Simulation study of zone-height limit by electron beam lithography for 30 nm Fresnel zone plates in X ray optics

Diffractive Fresnel zone plates with rectangular zones in noble metals are the key components in X-ray microscope as focusing/imaging lenses, which are mainly fabricated by electron beam lithography, followed by Au electroplating. For obtaining high quality focus and/or images in hard-X-rays, the zone height should be high enough to achieve coherent focusing/imaging on the focal plane. However, when the focusing/imaging resolution advances from 50 nm to 30 nm, the required height becomes enormously difficult owing to proximity effect in electron beam lithography, resulting in reduced efficiency. This work carried out systematic simulations based on Monte Carlo model to explore the zone height achievable by electron-beam lithography for 30 nm zone plates under 100 keV in ZEP520A which has higher contrast than PMMA. The designed exposure width for the outermost trench of the zone plate was used as the processing variable to work out the dose latitudes in which the maximum height was replicated. Both the sidewall verticality and the duty ratio were used as criteria to evaluate the lithography quality. By this way, the height limits of electron beam lithography for 30 nm zone plates were theoretically predicted, corresponding to various resist thicknesses from 50 nm up to 600 nm. The simulation results obtained in this work are instructive to the development of nanofabrication process of zone plates with 30 nm resolution.