Carbon Nanotubes as Etching Masks for the Formation of Polymer

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Carbon Nanotubes as Etching Masks for the Formation of Polymer Nanostructures Woongbin Yim, Sae-June Park, Sung Yong Han, Yong Hyun Park, Sang Woon Lee, Hui Joon Park, Yeong Hwan Ahn, Soonil Lee, and Ji-Yong Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18035 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Applied Materials & Interfaces

Carbon Nanotubes as Etching Masks for the Formation of Polymer Nanostructures Woongbin Yim,§,† Sae June Park,§,† Sung Yong Han,† Yong Hyun Park,§,† Sang Woon Lee,§,† Hui Joon Park,† Yeong Hwan Ahn,§,† Soonil Lee,§,† and Ji-Yong Park*,§,† §

Department of Physics, Ajou University, Suwon 16499 Korea

†

Department of Energy Systems Research, Ajou University, Suwon 16499 KOREA

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

We investigate the interaction of Carbon Nanotubes (CNTs) embedded in a polymer matrix [Poly(methyl methacrylate) (PMMA)] with Ar plasma, which results in the formation of PMMA nanostructures as CNTs act as an etching mask. Due to the large differences in the Ar ion sputtering yields between CNTs and PMMA, PMMA lines with the width comparable to that of CNTs and as high as 20 nm (for single-walled CNTs) or 80 nm (for multi-walled CNTs) can be obtained after repeated exposure of CNT/PMMA films to Ar plasma. We also follow the etching process by investigating changes in IV characteristics and Raman spectra of CNTs after each exposure to Ar plasma, which shows progressive defect generations in CNTs while they maintain structural integrity long enough to act as an etching mask for PMMA underneath.

We

demonstrate that the PMMA nanostructure patterns can be transferred to a different polymer substrate using nanoimprinting.

TOC GRAPHICS

KEYWORDS: carbon nanotube, plasma, sputtering, polymer, nanostructure


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■ INTRODUCTION

Interaction of ions with materials has been widely utilized for various purposes such as ion implantation, functionalization or cleaning of surface, fabrication of nanostructures and so on.1 Such interactions of energetic ions with bulk materials have been extensively studied both theoretically and experimentally.2-3 Interactions of energetic ions with nanomaterials such as carbon nanotubes (CNTs) have been also of interest recently since such interactions can be different from those with bulk materials and they can be utilized to control various characteristics of nanomaterials.4-6 When CNTs are irradiated by ions, defects are usually generated as carbon atoms are either removed or displaced. Formations of defects in CNTs depending on the kinds of ions or their kinetic energies have been studied.4-5, 7-13 Changes in electrical or mechanical properties of CNTs induced by ion irradiation have also been investigated with possible applications.6,

10, 14-16

However, relatively few studies have been

performed on the interaction of ions with CNTs on different substrates. One of the applications of such interaction is nanowire lithography (NWL).17-30 NWL is a method to fabricate one-dimensional (1D) nanostructures using NWs as an etching mask.18 In such applications, NWs are usually placed or fabricated on a metal or SiO2 substrate and 1D nanostructures are generated in the substrate by subsequent etching.25, 31-32 Multi-walled CNTs (MWCNTs) also have been demonstrated as an etching mask for metal or dielectric lines with ion sputtering.33-35 When polymers are irradiated by low energy ions at low dose, the dominant effect is either cross-linking or chain-scission depending on the properties of the polymers.36 For polymers which are usually used as positive resists such as Poly(methyl methacrylate) (PMMA), chain-


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scission is the dominant effect.36-38 Also, carbon-oxygen and carbon-hydrogen bonds are more easily sputtered compared to carbon-carbon bonds.38-42 Therefore, PMMA which has large oxygen contents can be a good substrate for NWL with CNTs as an etching mask with large difference in etching speed when interacting with energetic ions. In this contribution, we investigate the effect of Ar ion irradiation on CNTs embedded in the PMMA matrix. We show that even single-walled CNTs (SWCNTs) with diameter as small as ~ 1 nm, which is the thinnest 1D material can still act as an effective etching mask for PMMA against Ar plasma. In this way, we demonstrate that PMMA lines with high aspect ratio can be formed. We also follow the corresponding evolutions in electrical transport properties and Raman signals of CNTs after each Ar plasma exposure step during the formation of PMMA nanostructures to investigate the interaction between Ar ions and CNT/PMMA. As a possible application, we successfully create nanostructures on a different polymer which is inverse to the PMMA lines by nanoimprinting.

■ RESULTS AND DISCUSSION

Figure 1 shows the schematic process of NWL with CNTs as an etching mask against Ar plasma.

In order to monitor the progress of etching by Ar plasma at the same position, CNT

device structures (with electrodes for easy locating and electrical characterization at the same time) embedded in a PMMA substrate is also used. We employ the whole device transfer scheme as shown in Figure 1a-d for the preparation of the sample.43 Briefly, after CNTs are either deposited or grown on a SiO2/Si substrate, CNT devices are fabricated with metal


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electrodes using conventional photolithography. (Figure 1a-b) Then, PMMA is spin-coated on top of the devices and cured. The whole CNT device along with PMMA is detached from the Si substrate as a sacrificial SiO2 layer is etched by buffered oxide etch solution. (Figure 1c) In this way, CNT devices which are half-buried (embedded) in PMMA can be prepared. (We refer this sample as a CNT/PMMA film in this paper) After the CNT/PMMA film is placed on a different substrate with the device side facing up (Figure 1d), it is subsequently exposed to Ar plasma as shown in Figure 1e. By employing this scheme, we can monitor topographic changes in the same position as well as electrical characteristics of the CNTs after each Ar plasma exposure. If CNTs act as etching masks as expected, PMMA lines are expected to form as the schematic sideview in Figure 1e since the region of PMMA that is protected by CNTs can withstand Ar ion sputtering. Figure S1 in the Supporting Information shows examples of atomic force microscopy (AFM) images of the CNT/PMMA samples after Ar plasma exposure with different conditions. Although the typical diameter of CNTs grown with CVD is 1 ~ 2 nm, the CNT-like features in Figure S1 show varied heights as high as ~15 nm depending on Ar plasma power and exposure time. Such observations clearly show that CNTs act as a mask for Ar plasma etching. Since we used Ar plasma, the dominant etching mechanism is expected to be the physical sputtering by Ar ions, although UV generated inside plasma may have some minor effect.44 In the following experiment, we investigate the progress of the PMMA nanostructure formation. Figure 2 shows a series of AFM images of a CNT device (embedded in PMMA) showing evolution of surface topographic features with Ar plasma exposure for 3 seconds each time. The whole sets of AFM images for each 3s exposure are also available in Figure S2. When CNT devices are first transferred and embedded in the PMMA substrate as schematically shown in Figure 1, CNTs are barely visible in the AFM image as in Figure 2a, which only shows the edges


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of electrodes. However, when the sample is exposed to Ar plasma, CNTs start to show up and the height of the CNT-like structures grows with each Ar plasma exposure as shown in Figure 2b-d. The measured height of the structure after each Ar plasma exposure time is shown in Figure 2e. The height of the structure grows linearly up to ~ 20 nm, and then starts to decrease with further Ar plasma exposure. From these observations, CNTs seem to effectively act as an etching mask up to 21 seconds, while PMMA is etched almost ~1nm per each 1s Ar plasma exposure, afterwards CNTs no longer are effective as etching masks and the whole PMMA substrate seems to be etched including the nanostructures. In this way, nanoscale PMMA lines following the shapes of CNTs can be prepared. Since the whole CNT device is embedded in PMMA, IV characteristics can be measured after each Ar plasma exposure along with Raman measurement. One example of such measurements is shown in Figure 3. Electrical transfer characteristics (IVG) of a CNT device after exposure to Ar plasma for 2 seconds each time are measured until no current can be detected in Figure 3a-b. Corresponding AFM images of the device are shown in Figure S4, which also shows the progress of PMMA nanostructure formation. As can be seen in Figure S4a, the starting CNT device consists of a network of CNTs. After 2s exposure to Ar plasma, the networks of CNTs start to be visible in Figure S4a. As this CNT device is exposed to Ar plasma for 2 s each time, the CNT-like features further develop and their height reached ~8 nm after total exposure time of 10 seconds as shown in Figs. S4f.

Corresponding electrical transfer characteristics show

originally p-type semiconducting behavior in Figure 3a suggesting the current path is made through networks of semiconducting CNTs in Figure S4.

The current path through this CNT

network survives up to 10 s of Ar plasma exposure, although the current level of the on-state decreases with each Ar plasma exposure. After 12 seconds, no current can be detected. While


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the on-state current decreases, there is almost no change in the semiconducting behavior such as the threshold voltage as in Figure 3b, suggesting semiconducting behavior of CNTs are preserved even with the creation of many defects. The observed exponential decrease of the onstate current with Ar ion plasma exposure time as shown in the inset in Figure 3b is consistent with the emergence of strong localization induced by defects created by Ar ions.14, 45-47 Similar evolution of IV characteristics was observed for the device in Figure 2 as shown in Figure S3 in the Supporting Information. Figure 3c shows representative Raman spectra taken from the same sample (at different positions) after each exposure to Ar plasma. Before the Ar plasma exposure, Raman spectrum shows typical G peak of CNTs along with signals originated from PMMA (marked by * in the Figure 3c) in the Raman shift range of 1400 ~ 1600 cm-1 while almost no D peak is observed indicating good quality of CVD-grown SWCNTs. As this device is exposed to Ar plasma, the D peak starts to grow while G peak reduces as shown in Figure 3c. After exposure for 6 seconds, the D peak becomes larger than G peak. After 8 seconds, no discernible Raman peaks related to D or G could be detected from this position. Ar ions impinging on CNTs are known to create various defects such as mono or di-vacancies4-5, 9, 11-12 and such defects are manifested as the development of D peak in the Raman spectrum. The measurements in Figure 3 show that CNTs mostly retain their characteristics until Ar plasma exposure time of 8 ~ 10 seconds.

After this, CNTs are most likely fragmented or amorphized, but still can act as an

etching mask as shown in Figure 2. These results obtained with SWCNTs demonstrate that even SWCNTs with diameter as small as ~1 nm can be used as an etching mask for PMMA for the nanostructure formations with Ar plasma. Although we may assume that the width of such created PMMA lines to be comparable to the diameter of CNTs, AFM images such as in Figure S1 cannot give the exact width since


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lateral resolution of an AFM image is determined by the convolution of the tip size and the sample feature size. We were not able to confirm the exact widths and shapes of the PMMA lines prepared with CVD-grown SWCNTs by taking cross-sectional images with a transmission electron microscope (TEM) either, since imaging such thin polymer structure with a TEM without damaging is very challenging and conductive coating process also damages the nanostructures due to inter-diffusion. Therefore, we try to estimate the width of the PMMA line structures from the AFM images as in Figure S5 using simple geometrical considerations. First, we compared the measured width of as-grown SWCNTs on a SiO2/Si substrate with that of etched PMMA lines with the same tip. Given the typical radius of the AFM tip (r = 8 ~ 10 nm), we expect 6 ~ 8 nm broadening when the height of the structure is comparable to that of the tip radius as shown in Figure S5a-b. When the height of the structure exceeds the tip radius, we found steeper sidewall profile as in Figure S5c of which slope is limited by that of the tip. Although the exact profile of the PMMA lines, especially near the base of the lines cannot be confirmed with AFM images, minimal broadening around the top of the PMMA lines until it is etched down to ~ 20 nm can be inferred from AFM images. From these estimations, we believe that we can fabricate PMMA lines with the width comparable to that of CNTs and aspect ratio 10 or larger using SWCNTs as an etching mask with Ar plasma. We also applied the same procedure to multi-walled CNTs (MWCNTs) and bundled SWCNTs which have larger diameters than CVD-grown SWCNTs. Results for MWCNTs are shown in Figure 4 while those for bundled SWCNTs are shown in Figure S6. Diameters of MWCNTs used for this measurement are typically 5 to 10 nm. They are usually bundled when deposited on a substrate as shown in Figure 4a, which shows the surface features after exposure to Ar plasma for 15 seconds. When exposed to Ar plasma, MWCNTs also act as an etching


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mask and the evolution of PMMA nanostructures is also observed. With more exposure to Ar plasma, the height of the PMMA nanostructure reached up to ~80 nm as shown in Figure 4b. In this case, the cross-section of the nanostructure could be examined by TEM as shown in Figure 4c. This image shows the width of the nanostructure (~18 nm) and the height (~28 nm). Deposited SWCNTs from dispersion also bundled and form an etching mask with the larger diameter as shown in Figure S6. These bundled SWCNTs can act as more effective etching masks than individual SWCNTs as in Figure 2.

In this case, the height of the PMMA

nanostructures can reach up to 40 nm as shown in Figure S6. Further patterning applications using current method is quite limited since only a few materials will have enough contrast in the sputtering yields against CNTs. However, PMMA lines created this way can be further utilized as a template for the creation of lines or trenches of different materials using such method as nanoimprinting.

In order to demonstrate such

application, we used the PMMA nanostructure as a master stamp and made a mold using high modulus Polydimethylsiloxane (PDMS) forming nanoscale trenches which is inverse to the original PMMA lines as shown in Figure 5 following the procedure as explained in the experimental section. Higher modulus PDMS is necessary to effectively prevent the collapsing of the replicated nanostructures.48 We expect that such a mold can be further utilized to make another polymer nanostructures, replicating the original PMMA lines. The patterning capability is also limited by the random nature of as-grown CNTs as geometries and size of CNTs are not easy to control. However, for example, some patterning such as line and space can be possible using horizontally aligned CNTs.49-50


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■ CONCLUSION

We investigated the etching process of PMMA with CNTs as an etching mask under Ar plasma exposure. We found large contrast in the etching efficiency between PMMA and CNTs for Ar ions. With the exposure, PMMA lines with high aspect ratio [width comparable to the mask (CNT) and height as high as ~20 nm (for individual SWCNTs) or ~ 80 nm (for bundled MWCNTs)] are formed. We found that with the Ar plasma exposure, PMMA is etched while defects are formed in CNTs. Although the created defects in CNTs work as scatterers for electrical conduction, the structural integrity of CNTs is preserved while PMMA is etched efficiently. This work demonstrates that CNTs can work as an effective etching mask for PMMA and show the possibility of transferring the 1D profile of CNTs to different materials. As a possible application, we demonstrate that the inverse nanostructures in PDMS using the PMMA nanostructures as stamps can be formed.

■ EXPERIMENTAL SECTION

Preparation of CNTs and Device fabrications: CVD growth of CNTs Catalysts are deposited on Si/SiO2 substrates by dipping in IPA solution, dissolved with Fe(NO)3·9H2O (0.02mg/ml). SWCNTs are grown by thermal chemical vapor deposition (CVD) at 800℃ under flows of Ar, H2, and C2H4 (700, 100, and 9 sccm) for 20 minutes. MWCNTs were grown by low-pressure CVD on a Si substrate with a 1nm-thick Fe catalyst layer on top of


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20 nm-thick Al layer which is prepared by sputtering (Al layer is annealed for 5 minutes at 500℃ before Fe deposition). MWCNTs were grown under flows of Ar and C2H2 gases (795, 5 sccm, respectively) at 800℃ and 30 Torr. Deposition of CNTs from dispersion solution Arc-made SWCNTs (from Iljin Nanotech) were vacuum-dried at 100 ℃ for overnight to remove the water molecules attached to the sidewalls of CNTs. The SWCNTs were dispersed in 200 ml of DCE (dichloroethane) via ultrasonication (100 W at 42 kHz) for 12 hours.

Then, the

SWCNT-dispersed DCE solutions were centrifuged at 48,000 g for 20 minutes to precipitate large CNT bundles and other carbonaceous materials to obtain SWCNT-dispersion solution (~0.005 mg/ml) of supernatant. As-grown MWCNT is dispersed in DI water with 0.3wt% of SDBS (Sodium Dodecyl Benzene Sulfonate) and sonication. MWCNTs or SWCNTs in the dispersion solutions are deposited on a Si/SiO2 substrate by spin coating. Device fabrication Source and drain electrode patterns (gap size: 2 ~ 10 µm, width of electrode: 8 ~ 50 µm) are produced by photolithography. The Ti/Au (1.5 and 30 nm thick, respectively) electrodes are prepared by electron beam evaporation followed by the lift-off process.

Transfer process and Ar plasma etching process: PMMA [poly(methyl methacrylate), MicroChem, 495 A6 (MW 495,000, 6% in Anisole)] is spin-coated on the fabricated CNT devices and baked at 180 ℃ for 2 min. Then the SiO2 layer is removed by dipping in a buffered oxide etch solution for ~24 hours. Once the SiO2 layer is removed, the PMMA/CNT film floats up on the solution. After rinsing with Di water, the PMMA/CNT film is put on a new Si/SiO2


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substrate with the CNT-side up for easy handling. The CNT/PMMA film was etched with Ar plasma using an RIE system. (Plasmalab System 100, Oxford Instruments) Etching was carried out with Ar gases at a pressure of 30 mTorr and a flow of 5 sccm. The samples were etched at room temperature with 30 W power unless stated otherwise for the desired time at each step.

Characterization of CNTs and devices: The formation of PMMA nanostructures after the Ar plasma etching process is observed by taking AFM (XE-100, Park Systems) topographic images. For device characterizations, electrical transfer characteristics (source-drain current vs. gate voltage) are measured in ambient conditions. Raman spectra were obtained with 531.6 nm laser.

Preparation for TEM observation: For the TEM sample, Al2O3 was deposited on the prepared PMMA nanostructures by atomic layer deposition at 150 ℃, which serves as a barrier layer, before depositing Pt (~2 µm) by sputtering in FIB. The sample was cut perpendicular to the nanostructure for the cross-sectional imaging in TEM. Procedures for PDMS molds: The PMMA film with nanostructures is first treated with surfactant, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (from GELEST), for easy demolding, as a mold. After treating the substrate by vinyltrimethoxysilane (from SigmaAldrich) working as an adhesion promoter, high modulus PDMS solution48 was dropped on the substrate and subsequently covered by the PMMA film. The PDMS was cured at 90 °C for 10 min without additional pressure and the PMMA film is detached with a tweezer. The schematic of the replicating process is shown in Figure 5a.


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■ ASSOCIATED CONTENT

Supporting Information. Additional data such as AFM images (Figure S1, 2, 4, 6), IV characteristics (Figure S3), and analysis of high-resolution AFM images (Figure S5) are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy (No. 2016403020138C) and Basic Science Research Program (NRF-2015R1D1A1A01057417) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Republic of Korea.

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Figure 1. A schematic of the experimental procedures. (a)~(d) The process for the preparation of CNTs embedded in the PMMA layer. Please refer to the main text and the previous work43 for the full explanation of the process. (e) A schematic Ar plasma etching with CNTs as masks. Images below (d) and (e) show the schematic side views of the CNT/PMMA sample before and after exposure to the Ar plasma.


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Figure 2. Progress of nanostructure formation in a CNT/PMMA film with exposure to Ar plasma for 3 seconds each. AFM topographic images (a) before etching, after etching (b) for 9 s, (c) for 21 s, and (d) for 33 s. Insets show the cross-sectional line profiles along the dotted line in (b), (c), and (d). (e) Heights of the nanostructures as a function of Ar plasma etching time.


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Figure 3. Electrical transfer characteristics of a CNT device with exposure to Ar plasma (a) before and after Ar plasma etching for 2 seconds. (b) After subsequent Ar plasma etching for intervals of 2 seconds up to total etching time of 12 seconds with decreasing currents. (Black line for 2 s, Red for 4 s, Green for 6 s, Blue for 8 s, Cyan for 10 s, and Magenta for 12 s) Inset shows the on-state current in log scale as a function of etching time. The solid line is for guidance. Bias voltage was 1V. (c) Corresponding evolution of Raman spectra with Ar plasma etching. Peaks related to the PMMA substrate are marked by *.


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Figure 4. (a) An AFM topographic image of MWCNT/PMMA after Ar plasma etching for 15 s. (b) Heights of the nanostructures as a function of Ar plasma etching time. (c) A TEM crosssection image of a PMMA nanostructure formed by etching MWCNT/PMMA such as in (a).


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Figure 5. (a) A schematic for nanoimprinting inverse pattern transfer to a high modulus PDMS (h-PDMS) using the PMMA nanostructures as a master stamp. (b) An AFM topographic image of a h-PDMS showing nanoscale trenches. The original PMMA nanostructures which were used as a master stamp have heights of ~ 6 nm after 6s etching with Ar.


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Figure 1. 402x226mm (300 x 300 DPI)


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Figure 2. 375x249mm (300 x 300 DPI)



Figure 3. 357x277mm (300 x 300 DPI)


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Figure 4. 289x255mm (300 x 300 DPI)



Figure 5. 366x170mm (300 x 300 DPI)


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Carbon Nanotubes as Etching Masks for the Formation of Polymer

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