3D Volumetric Energy Deposition of Focused Helium Ion Beam Lithography: Visualization, Modeling, and Applications in Nanofabrication

Abstract

COMMUNICATION (1 of 8)charged-particle beam lithography technique, has shown great potential in 3D nanoscale fabrication with high precision at the submicron scale. 3D nanofabrication has been demonstrated using low-energy EBL based on the saturation of exposure depth and width, [24] multiple electron beam exposures at different exposure energies, [20,25] continuous overlay electron beam exposures, [26][27][28] and grayscale EBL [29,30] among others. To the same end, 3D nanostructures have also been fabricated in bilayer resist stacks by EBL, [31] which was enabled by the difference in dose sensitivity of two different resists. However, in this approach, multiple coatings of resists with different sensitivities are required. Moreover, there is a limited selection of suitable combinations of high-resolution resists. The recently introduced scanning helium ion microscope (HIM) is a superior tool in high-resolution imaging, thanks to its minimized sample charging through flood charge neutralization, enhanced depth of field, and short De Broglie wavelength. As a scanning focused-ion-beam instrument, HIM can perform high-resolution nanofabrication by milling, [32,33] etching, [34] deposition, [35,36] modification of materials, [37,38] and lithographic patterning in resists. [39,40] When used as a high-resolution lithography tool, helium ion beam lithography (HIBL) has achieved record-high-resolution patterning of 8 nm pitch dense lines. [41] Moreover, the narrow interaction volume of helium ions in resist materials makes HIBL a promising candidate for In this paper, 3D volumetric energy deposition and local crosslinking of hydrogen silsesquioxane (HSQ) are experimentally and numerically explored in focused helium ion beam lithography (HIBL). In particular, a throughmembrane exposure method is developed to make visible and subsequently to measure the 3D interaction volume and energy deposition of helium ions in HSQ. By comparing the actual dimensions of the crosslinked HSQ structures with Monte Carlo modeling of the spatial distribution of the energy deposition, the critical energy density for crosslinking HSQ is obtained. Finally, 3D nanofabrication of complex crosslinked HSQ nanostructures such as embedded nanochannels and suspended grids is demonstrated using two different exposure configurations. The proposed method expands the 2D point spread function of HIBL into three dimensions, thus opening a new avenue for nanoscale 3D fabrication. Helium Ion Beam LithographyThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201800203.3D micro/nanostructures are important for electronic, [1] photonic, [2] plasmonic, [3] microfluidic, [4] bio-MEMS, [5] and chemical synthesis [6] applications. Directly patterning materials into 3D micro/nanostructures has recently attracted considerable attention. Significant effort has been devoted to investigating methods for defining functional 3D micro/nanoscale structures, including direct material deposition, [7][8][9][10] ...

A review of defect engineering, ion implantation, and nanofabrication using the helium ion microscope

“…Modeling and experimental measurements of the 2D point-spread function for HIBL (i.e., the spatial distribution of energy deposition, which determines the proximity effect) can be found in the same reference. Cai et al extended this work to a 3D visualization of the point-spread function by performing point exposures on HSQ through a thin layer of silicon nitride and then developing the resist to remove non-exposed regions, leaving drop-shaped cross-linked structures that define the 3D exposed volumes [ 108 ] ( Figure 5c ). The authors also directly exposed substrate-supported resists from the front side and fabricated hollow and suspended nanostructures (see Figure 5d ) by appropriate choice of beam energy and dose during the patterning process.…”

“…(c) Procedure for the 3D measurement of the point-spread function for HIBL using a through-membrane exposure technique: The SEM image shows a cross-linked volume of HSQ upon development of the resist following the point exposure. Adapted from [ 108 ]. Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.…”

“…(d) Fabrication of a suspended mesh structure by direct exposure of the resist from the front side. Adapted from [ 108 ]. Copyright © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.…”

Focused Ion Beam Processing for 3D Chiral Photonics Nanostructures

“…The focused ion beam (either with heavy ions like Ga + [ 32 ], or light ions like He + [ 33 ]) can be also employed as a scanning ion probe for lithographic patterning in resists, with position and timing controlled by a pattern generator. The ion beam lithography reaches higher resolution as compared to EBL, even if with the same spot size, thanks to the absence of backscattering effects together with a weaker forward scattering and a smaller lateral diffusion of secondary electrons [ 34 ].…”

Quantitative mechanics of 3D printed nanopillars interacting with bacterial cells

“…An increasing number of advanced nanotechnological applications require highly repeatable fabrication of 'three-dimensional' devices with complex and precisely-controlled geometries. Depending on the targeted feature size and the complexity of the desired geometry, nanofabrication techniques, such as two-photon polymerization (TPP), 1 imprint lithography, [2][3][4][5] interference lithography, 6 3D molding, 7 electron beam lithography (EBL), [8][9][10] electron beam-induced deposition (EBID), [10][11][12][13] ion beam lithography (IBL), 14 or focused ion beam (FIB) 10,15,16 could be used to fabricate such 3D structures.…”