FIB Secondary Etching Method for Fabrication of Fine CNT Forest Metamaterials
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Anisotropic materials, like carbon nanotubes (CNTs), are the perfect substitutes to overcome the limitations of conventional metamaterials; however, the successful fabrication of CNT forest metamaterial structures is still very challenging. In this study, a new method utilizing a focused ion beam (FIB) with additional secondary etching is presented, which can obtain uniform and fine patterning of CNT forest nanostructures for metamaterials and ranging in sizes from hundreds of nanometers to several micrometers. The influence of the FIB processing parameters on the morphology of the catalyst surface and the growth of the CNT forest was investigated, including the removal of redeposited material, decreasing the average surface roughness (from 0.45 to 0.15 nm), and a decrease in the thickness of the Fe catalyst. The results showed that the combination of FIB patterning and secondary etching enabled the growth of highly aligned, high-density CNT forest metamaterials. The improvement in the quality of single-walled CNTs (SWNTs), defined by the very high G/D peak ratio intensity of 10.47, demonstrated successful fine patterning of CNT forest for the first time. With a FIB patterning depth of 10 nm and a secondary etching of 0.5 nm, a minimum size of 150 nm of CNT forest metamaterials was achieved. The development of the FIB secondary etching method enabled for the first time, the fabrication of SWNT forest metamaterials for the optical and infrared regime, for future applications, e.g., in superlenses, antennas, or thermal metamaterials.
KeywordsCarbon nanotubes Metamaterial FIB patterning Secondary etching method Chemical vapor deposition
A newly developed FIB secondary etching method enables the precise fabrication of SWNT forest patterns in sizes from hundreds of nanometers to several micrometers, for the first time. This method fills the previous fabrication gap in this range.
The precise control of the CNT forest structure is achieved by controlling the FIB processing parameters.
By applying FIB secondary etching, the growth of the FIB-patterned CNT forest is significantly improved, enabling them to be used in metamaterial, electronic, photonic, and thermal applications.
Carbon nanotubes (CNTs) possess extraordinary electrical, physical and optical properties [1, 2, 3, 4, 5] due to their unique structure, such as the anisotropic electrical conductivity of horizontally aligned CNTs  and the anisotropic optical absorption of vertically aligned CNTs . The extraordinary properties of CNT forests were recently used for the fabrication of metamaterials and nonlinear photonic devices in the form of multi-walled CNT (MWNT) arrays [6, 7, 8], slits cut in the CNT films , or spray-coated CNT films on ceramic metamaterials [10, 11]; however, none of these examples utilized the unique properties of a CNT forest. To fully utilize the properties of a CNT forest with the high alignment, density, and the CNT forest structure, a different method of patterning is required. Patterned growth of CNTs on the scale of tenths of micrometers has been successfully reported on silica [12, 13, 14], silicon substrates , and also on predeposited catalyst [16, 17, 18]. On the other hand, the patterning of predeposited catalyst at the nanoscale, which would enable uniform, vertically aligned SWNT forest growth on various shapes of nanosize structures, is still challenging and is yet to be achieved [17, 19, 20, 21]. High-quality CNT structures with various shapes were achieved by utilizing different fabrication methods. Standard lithography methods, like photolithography [16, 22], electron beam lithography (EBL) , or soft-lithography  enabled the growth of fine CNT patterns. A laser machining method was used for the fabrication of one-dimensional grating patterns , while a laser etching method was utilized to obtain microstructures of CNT brushes . Moreover, the patterned growth of CNTs was also achieved using anodic aluminum oxide (AAO) templates  and inkjet printing of the catalyst . Finally, a focused ion beam (FIB) and electron beam (EB) were used for the patterning of catalyst nanodots and enabled the fabrication of arrays of individual MWNTs [6, 8, 27].
For the patterning of nanostructures on the predeposited catalyst, high precision at the nanoscale is required. The ability to pattern thin films by the FIB method has been presented before and has been successfully used to fabricate metamaterial nanostructures from metals . The FIB method is maskless and allows the fabrication of nanoscale patterns of various shapes and sizes; however, due to sputtering, the redeposited material is observed on the surface outside the patterned area . In this study, in order to overcome the major disadvantage of the redeposition of material, a FIB secondary etching method was developed to clean the patterned surface from the redeposited material by low-depth ion beam irradiation  and to improve the growth of CNTs in patterned areas.
The purpose of this study is to provide a reliable method for the precise fabrication of CNT patterns for metamaterials, with sizes ranging from around 150 nm to single micrometers, for the optical and infrared regime, and future applications, e.g., in superlenses, antennas, and thermal metamaterials. In this work, the influence of FIB fabrication parameters on the catalyst surface morphology and CNT forest internal structure (uniformity, alignment, etc.) were investigated. By controlling the FIB processing, a synergy between the top-down (shape of CNT metamaterial nanostructures) and bottom-up (structure of CNT forest) approaches is achieved.
A 30-nm-thick AlOx support layer and a 0.9-nm-thick Fe catalyst layer were deposited using radio frequency (RF) magnetron sputtering on p-type (100) silicon substrates. The detailed procedure has been described previously . The growth of CNTs was conducted using thermal chemical vapor deposition (CVD) on the deposited AlOx/Fe catalyst. The annealing process was performed at 730 °C in a hydrogen atmosphere with a gas flow rate of 65 sccm for 2.5 min (p = 29 Pa). After annealing, the CNT forest was grown in an acetylene (15 sccm) and hydrogen (15 sccm) atmosphere (p = 110 Pa), for a precisely controlled period of time.
Parameters of FIB patterning
Beam current (pA)
Acceleration voltage (kV)
Ion beam source
FIB patterning depth (nm)
1, 1.5, 2, 4, 10
Secondary etching depth – whole area (nm)
0.2, 0.3, 0.4, 0.5
Ion beam diameter (nm)
Dwell time (s)
1.0 × 10−6
The morphology of the catalyst surface was determined by using an atomic force microscope (AFM, SPI3800 N/SPA400, SII Nanotechnology Inc.), in the dynamic force microscopy (DFM) mode (tip curvature radius of 10 nm). The structure of patterns and morphology of CNT forests were characterized by using a field emission scanning electron microscope (FE-SEM, JEOL JSM-5310). The analysis of the type and quality of grown CNTs was performed by using a micro-Raman HORIBA JOBIN–YVON HR-800 spectrometer with laser excitation of 532.08 nm. A transmission electron microscope (TEM, JEOL JEM 2100 M) was used to observe the diameter of CNTs in the patterned arrays.
4 Results and Discussion
As shown in Fig. 4b (w/o secondary etching), on both sides of the profile with around 50 and 350 nm, the formation of ridges was noticed as a result of resputtering during the FIB patterning. Similar ridges were observed in Refs. 15 and 29 after the FIB patterning process. By applying secondary etching to the surface, the top layer of material was removed, and the height of the ridges diminished to the point where they were no longer observable (for 0.4 and 0.5 nm etching depths). A more detailed analysis of the average roughness of the Fe catalyst surface, after FIB patterning and secondary etching, was conducted and is presented in Fig. 4c. For comparison, the average roughness of the as-prepared catalyst is also shown. The FIB patterning resulted in a decrease in the average roughness. It was assumed that the sputtered material redeposition on the patterns resulted in a smooth surface via uniform redeposition. Furthermore, by applying the FIB secondary etching process, the average roughness was further decreased, from R a = 0.45 nm for an as-prepared catalyst surface to 0.15 nm for a secondary etching depth of 0.5 nm. Lower roughness supported the formation of uniform catalyst particles and resulted in the growth of high-density CNT forests.
The reason for the differences between the cases, i.e., with and without secondary etching, was related to the annealing process at high temperatures. For the case without secondary etching, the surface diffusion of catalyst particles was limited by the layer of redeposited material at the ridges. The catalyst particles agglomerated in the area before the ridges, which became a barrier for further catalyst surface diffusion to the area outside the ridges, resulting in a higher area density of the particles, allowing the growth of CNTs on the edges.
On the other hand, the effect of secondary etching was determined by the amount of removed material and the decreased surface roughness. During the patterning of SRR arrays, the sputtered material was mainly redeposited on the edges of patterns, forming ridges (Fig. 4b), and, to a lesser degree, in the middle of the walls. Furthermore, it was assumed that the mass density of the redeposited film was lower than that of the catalyst film. So, the sputtering rate was higher, resulting in faster etching at the ridge area as shown in Fig. 4b. By applying secondary etching of 0.5 nm, the majority of resputtered material and the top layer of Fe catalyst film were removed. By additional ion irradiation during the secondary etching process, cleaning of the surface and a decrease in the thickness of the Fe catalyst is to be expected, and these result in a decrease in the roughness and a consequent improvement in the height, uniformity, and quality of CNTs. Finally, as a result of the etching, due to the thinner catalyst and significantly decreased roughness, the growth of thin SWNTs with a higher growth rate was achieved , which was also observed in Fig. 3 A(I–II).
In the present study, the 30 kV accelerating voltage of the Ga ion was the direct cause of sputtering and redeposition of the material in the patterned areas; however, the other possible effects should also be noted. High accelerating voltages may cause implantation and diffusion of the Ga ion caused by ion beam irradiation. During the FIB patterning process, the depth of patterning was sufficient to remove the catalyst from the designated area, resulting in no growth of CNTs. On the other hand, the FIB secondary etching treatment of the patterned catalyst was used to clean the surface of the redeposited material. During the process, most of the deposited catalyst remained, and despite a relatively low ion energy, the implantation of Ga ions was possible. However, due to the significant improvements in the CNT forest observed after the secondary etching, the possible influence of Ga ions was negligible and could be ignored.
Finally, as can be seen in Figs. 2 and 5, the size of the patterns was significantly decreased after FIB secondary etching. It was assumed that this shrinking effect was related to the FIB patterning process. During patterning, the ion beam was oriented perpendicularly to the surface and the surface material was sputtered. Typical patterns milled using FIB do not possess vertical walls and flat bottoms, but are V-shaped with a maximum depth in the middle [29, 32, 33]. Furthermore, the mouth width of these V-shaped patterns is usually much larger than the diameter of the ion beam. This effect can be explained by the high intensity of the ions outside the core region, which causes a relatively higher sputtering of the material on the sides of the walls, thus resulting in the creation of slopes. Therefore, with an increase in the etching depth, the duration of ion beam irradiation is also increased, and more material from the walls is sputtered. During the sputtering, a part of the surface with the pristine catalyst is also removed from the edges of the walls, causing overall shrinking of the patterns. This shrinking effect should be taken into consideration during the design of fine patterns of CNT forest metamaterials in the visible region.
This work presents the combination of precise FIB patterning with additional secondary FIB etching in order to remove redeposited material from the surface and improve the growth of a CNT forest in the SRR array nanostructures. This method enables, for the first time, the fabrication of nanoscale metamaterial patterns and the catalytic growth of high-density CNT structures as small as about 150 nm on a predeposited catalyst film in designated areas. The patterning depth of 10 nm and the secondary etching depth of 0.5 nm allowed the growth of uniform, high-density, and highly aligned CNT forest metamaterials. The FIB secondary etching method decreased the average roughness of the catalyst surface, resulting in a significant improvement in the CNT forest quality. The top-down process of FIB patterning, allowed the precise fabrication of SRR structures; further, in the bottom-up approach for the fabrication of CNT forest metamaterials, the CNT forest growth could be controlled depending on the catalyst preparation. In the future, the effects of the redeposited material and of the catalyst film and the mutual interactions between them should be investigated to assist in the design of light wave-sized CNT metamaterials for optical, thermal, and optronic devices.
This work was supported by JSPS KAKENHI Grant (No. 24560050). The authors would like to thank Prof. Hideo Kohno for the TEM analysis, Mr. Hiroki Miyaji for experimental support, and Ms. Justyna Pander and Mr. Mark Hebden for English guidance and support.
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