Rapid Focused Ion Beam Milling Based Fabrication of Plasmonic Nanoparticles and Assemblies via “Sketch and Peel” Strategy
Yiqin Chen,†,# Kaixi Bi,†,‡,# Qianjin Wang,§ Mengjie Zheng,† Qing Liu,† Yunxin Han,‡ Junbo Yang,‡ Shengli Chang,‡ Guanhua Zhang,∥ and Huigao Duan*,∥,⊥ †
School of Physics and Electronics, State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, ∥College of Mechanical and Vehicle Engineering, and ⊥State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, People’s Republic of China ‡ College of Science, National University of Defense Technology, Changsha 410073, People’s Republic of China § College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *
ABSTRACT: Focused ion beam (FIB) milling is a versatile maskless and resistless patterning technique and has been widely used for the fabrication of inverse plasmonic structures such as nanoholes and nanoslits for various applications. However, due to its subtractive milling nature, it is an impractical method to fabricate isolated plasmonic nanoparticles and assemblies which are more commonly adopted in applications. In this work, we propose and demonstrate an approach to reliably and rapidly define plasmonic nanoparticles and their assemblies using FIB milling via a simple “sketch and peel” strategy. Systematic experimental investigations and mechanism studies reveal that the high reliability of this fabrication approach is enabled by a conformally formed sidewall coating due to the ion-milling-induced redeposition. Particularly, we demonstrated that this strategy is also applicable to the state-of-the-art helium ion beam milling technology, with which high-fidelity plasmonic dimers with tiny gaps could be directly and rapidly prototyped. Because the proposed approach enables rapid and reliable patterning of arbitrary plasmonic nanostructures that are not feasible to fabricate via conventional FIB milling process, our work provides the FIB milling technology an additional nanopatterning capability and thus could greatly increase its popularity for utilization in fundamental research and device prototyping. KEYWORDS: nanofabrication, FIB milling, helium ion beam, plasmonics, sketch and peel
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has several advantages for patterning metallic structures. First, it is an all-dry and resistless one-step fabrication process without involving any spin-coating, development, and extra pattern transfer steps. Second, it can work on insulating substrates without the charging effect that EBL usually suffers from. Considering most optical substrates such as quartz, CaF2, and MgF2 are insulating, this advantage is of particular importance in practical applications. Third, with the current state-of-the-art focused helium ion beam, FIB milling has the capability to fabricate sub-10 nm features in plasmonic metallic structures that are not feasible or extremely difficult to fabricate with
eliable top-down fabrication of plasmonic nanostructures with controlled size, shape, arrangement, and position is essential for broad plasmonic applications in sensing,1,2 integrated optical circuits, and optoelectronic devices.3−7 Due to the nanometric dimension and precision requirements, the fabrication of plasmonic nanostructures and the prototyping of plasmonic devices heavily rely on highresolution maskless lithographic techniques, mainly including electron-beam lithography (EBL) and focused ion beam (FIB, e.g., Ga+ and He+) milling.8−12 Among these two methods, EBL is more commonly used in practice because it has the flexibility to produce arbitrary planar plasmonic nanostructures including both particles and inverse structures by combining subsequent metallic lift-off or an etching process.13,14 Compared to the EBL-based approach, FIB milling can directly create plasmonic nanostructures starting from a metallic layer and thus actually © 2016 American Chemical Society
Received: September 17, 2016 Accepted: November 21, 2016 Published: November 21, 2016 11228
DOI: 10.1021/acsnano.6b06290 ACS Nano 2016, 10, 11228−11236
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Figure 1. Comparison of “sketch and peel”-based FIB milling process with conventional FIB milling process to fabricate a plasmonic nanoparticle. (a) Conventional process, in which almost the whole starting film should be milled to obtain an isolated particle. (b) “Sketch and peel” approach, in which only a narrow trench is required to mill to obtain a particle by selectively peeling off the majority of the metal film.
EBL.9,15,16 With the fast improved performance of FIB technology based on new ion sources, including He+, Be+, Si+, and Ne+,17−19 it is believed that FIB milling would be more and more accessible and popular for nanopatterning applications. Though FIB milling possesses many merits and is becoming highly accessible in common laboratories, unfortunately, however, it is currently still far less used in patterning plasmonic structures compared to EBL-based processes. The main reason is that FIB milling is intrinsically a subtractive fabrication technique which is only favorable for creating inverse structures such as nanoholes,20 nanoslits, and nanotrenches.21,22 To directly create isolated plasmonic nanostructures which are more commonly adopted in practical applications due to their more tunable optical properties, removal of the majority of the starting metallic layer is required,3,23 leading to an unacceptable long fabrication time in practice, as indicated by Figure 1a. Such a subtraction− fabrication nature significantly limits the functionality of FIB milling for broader applications because it makes FIB milling unfeasible to create isolated plasmonic particles.
In this work, we propose and demonstrate a simple but highly robust approach to address the above challenge of FIB milling technology via the “sketch and peel” concept that has been recently developed in EBL.24 With this simple approach, arbitrary plasmonic nanoparticles and their arrays, which are not feasible to fabricate with conventional FIB milling, can be rapidly and reliably defined. Particularly, we demonstrate that the concept is also applicable to current state-of-the-art helium ion beam technology, enabling ultrafast prototyping of highresolution plasmonic assemblies with tiny gaps. By providing FIB milling an additional capability, our work represents a significant progress for FIB direct patterning technology and to some degree revolutionizes the functionality of FIB for various applications.
RESULTS AND DISCUSSION Basic Concept. The basic concept of the “sketch and peel” fabrication strategy to obtain plasmonic nanoparticles is schematically shown in Figure 1b. In a typical process, starting from an initially evaporated metallic (e.g., Au) film without any adhesion layer (i) on a Si or SiO2 substrate, we first “sketch” the 11229
DOI: 10.1021/acsnano.6b06290 ACS Nano 2016, 10, 11228−11236
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ACS Nano
Figure 2. Experimental demonstration of the fabrication process flow for obtaining plasmonic nanoparticles using FIB milling via the “sketch and peel” strategy. (a−d) Four photographs showing the samples during the fabrication process: after “sketching” the outlines with FIB milling on a 30 nm thick gold film (a), after pasting transparent scotch tape (b), peeling off the tape (c), and after complete peeling off (d). (e,f) SEM images showing the fabricated structures before (e) and after (f) the peeling step. The periodic gold disks were defined on a 1 cm × 1 cm silicon die with a diameter of 800 nm and a pitch of 2.5 μm. Scale bar: 2 μm.
after complete peeling (Figure 2d). The scanning electron microscopy (SEM) image of the as-FIB-milled structures is given by Figure 2e, showing that the target disks were isolated from the entire gold film by the well-defined outlines. After the peeling-off process, the entire gold film was cleanly removed from the silicon substrate and transferred onto the adhesive tape (Figure 2d), and only the isolated gold disks remained, as depicted by Figure 2f. That demonstrated the reliable fabrication of plasmonic particles using the “sketch and peel” approach. Note that complete isolation of target particles from the entire gold film without any connections is necessary to enable reliable realization of this approach. Thus, the depth of the milled outline (D) has to be larger than that of the starting gold film (T). If D is smaller than T or there are any bridges connecting the inside disks with the rest of the gold film, the disks would have a high probability to be stripped away along with the entire gold film during the peeling process, as indicated by Figure S1 in the Supporting Information. Arbitrary Geometries. Such a simple approach demonstrated extremely high fidelity in patterning plasmonic particles with arbitrary geometries, as shown in Figure 3, in which all structures were obtained with the Ga+-based FIB milling process via the “sketch and peel” approach. Figure 3a−c demonstrates the fabrication of three types of plasmonic structures with different shapes including nanosquares, nanotriangles, and nanorods. Together with the nanodisks, these nanostructures represent the most commonly used plasmonic building blocks due to their tunable optical properties. From the SEM images, we can see that the designed structural geometries such as the sharp corners of the nanotriangles were
outline of the target particle using FIB milling to obtain a circular trench (ii and iii), which completely separates the particle and the rest of the film. Afterwards, a transparent adhesive tape is conformally adhered onto the metallic surface (iv) and then peeled off from the substrate surface, resulting in isolated particles on the surface (v). The key of this fabrication approach is that the rest of the metallic film can be selectively peeled off while the isolated particle can remain during the peeling process. The mechanism of this phenomenon will be discussed in the Mechanism Analysis section. Note that the weak adhesion between the metallic film and the substrate is the prerequisite for this process. Compared to the conventional FIB milling method, which requires point-by-point removal of almost the whole metallic film to obtain a particle (Figure 1a) and thus is actually impractical for real applications,3,25 this “sketch and peel” strategy can fabricate isolated particles by only milling a very small part (i.e., the outline) of the metallic layer, and thus the fabrication efficiency is dramatically improved with mitigated ion-bombardment-induced substrate damage. Concept Demonstration. As a proof of concept, Figure 2 demonstrates the fabrication of a gold disk array by Ga+-based FIB milling via the “sketch and peel” approach. The diameter of the designed disks was 800 nm. Starting from a gold film with a thickness 30 nm on silicon substrate, Figure 2a−d shows the scenarios of the sample after FIB milling (Figure 2a), after pasting a tape (Scotch-810, tack value = 2.41 N/cm) that was commonly used for dry pattern transfer in some lithographic process such as nanosphere lithography and edge lithography (Figure 2b),26,27 during the peeling process (Figure 2c), and 11230
DOI: 10.1021/acsnano.6b06290 ACS Nano 2016, 10, 11228−11236
Article
ACS Nano
Figure 3. Plasmonic particles with arbitrary geometries fabricated by Ga+-FIB milling based on the “sketch and peel” strategy. (a−c) Commonly used periodic plasmonic structures with square (a), triangle (b), and rod (c) shape. The pitch of the structures was 800 nm. The average edge length of the square and triangle was 273 and 185 nm, respectively. The length and width of each rod were 320 and 90 nm, respectively. (d) “L”-shaped chiral structures with a pitch of 2 μm. (e) Gold disks with varied diameter from 100 nm to 1 μm. (f) Arrayed circular gold pads embedded with nanoholes. The diameter of each gold pad was 4.5 μm. The diameter of the embedded nanoholes was set to be 300 nm. Note that the elliptical geometry was induced by the astigmatism of the ion beam during our fabrication process. The starting gold film was 30 nm. All structures were defined on the silicon substrate with a 285 nm SiO2 layer. The black edges in each SEM image were ascribed to the relative lower intensity of a secondary electron signal in the overmilled trenches during the imaging. All scale bars: 1 μm.
Figure 3e and Figure S2 show a series of fabricated gold disks with varied diameter from 100 nm to 120 μm, indicating the generality of this approach for fabricating structures with different scales. In addition, with this “sketch and peel” strategy, FIB milling exhibits the excellent capability in prototyping components for optoelectronic and electronic devices, as well. As an example, Figure S2f,g demonstrates the rapid and direct fabrication of nanogap electrodes. Particularly, by combining the direct milling advantage of FIB with the current “sketch and peel” approach, functional structures that are not feasible or extremely challenging to fabricate by any other methods can be readily fabricated. As an example, Figure 3f shows an array of pads in which each nanohole array was fabricated. Such a multiscale plasmonic array has demonstrated intriguing optical properties for sensing and lasing applications,32,33 but its direct maskless fabrication has remained a challenge for a long time because both common EBL and FIB have difficulties realizing structures including densely packed complementary elements. One possible limit of this process is that it would be difficult to directly obtain closely packed periodic metallic structures. As demonstrated in Figure S3, dense structures with a too narrow space (
