Helium ion beam lithography and liftoff

Abstract

We introduce a helium ion beam lithography and liftoff process to fabricate arbitrary nanostructures. Exploiting existing high-resolution positive tone resists such as poly (methyl methacrylate) (PMMA), the process offers three significant advantages over electron beam lithography: (a) the exposing helium ion beam produces a high secondary electron yield leading to fast patterning, (b) proximity effects are negligible due to the low count of backscattered helium ions from the substrate, and (c) the process is transferrable with minimal alteration among different types of substrates (e.g. silicon, fused silica). The process can be used to pattern any material compatible with liftoff such as evaporated metals or dielectrics, and allows overlay of nanostructures precision-aligned to microstructures realised beforehand on the same substrate. The process is demonstrated for several PMMA thicknesses to liftoff different thicknesses of deposited material. Resolution trials are conducted to determine the limits of the process for each PMMA thickness. Isolated lines as narrow as 14 nm, and line-space gratings of 40 nm pitch (50% duty cycle), are produced as resolution tests by lifting off a 20 nm thick Au film. Nanostructures of aspect ratio up to ∼3:1 have been realised. Plasmonic nanoantenna arrays overlaid to microscale contacts are produced as device demonstrators, for which optical measurements are in excellent agreement with theory.

Optical Beam Steering Using Tunable Metasurfaces

“…The studies reviewed addressed these challenges to varying degrees. Plasmonic − and dielectric , metasurfaces exploiting carrier refraction effects have been explored for beam steering.…”

“…Optimisation of all geometric parameters yields designs where shifts in phase of 300° to 330° are possible, with an approximately constant reflection coefficient magnitude of |Γ| ∼ 40% (reflectance |Γ| 2 ∼ 16%), using a single drive voltage scheme. Figure e gives helium ion microscope (HIM) images of an electrical fan-out structure, the contacted dipole nanoantenna array as the high-magnification left inset, and a connection point between a nanoantenna contact line and a fan-out finger as the high-magnification right inset . On this image, the dipole gaps are 30 nm long, and the dipole arms and connector widths are 50 nm.…”

“…The nanoantennas were arranged in a 1D array, to eventually enable horizontally polarized light to be steered in the horizontal plane. This array with connectors (Figure e) were fabricated using a deposition and lift-off technique based on helium ion beam exposure of resist . This novel patterning process is advantageous, as it produces negligible proximity effects, enabling complex nanoscale features to be defined accurately and rapidly.…”

“…The dipole gaps are 30 nm long, and the dipole arms and connector widths are 50 nm. Reprinted with permission from . Copyright 2021 IOP Publishing Ltd.…”

Topology optimization of dispersive plasmonic nanostructures in the time-domain

“…Advances in nanofabrication technologies have enabled such technologies by allowing unprecedented design complexity at the nanoscale. 5,10 The opportunities offered by theoretical research and nanofabrication facilities raise the need for new methods to efficiently design and optimize such nanophotonic systems. 11,12 Advances in computing capabilities and numerical methods, such as the finite-difference time-domain (FDTD) 13 and finite-element methods (FEM), 14 empowered the design of nanophotonic devices while shortening their design cycle.…”