  The blasting of brittle materials with sharp particles is an important
fabrication technology in many industrial processes. In particular, for
micro-systems, it allows the production of devices with feature sizes down to
few tens of microns. An important parameter of this process is the surface
roughness of post-blasted surfaces. In this work the scaling properties of
Pyrex glass and silicon surfaces after bombardment with alumina particles is
investigated. The targets were bombarded at normal incidence using alumina
particles with two different average sizes, $29\mu m$ and $9\mu m$,
respectively. This investigation indicates that the resulting surfaces have
multifractal properties. Applying multifractal detrended fluctuation analysis
(MFDFA) allowed us to determine the singularity spectrum of the surfaces. This
spectrum did not depend on the target material or on the size of the particles.
Several parameters quantifying relevant quantities were determined. We argue
that for scales below $5 \mu m$, fracture processes are dominant while at large
scales long range correlations are responsible for the multifractal behaviour.
