  We present a lab-on-chip experiment to accurately measure losses of
superconducting microstrip lines at microwave and sub-mm wavelengths. The
microstrips are fabricated from NbTiN, which is deposited using reactive
magnetron sputtering, and amorphous silicon which is deposited using
plasma-enhanced chemical vapor deposition (PECVD). Sub-mm wave losses are
measured using on-chip Fabry-P{\'e}rot resonators (FPR) operating around $350\
$GHz. Microwave losses are measured using shunted half-wave resonators with an
identical geometry and fabricated on the same chip. We measure a loss tangent
of the amorphous silicon at single-photon energies of $\tan\delta
=3.7\pm0.5\times10^{-5}$ at $6\ $GHz and $\tan\delta = 2.1\pm 0.1\times10^{-4}$
at $350\ $GHz. These results represent very low losses for deposited
dielectrics, but the sub-mm wave losses are significantly higher than the
microwave losses, which cannot be understood using the standard two-level
system loss model.
