  The microwave surface resistance Rs of MgB2 films with the zero-resistance
temperature of - 39 K was measured at 8.0 - 8.5 GHz. The MgB2 films were
prepared by deposition of boron films on c-cut sapphire, followed by annealing
in a vaporized magnesium environment. The Rs appeared significantly reduced by
ion milling of the as-grown MgB2 film surface, with the observed Rs of ~ 0.8
mohm at 24 K for an ion-milled MgB2 film as small as 1/15 of the value for the
corresponding as-grown MgB2 film. The reduced Rs of the ion-milled MgB2 films
is attributed to the effects of the Mg-rich metallic layer existing at the
surfaces of the as-grown MgB2 films.
