  A method to fabricate GaAs microcavities using only a soft mask with an
electrolithographic pattern in an inductively coupled plasma etching is
presented. A careful characterization of the fabrication process pinpointing
the main routes for a smooth device sidewall is discussed. Using the final
recipe, optomechanical microdisk resonators are fabricated. The results show a
very high optical quality factors of $Q_\text{opt}>2\times 10^{5}$, among the
largest already reported for dry-etching devices. The final devices are also
shown to present high mechanical quality factors and an optomechanical vacuum
coupling constant of $g_{0}=2\pi\times 13.6$ kHz enabling self-sustainable
mechanical oscillations for an optical input power above $1$ mW.
