Substrate-adhered semiconductor nanowires can be prepared by locally confined laser ablation of high-quality thin films. Wurtzitic GaN films with thicknesses of several tens of nanometers were deposited by ion-beam-assisted molecular beam epitaxy on 6H-SiC. Diffraction-mask-projection laser ablation (at a wavelength of 248 nm) was used to convert the thin film into a well-ordered arrangement of parallel, substrate-adhered nanowires. The ablation profile generated by the stripe-patterned phase mask was shown to sharpen upon multipulse application. Hence, structural widths of the remaining semiconductor banks below 200 nm can be achieved. Beyond substrate adherence, the introduced methodology makes the preparation of growth-direction-independent nanowire orientations feasible.