  Quantum computers are close to become a practical technology. Solid-state
implementations based, for example, on superconducting devices strongly rely on
the quality of the constituent materials. In this work, we fabricate and
characterize superconducting planar resonators in the microwave range, made
from aluminum films on silicon substrates. We study two samples, one of which
is unprocessed and the other cleaned with a hydrofluoric acid bath and by
heating at $880^{\circ}$C in high vacuum. We verify the efficacy of the
cleaning treatment by means of scanning transmission electron microscope
imaging of samples' cross sections. From 3 h-long resonator measurements at
$\approx 10$ mK and with $\approx 10$ photonic excitations, we estimate the
frequency flicker noise level using the Allan deviation and find an
approximately tenfold noise reduction between the two samples; the cleaned
sample shows a flicker noise power coefficient for the fractional frequency of
$\approx 0.23 \times 10^{-15}$. Our preliminary results follow the generalized
tunneling model for two-level state defects in amorphous dielectric materials
and show that suitable cleaning treatments can help the operation of
superconducting quantum computers.
