are tuned by nearly 1.5 THz, a 5× improvement compared to equivalent devices with lateral heaters. Finally, we demonstrate broadband dissipative Kerr soliton generation in this platform and show how the heaters can be utilized to aid in bringing relevant lock frequencies within a detectable range.
