Predicting the structure of ultra shallow EUV gratings fabricated via ion irradiation

The controlled and precise fabrication of ultra-shallow structures for gratings operating near-normal incidence in the extreme ultraviolet and soft X-ray range is an important step towards compact tabletop devices. For this purpose, ion irradiation induced swelling has emerged as alternative to conventional etching processes for the precise fabrication of gratings with heights in the nanometre range. Here, we expand on this by presenting a model to predict the structure shape for gratings fabricated with a variety of ion species. Our model applies a Gaussian approximation in a convolution approach to calculate the point spread function of the resulting swelling. This approach is suitable to model the swelling caused by the irradiation of silicon with molecular nitrogen ions at 20 keV, and helium ions at 10 keV and 30 keV, respectively. We verified the model by the deconvolution solution via Fourier transformation and experiments at smaller pitch sizes. By predicting structure shapes for pitch sizes between 250 nm and 2 μm the model extends the range of applications both in the EUV and soft X-ray range. This enables the prediction of structure shapes from masks with arbitrary pitch size and duty cycle from the morphology of the mask measured from focussed ion beam cross sections.