  Lossy dielectrics are a significant source of decoherence in superconducting
quantum circuits. In this report, we model and compare the dielectric loss in
bulk and interfacial dielectrics in titanium nitride (TiN) and aluminum (Al)
superconducting coplanar waveguide (CPW) resonators. We fabricate isotropically
trenched resonators to produce a series of device geometries that accentuate a
specific dielectric region's contribution to resonator quality factor. While
each dielectric region contributes significantly to loss in TiN devices, the
metal-air interface dominates the loss in the Al devices. Furthermore, we
evaluate the quality factor of each TiN resonator geometry with and without a
post-process hydrofluoric (HF) etch, and find that it reduced losses from the
substrate-air interface, thereby improving the quality factor.
