  Superconducting quantum circuits are one of the leading quantum computing
platforms. To advance superconducting quantum computing to a point of practical
importance, it is critical to identify and address material imperfections that
lead to decoherence. Here, we use terahertz Scanning Near-field Optical
Microscopy (SNOM) to probe the local dielectric properties and carrier
concentrations of wet-etched aluminum resonators on silicon, one of the most
characteristic components of the superconducting quantum processors. Using a
recently developed vector calibration technique, we extract the THz
permittivity from spectroscopy in proximity to the microwave feedline. Fitting
the extracted permittivity to the Drude model, we find that silicon in the
etched channel has a carrier concentration greater than buffer oxide etched
silicon and we explore post-processing methods to reduce the carrier
concentrations. Our results show that near-field THz investigations can be
applied to quantitatively evaluate and identify potential loss channels in
quantum devices.
