The smooth surface of the metallic nanostructure is essential for thepropagation of surface plasmon polaritons. In this paper, we present a novel method to fabricate the metallic nanopatterns with ultra-smooth surface on various substrates. By using a silica film as the sacrificial layer, we show that the prefabricated metallic nanopatterns produced by electron beam lithography and film deposition can be hydrolyzed and transferred onto a designated substrate. The ultra-smooth surface morphology of nanopatterns has been characterized and verified by scanning electron microscopy and atomic force microscopy. More importantly, we demonstrate that this method can successfully produce a variety of nanostructures with high product yield, even onto the uneven substrate. The results indicate that our proposed method is a promising and versatile means to fabricate multiplicate smooth metallic nanostructure on various substrates for the application of nanophotonic devices.
© 2013 Optical Society of America
Surface plasmon polaritons (SPPs) are light waves coupled with free electron oscillations bound to a metal-dielectric interface . The fascinating features of SPPs, such as strong field confinement at the nanoscale, intensive local field enhancement, and interplay between strongly localized and propagating SPPs, have found a variety of great applications [2–4]. In particularly, the propagation of SPPs in metal strips, waveguides, and arrays has been considered as a means to transport and process optical signals in nanophotonic devices [5–9]. The common methods used for the fabrication of plasmonic devices include the focused ion beam (FIB) [10–12] lithography, electron beam lithography (EBL) [13, 14], nanoimprint lithography (NIL) [15–17], and stencil lithography [18–20], to name a few. Each method has its own distinct advantages, however, can also constantly introduce the deformation of designed patterns and related roughness surface, resulting in the poor performance of the SPPs propagation due to severe Ohmic dissipation and scattering losses from the surface roughness, grain boundaries, and other imperfections .
Previous reports by Wagner , Ederth  and Hegner  have shown that the template stripping (TS) method can be used to fabricate metallic film with ultra-smooth surface. The method starts with the deposition of metal film on the substrate with very smooth surface such as silicon, silicon oxide and mica, then the metal film is peeled off using a back layer such as epoxy, resulting in the reversed surface of metal film as smooth as that of the substrate. By combining with other nanofabrication techniques such as UV lithography, EBL and nanosphere lithography, the TS method could be more efficient and versatile for producing the ultrafine and smooth plasmonics nanostructures [11, 21, 25–28]. Unfortunately, the deformation and the cracks of the metallic nanostructure occur frequently during the peeling process with a back support of epoxy layer. Herein, instead of the stripping processing, we introduce a new method to fabricate the intact metallic nanopatterns onto various substrates through hydrolysis of a sacrificial layer and transfer process (HSLT). As compared to the TS method, our proposed HSLT method cannot only produce less cracks and deformation of the nanopatterns, but also result in a much higher yield of products with ultrasmooth surface morphologies. Furthermore, we demonstrate that the HSLT method can also be applied onto uneven substrates and therefore have a potential application in the future 3-D plasmonics nano-devices.
2. Fabrication method
The main process of the HSLT method is presented schematically in Fig. 1. It started with the spin-coating of poly (methyl methacrylate) (PMMA) (950 K A4, 4 wt%, Micro Chem) resist on 1cm × 1cm silicon substrate covered with 300 nm thickness thermal oxide silica film (Silicon Valley Microelectronics Inc.), as shown in Fig. 1(a). The thickness of the PMMA resist is 280 nm obtained from 2000 rpm spin-coating followed by baking on a hot-plate for 2 minutes at 180 °C. The resist template patterns shown in Fig. 1(b) were produced by EBL system (Raith e-Line) at an extra high tension (EHT) of 21 kV with the exposure dose of 130 uC/cm2, and then developed in MIBK: IPA = 1:1 for one minute and stopped in IPA for 40 seconds. The template was then covered by sputtering a gold film with 500 nm thickness at the speed of 3 nm/min, as shown in Fig. 1(c). Because Au film was thicker than PMMA resist, a continuous Au film was formed above the PMMA resist even in the hollow areas. The sample was then immersed into KOH solution (1M) at the temperature of 95 °C (Fig. 1(d)). After about 20 minutes soaking, the metallic film detached from the substrate due to the hydrolysis of SiO2 layer in KOH solution. After rinsed in DI water for several times to clean off the remained KOH, the film was then transferred to an arbitrary substrate, as shown in Fig. 1(e). It should be noted that during the transfer process, the metallic film was reversed in order to turn the ultrasmooth metallic nanopatterns face up. After the removal of PMMA resist in acetone, the full metallic nanostructures were remained on the designated substrate, as shown in Fig. 1(f). Considering that, in our designed metallic pattern, the most of SiO2 surface is covered by the PMMA film. Therefore, we propose one possible mechanism for the hydrolysis process as follow: the PMMA film will form hydrogen bonds with the SiO2 surface through C-H and O-Si interaction when it is spun onto the substrate. When PMMA/SiO2 immersed into KOH solution, the ion of OH- will break the hydrogen bonds quickly and detaches the PMMA from SiO2, while KOH can react with the SiO2 and result in partially etching of its surface.
3. Results and discussions
The scanning electron microscope (SEM) images of several Au nanostructures fabricated by HSLT method are shown in Fig. 2. As seen, all nanostructures demonstrate perfectly intact and retain high resolution, while their shape and thickness can be controlled readily with the negative PMMA resist template produced by EBL. Figure 2(a) shows a nano-triangle cavities with the height of 280 nm, while the side length and width of the triangle are 1.5 μm and 500 nm, respectively. We also produced a bowtie nanostructure with 20 nm narrow gap, as seen in Fig. 2(b), which has widely applications in plasmonics device [29, 30]. Figure 2(c) shows a square split-ring resonator (SRR) with 250 nm gap, which can be used as a metamaterial device for the frequency tunable optical sensors [31–33]. Moreover, more intricate G-shape metallic nanostructures are also produced, as shown in Fig. 2(d). Figures 2(e)-2(h) are the side view SEM images of above mentioned nanopatterns.
It is noteworthy that the smooth surface of the nanostructure is prerequisite for realizing the efficient interference and the propagation of SPP, which is unfortunately rather difficult to achieve with conventional FIB, EBL and NIL techniques . Remarkably, such a difficulty can be simply overcome by our proposed HSLT method because of the following two reasons. On the one hand, the upper surface of the nanopatterns is directly contacted with the SiO2 substrate before hydrolysis, making it as a surface replica of the substrate to enable inheriting the substrate’s smooth feature. On the other hand, the side wall of the nanopatterns can also keep fine and flat as well as that of PMMA resist, due to its tightly contacting with the resist before the removal of PMMA.
In order to evaluate the morphology character of the fabricated nanostructure, we investigated the surface roughness with atomic force microscopy (AFM) (SPA-300HV Seiko Instruments Co.). The characterization was performed with tapping model at 180 kHz. The same Si cantilever and the tip with 30 nm radius were used for all samples. Figure 3 shows a typical result of the triangle cavity nanostructure. The whole morphology image of the nanostructure is shown in Fig. 3(a), in which the surfaces on the top of triangle cavity and the base region are the replica ones of SiO2 substrate and PMMA resist, respectively. Figure 3(b) and Fig. 3(c) show the topographies of selected areas of top triangle cavity (red square in Fig. 3(a)) and base area (green square in Fig. 3(a)), and the related root-mean-square roughness (RMS) of the surface is estimated to be about 0.25 nm and 0.87 nm, respectively. The result is on a par with, or ahead of the TS method . The finding also demonstrates that the surface roughness of the metallic nanostructure is strongly dependent on the morphologies of the substrates, and the replica surface from SiO2 is much smoother than that from PMMA. Nevertheless, both surfaces possess better smoothness than those of the films fabricated by the direct deposition method, with typical RMS ~5.2 nm , indicating that the HSLT method is a very promising approach to produce high quality plasmonic nanostructures.
With the hydrolysis process of a sacrificial layer in HSLT method instead of the stripping process in TS method, the disadvantage of the fractures and loss of the metallic patterns happened in the TS method can be significantly suppressed. As a consequence, the successful product yield of metallic nanopatterns is also obviously improved. As shown in the SEM image of Fig. 4(a), a variety of metallic nanopatterns transferred from the SiO2 substrate demonstrate intact feature. For well understanding the distinguished difference between TS and HSLT methods, we schematize the key process of both methods in Fig. 4(b) and compare the possible factors that may influence the product yield. As seen, two adhesive forces are mainly involved in TS method during the stripping process with back adhesive layer (here is the epoxy): F1 existed between the side walls of metallic patterns and PMMA resist, while F2 is between the metallic surface and the substrate. If either F1 or F2 is larger than the cohesive force of the metallic nanostructures, the edge cracks or the loss of nanopatterns occurs. By contrast, there are no forces involved in the HSLT method. With SiO2 as a sacrificial layer, both the crack and the loss of nanopatterns can be avoided in the mild hydrolysis process in KOH solution. Note that this peculiarity of HSLT method makes it more suitable to fabricate ultrafine nanopatterns. As shown in Fig. 4(c), we have successfully fabricated nano-rectangle cavities array with the width as smaller as 90 nm, which is very difficult to achieve with TS method .
In order to evaluate the quality of our fabricated nanocavity, we measured the optical spectra of three kinds of designed plasmonic nanocavities by the cathodoluminescence (CL). The CL spectra were collected by the use of a Gatan Mono-CL3 system attached to a FEI SIRION-200 FESEM through the electron beam excited on the cavity center with 30 kV acceleration voltages. The typical result is shown in Fig. 5. As seen, several resonant peaks can be observed clearly from each Ag nanocavity. Herein, we only focus on three modes in the square nanocavity with the resonant wavelength at ~420 nm, 500 nm and 620 nm. The resonant condition of (m, n) mode for the square cavity can be simply determined with the expression :35]. As to square cavity with L ~940 nm, the calculation result demonstrates that the SPP resonance with the wavelength at ~420 nm, 500 nm and 620 nm can be ascribed to the (3, 4), (1, 4) and (1, 3) mode. Moreover, their corresponding Q factor is estimated to be about 14, 16 and 13, respectively. The result is comparable to the previously reported values from the ultrafine plasmonic cavity by Yu group , indicating that the energy dissipation is very weak in our fabricated Ag nanocavity and its ultrasmooth surface can facilitate the SPP propagation and resonance. Moreover, the mode volume of the square nanocativy can be written as :35], so the calculated V = 0.003 μm3, and the figure of merit (defined as Q/V ) of this cavity can be estimated to be about 5330 μm-3.
Plasmonic nanostructures applied in biosensor [28, 36, 37] and optics devices  often refer to an uneven substrate, which often makes it very difficult to employ the traditional nanotechnologies because the failure of resist coating and the beam focusing. Only a few methods such as NIL , FIB lithography [11, 40] and self-assembly  can fabricate nanostructures on certain uneven substrates. Thanks to the absence of back layer and the softness of metallic and PMMA film in HSLT method, the film with prefabricated nanostructure can now be readily transferred and printing onto any uneven substrates. Additionally, during the process, the capillary force between the film and the substrate can spontaneously pull the film onto the substrate tightly. For instance, Fig. 6 shows the SEM images of various metallic nanopatterns on the cylinder surface of glass fiber with different diameter 20 ~200 μm. As seen, all these nanopatterns have smooth surface and no obvious cracks and deformation after the transfer process, indicating that the HSLT method possesses great potential for fabricating the nanopatterns on various uneven substrates.
In this letter, we propose and verify experimentally a novel HSLT method to fabricate the ultrasmooth and ultrafine metallic nanopatterns on various substrates. SEM observations indicate that the side wall of the nanopattern is sharp and the upper surface is very smooth. AFM characteristics demonstrate that the RMS of the pattern surface is only sub-nanometer, much smaller than that of the film produced by direct depositions. As compared to the TS method, the HSLT method is capable of fabricating the nanopatterns with high product yield and sub-100 nm nanostructures. More importantly, the softness of metal/PMMA film makes the HSLT method even suitable for the fabrication of the nanopattern on the nonplane substrate. All these results indicate that the HSLT method offer a simple, but powerful way to fabricate multiplicate metallic nanostructures with ultrasmooth surface for various plasmonic applications.
This work is supported by MOST of China (2011CB921403), NSFC (under Grant Nos. 11374274, 11074231, 11004179 and 21121003) as well as by CAS (XDB01020000).
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