  Here we demonstrate that adding a chemo-mechanical polishing (CMP) procedure
to conventional photolithography, a silica microdisk with ultra-high quality
factors ($>10^8$) can be fabricated. By comparing with the intrinsic optical
quality factor (Q) measured at 970~nm, we observe that due to the significantly
reduced surface roughness, at 1550~nm wavelength the water molecule absorption
at the cavity surface supersedes Rayleigh scattering as the dominant factor for
Q degradation.
