We demonstrate micro structuring of fused-silica laser mirror substrates by Inverse Laser Drilling. Slits of a width down to ~80 µm and circular holes with diameters down to ~50 µm have been structured into quarter-inch thick substrates. Except for chipping, the surface areas around these openings have not been irreversibly affected by the manufacturing process. The micro structured mirrors can be used for geometrical output coupling of coherent EUV radiation from cavity-enhanced high harmonic generation.