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. 2020 Jan 16;11(1):95.
doi: 10.3390/mi11010095.

A Programmable Nanofabrication Method for Complex 3D Meta-Atom Array Based on Focused-Ion-Beam Stress-Induced Deformation Effect

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A Programmable Nanofabrication Method for Complex 3D Meta-Atom Array Based on Focused-Ion-Beam Stress-Induced Deformation Effect

Xiaoyu Chen et al. Micromachines (Basel). .
Free PMC article

Abstract

Due to their unique electromagnetic properties, meta-atom arrays have always been a hotspot to realize all kinds of particular functions, and the research on meta-atom structure has extended from two-dimensions (2D) to three-dimensions (3D) in recent years. With the continuous pursuit of complex 3D meta-atom arrays, the increasing demand for more efficient and more precise nanofabrication methods has encountered challenges. To explore better fabrication methods, we presented a programmable nanofabrication method for a complex 3D meta-atom array based on focused-ion-beam stress-induced deformation (FIB-SID) effect and designed a distinctive nanostructure array composed of periodic 3D meta-atoms to demonstrate the presented method. After successful fabrication of the designed 3D meta-atom arrays, measurements were conducted to investigate the electric/magnetic field properties and infrared spectral characteristics using scanning cathodoluminescence (CL) microscopic imaging and Fourier transform infrared (FTIR) spectroscopy, which revealed a certain excitation mode induced by polarized incident IR light near 8 μm. Besides the programmability for complex 3D meta-atoms and wide applicability of materials, a more significant advantage of the method is that a large-scale array composed of complex 3D meta-atoms can be processed in a quasi-parallel way, which improves the processing efficiency and the consistency of unit cells dramatically.

Keywords: 3D meta-atom array; 3D nanofabrication; CL imaging; FIB-SID; FTIR spectroscopy.

Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
The designed structure of 3D meta-atom. (a) Oblique and (b) overhead views of a 3D meta-atom unit cell, which contains a pair of coupled claws with a gap of 0.1 μm, a length of 0.9 μm and a height of 0.6 μm. The whole array is 90 μm by 60 μm with a period of 3 μm in Y-axis direction and 2 μm in X-axis direction.
Figure 2
Figure 2
The unfolding 2D pattern figure of the designed 3D unit cell, which is the initial state derived reversely from the forming process of 3D meta-atom by FIB-SID bending. The FIB-SID bending areas for transformation processes are marked as green lines and red lines, respectively.
Figure 3
Figure 3
The electric/magnetic field distribution of 3D meta-atom unit cells with different claw gap distance. (a) Electric field and (b) magnetic field distribution simulation results of the 3D meta-atom array with larger claw gap, (c) electric field and (d) magnetic field distribution simulation results of the 3D meta-atom array with smaller claw gap.
Figure 4
Figure 4
Simulated reflection spectra of the 3D meta-atom array with a smaller claw gap. Rx and Ry are the reflection spectra of the array under normally incident IR light with the polarization direction parallel to X-axis and Y-axis, respectively.
Figure 5
Figure 5
MEMS process diagram of the suspended gold film. (a) LPCVD for silicon dioxide and silicon nitride layer on both sides separately, (b) Lithography and RIE to expose square windows on the backside, (c) Wet etching to remove exposed silicon substrate using KOH solution, (d) RIE to remove all the silicon nitride layer on the front side, (e) Gold film deposition of 120 nm using PVD process, and (f) Release-etching of silicon dioxide layer to complete the process with BHF solution.
Figure 6
Figure 6
Main fabrication steps and process details of the designed 3D meta-atom array based on the FIB-SID technique. (a) unfolding 2D pattern figure and (b,c) the 3D transformation figures for FIB process, (d) unfolding 2D patterning using the FIB milling function to irradiate as (a) drawing, (e,f) 3D transformation process using the FIB-SID technique to irradiate as (b,d) drawing, (g) a unit cell of unfolding 2D patterned structure which marked irradiation area of FIB-SID process as dotted line, (h) a unit cell of completed 3D meta-atom array with two coupled claws facing to each other, (i) a part of large scale 3D meta-atom array fabricated using this method. The scale bars are 2 μm for (df), 500 nm for (g,h) and 10 μm for (i).
Figure 7
Figure 7
SEM images of the fabricated 3D meta-atom arrays with different cell sizes and claw distances. The inset images are the top view of each 3D meta-atom array, respectively. The overall size is around 60 μm × 90 μm. (a,b) are arrays of the same size with a larger claw distance of ~500 nm (a) and a smaller distance of ~150 nm (b), both with a claw length of ~500 nm. (c,d) are arrays of the scaled down size with larger claw distance of ~250 nm (c) and smaller distance of ~70 nm (d), both with a claw length of ~330 nm. The scale bars are 1 μm.
Figure 8
Figure 8
CL (cathodoluminescence) images of the 3D meta-atom arrays with different claw gaps. (ac) CL images of the same 3D meta-atom arrays with larger claw gaps in different magnifications, and (df) CL images of the same 3D meta-atom arrays with smaller claw gaps in different magnifications. The scale bars are 2 μm for (a,d), 1 μm for (b,e) and 500 nm for (c,f).
Figure 9
Figure 9
Measured IR reflection spectra of the 3D meta-atom arrays under normal incidence, with the polarization along the X-axis (A1x, A2x) and Y-axis (A1y, A2y). The tested samples are Array1 (A1x, A1y) and Array2 (A2x, A2y).
Figure 10
Figure 10
The reflectance ratio spectra of the 3D meta-atom array for the incident light with different polarized directions. RXratio and RYratio describe how much the reflectance of smaller claw gap array (Array2) varies with respect to that of larger claw gap array (Array1) under IR incident light with linear polarization direction of X-axis and Y-axis, respectively.

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