  Ruthenium dioxide (RuO$_2$) thin films were synthesized by Chemical Solution
Deposition (CSD) on silicon substrates using only water and acetic acid as
solvents. The microstructure, phase-purity, electrical and optical properties
as well as the thermal stability of the thin films have been characterized. The
microstructure of the thin films strongly depends on the annealing temperature:
A smooth thin film was achieved at an annealing temperature of 600$^\circ$C.
Higher annealing temperatures (800$^\circ$C) led to radial grain growth and an
inhomogeneous thin film. A very low resistivity of 0.89 {\Omega}m was measured
for a 220 nm-thick thin film prepared at 600$^\circ$. The resistivity of the
thin films increases with temperature, which indicates metallic behavior.
Phase-purity of the thin films was confirmed with X-ray Diffraction (XRD)
measurements, X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy.
Transmission and reflectivity measurements indicate that RuO$_2$ efficiently
blocks the UV-VIS and IR wavelengths. The optical constants determined via
spectroscopic ellipsometry show high absorption in the near-IR region as well
as a lower one in the UV-VIS region. The thermal stability was investigated by
post-annealing, confirming that the thin films are stable up to 750$^\circ$C in
synthetic air.
