Nano-proximity direct ion beam writing

Abstract

Nanofabrication 2015; 2: 54-62 the required resolution and footprint of structures these two techniques can be complemented making pattern over-lays [1]. However, both EBL and photolithography have limitations due to several processing steps of resist exposure, etching, metallization required to make a final device. These limitations were overcome with an emergence of focused ion beam lithography (IBL) which enables direct 3D writing. Even though focused ion beam (FIB) milling technology was first demonstrated in the mid-1970s [2,3], it is still mainly used for sample slicing and lamella preparation for transmission electron microscopy.Among several reasons, lack of stable ion sources has hindered FIB applications in high resolution large scale fabrication, especially where He ion sources are implemented. Nonetheless novel ion sources and column geometries providing high temporal beam stability have been developed over the last decade and opened new avenues for the direct writing at the nanoscale. Stateof-the-art ion beam lithography tools are capable of patterning relatively large areas with cross sections of 100 × 100 μm 2 in tens of minutes [4] depending on complexity and are comparable in speed with mature EBL which is producing 22-nm node lithography masks for the latest microelectronics industry at a throughput ̴ 5 × 10 5 μm 2 h -1 [5]. High precision direct IBL writing found its way in application fields of photonic crystals [6], single molecule sensors [7], nanopores for DNA probing [8,9] to mention a few. Recent development of new Au, Si, Ge ion sources allows not only milling but also a controlled implantation useful for other nanoscale etching and material growth techniques [10][11][12].In plasmonics and nano-photonics, IBL is often used for fabrication/reshaping of nanoscale antennas for the light field confinement and enhancement [13,14] as well as for milling metasurfaces into metal layers [15,16]. Material properties are altered by projectile ion implantation (usually Ga + ) around the cuts and this dampens plasmonic or optical resonances of the fabricated structures. With Abstract: Focused ion beam (FIB) milling with a 10 nm resolution is used to directly write metallic metasurfaces and micro-optical elements capable to create structured light fields. Surface density of fabricated nano-features, their edge steepness as well as ion implantation extension around the cut line depend on the ion beam intensity profile. The FIB beam intensity cross section was evaluated using atomic force microscopy (AFM) scans of milled line arrays on a thin Pt film. Approximation of two Gaussian intensity distributions describes the actual beam profile composed of central high intensity part and peripheral wings. FIB fabrication reaching aspect ratio of 10 in gold film is demonstrated.

Tipping solutions: emerging 3D nano-fabrication/ -imaging technologies

“…Figure 6B shows a phase mask with the apparent departure of the pixel phase from the intended fixed depth, especially when large phase jumps between the adjacent pixels had to be made. Milling of closely spaced grooves into the surface of a metal film can be used to estimate the width of the central part of the ion intensity distribution and side lobes [89]. It is critical to have the narrowest central part of ion beam intensity within 10 nm at 1/e 2 for close proximity milling.…”

“…[89]. SEM image of a 45°-tilted 20-nm-wide groove milled into a 220-nmthick sputtered Au film on a glass substrate.…”

Helium focused ion beam direct milling of plasmonic heptamer-arranged nanohole arrays

“…An HeFIB can have a beam spot size of less than 0.5 nm [9] and a direct milling resolution of 3.5 nm [10] due to the atomically defined metal tip used to form the He beam and the small de Broglie wavelength of He ions. Unlike liquid metal ion sources, such as Ga, He ions are pulled by high extraction voltages from a sharp, atomically defined metal tip, so they have a higher axial directionality, enabling high aspect ratio features to be defined by HeFIB direct milling [11,12]. In addition, He ions produce less contamination by unintentional implantation, as He is a noble gas, and it has a high propensity to diffuse out of materials [13].…”

High-throughput synthesis of modified Fresnel zone plate arrays via ion beam lithography

“…1 , and involves drift correction steps in between cycles [ 29 , 35 – 36 ]. In some cases, the drift correction can be unnecessary, but the MP-E can still be desired when a better dose distribution or a well-defined wall geometry is aimed for in structures with higher aspect ratio [ 38 ]. We have shown that a much faster process can be devised by employing a single-pass-exposure (SP-E, Fig.…”