Fabrication of ultra-shallow EUV gratings in silicon via ion irradiation

Operation of diffraction gratings at near-normal incidence in the extreme ultra violet range (EUV) requires ultra-shallow gratings. These are extremely challenging to produce via conventional dry etching techniques. As alternative, the approach presented here utilizes swelling caused by ion irradiation to fabricate ulta-shallow gratings. To increase the processing speed by at least one order of magnitude in comparison to direct write processes with focussed ion beams, we utilize a broad ion source which can structure areas in the range of tens of square-cm via irradiation through a mask of photoresist. Here, we focus on the irradiation of crystalline silicon with molecular nitrogen ions with an energy of 40 keV. By utilizing a variation in the conical angle of irradiation we fabricated structures with heights ranging from 1.7 nm to 5.5 nm, while maintaining stable source and irradiation conditions, highlighting a parameter for the tailoring of processing times. Next to that, a variation of non-conical angle of irradiation paths the way towards a fabrication of arbitrary gratings shapes. In combination, this enhances the control of structure height and shape in the angstrom range, which is highly relevant to applications in the EUV.