Universal Nanopatterning Technique Combining Secondary

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Universal Nanopatterning Technique Combining Secondary Sputtering with Nanoscale Electroplating for Fabricating Size-controllable Ultrahigh-resolution Nanostructures Tae-Eun Song, Chi Won Ahn, and Hwan-Jin Jeon Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00950 • Publication Date (Web): 30 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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Universal Nanopatterning Technique Combining Secondary Sputtering with Nanoscale Electroplating for Fabricating Size-controllable Ultrahighresolution Nanostructures

Tae-Eun Song,a,b Chi Won Ahn*,a, and Hwan-Jin Jeon*,c,d

a

Department of Nano-Structured Materials Research, National NanoFab Center (NNFC), 291

Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea. b

c

Institute für Materialphysik, Wilhelm-Klemm-Straße 10, D-48149 Münster, Germany.

Department of Chemical Engineering and Biotechnology, Korea Polytechnic University, 237.

Sangidaehak-ro, Siheung-si, Gyeonggi-do 15073, Republic of Korea. d

Department of Information Device and Fusion Materials Engineering, Korea Polytechnic

University, 237. Sangidaehak-ro, Siheung-si, Gyeonggi-do 15073, Republic of Korea. E-mail: [email protected], [email protected]

KEYWORDS: Nanopatterning, Secondary Sputtering Lithography, pulse-mode electroplating

ACS Paragon Plus Environment

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Abstract Here, we describe a next-generation lithographic technique for fabricating ultrahigh-resolution nanostructures. This technique makes use of the secondary sputtering phenomenon of plasma ion etching and of nanoscale electroplating to finely control the resolution of the fabricated structures from ten nanometers to hundreds of nanometers from a single microsized master pattern. In contrast to previously described techniques that incorporate a recently developed secondary sputtering lithography (SSL) patterning approach, which could only yield 10-nmresolution structures, in the current technique, we used an improved SSL approach to produce various-sized, high-resolution structures. Additionally, this improved SSL approach was used to fabricate size-controllable 3D patterns on various types of substrates, in particular, a silicon wafer, transparent glass and flexible polycarbonate (PC) film. Thus, this method can serve as a new-concept patterning method for the efficient mass production of ultrahigh-resolution nanostructures.

Introduction A high-resolution, high-aspect-ratio nanostructure patterning technology has the potential to be of significant value in various fields such as nanoelectronics, optics, biosensors, energy devices and display devices owing to the optical and electric peculiarities of nanostructured materials1–9. For example, the nano- and microstructural characteristics of patterned materials can be adjusted to make these materials superhydrophobic/superoleophobic and hence potentially applicable in oil/water separation,

anti-fogging,

water desalination, and heat transfer (anti-icing)


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applications10–12. Additionally, changing the structure and size of nanostructured materials can alter their dynamic behavior as well as the alignment of molecular building blocks. Photolithography is a well-known conventional lithographic technique that has been widely used for the economically efficient and commercially scalable mass production of nanoand microsized structured materials for various applications. The use of photolithography for fabricating large-area patterned materials at sub-100-nm resolution is, however, critically limited because this technique requires very costly processing conditions. It has recently been suggested that these limitations may be overcome by using nanoimprint, capillary force, colloidal, block copolymer, photo-roll and edge lithography approaches13–19. While these approaches can yield high-resolution patterns, their use still faces some critical obstacles: they require high-cost, specialized equipment, and they suffer from processing defects and low throughput. The Jung group recently introduced “secondary sputtering lithography” (SSL)20–26, a patterning technique that can be used to fabricate ultrahigh-resolution (10 nm), threedimensionally complex structures with high aspect ratios over large areas by utilizing plasma processing and without requiring specialized equipment. The secondary sputtering in this lithographic technique involves a plasma ion etching process in which the etched particles on the bottom layer of the target materials are sputtered with plasma at a wide angular distribution. This technique has many advantages. First, SSL can enable the fabrication of high-resolution 10-nmscale 3D structures from a microsized master pattern (100 nm ~ 10 ㎛), as this technique is a one-step process in which target materials on the bottom layer are etched and deposited on the side surfaces of a prepattern at the same time that plasma ion etching is carried out, without requiring any further deposition process. In addition, SSL can be used to produce various complex 3D nanostructures, such as line patterns and cup/hollow-cylinder-shaped patterns, at a


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resolution of 10 nm and an aspect ratio of ~30 because this method can transfer the 3D side surface shape of organic prepatterns to 3D inorganic structures with walls having widths of 10 nm. Lastly, SSL can be applied to most inorganic materials, as this technique involves the physical etching of plasma ions and not a chemical process. Although the SSL technique is a very promising approach for fabricating 10-nm-scale structures, the patterns of the products of this technique are limited to this specific 10-nm resolution, and thus, this SSL technique cannot be considered to be a universal lithographic technique. A next-generation lithographic technique should be able to fabricate patterns having a wide range of resolutions from ten nanometers to hundreds of nanometers. In this work, we developed a new lithographic technique that, using a microsized master pattern, can fabricate a 3D patterned nanostructure with a high aspect ratio over a large area – and whose resolution can be tuned from ten nanometers to a few hundred nanometers – by combining the plasma ion etching process and the nanoscale electroplating technique. To control the widths of the structures, we utilized square-pulse-mode electroplating, which enabled a uniform and fine grain deposition onto the 10-nm-scale SSL structures. The electroplating method we used can increase and control the resolution of the structures very effectively, as this technique involves a simple solvent process requiring only a small reaction bath and short processing time27–30. Electroplating of the high-aspect-ratio nanostructures required a specific current flow across the surface to facilitate deposition without damaging the nanostructures. Pulse electroplating usually yields finer grain deposits than dc electroplating because of the high pulse current density and resulting high nucleation rate31. Low porosity has been observed in the pulse electroplating of Au due to grain refinement32,33. After a short (pulse on and off, total time: 10 min ~ 1 hour) and simple (one-step solution-immersion process) pulse electroplating process,


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the widths of the structures were easily controlled in the nanosize range by adjusting the electroplating processing time. In addition, this improved secondary sputtering lithography was shown to be applicable to various 3D patterns, including line and hole-cylinder structures, and various types of substrates, such as silicon wafers, transparent glass, and flexible polycarbonate (PC) films. Furthermore, we demonstrated this technique to be a potential candidate for nextgeneration lithographic techniques to replace conventional photolithography and optical lithography.

Experimental Section Preparation of the polydimethylsiloxane (PDMS) mold pattern. To prepare the master pattern, an array of line shapes with a periodicity of 1 µm and widths of 200 nm and 500 nm was fabricated in silicon using e-beam lithography. A PDMS mold was replicated from the silicon master. The PDMS was prepared by mixing the PDMS prepolymer (Sylgard 184A/B = 10:1, Dow Corning) and pouring the mixed PDMS onto the silicon master. After removing bubbles from the mixture, the PDMS mold was cured at 80°C for two hours. Fabrication of 10-nm-scale nanostructures using the secondary sputtering phenomenon during plasma ion etching. A predominantly Au layer with a thickness of either 20 nm or 100 nm and a Ti adhesion layer with a thickness of either 2 nm or 5 nm were deposited on a Si wafer, glass, or PC film substrate using e-beam evaporation. Then, a polystyrene (PS) thin film was prepared on the Au-coated substrate by spin-coating 6 wt% polystyrene (PS 18,000 g mol-1, Sigma Aldrich) in toluene for 45 s at 3000 rpm onto the Si, glass or PC film substrate. A cured PDMS mold with the topographic features described above was placed and pressed onto the PScoated Au-coated substrate. Then, the PDMS-covered substrate was heated at 135°C for 45 min


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in vacuum conditions, which induced the PS layer to form the PS nanopatterned structures. These PS prepatterns played an important role as a framework during secondary sputtering. The residual PS layer, i.e., that remaining on the bottom layer, was removed by reactive ion etching (RIE) using 40 sccm O2 and 60 sccm CF4 at a chamber pressure of 20 mTorr and a power density of 80 W. In addition, the Au layer that was not covered with a PS prepattern was simultaneously etched and deposited onto the side surfaces of the PS prepatterned structures via secondary sputtering during the Ar plasma ion etching (ion-bombardment) process. Residual PS layers in the Au pattern were removed during a second round of RIE using 100 sccm O2 for 20 min. Controlling the resolution of Au patterned structures using pulse-mode electroplating. Many Au plating baths and additives are used industrially. In our current work, we used a neutral, buffered Au cyanide bath (HoplaGold PG-100M, Hojin Platech Co., Ltd., Gyeonggi-do, Korea). High-purity potassium Au cyanide (KAu(CN)2) with 68 wt% Au metal was obtained from commercial vendors (Yoo Chang Metal Ind. Co., Gyeonggi-do, Korea). Au was pulse electroplated from a cyanide bath containing KAu(CN)2, whose concentration when used in a low-current-density bath generally ranges from 2 ~ 12 g/L; we used a concentration of 8 g/L. The electroplating also used a citrate buffer at pH 6.0-6.5 and operated with an external power supply (WPG100e potentiostat/galvanostat, WonATech Ltd., Korea), in which the current was controlled. The periodic array of prepatterned Au lines was placed in the cathode (working electrode), and the voltage was measured between it and the anode (with a platinum mesh counter electrode) at room temperature. In pulse electroplating, there are numerous process variables, including pulse amplitude, pulse time, and relaxation time34, whose values affect the morphology of the coating and its surface. Each pulse that we applied consisted of a pulse time (Ton) during which the current was applied and a relaxation time (Toff) during which zero current


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was applied, as shown in Supporting Information Fig. S1. The duty cycle (γ) of the pulse amplitude, i.e., the percentage of the total time of a cycle spent in the on current state, was calculated using the equation35-36

The average current density (Iavg) was calculated using the equation

where Ip is the pulse current density. Turning off the current of the pulsed electroplating process for the relaxation time (Toff) in general decreases the negative electrical potential at the surface and in our case consequently permitted the diffusion of Au+ from the bulk solution to the prepatterned Au line, where the negative electrical potential applied for a pulse time of Ton to the prepatterned Au line drove the reduction of Au+ and consequent deposition of Au on the prepatterned Au line.


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Results and Discussion Fig. 1 illustrates the overall procedure, including the secondary sputtering phenomenon (SSP) and pulse electroplating, used to produce the highly periodic and high-aspect-ratio micro/nanostructured patterns. The electron-beam evaporation of Au, with Ti used as an adhesive layer, generated a uniform gold layer on the Si, glass, and PC film surfaces. A prepatterned PS film was deposited via spin coating in anhydrous toluene. Two types of patterns, one whose repeating structures each had a width of 200 nm and the other each with a width of 500 nm, were created by carrying out a pattern transfer using a PDMS mold placed on the PS surface, which was then heated above the glass transition temperature of the PS polymer to drive the polymer into the void spaces of the mold patterns by the capillary effect. The PS on the bottom was removed by RIE (Fig. 1a). Then, an Au layer on the bottom was etched and sprayed onto the side surfaces of the PS prepattern with a wide-angle distribution, as a result of the SSP during the Ar ion bombardment process (Fig. 1b, 1d). The PS residue was then removed by carrying out an oxygen RIE process under vacuum conditions (Fig. 1c, 1d). Arrays of periodically repeating Au lines with ~10 nm cross-sectional dimensions and heights of ~470 nm were generated. For the pulse electroplating of prepatterned Au lines with a potential-driven electrolytic cell, reduction at the cathode led to the deposition of Au. Reduction was achieved via electron transfer from the polarized substrate surface to the dissolved Au(CN)2- ionic complexes, resulting in the splitting of the complex and the deposition of solid Au on the prepatterned Au lines. This electrolytic reaction occurred at the interface of the solution and prepatterned Au line surface. At this interface, the CN- and Au+ reactive species reacted according to the reaction37 (Fig. 1d)


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(3)

The reduction occurred at the cathode (prepatterned Au line array) according to the reaction (4)

In the subsequent step, a thick Au nanostructure was electrodeposited onto the periodic prepatterned Au lines using pulse electroplating (Fig. 1e). The thickness of the Au nanostructure increased as a function of pulse electroplating time (Fig. 1f). In Fig. 2, scanning electron microscope (SEM) images of the patterns of Au lines obtained at each step in the fabrication process (secondary sputtering lithography and electroplating) are shown. First, PS prepatterns with periodically repeating structures separated by spacings of 500 nm and each having a width of 500 nm and height of 400 nm were fabricated on the Au-coated Si wafer by carrying out a pattern transfer from PDMS (Fig. 2a). After secondary sputtering during the plasma ion etching process, Au particles on the bottom were etched and deposited simultaneously onto the side surfaces of PS prepatterned structures (Fig. 2b). As seen in the sideview image in Fig. 2a, an ultrathin Au layer (15-nm-scale) was formed on the side surface of each PS structure. Then, after removing PS residue via reactive ion etching, 15-nm-scale ultrathin Au lines separated by spacings of 500 nm and with heights of 400 nm were successfully fabricated on the Si wafer (Fig. 2c). Both Au line patterns with (Fig. 2b) and without (Fig. 2c) PS residues can be applied to the controllable nanoscale electroplating process. In the Au squarewave-pulse electroplating process, a current density of ~0.5 mA/cm2 was applied onto the Au prepatterned lines over a period of time. Figs. 2d and 2e show SEM images of Au nanostructures with thicknesses of approximately 110 nm. These structures were constructed by plating the gold particles onto the Au prepatterned lines (Figs. 2b and 2c) using a plating time of 60 min (Ton: 1


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ms, Toff: 9 ms, γ: 10 %). Analysis of these SEM images indicated that adjusting the parameters of the electroplating process can control the resolution of the Au line pattern and change the widths of the lines. Additionally, it is noteworthy that Au structures can be applied to our method regardless of the presence of PS residue. That is, even though PS residue can act as an inhibitor of electroplating growth, more complex nanostructures can be fabricated due to the presence of some PS residue. Energy dispersive X-ray (EDX) spectroscopy, carried out after electroplating, showed that 99.18 % of the Au was electroplated onto the wall of the Au line (Fig. 2f). In addition, we found that the sizes of the features of the resulting Au line-patterned structures can be controlled by changing the duration of the electroplating (Fig. 3). In Au pulse-mode electroplating, a current density of ~ 0.5 mA/cm2 was applied to the Au prepatterned lines on a Si substrate. The thickness of the deposited Au nanostructure line was measured as a function of electroplating time and focused ion beam-scanning electron microscopy (FIB-SEM) images were acquired for the pulse electroplated sample (on a Si substrate), and the periodically repeating structures showed a periodicity of 1 µm, spacings of 800 nm, a height of ~470 nm and a width of 200 nm. Au nanostructure lines were constructed by using electroplating times of 20 min and 40 min (Ton: 1 ms, Toff : 9 ms, γ: 10 %). Fig. 3a shows a plot of the thickness of the Au nanostructure line against electroplating time. As the electroplating time with PS was increased from 20 min to 40 min, the average thickness of the Au nanostructure line increased from 24 nm to 42 nm. These results showed that increasing the electroplating time can increase the thickness of the Au nanostructure line. It is very important that various nanostructures with different resolutions (from ten nanometers to hundreds of nanometers) can be fabricated easily, inexpensively, and with high control from a single 500-nm-sized prepattern.


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Our approach can be applied to various substrates that are not damaged by the electroplating solution. The substrates we tested for this purpose included a Si wafer, glass (transparent) and a PC film (transparent and flexible) (Fig. 4). Au prepatterned lines were constructed on these substrates using the SSL method. PS was then entirely removed from the Au-covered PS patterns by RIE treatment with O2 plasma, and Au was pulse electroplated, with a current density of ~0.5 mA/cm2, onto the Au prepatterned lines. Fig. 4 shows the Au nanostructure lines constructed on the Si (Fig. 4a), glass (Fig. 4b), and PC film (Fig. 4c) substrates using a plating time of 60 min (Ton: 1 ms, Toff: 9 ms, γ: 10 %). We were able to exercise good control over the resolution of the Au line structures by adjusting the duration of the electroplating (Figs. 4d-4g). It was confirmed that the electrical conductivity of the samples differed for different Au line widths and process times (Supporting Information Fig. S3). Although nucleation and binding during crystal growth depend on the substrate conductivity, most research has concentrated on the influence of the deposition conditions (rather than the substrate conditions) during pulse-mode electroplating on the morphology of the walls of prepatterned Au lines.

Conclusion We fabricated size-controllable 3D patterned nanostructures with a wide range of resolutions (from ten nanometers to a few hundreds of nanometers) and a high aspect ratio over a large area through controlled plasma ion reactions and an electroplating process. This technique can produce various 3D patterns with ultrahigh resolution by utilizing (i) secondary sputtering during the plasma ion process, which can enable the fabrication of 3D structures on the 10 nm scale, and (ii) finely controlled electroplating (square-pulse-mode electroplating), which can control the resolution of structures through simple solution reactions by increasing the thickness of the


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original 10-nm-scale SSL-derived structures. Additionally, this technique can be extended to produce features with various 3D shapes (lines, hole-cylinders) from various other conductive materials (Cu, Al, Ag, etc.) on various types of substrates (silicon wafers, transparent glass and flexible PC films). Accordingly, this new lithographic technique can serve as a next-generation patterning method for fabricating complex high-aspect-ratio patterned structures with ultrahigh resolution.


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Figure 1. Schematic illustration of the high-aspect-ratio Au electroplating technology and secondary sputtering phenomenon (SSP) used to increase the thickness of Au nanostructures. Using a PDMS mold, a PS prepattern was fabricated on the target material by depositing the substrate on the target material (a). An Au layer was applied on the whole sample (b) or on part of the sample (d) by a plasma etching process, and secondary sputtered Au particles were attached to the sides of the walls of the prepattern (b, d). The 10-nm-scale patterned structures were fabricated uniformly on the substrate surface (c) and the surface of the coated gold layer (e) after removal of the PS prepattern by RIE. Pulse-square-wave Au electroplating was carried out (f). Au was nucleated and grown on the prepattern of an Au line via pulse electroplating (g). Influence of the electroplating time on the thickness of the structure (h).


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Figure 2. The scanning electron microscopy (SEM) images of line-shaped PS patterns with a width of 500 nm, spacing of 500 nm and periodicity of 1 µm fabricated by SSP and pulse electroplating over a large area (5 mm × 5 mm). Ultrathin Au line pattern with a width of ~15 nm and height of ~470 nm. (a) The PS prepattern is fabricated on the target-material-deposited substrate from a PDMS mold and (b) secondary sputtered Au particles are attached to the


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sidewall of the prepattern. (c) The PS prepattern is removed entirely by O2 plasma treatment for 40 min. (d) ~110-nm-scale Au nanostructure line prepared by pulse electroplating of the sample (e) and ~110-nm-scale Au nanostructure line prepared by pulse electroplating after removing the PS prepattern. (f) The Au peak in the EDX graph demonstrates that the Au nanostructure line pattern is present on the substrate.


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Figure 3. (a) Plot of the thickness of the deposited (linear) Au nanostructure versus pulse electroplating duration. More Au was deposited with increasing electroplating time. (b-d) Images of the Au nanostructures constructed on the Si substrate using electroplating times of (b) 0 min, (c) 20 min, and (d) 40 min (Ton: 1 ms, Toff: 9 ms, γ: 10 %).


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Figure 4. (a-c) Digital camera photographs of the Au deposited on (a) the Si wafer, (b) glass and (c) PC film after 60 min of pulse electroplating. (d-g) Images of the patterns of the Au nanostructures constructed on the glass substrate using electroplating durations of (d) 0 min, (e) 20 min, (f) 40 min, and (g) 60 min (Ton: 1 ms, Toff: 9 ms, γ: 10 %). More Au was deposited with increasing electroplating time


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ASSOCIATED CONTENT Supporting Information. Details of methods and supplementary results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.A.) and [email protected] (H.-J.J.)

ACKNOWLEDGMENT This research was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (NRF-2015K1A4A3047100). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT and Future Planning) (NRF-2017R1C1B1006807).


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Table of Contents Graphic We fabricated size-controllable 3D patterned nanostructures with a wide range of resolutions and a high aspect ratio over a large area.


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Table of Contents Graphic We fabricated size-controllable 3D patterned nanostructures with a wide range of resolutions and a high aspect ratio over a large area.


Universal Nanopatterning Technique Combining Secondary

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