The application of focused ion beam (FIB) technology in microfabrication has become increasingly popular. Its use in microfabrication has advantages over contemporary photolithography or other micromachining technologies, such as small feature resolution, the ability to process without masks and being accommodating for a variety of materials and geometries. An overview of the recent development in FIB microfabrication technology is given. The emphasis will be on direct milling, or maskless techniques, and this can distinguish the FIB technology from the contemporary photolithography process and provide a vital alternative to it. After an introduction to the technology and its FIB principles, the recent developments in using milling techniques for making various high-quality devices and high-precision components at the micrometer scale are examined and discussed. Finally, conclusions are presented to summarize the reviewed work and to suggest the areas for improving the FIB milling technology and for future research.

https://iopscience.iop.org/article/10.1088/0960-1317/14/4/R01/pdf

Recent developments in micromilling using focused ion beam technology

Review: Developments in micro/nanoscale fabrication by focused ion beams

Helical structures have attracted considerable attention due to their inherent optical chirality. Here, we report a unique type of 3D Janus plasmonic helical nanoaperture with direction-controlled polarization sensitivity, which is simply fabricated via the one-step grayscale focused ion beam milling method. Circular dichroism in transmission of as large as 0.72 is experimentally realized in the forward direction due to the spin-dependent mode coupling process inside the helical nanoaperture. However, in the backward direction, the nanoaperture acquires giant linear dichroism in transmission of up to 0.87. By encoding the Janus metasurface with the two nanoaperture enantiomers having specified rotation angles, direction-controlled polarization-encrypted data storage is demonstrated for the first time, where a binary quick-response code image is displayed in the forward direction under the circularly polarized incidence of a specified handedness, while a distinct grayscale image is revealed in the backward direction under linearly polarized illumination with a specified azimuthal angle. We envision that the proposed Janus helical nanoapertures will provide an appealing platform for a variety of applications, which will range from multifunctional polarization control, enantiomer sensing, data encryption and decryption to optical information processing.

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3D Janus Plasmonic Helical Nanoapertures for Polarization-Encrypted Data Storage

Focused ion beam (FIB) milling is receiving increasing attention for nanostructuring in silicon (Si). These structures can for example be used for photonic crystal structures in a silicon-on-insulator (SOI) configuration or for moulds which can have various applications in combination with imprint technologies. However, FIB fabrication of submicrometre holes having perfectly vertical sidewalls is still challenging due to the redeposition effect in Si. In this study we show how the scan routine of the ion beam can be used as a sidewall optimization parameter. The experiments have been performed in Si and SOI. Furthermore, we show that sidewall angles as small as 1.5◦ are possible in Si membranes using a spiral scan method. We investigate the effect of the dose, loop number and dwell time on the sidewall angle,interhole milling and total milling depth by studying the milling of single and multiple holes into a crystal. We show that the sidewall angles can be as small as 5◦ in (bulk) Si and SOI when applying a larger dose. Finally, we found that a relatively large dwell time of 1 ms and a small loop number is favourable for obtaining vertical sidewalls. By comparing the results with those obtained by others, we conclude that the number of loops at a fixed dose per hole is the parameter that determines the sidewall angle and not the dwell time by itself.

http://iopscience.iop.org/article/10.1088/0957-4484/18/19/195305/pdf

Focused ion beam scan routine, dwell time and dose optimization for submicrometre period planar photonic crystal components and stamps in silicon

This paper reviews the applications of focused ion beam (FIB) sputtering for micro/nano fabrication. Basic principles of FIB were briefly discussed, and then empirical and fundamental models for sputtering yield, material removal rate, and surface roughness were presented and compared. The empirical models were more useful for application compared to fundamental models. Fabrication of various micro and nano structures was discussed. Trimmed atomic force microscope (AFM) tips were tested in measurement and imaging of high aspect ratio nanopillars where higher accuracy and clarity were observed. Micromilling tool fabricated using FIB sputtering was used to machine microchannels. Slicing and dwell time control approaches on FIB sputtering were presented for the fabrication of three dimensional microcavities. The first approach is preferred for practical applications. The maximum aspect ratio of 13:1 of the microstructures was achieved. The minimum size of the nanopore was in the range of 2–10 μm. Cavities of microgear of 70 μm outside diameter were sputtered with submicrometer accuracy and 2–5 nm average surface roughness. The microcavities were then filled with polymer in a subsequent micromodling process. The replicated microcomponents were inspected with scanning electron microscope where faithful duplication of accuracy and surface texture of the cavity was observed.

A review of focused ion beam sputtering

The influence of the redeposition effect on the focused ion beam 3D micromachining process of silicon is discussed. Milling of some typical patterns in which redeposition is serious has been carried out. Experimental results are analysed in combination with a theoretical model. It can be seen from the analysis that the order of recycle-milling is very important for 3D microfabrication owing to redeposition during the process. In addition, the parameters of beam current, ion beam spot size, etc. are also key factors in the process. A groove and dome shape will be formed at the root of the sidewall and the bottom of the pattern, respectively, because of the ion sputtering yield variation and sidewall redeposition. Finally, avoidance methods are suggested as a result of the analysis.

Influence of the Redeposition effect for Focused Ion Beam 3D Micromachining in Silicon

Submicron linewidth direct deposition is a challenge for focused ion beam technology. A high-resolution image is the first necessary condition in order to obtain such a fine deposited line. Besides this, dwell time, X and Y pixel space, beam current, residual gas pressure, and work chamber vacuum are also important parameters for the deposition process. The quality of the deposited line depends on reasonable selection and optimisation of these parameters. Based on influence factor analysis, suitable parameters are arrived at for submicron linewidth deposition. The procedure was tested by its application of high-density IC chip modification.

O.N. Shing

Papers

Influence of the Redeposition effect for Focused Ion Beam 3D Micromachining in Silicon

Investigation of submicron linewidth direct deposition for high-density IC chip modification by focused ion beam