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Fabrication of polymer nanowires via maskless O2 plasma etching

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 165301 (http://iopscience.iop.org/0957-4484/25/16/165301) View the table of contents for this issue, or go to the journal homepage for more

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Nanotechnology Nanotechnology 25 (2014) 165301 (10pp)

doi:10.1088/0957-4484/25/16/165301

Fabrication of polymer nanowires via maskless O2 plasma etching Ke Du1 , Ishan Wathuthanthri1 , Yuyang Liu1 , Yong Tae Kang2 and Chang-Hwan Choi1,3 1 2 3

Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA School of Mechanical Engineering, Korea University, Seoul 136-701, Korea Department of Mechanical Engineering, Kyung Hee University, Yong In, Gyeong-Gi, 446-701, Korea

E-mail: [email protected] Received 13 August 2013, revised 26 February 2014 Accepted for publication 27 February 2014 Published 26 March 2014

Abstract

In this paper, we introduce a simple fabrication technique which can pattern high-aspect-ratio polymer nanowire structures of photoresist films by using a maskless one-step oxygen plasma etching process. When carbon-based photoresist materials on silicon substrates are etched by oxygen plasma in a metallic etching chamber, nanoparticles such as antimony, aluminum, fluorine, silicon or their compound materials are self-generated and densely occupy the photoresist polymer surface. Such self-masking effects result in the formation of high-aspect-ratio vertical nanowire arrays of the polymer in the reactive ion etching mode without the necessity of any artificial etch mask. Nanowires fabricated by this technique have a diameter of less than 50 nm and an aspect ratio greater than 20. When such nanowires are fabricated on lithographically pre-patterned photoresist films, hierarchical and hybrid nanostructures of polymer are also conveniently attained. This simple and high-throughput fabrication technique for polymer nanostructures should pave the way to a wide range of applications such as in sensors, energy storage, optical devices and microfluidics systems. Keywords: polymer nanowires, hierarchical nanostructures, interference lithography, plasma etching S Online supplementary data available from stacks.iop.org/Nano/25/165301/mmedia (Some figures may appear in colour only in the online journal)

1. Introduction

technique which can pattern polymer nanostructures at a high throughput [8]. However, to achieve sub-50 nm nanopatterns, expensive EUV light sources and complex systems such as high-index immersion techniques are typically required [9]. Nanoimprint lithography is another parallel technique which can create polymer nanostructures at a low cost [10, 11]. However, the preparation of a mold substrate increases the complexity of the whole fabrication process, as such mold patterns are typically made by using sophisticated lithography techniques such as electron beam lithography or interference lithography [12, 13]. Alternatively, large-area nanopatterns can be obtained with low-cost self-assembled mask layers, such as by the use of nanosphere lithography [14]. However, the uniformity of nanostructures remains a concern in such non-conventional lithographic techniques. Electrospinning

In recent years, there has been a high demand for the fabrication of uniform and dense arrays of high-aspect-ratio polymer nanostructures with potential applications to energy storage [1], organic light-emitting diodes (OLEDs) [2], and lab-on-a-chip systems [3]. Furthermore, amorphous carbon nanowires, which can be fabricated by using a pyrolyzed process with polymer nanostructures, have a great range of applications in biosensors [4], chemical probes [5] and heat exchangers [6]. However, most nanofabrication techniques for such polymer nanostructures rely on expensive and time-consuming lithography methods, such as serial-type electron beam lithography [7]. In comparison, parallel-type interference lithography is a low-cost maskless lithography 0957-4484/14/165301+10$33.00

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c 2014 IOP Publishing Ltd

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Figure 1. Schematics of the fabrication processes of polymer nanowire structures. (a) Fabrication of single-level uniform nanowire structures. Carbon-based polymer film (e.g., photoresist) is spun on a substrate (e.g., silicon wafer) and hard-baked. The polymer film is then directly etched by O2 plasma with no use of any mask layer. With the self-masking effects associated with the plasma etching process, the maskless O2 plasma etching results in the formation of an array of polymer nanowire structures. (b) Fabrication of hierarchical polymer nanostructures. An anti-reflective coating (ARC) and a polymer layer (photoresist) are spun on a substrate, respectively, and hard-baked. The ARC is used to reduce scalloping effects in the photoresist sidewalls. Nanolithography (laser interference lithography in this study) is applied to create lithography-patterned nanostructures of a larger size. Then, maskless O2 plasma etching is applied to create self-patterned nanostructures of a smaller size on top of the lithography-patterned nanostructures. Such combined processes result in the formation of hierarchical polymer nanostructures.

in the etching chamber (e.g., antimony, aluminum, fluorine, silicon, or their compounds) are the main reason for the formation of a nanoscale self-mask layer, allowing us to etch for high-aspect-ratio polymer nanowires. We show that a uniform and high-density array of polymer nanowire structures with a sub-50 nm feature size and a high aspect ratio greater than 20 is attainable, for the first time, by using the maskless one-step oxygen plasma etching process. We further apply the fabrication scheme to pre-patterned photoresist films and demonstrate the formation of hierarchical and hybrid polymer nanostructures, which will greatly enhance the functionality and applicability of polymer nanostructures. Although hierarchical nanostructures patterned on SU-8 microstructures have previously been reported with the help of conventional microscale photolithography [20], we show that by using NR-7 series photoresist material, combined with nanoscale laser interference lithography, the feature size of the hierarchical nanostructures can be further minimized.

is a non-lithographic simple way to fabricate high-aspectratio nanowire and nanofiber structures at a low cost [15]. However, it is not practical to fabricate vertically aligned nanowire structures and it is difficult to regulate the structural uniformity [16]. In this paper, we report a new fabrication method which can create large-area dense-array high-aspect-ratio polymer nanostructures of various types of photoresist material (NR-7, SU-8 and PMMA) by using a simple oxygen plasma etching process with no artificial mask layer introduced. Previously, there have been attempts to create polymer nanowires on various substrates by using oxygen-based plasma etching [2, 17–20]. Such techniques were further employed to create three-dimensional (3D) hierarchical nanostructures with various material processes such as pyrolysis and chemical functionalization [20]. The 3D hierarchical nanostructures demonstrated several advantages and new applications for advanced devices and systems, such as a significant decrease of the transport losses in supercapacitor electrode materials [1], strong and reversible dry adhesives [21] and increased sensitivity in biosensors [22]. However, it should be noted that etching mask layers or coatings were artificially introduced in most of the works [17–19]. Although self-masking effects in an oxygen plasma process have also been employed in other studies [2, 20], the mechanism of such self-masking effects has not been clearly revealed yet. Furthermore, the lateral and vertical dimensions of the polymer nanowire structures were limited so that nanowire structures with diameters of less than the 50 nm level or aspect ratios greater than 20 have not been attained so far. In this work, we study the main mechanism of such a maskless etching process of oxygen plasma for the formation of a self-ordered polymer nanostructure array and show the characteristics of the etching process for different photoresist materials. We find that various types of nanoparticulate impurities self-generated

2. Fabrication schemes

Figure 1 shows the schematics of the fabrication processes of the polymer nanowire structures. First, figure 1(a) shows the fabrication scheme of a single-level (non-hierarchical) array of nanowire structures. The key idea of the process is to directly etch the polymer film by oxygen plasma in a reactive ion etching (RIE) mode without using any artificial mask layer on it. Oxygen plasma can readily etch carbon-based polymer materials [17–20]. It is known that residual nanoparticles retained or self-generated inside the etching chamber walls are often released and re-deposited autonomously on the sample surface during oxygen or fluorine plasma etching. For example, it has been reported that fluorine (F) would be deposited on sample surfaces when fluorine-based gases are introduced in the etching species [2, 17]. Sodium (Na) and potassium (K) can also be intentionally introduced to serve 2

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as the self-etching materials by using a dummy material such as glass [17]. Metal impurities such as aluminum (Al) and antimony (Sb) were also found as significant elements of the self-masking materials in the etching process, originating from the metallic etching chamber or polymer (e.g., photoresist) films containing such elements [20]. Then, the self-deposited nanoparticles are employed as etch masks to form vertically aligned polymer nanowire structures in the oxygen plasma etching. Further, figure 1(b) shows the fabrication scheme of hierarchical nanostructures of the polymer nanowires. The base-level nanostructures of a larger dimension can be prepared by using a lithographic method, such as laser interference lithography. Among various nanolithography techniques, laser interference lithography is one of the most efficient optical lithography techniques to create uniform nanopattern arrays over a large substrate area (wafer-level) with good control of the pattern periodicity [23, 24], allowing many new applications in nanofabrication with superior simplicity and convenience [25–34]. In this optical lithography, vertical standing wave effects due to reflection of the incident light from the substrate surface cause serious scalloping profiles along the sidewalls of the patterned photoresist structures [35]. In nanoscale patterning, such scalloping effects severely undermine the structures and preclude the formation of high-aspect-ratio nanostructures. Thus, in order to avoid such issues and obtain high-aspect-ratio hierarchical nanostructures, an anti-reflective coating (ARC) is applied on the substrate before coating the polymer layer. The use of an ARC interlayer minimizes the reflection of the incident light from the substrate during exposure and leads to reduction of the vertical standing wave effects on the sidewalls of the lithographically patterned polymer nanostructures. After the base-level nanostructures have been created by the lithography method, they are then directly etched by oxygen plasma in an RIE mode with no introduction of any artificial etch mask layer. Then, due to the self-masking effects of the self-deposited nanoparticles discussed earlier, polymer nanowire structures of a smaller dimension are formed on the lithographically patterned base-level nanostructures of a larger dimension and result in hierarchical nanostructures. Depending on the shapes (e.g., pillar versus pore) and dimensions of the lithographically patterned base-level nanostructures as well as the etching conditions of the following oxygen plasma process (e.g., etch rate, total etching time, and anisotropy), various types of hierarchical and hybrid nanostructures can be formed, such as pillar-on-pillar or pillar-on-pore nanostructures for various applications [36].

Then, carbon-based photoresist polymer material was spun on the silicon wafer. Three different types of photoresist material were examined, including NR-7 (Futurrex Inc.), SU-8 (Microchem) and PMMA (Microchem). Various thicknesses of the photoresist polymer films were prepared, including 1.5 µm for NR-7, 50 µm for SU-8 and 300 µm for PMMA. They were all made to be thicker than 1 µm in order to allow the fabrication of high-aspect-ratio nanostructures. In the cases of NR-7 and SU-8, the substrates were soft-baked on a hotplate, at 150 ◦ C for 1 min for NR-7 and at 95 ◦ C for 20 min for SU-8, respectively, after the spin-coating. In the case of the thick (300 µm) PMMA film, after the spin-coating, it was dried at room temperature for 2–3 h to avoid the excessive out-gassing of toxic fumes from the solvent that would be generated by heating at high temperature. For the O2 plasma etching of the polymers, a reactive ion etcher (Phantom III, Trion Technology) was used; the RIE power was set at 50 W and the oxygen gas flow rate at 30 sccm. The etcher system had previously been used for typical RIE processes using fluorine-based etching gases such as CF4 and SF6 . Oxygen and nitrogen gases were also employed in the system for the cleaning and purging processes of the chamber. The chamber walls were made of aluminum. In order to examine the effect of the cleanliness of the chamber on the self-masking and the formation of polymer nanowire structures, the etching chamber was cleaned by using oxygen plasma for 10–60 min before loading each sample into the chamber for patterning. During the etching process, the temperature of the etching chamber was controlled to be maintained at 25 ◦ C. To study the etching characteristics, four different etching times (180, 300, 420 and 600 s) were tested for the photoresist polymers. 3.2. Fabrication of hierarchical and hybrid nanostructures

The experimental details of the fabrication of hierarchical nanostructures (figure 1(b)) are summarized in the following. A polished new silicon wafer (4 in) was cleaned by acetone and de-ionized water and then dried by nitrogen gas (filtered). Anti-reflective coating (ARC) (XHRiC 16, Brewer Science) was first spun on the silicon wafer for a film thickness of 200 nm (2000 rpm for 10 s) and baked on a hotplate at 175 ◦ C for 1 min. Then, photoresist polymer was spun on the ARC layer and baked. In the fabrication of hierarchical nanostructures, NR-7 photoresist polymer material was mainly tested for demonstration and prepared to have a film thickness of 1.5 µm (2000 rpm for 40 s), followed by baking at 150 ◦ C for 1 min on a hotplate. Then, for the fabrication of the base-level nanostructures of a larger dimension, customized laser interference lithography systems for wafer-scale large pattern coverage area were used [23, 24]. The NR-7 polymer is a negative-tone photoresist material. Thus, normal exposure dosage results in a pore array by double exposure (with interim rotation of the substrate between each exposure). In order to create both pore and pillar patterns for the lithographically patterned base-level nanostructures, the exposure dosage was regulated. In the case of preparing for an initial smaller pore dimension that would still retain the pore pattern even

3. Experimental details 3.1. Fabrication of single-level uniform nanostructures

The experimental details of the fabrication of single-level (non-hierarchical) uniform nanostructures (figure 1(a)) are summarized in the following. A polished new silicon wafer (4 in) was used as a substrate for polymer coating. Before polymer coating, the substrate was cleaned by acetone and de-ionized water and then blow-dried by nitrogen gas (filtered). 3

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Figure 2. Scanning electron microscope (SEM) images of the self-patterned polymer nanowire structures. The three columns represent the different photoresist polymer materials, namely NR-7 (a), SU-8 (b) and PMMA (c). The four rows represent different etching times of the oxygen plasma, namely 180 s (i), 300 s (ii), 420 s (iii) and 600 s (iv). The scale bar in each figure indicates 1 µm.

with no introduction of any artificial etch mask layer. To compare to the structural morphology of the single-level (non-hierarchical) polymer nanowire structures, four different etching times (180, 300, 420 and 600 s) were also applied for the fabrication of hierarchical nanostructures.

after being undercut by the following O2 plasma process, the photoresist layer was over-exposed; the exposure dosage was 9 mJ cm−2 for each exposure. In the case of the larger pore dimension that would allow the formation of a pillar pattern for the base-level nanostructures after being over-etched by O2 plasma, the photoresist layer was under-exposed; the exposure dosage was 5 mJ cm−2 for each exposure. In the double exposure, the angle of the interfering beams in each exposure was controlled to result in a pattern periodicity of 935 nm. The second exposure was made after a 90◦ interim sample rotation following the first exposure, in order to result in a square array. After exposure, the samples were baked on a hotplate at 100 ◦ C for 1 min and developed in RD6 (Futurrex Inc.) for 10 s without dilution. In order to make hierarchical nanostructures of a smaller dimension on top of the lithographically patterned base-level nanostructures, the samples were etched by O2 plasma in the RIE mode (Phantom III, Trion Technology)

3.3. Characterization of the fabricated polymer nanowire structures

After the fabrication of the polymer nanowire structures using the oxygen plasma etching processes, the morphology and dimensions of the polymer nanostructures were examined by using scanning electron microscopy (SEM). In the SEM imaging, a field-emission SEM system (Auriga, Zeiss) was used with a voltage of 2 kV and a working distance of 5 mm in a secondary electron (SE) mode. In order to analyze the changes of surface elements of the polymer films, x-ray photoelectron 4

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charge neutralization. Photoemission electrons were collected at 55◦ emission angle and the hemispherical analyzer used 150 V pass energy for the survey scan and 50 V pass energy for the high resolution scans. The sensitivity of the XPS was ∼0.1 atomic per cent (%). 4. Results and discussion

Figure 2 shows the SEM images of the single-level (nonhierarchical) polymer nanowire structures fabricated on un-patterned planar films of the photoresist materials, following the scheme illustrated in figure 1(a). From the SEM images, the structural dimensions of the nanostructures were analyzed; these are summarized in figure 3. Although the chamber walls were pre-cleaned for varying intervals (10–60 min) by oxygen plasma before loading the samples in order to examine the effect of the cleanliness of the chamber on the formation of the polymer nanowire structures, no significant difference of the morphology or dimensions of the fabricated polymer nanowire structures was observed for the different pre-cleaning periods of the chamber walls. Overall, the height of the patterned nanostructures increased with etching time, while the diameter showed only a slight decrease with time. In the case of NR-7, the etch rate in the vertical direction gradually increased with the etching time (∼0.83 nm s−1 in 0–180 s, ∼2.5 nm s−1 in 180–300 s and then ∼2.91 nm s−1 in 300–420 s). The height increased up to 800 nm in 420 s, and then slightly decreased by ∼50 nm in 600 s. In the meantime, the diameter of the NR-7 polymer nanowires was constant at 40–50 nm until 300 s. Afterward, it decreased slightly to be 30–40 nm, being constant until 600 s. Thus, the aspect ratio (height to diameter) increased exponentially with etching time until 420 s, and then slightly decreased in 600 s. The aspect ratio obtained in 420 s was greater than 20. As the aspect ratio increased, the nanowire structures were inclined to bundle with each other. The decrease of the structural height and aspect ratio in 600 s is attributed to the small diameter of the polymer nanowires. Such slender nanostructures are more vulnerable to the lateral etching and undercut, which prevent further increase in height and aspect ratio. In the case of SU-8, the vertical etch rate was almost constant at ∼0.3 nm s−1 , which was significantly lower than those of the other photoresist polymers. The low etch rate resulted in relatively short nanostructures (e.g., ∼180 nm in height in 600 s). Until 300 s, the diameter of the SU-8 nanowires was similar to that of NR-7 (40–50 nm). Afterward, it reduced a little (∼5 nm), which is, however, less than that of NR-7 (∼10 nm). The structural aspect ratio of the SU-8 nanowires increased linearly, being ∼5 in 600 s. No significant bundling of the SU-8 nanowires was observed due to the relatively low aspect ratio. In the case of PMMA, the etch rate was ∼2 nm s−1 for 180–300 s. Afterward, it dropped significantly to be ∼1 nm s−1 , resulting in the formation of nanowire structures of ∼900 nm in height in 600 s. In comparison to the NR-7 and SU-8, the diameter of the PMMA nanowires was much larger. It was close to 100 nm in the early stage of the etching and then reduced slightly, being ∼90 nm in 600 s. The aspect ratio increased almost linearly, being ∼10

Figure 3. Characterizations of the geometric parameters of the self-patterned polymer nanowire structures over etching time: (a) height; (b) diameter; (c) aspect ratio (height to diameter). In (a) and (b), the average (symbol) and standard deviation (error bar) values are shown, measured at five different points on each sample. In (c), the average values of the height and diameter are used to estimate the aspect ratio.

spectroscopy (XPS) was also performed before and after the oxygen plasma etching. The XPS was performed using a Surface Science Instruments SSX-100 system with aluminum Kα x-rays and a 1000 µm beam diameter. The operating pressure was 2 × 10−9 Torr and a flood gun was used for 5

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in 600 s. Compared to the other photoresist polymers, the density of the PMMA nanowire array was significantly lower and the bundling effect was more pronounced. As shown in the SEM images (figure 2(c)), the slowdown in the increase of structural height, accompanied by the prominent decrease of the number density of the PMMA nanowire array over the etching time, is also attributed to the significant undercut and over-etching of the PMMA nanowire structures by the oxygen plasma. In previous reports, the main mechanisms for the formation of polymer nanowires in the maskless O2 plasma etching process were attributed to two reasons, including the non-planar protrusive nanoscale structures on the polymer surface [2] and the external impurities introduced as the sources for self-masking layers [17, 20]. In the previous work of Morber et al [2], protrusions of tens of nanometers were intentionally made on the polymer surface, and the masking effect and the spatial variation in etching were attributed to the changes in interaction volume of the ions with surface curvature and the formation of polymer nanowires. In their subsequent works [18, 19], gold was also sputtered intentionally onto the polymer film before the plasma etching in order to create surface undulations such as bumps and facilitate such effects. However, in this study, no protrusive structures or bumps were artificially added to the polymer films and the polymer surfaces retained their original planarity and smoothness before the plasma etching so that such a surface curvature would not be the main reason for the self-formation of the polymer nanowire structures shown in this study. The average surface roughness values of the NR-7, SU-8 and PMMA after spin-coating were all less than 1 nm, measured by an atomic force microscope (AFM) in tapping mode. Furthermore, in the previous work [17], impurities such as potassium (K) and sodium (Na) were normally introduced for the provision of the sources of self-masking elements by using dummy materials such as glass. Fluorine (F) was also intentionally introduced externally as one of the etching species (gases), in addition to oxygen gas [2, 17–19]. Thus, the fluoride gas might also partially contribute to the self-masking and the creation of polymer nanowires in the previous works [2, 18, 19]. On the other hand, it was also reported that the presence of metallic impurities (e.g., antimony and aluminum) could be the source of in situ mask generation during oxygen plasma processing [20, 37], originating from the photoresist polymer film or the metallic etching chamber material containing such elements. In this study, no external impurities such as K and Na were introduced before etching, and only oxygen gas was employed as the etching species during the etching process. Since the chamber employed in this study was made of aluminum and used for fluorine-based plasma etching processes for silicon-based substrate materials, it was anticipated that the presence of metallic and inorganic impurities such as aluminum (Al), fluorine (F), silicon (Si) or their compounds (e.g., aluminum fluoride and silicon nitride) would be the most probable elements for the source of self-masking materials and for the formation of polymer nanowires. Furthermore, negative photoresist materials such as SU-8 contain antimony (Sb)

Figure 4. X-ray photoelectron spectroscopy (XPS) analysis results

of the changes of atomic percentages of surface elements before and after etching: (a) NR-7; (b) SU-8; (c) PMMA. See the supplementary data for the raw data in more details (available at stacks.iop.org/Nano/25/165301/mmedia).

in their chemical composition, which can also work as the self-masking element during the oxygen plasma etching. In order to confirm the surface elements that would be associated with the self-masking effects, the XPS measurement of the polymer surfaces was performed both before and after the O2 plasma etching (420 s and 600 s etching for NR-7 samples, and 600 s for SU-8 and PMMA). The raw data of the XPS measurement are shown in figures S1–S7 in the supplementary data (available at stacks.iop.org/Nano/25/165301/mmedia). Figure 4 summarizes the changes of the atomic percentages of the main surface elements after the O2 plasma etching. Before etching, the XPS result for the NR-7 photoresist (figure 4(a)) shows the presence of carbon (59.43%), oxygen (21.34%), fluorine (14.10%), antimony (2.26%) and nitrogen (1.58%). 6

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robust self-mask materials [40]. When the fluorine-based and nitrogen gases are absorbed onto the aluminum-based chamber walls or combined with the antimony element released from the photoresist materials, aluminum compounds (e.g., aluminum fluoride and aluminum nitride) and antimony compounds (antimony fluoride and antimony nitride) can also play a significant role in the oxygen plasma etching processes as effective self-masking materials. Meanwhile, it should be noted that antimony (Sb) is a unique element that is found only in the cases of NR-7 (figure 4(a)) and SU-8 (figure 4(b)), while the aluminum (Al) element is only found in the case of PMMA (figure 4(c)). Figure 3(b) shows that the diameters of the polymer nanowire structures for the NR-7 and SU-8 are around the same at 40–50 nm, while that for the PMMA is 90–100 nm. Figure 2 also shows that the arrays of polymer nanowire structures of NR-7 (figure 2(a)) and SU-8 (figure 2(b)) are dense to a similar degree, while that of PMMA (figure 2(c)) is less dense. It is speculated that the size (diameter) and the number density of the polymer nanostructures would be associated with the difference of the elements (i.e., Sb versus Al) found on the polymer surfaces. For example, it would be possible that the particle size of aluminum compounds would be larger than that of antimony compounds so that it would result in the formation of polymer nanostructures with a larger size (diameter) by etching. Note that the antimony elements would mostly originate from the photoresist polymer materials (NR-7 or SU-8), while the aluminum elements would be from the chamber walls. This difference would also cause the different number densities of the nanoparticles of impurities formed/deposited on the surfaces (e.g., the low number density for the aluminum compounds on PMMA). In addition to the over-etching and undercut effects discussed earlier, the lower number density of the PMMA nanowire structures shown in figure 2(c) would also be caused by the difference of the number density of the deposited impurities. It should be noted that antimony is highly toxic and harmful to many biospecies [37]. Thus, bio-related applications of polymer nanostructures of NR-7 and SU-8 fabricated by the oxygen plasma etching process based on the self-masking effects should be made with caution. In this study, the fabrication results for the polymer nanowire structures through the oxygen plasma etching process were reproducible and uniform over the whole sample area regardless of the previous loading (run) history or the cleaning procedures. This is attributed to the fact that the dense residues adsorbed in the chamber walls can easily be activated by the oxygen plasma and uniformly occupy the whole etching chamber [20]. Then, the directional RIE RF power is able to drive and deposit the impurities onto the sample surface to work as a robust self-masking layer in the oxygen-based polymer etching process [22]. In order to confirm whether the self-masking effects are reproducible and universal in other RIE systems, a different RIE system (Hi-Etch, BMR Technology Corporation) was also employed for the fabrication of polymer nanowire structures of the three different photoresist materials (NR-7, SU-8, and PMMA) using the maskless oxygen plasma etching process. This RIE system had also been used for typical RIE processes using

After the oxygen plasma etching for 420 s, the result shows the new presence of silicon (5.12%) and the significant increase of antimony (7.19%). Even after the oxygen plasma etching for 600 s, silicon (5.65%) and antimony (6.86%) were retained with little change of the atomic percentage. The significant appearance and increase of the atomic percentage of silicon and antimony after the oxygen plasma etching suggests that they should be the main elements causing the self-masking effects in the case of the etching of the NR-7 polymer material. For SU-8 photoresist (figure 4(b)), the presence of carbon (71.22%), oxygen (25.28%), silicon (3.24%), nitrogen (0.21%) and fluorine (0.05%) was initially detected before etching. After the oxygen plasma etching for 600 s, the new presence of antimony (5.88%) and an increase of the atomic percentages of silicon (6.81%), nitrogen (0.52%) and fluorine (0.96%) are shown. This suggests that antimony, silicon, fluorine or their compounds (e.g., antimony fluoride, antimony nitride, silicon fluoride and silicon nitride) should be the main elements causing the self-masking effects in the case of the etching of SU-8 polymer material. In the case of PMMA photoresist (figure 4(c)), the presence of carbon (71.88%), oxygen (27.85%), antimony (0.13%), fluorine (0.08%) and nitrogen (0.06%) was detected initially before etching. After the oxygen plasma etching for 600 s, the new presence of silicon (3.52%) and aluminum (3.06%), and an increase of the atomic percentages of fluorine (1.19%) and nitrogen (1.01%) are shown. This suggests that silicon, aluminum, fluorine or their compounds (e.g., silicon fluoride, silicon nitride, aluminum fluoride and aluminum nitride) should be the main elements causing the self-masking effects in the case of the oxygen plasma etching of PMMA polymer material. The XPS measurement results suggest that various sources of species including aluminum, antimony, silicon, fluorine, and nitrogen can cause the self-masking effects in the oxygen plasma etching of various types of photoresist polymer films. The etching chamber walls employed in this study were made of aluminum-based metal so that the aluminum element should have originated from the chamber walls, working as an effective self-masking element in the oxygen plasma process. Although the etching chamber had a glass view port, no sodium (Na) or potassium (K) element was detected in our XPS measurement, suggesting that Na or K should not be the major sources for the self-masking effects shown in this study. As reported by De Volder et al [20] and Rasmussen et al [37], some photoresist materials such as SU-8 contain antimony in their chemical compositions, which can accumulate on the sample surfaces during plasma etching processes. Oxygen plasma cannot etch metallic particulates such as antimony so that antimony can also work as an effective self-mask element in the oxygen plasma process. Since the RIE system was mostly used to etch silicon substrates using fluorine-based gases and a nitrogen gas was also used for the purging step, the species of silicon, fluorine or their compounds (e.g., silicon fluoride and silicon nitride) should also be pre-absorbed onto the chamber walls during previous runs and re-deposited on the sample surfaces in the oxygen plasma etching [38, 39]. Silicon and its compounds are also not effectively etched by oxygen plasma so that their nanoparticulates can work as 7

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Figure 5. SEM images of hierarchical polymer nanostructures; the two columns represent NR-7 photoresist layers with different pore sizes ((a) ∼300 nm, (b) ∼600 nm). The five rows represent different etching times by oxygen plasma: no etching (i); 180 s (ii); 300 s (iii); 420 s (iv); 600 s (v), respectively. The scale bar in each image indicates 1 µm.

fluorine-based etching gases. The chamber walls were also made of aluminum. The samples were etched with an oxygen gas flow rate of 30 sccm, an RF power ranging from 50 to 100 W and an etching time ranging from 180 to 600 s. The fabrication results are included in the supplementary data (figures S8 and S9 available at stacks.iop.org/Nano/25/ 165301/mmedia). Despite using the different RIE system and chamber, similar fabrication results (compared to the results shown in figures 2 and 3) were obtained for all the three different photoresist materials, showing the increase of the aspect ratio of the nanostructures with the etching time. It is also shown that the aspect ratio of the polymer nanostructures

increases with higher RF power. Meanwhile, the diameter of the polymer nanowires did not change much with the etching time or RF power, ranging from 50 to 100 nm, which agrees with the results obtained with the primary RIE system (Phantom III, Trion Technology). This additional test result illustrates that the self-masking effects should be universal and reproducible in most RIE systems made of the same chamber material (aluminum-based metal material in this study) and used for similar etching processes (fluorine-based gases for silicon-based substrates in this study). If the oxygen plasma can react with the etching chamber walls (e.g., chamber walls made of carbon-based materials), the release and re-deposition 8

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surface (figure 5(a-v)). Only the nanowire structures at the junction points where four pores in a square array intersected mostly remained. However, they were seriously bundled with each other and the majority of them were collapsed (figure 5(a-v)). In the case of using the initially large-size pore pattern, the pore walls disappeared with a relatively short etching time (e.g., in 300 s). Then, pillar-type structures decorated with nanowire structures of smaller sizes formed, resembling the shape of buds. When the etching was prolonged (figures 5(a-iv) and (a-v)), these pillar-on-pillar hierarchical nanostructures became slenderer with a smaller number of the top (self-generated) nanowire structures remaining on the base (lithographically patterned) pillar structures. In the earlier work of De Volder et al [20], a microarchitecture of hierarchical carbon nanowires was introduced, using oxygen plasma etching and thermal treatment with SU-8 polymer material. Compared with the method [20], ultra-fine polymer nanowires (diameter smaller than 30 nm) on a well-ordered nanopillar or nanopore architecture were attainable in this study. Our results show that other polymer materials such as NR7 photoresists can be employed to form a nanoarchitecture of hierarchical nanowires. Resembling the 3D architectures often found in natural systems [20, 21], such hierarchical and hybrid polymer nanostructures would be of great use in many scientific and engineering applications, such as control of the wettability [41–44], adhesion [45–52] and friction properties of surfaces [53–56].

process of the impurities may not occur [38]. However, if the etching chamber wall materials cannot be etched by the oxygen plasma (e.g., made of metal materials), it is much easier for the sputter and re-deposition process to occur. Compared to previous works [2, 17–20], the new results reveal that the presence of common impurities associated with RIE systems/materials/processes such as aluminum, antimony, silicon, fluorine and nitrogen can all work as self-generated etch mask elements, with no necessity of the introduction of any other impurity species. Further, our results suggest that similar, reliable and repeatable results can be obtained with proper conditioning of new or other chamber systems with similar impurities that oxygen plasmas cannot etch. Such advantages make the maskless oxygen plasma etching process attractive and useful for several applications in nanofabrication. Another advantage of the fabrication of nanowire structures by using the maskless oxygen plasma etching process is that it can also be simply applied on pre-patterned polymer layers to form hierarchical nanostructures, as illustrated in figure 1(b). Figure 5 shows the fabrication results of such nanostructures. In the preparation of the pre-patterned polymer layer, nanopore patterns of NR-7 photoresist polymer material were mainly used, made by using laser interference lithography. As seen in figure 3, the NR-7 photoresist polymer material shows good etch characteristics (e.g., high-aspectratio polymer nanowire structures with uniform and high density arrays) in oxygen plasma. Furthermore, NR-7 is a negative-tone photoresist and does not require high exposure dosage in the laser interference lithography. Such merits improve the patterning efficiency. In this experiment, two pore patterns with the same inter-pore distance but different pore sizes were prepared as the pre-patterns by regulating the exposure dosage. Figure 5(a-i) shows the pore pattern of a smaller pore size (280 nm in diameter), made by over-exposure, while figure 5(b-i) shows the pore pattern of a larger pore size (560 nm in diameter), made by under-exposure. Due to the relatively thick layer of the film and the gradual decrease of the exposure dosage along the film thickness, the pore size gradually increases along the bottom direction. In the case of under-exposure (figure 5(b-i)), the pores in the bottom region (near the interface between the photoresist and the ARC layers) become connected laterally due to the effect. Then, figures 5(a-ii)–(a-v) and 5(b-ii)–(b-v) show the hierarchical polymer nanostructures formed with the two different pore patterns, with gradual increase of the etching time by oxygen plasma. Overall, the results show that the pore sizes gradually increase with the etching time, while the nanowire structures are formed on the top surface. This pore widening is attributed to the non-negligible lateral etching effect by the oxygen plasma. In the case of using the initially small pore pattern, the pore walls were still mostly connected (until 420 s); the pore walls were comprised of nanowire structures whose height (or aspect ratio) was gradually increased by the continuous oxygen plasma etching (figures 5(a-ii)–(a-iv)). Such structures possess both pore and pillar types of the patterns in the same layer, forming pillar-on-pore hybrid nanostructures. In 600 s, the nanowire structures were overetched and no clear pore walls remained on the substrate

5. Summary and conclusions

A simple and efficient fabrication process of a nanoarray of polymer nanowire structures has been successfully demonstrated, using oxygen plasma etching with various carbon-based photoresist materials. Caused by the reaction of the oxygen plasma and the chamber wall, the selfmasking phenomena of inorganic nanoparticulates on the organic polymer surfaces enabled us to create high-aspect-ratio (greater than 20) sub-50 nm level (lateral dimension) polymer nanowire structures uniformly over the wafer-level substrate area, without using any artificial etch mask layers. Integrated with laser interference lithography techniques, 3D hierarchical and hybrid polymer nanostructures have further been demonstrated. Compared to previous reports, there are several novelties and advantages in the method reported in this study. First, no artificial etch mask layer or coating was introduced, which simplifies the fabrication process significantly. Second, it was shown that the structural dimensions of the polymer nanowire structures of various types of photoresist material can be conveniently modulated by controlling the process parameters of the oxygen plasma etching, such as the etching time. Thus, uniform and high-density arrays of polymer nanowire structures of sub-50 nm feature size with high aspect ratio greater than 20 could be attained for the first time. Furthermore, by combining with interference lithography and manipulating the lithographic processes (e.g., exposure dosage and sidewall profile), novel hierarchical and hybrid polymer nanostructures were successfully demonstrated. The new fabrication process of polymer nanostructures with tailored structural geometry and hierarchy covering full wafer-scale 9

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substrate areas will be of great significance in many scientific studies and engineering applications including photovoltaics, optical devices, microfluidics systems, sensors, energy storage, etc with customized physical, chemical, mechanical and interfacial properties.

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Acknowledgments

This study was supported by the Stevens Innovation and Entrepreneurship Doctoral Fellowship program and the US Office of Naval Research (ONR) Young Investigator Program (Award No.: N00014-10-1-0751). This research was carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which was supported by the US Department of Energy, Office of Basic Energy Sciences, under contract No.: DE-AC02-98CH10886. The research effort used microscope resources partially funded by the National Science Foundation through Grant DMR-0922522 and the ONR Defense University Research Instrumentation Program (Award No. N00014-11-1-0841). The authors would like to thank Mr Jon Shu at Cornell Center for Materials Research for the XPS measurements and Mr Junjun Ding at Stevens Institute of Technology for preparing samples. References [1] Xu J J, Wangm K, Zu S Z, Han B H and Wei Z X 2010 ACS Nano 4 5019 [2] Morber J R, Wang X, Liu J, Snyder R L and Wang Z L 2009 Adv. Mater. 21 2072 [3] Jaramillo M D, Torrents E, Martnez-Duarte R, Madou M J and Juarez A 2010 Electrophoresis 31 2921 [4] Sun B, Colavita P, Kim H, Lockett M, Marcus M, Smith L and Hamers R 2006 Langmuir 22 9598 [5] Du R B, Ssenyange S, Aktary M and McDermott M T 2009 Small 5 1162 [6] Schueller O J, Brittain S T and Whitesides G M 1999 Sensors Actuators A 72 125 [7] Fujita J, Ohnishi Y, Ochiai Y and Matsui S 1996 Appl. Phys. Lett. 68 1297 [8] Smith H 2001 Physica E 11 104 [9] P¨aiv¨anranta B, Langner A, Kirk E, David C and Ekinci Y 2011 Nanotechnology 22 375302 [10] Zankovych S, Hoffmann T, Seekamp J, Brunch J U and Torres C M 2001 Nanotechnology 12 91 [11] Beck M, Graczyk M, Maximov I, Sarwe E L, Ling T G I, Keil M and Montelius L 2002 Microelectron. Eng. 61–2 441–8 [12] Chou S, Krauss P R and Renstrom P J 1996 J. Vac. Sci. Technol. B 14 4129 [13] Guo J 2007 Adv. Mater. 19 495 [14] Madaria A R, Yao M Q, Chi C Y, Huang N F, Lin C, Li R J, Povinelli M L, Dapkus P D and Zhou C 2012 Nano Lett. 12 2839 [15] Steach J K, Clark J E and Olesik S V 2010 J. Appl. Polym. Sci. 118 405 [16] Huang Z, Zhang Y, Kotaki M and Ramakrishna S 2003 Compos. Sci. Technol. 63 2223 [17] Chen M H, Chuang Y and Tseng F G 2008 Nanotechnology 19 505301 [18] Fang H, Wu W, Song J and Wang Z L 2009 J. Phys. Chem. C 113 16571 [19] Fang H, Yuan D, Guo R, Zhang S, Han R P, Das S and Wang Z L 2011 ACS Nano 5 1476 10

Fabrication of polymer nanowires via maskless O2 plasma etching.

In this paper, we introduce a simple fabrication technique which can pattern high-aspect-ratio polymer nanowire structures of photoresist films by usi...
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