UV sub-ps laser pulse patterning of Mo/Si and W/Si multilayers for soft x-ray gratings

The micropatteming of multilayer gratings (MLG) using ultraviolet sub-ps laser pulses is described. A micromachining system operating with a 0.5 ps KrF laser (248 nm) was used. Grating structures with a groove width in sub-pm region were created in Mo/Si, Si/Mo, W/Si and Si/W multilayers (MLs) with 5 (in one case 10) periods, each 7-10 nm thick. Grating area was up to 900 x 900 μm2. Laser fluence on the samples varied between 60 and 710 mJcm-2. Atomic force microscopy, scanning electron microscopy, X-ray reflectivity and X-ray diffraction were used to characterize multilayers and gratings. MIs were locally ablated up to the Si, oxidized Si or glass substrate, or deeper, using from 1 to 5 pulses. The roughness on the surface of lines and in grooves of MLG increased with the depth of ablation. It was caused first of all by debris. The ω-scans around the 1st Bragg maximum show symmetric satellites up to the 2nd or 3rd order, giving the evidence that the ML in MLG is preserved.