  Understanding the origins of power loss in superconducting interconnects is
essential for the energy efficiency and scalability of superconducting digital
logic. At microwave frequencies, power dissipates in both the dielectrics and
superconducting wires, and these losses can be of comparable magnitude. A novel
method to accurately disentangle such losses by exploiting their frequency
dependence using a multi-mode transmission line resonator, supported by a
geometric factor concept and a 3D superconductor finite element method (FEM)
modeling, is described. Using the method we optimized a planarized fabrication
process of reciprocal quantum logic (RQL) for the interconnect loss at 4.2 K
and GHz frequencies. The interconnects are composed of niobium ($\rm Nb$)
insulated by silicon dioxide made with a tetraethyl orthosilicate precursor
($\rm TEOS-SiO_2$). Two process generations use damascene fabrication, and the
third one uses Cloisonn\'{e} fabrication. For all three, $\rm TEOS-SiO_2$
exhibits a dielectric loss tangent $\tan \delta = 0.0012 \pm 0.0001$,
independent of $\rm Nb$ wire width over $0.25 - 4 \: \mu m$. The $\rm Nb$ loss
varies with both the processing and the wire width. For damascene fabrication,
scanning transmission electron microscopy (STEM) and energy dispersive X-ray
spectroscopy (EDS) reveal that Nb oxide and Nb grain growth orientation
increase the loss above the Bardeen Cooper Schrieffer (BCS) minimum theoretical
resistance $R _{BCS}$. For Cloisonn\'{e} fabrication, the $0.25 \: \mu m$ wide
$\rm Nb$ wires exhibit an intrinsic resistance $R_s = 13 \pm 1.4 \: \mu \Omega$
at 10 GHz, which is below $R_{BCS} \approx 17 \: \mu \Omega$. That is arguably
the lowest resistive loss reported for $\rm Nb$.
