Simulation and experimental study of aspect ratio limitation in Fresnel zone plates for hard-x-ray optics

Abstract

For acquiring high-contrast and high-brightness images in hard-x-ray optics, Fresnel zone plates with high aspect ratios (zone height/zone width) have been constantly pursued. However, knowledge of aspect ratio limits remains limited. This work explores the achievable aspect ratio limit in polymethyl methacrylate (PMMA) by electron-beam lithography (EBL) under 100 keV, and investigates the lithographic factors for this limitation. Both Monte Carlo simulation and EBL on thick PMMA are applied to investigate the profile evolution with exposure doses over 100 nm wide dense zones. A high-resolution scanning electron microscope at low acceleration mode for charging free is applied to characterize the resultant zone profiles. It was discovered for what we believe is the first time that the primary electron-beam spreading in PMMA and the proximity effect due to extra exposure from neighboring areas could be the major causes of limiting the aspect ratio. Using the optimized lithography condition, a 100 nm zone plate with aspect ratio of 15/1 was fabricated and its focusing property was characterized at the Shanghai Synchrotron Radiation Facility. The aspect ratio limit found in this work should be extremely useful for guiding further technical development in nanofabrication of high-quality Fresnel zone plates.

All-Dielectric Silicon Nanoring Metasurface for Full-Color Printing

“…The period of the metasurface can be described as P = D + g. The thickness of the whole nanoring layer is fixed at 582 nm, which contains a 220 nm thick silicon nanoring overlapping a 362 nm thick silica nanoring. We use the electron beam lithography (EBL) and inductively coupled plasma (ICP) etching 33,34 during the fabrication procedure. The fabrication process is schematically illustrated in Figure S1.…”

Theoretical modeling of ice lithography on amorphous solid water

“…On the other hand, electron beam lithography, which seriously suffers from the proximity effect, is limited when replicating dense patterns like short-pitched gratings with ultra-high resolution well below 100 nm. A good example is when replicating Fresnel zone plates as diffractive lenses in X-ray optics, 30,31 which need both high resolutions and high aspect ratios. In this paper, the patterning performance by IL was simulated and compared with EBL, trying to demonstrate the advantages of ice lithography.…”

Nanofabrication of 50 nm zone plates through e-beam lithography with local proximity effect correction for x-ray imaging*

“…Therefore, the extra exposure by backscattered electrons in the 4-µm PMMA/Si can be neglected as in the resist on the Si 3 N 4 membrane. [34] Based on the simulated PSF curve, the deposited energy distribution in the PMMA coated on the SiN x membrane was first calculated by convoluting the PSF with the designed zone plate pattern without PEC. Figure 2(a) presents the spatial distribution of the deposited charge at the depth of 625 nm in the PMMA for the three different regions of the zone plate as marked in Fig.…”