Optical waveguides have been fabricated and investigated that employ silicon nitride as core and polymer as surrounding cladding material. Buried single-mode waveguides showed a propagation loss of 0.7 dB/cm at 1550 nm. Depending on the thickness of the silicon nitride core, the optical mode can be designed to expand largely into the polymer cladding. Using such a low-confinement structure and a metal strip heater embedded in the polymer cladding material beneath the core region, efficient thermally tunable waveguide Bragg grating filters were demonstrated. Wavelength tuning >57 nm was achieved at 220 mW of electrical heating power.