Langmuir 2008, 24, 8413-8416
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Nonaqueous Nanoscale Metal Transfer by Controlling the Stickiness of Organic Film Kyeongmi Lee,†,§ Seung-Hwan Oh,† Nam-Goo Kang,† Jae-Suk Lee,† Dong-Yu Kim,† Heon Lee,*,‡ and Gun Young Jung† Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-Gwagiro, Buk-gu, Gwangju 500-712, South Korea, and Department of Materials Science and Engineering, Korea UniVersity, 5-1 Anam-Dong, Sungbuk-Ku, Seoul 136-701, South Korea ReceiVed April 1, 2008. ReVised Manuscript ReceiVed July 1, 2008 Nanoscale metal patterns were successfully reproduced on top of a functional organic layer by a direct metal-transfer technique (DMT). A gold film deposited on the protruding features of a stamp was transferred to the organic layer by controlling its stickiness through a two-step thermal treatment. The process was also suitable for the transfer of highly adhesive metal materials to the stamp surface by using an additional gold layer. Chromium nanowires at 70 nm half-pitch were faithfully produced without any damage to the organic active layer.
Organic electronic devices such as organic light-emitting diodes,1,2 organic field-effect transistors,3,4 hybrid solar cells based on polymer films,5 and memories using redox-active molecules6,7 have drawn attention for their potential applications in the next organic electronics era because of the availability of flexible, portable engineering and low-cost fabrication. It is important to fabricate metal patterns on the active organic layer without any damage to the reliable device characteristics. However, the conventional photolithography technique involving the lift-off process to define top metal patterns is likely to dissolve or degrade the active organic layer by aqueous solutions, resulting in a failure in device fabrication. Therefore, several nonaqueous processes have been proposed for the fabrication of metal patterns on the active organic layer through additive and subtractive techniques, cold welding,2 microcontact printing (µCP),8 nanoscale-transfer printing (nTP),9 and metal-transfer printing (MTP).10 Some of these methods were applied to the fabrication of organic devices in a single step to define microscale top electrodes and the demonstration of working devices. However, the cold-welding process requires enormous printing pressure to ensure the breakage of metal deposited on the active organic layer at the edge of the protruding features of the stamp, which could be easily broken, especially at nanoscale dimensions at such a high printing pressure. A * Corresponding authors. (H.L.) E-mail:
[email protected]. Phone: (+82)2-3290-3284. Fax: (+82)2-928-3584. (G.Y.J.) E-mail:
[email protected]. Phone: (+82)62-970-2324. Fax: (+82)62-970-2304. † Gwangju Institute of Science and Technology (GIST). ‡ Korea University. § Current address: Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama, Japan. (1) Tessler, N.; Harrison, N. T.; Friend, R. H. AdV. Mater. 1998, 10, 64. (2) Kim, C.; Burrows, P. E.; Forrest, S. R. Science 2000, 288, 831. (3) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (4) Kim, C.; Shtei, M.; Forrest, S. R. Appl. Phys. Lett. 2000, 80, 4051. (5) Gur, I.; Fromer, N. A.; Chen, C. P.; Kanaras, A. G.; Alivisatos, A. P. Nano Lett. 2007, 7, 409. (6) Chen, Y.; Jung, G. Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Williams, R. S. J. Nanotechnol. 2003, 14, 462. (7) Duan, X. F.; Huang, Y.; Lieber, C. M. Nano Lett. 2002, 2, 487. (8) Lackowski, W. M.; Ghosh, P.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 1419. (9) Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654. (10) Wang, Z.; Yuan, J.; Zhang, J.; Xing, R.; Yan, D.; Han, Y. AdV. Mater. 2003, 15, 1009.
Figure 1. (a) Molecular structure of WPF-oxy-F and (b) a digital image of a silicon stamp used for the DMT process.
poly(dimethylsiloxane) (PDMS) stamp has been intensively used in other processes because of its low surface energy for easy metal separation from the surface. Although the PDMS stamp is beneficial in terms of conformal contact at the interface between the stamp and the organic layer, its elasticity limits the minimum feature size that can be patterned. Processes using the PDMS stamp also require an additional step of selective polymer growth on the printed, self-assembled ink as an etching mask for subsequent metal etching8 as well as a monolayer coating of a chemical linker for chemical bonding between the gold and the substrate.9 In this study, a new direct metal-transfer (DMT) technique using a rigid stamp with nanoscale features is described for metal patterning on an organic active layer in a single process. This process is different from previous metal-transfer printing (MTP)10 using a PDMS stamp in that nanoscale metal patterning is achieved by controlling the stickiness of the organic film using a two-step thermal treatment. This technique is also available with highly adhesive metal materials using a bilayer metallic structure, which is advantageous for various top metal junctions to organic devices. The basic process of DMT is illustrated in Scheme 1. Prior to the DMT process, the stamp surface should be deposited with a monolayer of releasing material, CF3(CF2)5(CH2)2SiCl3 (tridecafloro-1,1,2,2-tetrahydrooctyltrichlorosilane) purchased from Gelest Inc., in the vapor phase.11 The stamp was loaded into a glass vacuum chamber and pumped down to 2 × 10-2 Torr, and then the vapor of the releasing agent at 60 °C entered the main (11) Jung, G. Y.; Wu, W.; Li, Z.; Chen, Y.; Wang, S. Y.; Tong, W. M.; Williams, R. S. Langmuir 2005, 21, 1158.
10.1021/la801019d CCC: $40.75 2008 American Chemical Society Published on Web 07/19/2008
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Scheme 1. Direct Metal-Transfer Process on the Organic Layer
chamber to reach a vapor pressure of 0.3 Torr. After 10 min of exposure, deionized water vapor was allowed to enter the main chamber carefully to attain a pressure of 0.8 Torr for the formation of a cross network between the releasing materials solely attached on the stamp surface by a condensation reaction. The stamp was kept for another 10 min in the combined gas atmosphere. This process was performed twice more to ensure perfect monolayer formation without defects. Finally, the stamp was cleaned with HEF-7100 solvent in an ultrasonic bath. After the releasing monolayer was formed on the stamp surface, a 40-nm-thick gold layer was deposited on the stamp surface by an electron beam evaporator at a deposition rate of 0.5 Å/s and a pressure of 10-5 Torr. A collimated beam was used to deposit metal selectively on only the protruding and recessed areas of the stamp, leaving the stamp sidewalls uncoated, as depicted in Scheme 1a. The stamp was then placed onto the organic active film, which had been previously spin coated on a silicon substrate, and these were loaded into a sample holder composed of a top aluminum circular plate with embedding copper lines for heating and a bottom elastic membrane serving as the uniform pressure applied to the sample during the process. The stamp with the metal film and the substrate coated with an organic active film were placed in initial contact by evacuating the sample holder. Subsequently, the chamber containing the sample was closed, and hydraulic pressure was applied to the chamber from a compressed nitrogen gas cylinder to bring the two substrates into conformal contact. The sample was annealed to make the organic film soft and to ensure that the contact at the interface was as conformal as possible. The system was then cooled to separation temperature, at which time the organic film possessed a sufficiently high stickiness to grab the metal film gently from the protruding stamp surface. After the applied pressure was released, the sample was separated into two parts with tweezers. The organic active material used in this study was synthesized as a switching material, which is a water-soluble polyfluorene derivative, poly[(9,9-bis((6′-(N,N,N-trimethylammonium)hexyl)2,7-fluorene)-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-fluorene)] dibromide (WPF-oxy-F), as shown in Figure 1a.12 The WPF-oxy-F is assumed to have a glass-transition temperature of around 60 °C because it has a similar repeating unit of poly(2,7(9,9-dioctylfluorene)).13 Interestingly, the glass-transition temperature of the thin film of the polyfluorene derivative in the thickness range of 60-160 nm was reported to be 18 °C higher than that of bulk polymer.14 Therefore, the glass-transition temperature of the WPF-oxy-F thin film would be around 78 °C. A 0.5 wt % solution in methanol and water with a volume ratio (12) Oh, S. H.; Na, S. I.; Nah, Y. C.; Vak, D.; Kim, S. S.; Kim, D. Y. Org. Electron. 2007, 8, 77. (13) Blondin, P.; Bouchard, J.; Beaupre, S.; Belletete, M.; Durocher, G.; Leclerc, M. Macromolecules 2000, 33, 5874. (14) Campoy-Quiles, M.; Sims, M.; Etchegoin, P. G.; Bradley, D. D. C. Macromolecules 2006, 39, 7673.
of 3:7 was prepared. A film with a thickness of 100 nm was produced after spin coating at a speed of 4000 rpm for 30 s on a silicon substrate precleaned with piranha solution and then baked at 120 °C for 5 min prior to the DMT process. Pressure and temperature were varied to find the optimum conditions for the DMT process with the above polymer film. The DMP process presented here used a silicon stamp, which had blocks consisting of line features at 70 nm half-pitch with an aspect ratio of 5 over the entire stamp surface as shown in Figure 1b. The blocks were separated by micrometer-scale features. Conformal contact between the organic film and the stamp surface was essential to achieving uniform metal transfer over the entire active area. The metal transfer was found not to occur below a printing pressure of 400 psi. The stickiness of the organic film at different temperatures was determined by the probability of the metal film transferred to the organic film surface. The sample was first heated to 100 °C and maintained for 10 min under a pressure of 550 psi, and then it was cooled to different temperatures before separation (100, 80, 50, and 30 °C). Figure 2a shows the effect of separation temperature on the DMT process using the silicon stamp deposited with a 40-nm-thick gold film. The yellow color of the images in Figure 2a indicates the region where gold metal was transferred from the stamp features to the organic film surface. The transferred metal area increases with the separation temperatures up to 80 °C, suggesting that the stickiness of the organic film is increasing and is maximized around its glass-transition temperature. At the separation temperature of 100 °C, however, the transferred metal area decreases, presumably because the organic film becomes too soft to grab the metal film from the stamp features. To prove the above assumption, the stickiness was measured physically using a tape test. A flat gold film was initially transferred onto the polymer film at the optimized separation temperature of 80 °C using the two-step thermal treatment mentioned above, and thermal tape was then applied on top of the transferred flat gold film. This sample was loaded into the chamber and annealed for 10 min at the aforementioned temperatures, followed by the removal of the tape from the organic film to investigate how much of the gold film survived on the organic film surface. The y axis in Figure 2b shows the percentage of remaining metal area after the tape experiment. The results of the tape experiment were similar to those seen in the separationtemperature experiment in the DMT process. Contact angle measurements showed that the releasing property of the releasing layer was intact up to 300 °C.11 The temperature effect on the adhesion force at the releasing layer/gold interface was also checked at different temperatures with the same tape method, demonstrating no notable difference. These experimental results clarify that the stickiness of the organic film plays an important part in successful metal transfer.
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Figure 2. (a) Percentage of metal-transfer area with different separation temperatures. Yellow illustrates the metal transferred to the organic layer by DMT, showing that the best result was achieved at 80 °C. The cross lines were transferred from the micrometer-scale features on the stamp. (b) Stickiness test using Scotch tape, illustrating that at 80 °C the organic layer best sticks to the metal, which is the driving mechanism for successful DMT.
Preheating the WPF-oxy-F material to 100 °C was essential for conformal contact between the stamp and the polymer layer because it made the polymer soft. After this step, the sample was cooled to the separation temperature of 80 °C, where the actual metal transfer was achieved using the maximized stickiness of the polymer film. Organic films that were not subjected to initial preheating to 100 °C showed poor metal transfer to the organic film, supporting the notion that the initial polymer softness is necessary to enhancing the intimate conformal contact at the interface prior to the actual metal transfer. The same phenomenon occurred with the other polymer material, poly(methyl methacrylate), such that two-step thermal treatment was required for faithful metal-pattern transfer. Figure 3 shows the optical and magnified SEM images of the metal patterns on the polymer film transferred from the stamp features. Many 70 nm half-pitch metal lines are depicted as purple blocks that are separated from others by brown boundaries (Figure 3a), where a sparse population of micrometer-scale metal patterns exists as shown in Figure 3c. Figure 3e is an SEM image of the 70 nm half-pitch gold nanowires transferred from the corresponding features of the stamp (Figure 3f). These two panels indicate that the DMT process transfers the dense nanoscale metal patterns to the organic film with high fidelity. The DMT process can be extended to other metals, such as aluminum, chromium, and titanium. Unfortunately, these metals adhered so strongly to the releasing layer during evaporation that they were not transferred to the organic film at all after the DMT process. To overcome this challenge, an additional metal layer was employed as shown in Scheme 2. A metal layer with poor adhesion was deposited on the releasing layer, and then the adhesive metal layer (e.g., chromium) was successively evapo-
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Figure 3. (a, b) Optical images and (c, d) magnified SEM images of the transferred metal pattern. (e) SEM image of 70 nm half-pitch metal wires transferred from (f) the corresponding silicon stamp. Scheme 2. DMT Process Using a Bilayer Metallic Structure
rated without breaking the vacuum of the electron-beam evaporator. Initial experiments with various thicknesses of the additional gold layer demonstrated that at least a 30-nm-thick gold layer was required for the successful DMT process with the highly adhesive metal materials. In this experiment, a 35-nmthick gold/40-nm-thick chromium bimetallic structure was used for the DMT process, where the chromium layer was transferred and formed an intimate junction with the WPF-oxy-F polymer layer with the help of easy detachment of the gold layer from the stamp surface. Figure 4a shows a tilted SEM image of the transferred chromium/gold metal lines at 70 nm half-pitch on top of the polymer layer after the DMT process. Its cross-sectional view as shown in Figure 4b demonstrates that the transferred metal patterns did not penetrate into the organic film at the DMT operating pressure of 550 psi. We utilized this technique to fabricate crossbar-type polymer nonvolatile memory devices in which heavily doped p-type polycrystalline bottom electrodes with a 2 µm line width were produced by conventional photolithography and a subsequent etching process using a siliconon-insulator-type substrate. A transparent glass stamp was also fabricated by photolithography with the same photomask for the alignment of top electrodes rotated 90° with respect to the bottom electrodes during the DMT process. Silver top electrodes (40 nm thick) were directly transferred to the organic active film by DMT using the additional gold layer (35 nm thick). Figure 4c
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the two electrodes. The bipolar switching properties from a cell in the fabricated polymer memory array are depicted in Figure 4d; the switching voltages from cells were observed to be 2.5-3.25 V along with an on/off ratio of ∼100. A detailed report on the device characteristics is being prepared. In summary, direct metal transfer (DMT) using two-step thermal treatment was capable of transferring metal lines at 70 nm half-pitch to an organic active layer. The stickiness of the organic layer was observed to be a key parameter in this technique. The best polymer stickiness permitted metal transfer on the polymer film surface at its glass-transition temperature. In addition, the use of an additional gold layer broadens the feasibility of the DMT technique with highly adhesive metal materials for the top contacts in various organic devices. The DMT process is economical because of its nonaqueous nature of processing, which will be useful for the next organic device era. Figure 4. (a) SEM image of chromium/gold nanowires at 70 nm halfpitch transferred to the organic layer with the help of the additional gold layer and (b) its cross-sectional image. (c) Optical image of an organic memory device with crossbar structure fabricated by DMT and (d) typical I-V characteristics of a cell.
depicts an optical image of a polymer memory device with a crossbar structure in which the WPF-oxy-F is sandwiched between
Acknowledgment. This work was supported by the Korea Research Foundation (grant KRF-2006-331-D00125), the Program for Integrated Molecular System (PIMS), and the System IC 2010 project of the Korea Ministry of Commerce, Industry and Energy. LA801019D
