pubs.acs.org/Langmuir © 2010 American Chemical Society
Fabrication of Copper Damascene Patterns on Polyimide Using Direct Metallization on Trench Templates Generated by Imprint Lithography Yasufumi Matsumura,*,† Yasushi Enomoto,† Takaaki Tsuruoka,‡ Kensuke Akamatsu,‡ and Hidemi Nawafune*,‡ †
Basic Research Laboratories, Nippon Steel Chemical Co., Ltd. 1-Tsukiji, Kisarazu, Chiba 292-0835, Japan, and ‡ Department of Nanobiochemistry, Frontiers of Innovative Research in Science and Technology (FIRST), Konan University, 7-1-20 Minatojimaminami, Chuo-ku, Kobe 650-0047, Japan Received April 6, 2010. Revised Manuscript Received May 24, 2010
Facile imprint and wet chemical processes were used to fabricate copper damascene patterns on polyimide substrate. Poly(amic acid) substrate with trench structures as template has been successfully prepared by imprint lithography using a poly(dimethylsiloxane) mold. The doped Ni2þ ions into a template through ion-exchange reaction were reduced by an aqueous NaBH4 solution, resulting in the formation of a nickel thin layer along the surface structure of the template. The resulting nickel films can act as catalyst for subsequent electrodeposition of copper. After electrodeposition, a polishing process was carried out for removing excess deposited copper films, followed by imidization of the substrate. The resulting damascene structured copper films exhibited fine and good adhesion with the polyimide substrate, and they could be utilized for good application in the fields of minute copper circuit patterns on insulating substrates.
Introduction The microfabrication of electronic components with high resolution onto various substrates has attracted much attention in the manufacturing processes of electronic, optical, and mechanical devices.1-4 Recently, a damascene process is mainly used to fabricate an ultrahigh-density copper wiring in the semiconductor industry, which involves fabrication of trench structures on the substrates using reactive ion etching (RIE) and metal deposition in trench patterns followed by removal of the superfluous copper film with a chemical mechanical polishing (CMP).5-7 The copper minute wiring obtained by conventional damascene approaches shows line width in the range of nanometers and is embedded into dielectric rigid substrate such as silicon dioxide, and thus, this structure provides an effective interconnection with high *To whom correspondence should be addressed. E-mail: matsumuraya@ nscc.co.jp (Y.M.);
[email protected] (H.N.). (1) Menard, E.; Meitl, M. A.; Sun, Y.; Park, J.-U.; Shir, D. J.-L.; Nam, Y.-S.; Jeon, S.; Rogers, J. A. Chem. Rev. 2007, 107, 1117–1160. (2) Whitesides, G. M.; Love, J. C. Sci. Am. 2001, 285, 32–41. (3) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171–1196. (4) del Campo, A.; Arzt, E. Chem. Rev. 2008, 108, 911–945. (5) Andricacos, P. C.; Uzoh, C.; Dukovic, J. O.; Horkans, J.; Deligianni, H. IBM J. Res. Dev. 1998, 42, 567–574. (6) Andricacos, P. C. Interface 1999, 32–37. (7) Nguyen, V. H.; Hof, A. J.; van Kranenburg, H.; Woerlee, P. H.; Weimar, F. Microelectron. Eng. 2001, 55, 305–312. (8) Ohba, T. Proc. - Electrochem. Soc. 2003, 183–193. (9) Lu, J.-Q. Proc. IEEE 2009, 97, 18–30. (10) Lanckmans, F.; Brongersma, S. H.; Varga, I.; Poortmans, S.; Bender, H.; Conard, T.; Maex, K. Appl. Surf. Sci. 2002, 201, 20–34. (11) Youn, S.-W.; Ueno, A.; Takahashi, M.; Maeda, R. Jpn. J. Appl. Phys. 2008, 47, 5189–5196. (12) Kawahara, J.; Nakano, A.; Kinoshita, K.; Harada, Y.; Tagami, M.; Tada, M.; Hayashi, Y. Plasma Sources Sci. Technol. 2003, 12, S80–S88. (13) Nomura, Y.; Ota, F.; Kurino, H.; Koyanagi, M. Jpn. J. Appl. Phys. 2005, 44, 7876–7882. (14) Geer, R. E.; Kolosov, O. V.; Briggs, G. A. D.; Shekhawat, G. S. J. Appl. Phys. 2002, 91, 4549–4555. (15) Schmid, G. M.; Stewart, M. D.; Wetzel, J.; Palmieri, F.; Hao, J.; Nishimura, Y.; Jen, K.; Kim, E. K.; Resnick, D. J.; Liddle, J. A.; Willson, C. G. J. Vac. Sci. Technol., B 2006, 24, 1283–1291. (16) Chao, B. H.; Palmieri, F.; Jen, W.-L.; McMichael, D. H.; Willson, C. G.; Owens, J.; Berger, R.; Sotoodeh, K.; Wilks, B.; Pham, J.; Carpio, R.; LaBelle, E.; Wetzel, J. Proc. SPIE 2008, 6921, 69210C-1–69210C-14.
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adhesion.8-10 This approach, although being widely employed for the practical fabrication of damascene structure using rigid substrate,11-16 was not utilized for the fabrication of metal damascene on flexible substrates. Thus, the development of new methodologies for fabrication of damascene structures using flexible polymer substrates is crucial for the flexible printed circuit (FPC) fabrication, multichip module (MCM) packaging, magnetic data storage, and ultralarge-scale integrated (ULSI) circuit technology with high performance. Successful microelectronic devices based on metal minute wiring embedded into flexible polymer substrates require the ability to construct three-dimensional structures. There are many approaches for preparing 3D polymer structures using conventional patterning techniques, including photolithography,17-20 ion beam lithography,21,22 and electron beam lithography.23-25 These approaches can create polymer patterns with high resolution but are generally limited in simplicity and cost. As an alternative to these approaches, an imprint lithography has been investigated for generating 3D polymers, in which trench and hole size are desirable in the range of micro/nanometers.26-30 (17) Wallraff, G. M.; Hinsberg, W. D. Chem. Rev. 1999, 99, 1801–1821. (18) Sanders, D. P. Chem. Rev. 2010, 110, 321–360. (19) Owa, S.; Nagasaki, H.; Ishii, Y.; Hirakawa, O.; Yamamoto, T. Solid State Technol. 2004, 47, 43–48. (20) Cerrina, F.; Bollepalli, S.; Khan, M.; Solak, H.; Li, W.; He, D. Microelectron. Eng. 2000, 53, 13–20. (21) van Kan, J. A.; Bettiol, A. A.; Ansari, K.; Teo, E. J.; Sum, T. C.; Watt, F. Int. J. Nanotechnol. 2004, 1, 464–479. (22) Ansari, K.; van Kan, J. A.; Bettiol, A. A.; Watt, F. Appl. Phys. Lett. 2004, 85, 476–478. (23) Yamazaki, K.; Namatsu, H. Microelectron. Eng. 2004, 73-74, 85–89. (24) Elsner, H.; Meyer, H.-G. Microelectron. Eng. 2001, 57-58, 291–296. (25) Bilenberg, B.; Scholer, M.; Shi, P.; Schmidt, M. S.; Boggild, P.; Fink, M.; Schuster, C.; Reuther, F.; Gruetzner, C.; Kristensen, A. J. Vac. Sci. Technol., B 2006, 24, 1776–1779. (26) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85–87. (27) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550–575. (28) Jung, G. Y.; Ganapathiappan, S.; Ohlberg, D. A. A.; Olynick, D. L.; Chen, Y.; Tong, W. M.; Williams, R. S. Nano Lett. 2004, 4, 1225–1229. (29) Lu, Y.; Chen, X.; Hu, W.; Lu, N.; Sun, J.; Shen, J. Langmuir 2007, 23, 3254– 3259. (30) Truong, T. T.; Lin, R.; Jeon, S.; Lee, H. H.; Maria, J.; Gaur, A.; Hua, F.; Meinel, I.; Rogers, J. A. Langmuir 2007, 23, 2898–2905.
Published on Web 06/17/2010
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This imprint lithography would be useful for the low cost and high throughput fabrication of 3D polymer patterns. In order to apply the damascene structure into flexible polymer to microelectronic devices, it is important to use polyimide as the flexible polymer substrate because it is one of the most attractive interconnecting low-k materials for microelectronics engineering.31 Polyimide with trench patterns can be readily prepared by casting and curing poly(amic acid) solutions onto the molds, followed by peeling away the cured materials without photolithographic steps. In addition, we have recently reported direct metallization of polyimide films including fabrication of precursor polyimide to form cation exchangeable groups (carboxylic acid groups) and subsequent loading of metal ions via an ion-exchange reaction followed by reduction of these incorporated metallic ions.32-34 This method is an all-wet chemical metallization process, which has advantages in terms of productivity because of lower energy consumption compared to conventional dry processes such as sputtering, physical vapor deposition (PVD), and chemical vapor deposition (CVD). Therefore, the ability to combine with the damascene process, imprint lithography, and direct metallization will open the way to a more advanced approach for low-cost fabrication of copper circuit patterns with high resolution onto flexible polymer substrates. In this study, we describe a novel process for the fabrication of copper damascene patterns on polyimide substrate. The present approach relies on fabrication of poly(amic acid) trench templates using imprint lithography and damascene processes including fabrication of a nickel thin layer for copper electrodeposition using direct metallization. The nickel thin layer is also available as an interfacial adhesion promoter and diffusion barrier. This strategy can be described as an additive process for the fabrication of damascene patterns on polyimide, providing an alternative process to our previous method and to conventional metallization methods.
Experimental Section Materials. All chemicals, 4,40 -biphthalic anhydride (BPDA), 1,3-bis(4-aminophenoxy)benzene (TPE-R), N,N-dimethylacetoamide (DMAc), Ni(CH3COO)2 3 4H2O, ammonia solution (30%), sodium borohydride, and hydrochloric acid (37%), were analytical grade and used as received. Poly(dimethylsiloxane) (PDMS) mold was purchased from Fluidware Technologies Inc. The PDMS mold consists of a sequence of grooves; the typical distance between grooves is 5.3 μm, their depth is 5.6 μm, and their width is 4.7 μm. Distilled water was used for the preperation of all aqueous solutions and washing steps. Synthesis of Poly(amic acid) Solution. The 4,40 -biphthalic anhydride/1,3-bis(4-aminophenoxy)benzene (BPDA/TPE-R) poly(amic acid) resin employed in this study was prepared by following procedure. TPE-R (15.0 g, 51.2 mmol) and DMAc (270 g) were placed into a flask with a mechanical stirrer under dry nitrogen. After dissolution of TPE-R, BPDA (15.0 g, 51.2 mmol) was added into the flask. After stirring at room temperature for 4 h, the BPDA/TPE-R poly(amic acid) with light yellow viscous solution was obtained. The inherent viscosity of the resulting solution applied in our present work was 26.3 Pa 3 s at 25 °C. Fabrication of Poly(amic acid) Trench Template. Figure 1 shows the schematic for fabricating poly(amic acid) trench templates using a PDMS mold. The obtained poly(amic acid) solution was poured onto the PDMS mold. Subsequently, the solution was (31) Polyimides: Fundamental Aspects and Technological Applications; Ghosh, M. K., Mittal, K. L., Eds.; Marcel Dekker: New York, 1996. (32) Matsumura, Y.; Enomoto, Y.; Sugiyama, M.; Akamatsu, K.; Nawafune, H. J. Mater. Chem. 2008, 18, 5078–5082. (33) Akamatsu, K.; Ikeda, S.; Nawafune, H.; Yanagimoto, H. J. Am. Chem. Soc. 2004, 126, 10822–10823. (34) Akamatsu, K.; Ikeda, S.; Nawafune, H. Langmuir 2003, 19, 10366–10371.
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Figure 1. Schematic illustration of poly(amic acid) trench template fabrication using imprint lithography. spread using a doctor blade followed by heat treatment at 130 °C for 10 min to obtain approximately 20 μm thick films. After the heat treatment, poly(amic acid) was removed from PDMS mold by being peeled. Fabrication of Damascene Pattern on Polyimide. The process for fabrication of copper damascene patterns onto polyimide is illustrated in Figure 2. In step (i), the poly(amic acid) trench template was immersed into aqueous solution containing nickel(II) acetate (Ni(CH3COO)2) (100 mM) and NH3 (600 mM) at 25 °C for 20 min to incorporate nickel ions into the poly(amic acid) through an ion-exchange reaction. In step (ii), the nickel-iondoped poly(amic acid) trench template was immersed into a 5 mM aqueous NaBH4 solution at 30 °C for 20 min to reduce the doped nickel ions. In step (iii), the metallic copper layer was electrochemically deposited from an acid copper sulfate plating bath comprising 0.44 M CuSO4, 2.04 M H2SO4, and a small amount of additives (82 mg L-1 NaCl, 5 mL L-1 TOP LUCINA SF made by Okuno chemical industries Co., Ltd.) at a current density of 0.05-2.0 A dm-2 at 25 °C. An approximately 5 μm thick copper was electrodeposited to overfill the imprinted features of poly(amic acid). In step (iv), the excess of copper layer formed on the surface of poly(amic acid) template except for part of trenches was removed by polishing techniques to fabricate damascene patterns. Removal of the extra copper layer was pruned by polycrystalline diamond suspension (diameter; 3 and 1 μm, Allied High Tech Products, Inc.). Finally, in step (v), the damascene pattern samples were dipped into 1 wt % aqueous HCl solution at 30 °C for 10 min to remove residual ions in the poly(amic acid) template and then heated at 300 °C for 10 min to convert polymers from poly(amic acid) to polyimide through imidization. After each step, the film was rinsed with copious amount of distilled water. Characterization. The surface morphology and cross-sectional microstructure of the films were observed by field emission scanning electron microscopy (FESEM, JEOL, JSM-6340F) and DOI: 10.1021/la101350f
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Figure 3. (A) SEM image of the poly(amic acid) film surface generated by imprint lithography using a PDMS mold. (B) Laser microscope image of the poly(amic acid) film surface. (C) Line scan taken from the laser microscope image shown in (B). Figure 2. Schematic illustration of copper damascene patterns onto polyimide substrate via direct metallization method. transmission electron microscopy (TEM, JEOL, JEM-2000EX). For cross-sectional SEM observation, the samples were embedded into epoxy resin and cross-sectionally polished with silicon carbide grinding paper and polycrystalline diamond suspension. For cross-sectional TEM observation, the samples were sectioned into ca. 100 nm thick slices with the conventional microtome technique using a diamond knife (Leica, Ultracut R). These thin sections floating on a water bath were mounted onto carbon-coated TEM copper grids. The microstructure of the poly(amic acid) template was measured using a laser scanning microscope (KEYENCE, VK-8500). The static contact angles of water were measured on the film surfaces using a contact angle goniometer (KYOWA interface measurement and analysis system FAMAS). The amount of adsorbed Ni2þ ions absorbed in poly(amic acid) film were determined by inductively coupled plasma atomic emission spectrometry (ICP, Perkin-Elmer Inc. OPTIMA 4300DV). The measurements were performed after immersing the ion-doped poly(amic acid) films in 1 N nitric acid for 12 h to extract adsorbed ions. The depth profiles of nickel, boron, and sodium in the film were measured using a glowdischarge optical emission spectrometer (GD-OES, Jobin Yvon JY5000RF-PSS). The nickel, boron, and sodium emission signals for the poly(amic acid) films were monitored as a function of Ar ion sputtering time. The sputtering rates obtained for the film thickness are a measurement of the true sample. The chemical state of nickel on the poly(amic acid) film was studied with X-ray photoelectron spectroscopy (XPS, PHI Quantum2000) with an Al KR X-ray source. The line profiling of damascene patterns was observed with an Auger electron spectroscope (AES, PHI SAM670) with an electron beam of acceleration voltage 10 kV and emission electric current 10 nA. The Fourier transform infrared (FT-IR, Japan Spectroscopic Co. FT/IR 620) instrument was equipped with an attenuated total reflection (ATR) attachment. Sheet resistance of the metallized surface was measured by the four-probe method using a low-resistivity meter (Mitsubishi 12450 DOI: 10.1021/la101350f
Chemical Co., MCP-T610). The measurement of adhesion strength was performed by using a peeling strength tester (TOYOSEIKI Co., STROGRAPH VES05D).
Results and Discussion Fabrication of Poly(amic acid) Trench Templates for Direct Nickel Metallization onto Trench Patterns. The patterning of the initial poly(amic acid) trench template is generated by the imprint lithography technique, which can be fabricated by casting and curing poly(amic acid) solution on a PDMS mold bearing the trench patterns (Figure 1). After solvent evaporation by heat treatment, the poly(amic acid) film can be readily peeled away from the PDMS mold. Figure 3A shows SEM image of the poly(amic acid) film surface. The micrograph shows that well-defined trench structures are formed on the poly(amic acid) film surface using a nonchemical treated PDMS mold, which are not observed defects in the form of merged or lacked the convex parts. This may be caused by the difference of surface polarity between the poly(amic acid) and the PDMS mold. To verify the surface energy of poly(amic acid) and PDMS substrates, we measured the water contact angles using the flat films, the values of which were 70.5° and 107.5°, respectively. This difference allows one to produce the poly(amic acid) films with excellent trench patterns. The laser microscopic image and line profile in Figure 3B and C show that the 5.5 4.4 5.6 μm3 (widthspaceheight) box shaped and vertical sidewalls trenches are clearly replicated into the poly(amic acid) surface. Although the width of convex parts changes slightly with solvent evaporation (shrinkage ratio: 6.4%), the motifs in the groove are the negative replica of the PDMS mold used and are consistent with the pattern structures of the PDMS mold. Regarding the pattern size of the poly(amic acid) trench structures, a width of 0.35 μm, space of 0.35 μm, and depth/width ratio of 1.1 can be fabricated by the present approach. Therefore, poly(amic acid) film with Langmuir 2010, 26(14), 12448–12454
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Figure 4. (A) Top view SEM image of the poly(amic acid) template after NaBH4 reduction treatment. (B) Cross-sectional SEM image of the template shown in (A).
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trench patterns can be simply formed by this process without photolithographic and electron beam lithographic processes. Deposition of Nickel Thin Films onto Poly(amic acid) Trench Templates. The present fabrication process of copper damascene patterns onto polyimide substrates via the direct metallization process is schematically presented in Figure 2. The poly(amic acid) trench templates are capable of ion doping by ion exchange reaction, because they have carboxyl groups, which can act as ion exchange sites, in the polymer backbone. Therefore, by immersing into aqueous Ni(CH3COO)2-NH3 solution, Ni2þ ions were readily incorporated in poly(amic acid) films. After immersion for 20 min, the color of the trench template changed from light brown to light green, indicating ion loading of Ni2þ ions into films. The amount of Ni2þ ions loaded into poly(amic acid) was approximately 1.9 mmol cm-3. Nickel thin films on poly(amic acid) trench patterns can be fabricated by wet chemical reduction of Ni2þ ions in the trench templates. When the ion-doped trench template was immersed into a 5 mM aqueous solution of NaBH4 at 30 °C for 20 min, the film color gradually changed from light green to the silver color with a metallic luster, indicating the formation of metallic nickel thin films on the trench patterns. SEM observation revealed that nickel films were formed onto the poly(amic acid) surface by reduction of doped ions using NaBH4 aqueous solution (Figure 4A). Additionally, a cross-sectional SEM image, as shown in Figure 4B, shows that the obtained film was smooth and uniform. These observations clearly demonstrate the formation of continuous nickel thin films on the trench patterns without the deformation of substrate. The results also demonstrate the possibility for the deposition of metal films onto complicated structures such as through holes. In an effort to gain more insight into the formation of continuous thin films, SEM and cross-sectional TEM observations were conducted for nickel films prepared by the different reduction time, and several important results were obtained.
Figure 5. SEM images of a nickel thin film on the poly(amic acid) surface obtained after aqueous NaBH4 treatment for 5 min (A), 10 min (B), and 20 min (C).
Figure 6. Cross-sectional TEM images of nickel thin film on the poly(amic acid) surface obtained after aqueous NaBH4 treatment for 5 min (A), 10 min (B), and 20 min (C). Langmuir 2010, 26(14), 12448–12454
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Figure 7. GD-OES depth profiles of nickel, boron, and sodium in the poly(amic acid) films before (A) and obtained after 5 mM aqueous NaBH4 treatment for 5 min (B) and 20 min (C).
In the early stages of reduction, the SEM image shows continuous nickel films consisting of small particles (mean diameter of ca. 50-80 nm) (Figures 5A and 6A). The sheet resistance of this film is 69.8 Ω sq-1, indicating that the obtained nickel films on trenches of poly(amic acid) substrate cannot act as a template for subsequent copper electrodeposition. However, as the reaction proceeded, the size of nickel particles gradually increased, and the deposited nickel film was 150 nm thick (Figures 5B,C and 6B,C), resulting in good surface conductivity of the obtained film. The sheet resistance of the nickel thin film after reduction for 20 min was 27.5 Ω sq-1, which is suitable for subsequent copper electrodeposition. Significantly, as shown in cross-sectional TEM images, the nickel deposition in the present process occurs around the poly(amic acid) surface. To obtain information related to changes in the distribution of nickel, boron, and sodium, depth profiling measurements were performed for samples with different NaBH4 treatment times, and typical representative results are shown in Figure 7. Before NaBH4 treatment, sodium and boron signals cannot be seen in the profiles, and Ni2þ ions are only observed to homogeneously distribute in the depth direction of the film (Figure 7A). However, upon NaBH4 treatment, the intensity of nickel gradually increases around the surface region. On the other hand, the intensity of nickel signals in the poly(amic acid) uniformly decreases (Figure 7B and C). For the sodium signal in the depth direction, the profile indicated that Naþ cations were almost uniformly incorporated into the polymer. Since the Ni2þdoped poly(amic acid) trench template before NaBH4 treatment does not include the Naþ cations, the observed changes in Naþ depth profile are essentially caused by ion-exchange reaction because the carboxylic groups after Ni2þ reduction can act as ion-exchangeable sites. Therefore, the intensity of the Naþ signal increased with increasing reduction time (Figure 7B and C). Additionally, the intensity of boron signals simultaneously increased around the surface region as the reduction time increased, whereas the presence of boron inside of poly(amic acid) was not confirmed. XPS for Ni 2p1/2, Ni 2p3/2, and B 1s in the obtained thin films clearly indicates the presence of Ni-B alloy, and the molar ratio of Ni2B and Ni0 in the films was approximately 1:0.75.35 The formation of Ni-B alloy is an advantage for fabrication of copper damascene structures on polymer substrates because it is well-known that this alloy shows the ability to prevent copper diffusion into dielectric substrates.36,37 (35) Since the Ni/B ratio of the nickel thin film was estimated to be ca. 2.6 by composition analysis, the resulting thin film consists of Ni2B and Ni0, at approximately 1:0.75 ratio. (36) Ikeda, A.; Sakamoto, A.; Hattori, R.; Kuroki, Y. Thin Solid Films 2009, 517, 1740–1745. (37) Yoshino, M.; Nonaka, Y.; Sasano, J.; Matsuda, I.; Shacham-Diamand, Y.; Osaka, T. Electrochim. Acta 2005, 51, 916–920.
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Figure 8. Proposed mechanism for direct metallization of an iondoped poly(amic acid) surface. (A) Reduction of nickel ions by BH4or electrons generated through hydrolysis of NaBH4 in aqueous solution. (B) Formation of metallic nickel and Ni2B followed by ion-exchange of the remaining carboxylate anion groups. (C) Ionexchange reaction between sodium cations and nickel ions through the generation of a concentration gradient of these ions in the poly(amic acid), and further reduction of nickel ions at the film surface. Langmuir 2010, 26(14), 12448–12454
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Figure 9. SEM images of the poly(amic acid) template after copper electrodeposition (A,B) and after subsequent removal of extra copper films (C,D) followed by heat treatment (E, F). Top view and cross-section images of each sample are shown.
From the above results, we suggest that the process for chemical metallization includes the local anodic reaction (eq 1) and the local cathodic reactions (eq 2-4) as follows. BH4 - þ 4H2 O f BO2 - þ 8Hþ þ 8e -
ð1Þ
2Ni2þ þ BH4 - þ 3e - f Ni2 B þ 2H2
ð2Þ
Ni2þ þ 2e - f Ni0
ð3Þ
2Hþ þ 2e - f H2
ð4Þ
Under the presence of NaBH4 in aqueous solution, the generation of electrons occurs by local anodic reaction (eq 1), followed by simultaneous reduction of Ni2þ ions at the interface between the film surface and solution containing BH4- and electrons by local cathodic reactions (eqs 2 and 3), resulting in the formation of Ni2B and Ni0 onto poly(amic acid) film as schematically shown in Figure 8A and B. The reduction is accompanied by the dissociation of Ni2þ ions from carboxylate anions followed by ion-exchange reaction to form sodium salts of carboxylic acid near the surface (Figure 8B). This would give rise to a concentration gradient of Naþ cations and Ni2þ ions into the poly(amic acid) template during NaBH4 treatment. The uniform distribution of Naþ and Ni2þ ions is energetically favorable in this system, and therefore, this gradient condition leads to diffusion of Ni2þ ions toward the surface region and Naþ ions in the depth direction through ion-exchange reaction between Naþ and Ni2þ ions. This can provide for an effective Ni2þ ion supply process to the polymer surface, generating the continuous reduction of Ni2þ ions to form nickel thin films (Figure 8C). The nickel thin films were highly adhesive with the poly(amic acid); the film readily passed the Scotch-tape test. This high adhesion is thought to be due to the interaction between poly(amic acid) and deposited nickel nanoparticles shown in Figure 6C. (38) Ikeda, S.; Yanagimoto, H.; Akamatsu, K.; Nawafune, H. Adv. Funct. Mater. 2007, 17, 889–897.
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A similar effect has been reported for the deposited copper films onto alkali-modified polyimide films using dimethylamine borane aqueous solution.38 This result suggests that the formation of nickel nanoparticles under nickel deposited thin films may be attributed to the high adhesion with the poly(amic acid) through a nanoscale mehanical interlocking effect. Fabrication of Copper Damascene Patterns on Polyimide. The present process for fabrication of copper damascene patterns involves the preparation of deposited nickel thin films on polymer substrate, followed by copper electrodeposition using a nickel film as the seed layer. Figure 9A and B show the top and crosssectional views of the polymer substrate obtained after copper electrodeposition. Conformal copper electrodeposition proceeds on nickel films with trench patterns, providing the copper deposited without voids and seams. However, a mechanical polishing for removal of excess deposited copper is important to form copper damascene patterns. After mechanical polishing, as shown in Figure 9C and D, only excess deposited copper is removed, whereas the deposited copper inside trench patterns remain unchanged, indicating the fabrication of copper damascene patterns onto poly(amic acid). Furthermore, the cross-sectional structure of the copper pattern consists of a sequence of squares; the copper patterns are of 5.5 μm width, the distance between copper patterns is 4.3 μm, and their height is 5.6 μm. The motifs in the copper patterns are nearly reproduced along the groove of the initial poly(amic acid). Thus, the size of copper damascene patterns can be controlled simply by altering the trench size of the PDMS mold. For practical applications, the poly(amic acid) substrate should be converted to polyimide in order to achieve reliable dielectric properties for metal/polyimide heterojunctions; thermal annealing is necessary after extracting remaining nickel ions and sodium cations from carboxylic groups to proceed imidization. The remaining nickel ions and sodium ions also cause serious problems in terms of the electrical conductivity and electromigration resistance. To accomplish this, the poly(amic acid) film with DOI: 10.1021/la101350f
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copper damascene pattern was immersed into 1 wt % aqueous HCl solution at 30 °C for 10 min and then rinsed in distilled water. In the FT-IR spectrum for the films after HCl treatment, since the bands newly appeared at 1720, 1410, and 1300 cm-1 (typical signals for carboxylic acid groups), the ions in the poly(amic acid) template were completely exchanged with protons by this treatment (results not shown). Subsequently, after the poly(amic acid) film was annealed under vacuum at the rate of 6 °C min-1 up to 300 °C and holding for 10 min to promote imidization, the feature size of copper patterns remained unchanged, but the typical distance between copper patterns decreased from 4.3 to 3.9 μm (ca. 9% shrinkage) and the polyimide surfaces and interface between polyimide and copper patterns became curved shapes (Figure 9E and F). This shrinkage and transformation of the polymer is caused by constriction of the polymer substrate during imidization. Therefore, for fabrication of desired copper patterns, the initial PDMS mold should be designed in consideration of polymer shrinkage during imidization. Figure 10 shows AES line profiles of the copper damascene patterns on polyimide. The nickel thin layer was observed around the interface between the copper and polyimide, but the nickel did not remain in the polymer. The resulting copper film has good resistivity of 1.69 μΩ cm, which is nearly consistent with the bulk resistivity of 1.67 μΩ cm, and is suitable for electronic devices. The copper damascene patterns after thermal treatment at 150 °C for 168 h under atmosphere (standard condition for practical application) were also highly adhesive to polyimide; the damascene patterns readily pass the Scotch-tape test (>0.32 kN/m). Additionally, to gain more insight information, we measured the peel strength of metallized copper films (thickness: 15 μm) on the nickel metallized poly(amic acid) substrate without trench patterns. The deposited copper films on the flat substrate showed an average adhesion strength of 0.76 kN/m. Therefore, the copper patterns embedded into the substrate may show a stronger adhesion as compared the film samples. This adhesion effect is due to not only a nanoscale mechanical interlocking effect of the nickel nanocomposite layer formed at the interfacial region, but also increasing of the interfacial region by fabrication of damascene structure, in addition to chemical interactions between nickel and polyimide.39 This fabrication process of copper damascene patterns on polyimide substrate may provide a (39) Faupel, F.; Willecke, R.; Thran, A. Mater. Sci. Eng., R 1998, 22, 1–55.
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Figure 10. (A) SEM image of the copper damascene patterns on polyimide. (B) AES line scan taken from the SEM image shown in (A).
promising alternative strategy for the fabrication of an extreme minute wiring on polyimide substrates.
Conclusion We have demonstrated the fabrication process of copper damascene wiring onto polyimide substrate involving the formation of trench patterns on the poly(amic acid) surface using imprint lithography, the fabrication of a nickel seed layer through the direct metallization method, and the formation of copper damascene structures by electrodeposition and polishing processes. Without any high-energy consumption, high-cost, and complicated process, fine copper damascene wiring with high adhesion to polymer substrate can be prepared by imprint technique and all-wet chemical metallization processes. Because the present approach allows fine-tuning of damascene structures, such as line and space width, we anticipate that this can be used as a general protocol to fabricate minute copper circuit patterns for the microelectronic industry.
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