  Integrated heaters are a basic ingredient within the photonics toolbox, in
particular for microresonator frequency tuning through the thermo-refractive
effect. Resonators that are fully embedded in a solid cladding (typically
SiO\textsubscript{2}) 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 similarly low loss and efficient integrated heater
integration through standard fabrication. Here, we develop a new approach in
which the integrated heater is embedded in SiO$_2$ 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 Si$_3$N$_4$ process flow that includes high-temperature
($>$1000~$^\circ$C) annealing. Microring resonators with a 1~THz free spectral
range and quality factors near 10$^6$ are demonstrated, and the resonant modes
are tuned by nearly 1.5~THz, a 5$\times$ improvement compared to equivalent
devices with lateral heaters\greg{.} 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.
