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Electroless Deposition of Nanoscale Copper Patterns via Microphase-Separated Diblock Copolymer Templated Self-Assembly Robert W. Zehner and Lawrence R. Sita*,† Searle Chemistry Laboratory, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 Received June 16, 1999 By use of an ultrathin film of microphase-separated poly(styrene-block-methyl methacrylate) as a template, nanometer-scale patterns of tetraalkylammonium passivated palladium colloids can be generated. This pattern serves, in turn, as the template for the electroless deposition of semicontinuous 20 nm wide copper nanostructures, which is approximately a factor of 2 smaller than feature sizes that can be generated by state-of-the-art lithographic processes.
The controlled fabrication of arrays of metallic structures with feature sizes in the sub-micrometer range (i.e., nanostructures) is a topic of current interest as such constructs can display technologically important sizedependent properties such as giant magnetoresistance (GMR) and single electron effects, to name just a few.1 For the eventual large scale production of devices that are based on such properties, however, methods must be developed that can faithfully produce these arrays in a parallel fashion and, preferably, under ambient conditions. In this regard, we recently introduced a procedure for generating 25 nm wide patterns of n-alkanethiol passivated gold nanoparticles that utilizes an ultrathin (∼100 nm thick) microphase-separated diblock copolymer film as the template.2,3 Herein, we now demonstrate extension of this patterning technique to tetraalkylammoniumpassivated palladium colloids which subsequently serve as catalysts for the electroless deposition of copper metal. Careful control of this metallization process results in the production of semicontinuous copper patterns that are of the same width as the original polymer domains, or roughly 20 nm, which is a factor of 2 smaller than feature sizes that can be generated by state-of-the-art lithographic techniques, such as near-field optical lithography.4 Figure 1a shows a transmission electron microscope (TEM) image of a microphase-separated poly(styreneblock-methylmethacrylate) (PS-b-PMMA) ultrathin film that is supported on a TEM-transparent silicon nitride substrate.5,6 At the film thickness and PS:PMMA ratio of 3:1 used in this study, the polymer film is believed to † Present address: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742.
(1) For some recent examples, see: (a) Martin, C. R. Science 1994, 266, 1961-1966. (b) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323-1325. (c) Hidber, P. C.; Heibig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (d) Piraux, L.; Dubois, S.; Duvail, J. L.; Ounadjela, K.; Fert, A. J. Magn. Magn. Mater. 1997, 175, 127-136. (e) Bradley, C. C.; Anderson, W. R.; McClelland, J. J.; Celotta, R. J. Appl. Surf. Sci. 1999, 141, 210-218. (2) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H.; Sita, L. R. Langmuir 1998, 14, 241-244. (3) For an alternative method for the nanoscale patterning of barium titanate on block copolymers, see: Lee, T.; Yao, N.; Aksay, I. A. Langmuir 1997, 13, 3866-3870. (4) Aizenberg, J.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Appl. Opt. 1998, 37, 2145-2152. (5) Morkved, T. L.; Lopes, W. A.; Hahm, J.; Sibener, S. J.; Jaeger, H. M. Polymer 1998, 39, 3871-3875. (6) The PS-b-PMMA (Polysciences) used in this study had a Mw of 84 300, a PDI of 1.08, and a PS weight fraction of 0.74.
adopt a morphology comprising half-cylinders of PMMA surrounded by PS, which gives rise to the apparent lamella structure of repeating stripes of PS and PMMA domains.7 Further, as the two types of polymer domains have differing electron beam damage thresholds, the PS appears darker in the TEM image than the lighter PMMA.8 Finally, the two-dimensional Fourier transform (2D FT) of the image in Figure 1a provides a measure of the repeat distance of the phase-separated pattern, which in this case is 41 ( 1 nm, as determined from the one ring that is visible in the 2D FT spectrum (see inset of Figure 1a). Treatment of the phase-separated PS-b-PMMA film with a tetraoctylammonium-passivated palladium colloid (TOA@Pd) solution in ethanol produced TEM images similar to those previously observed for n-alkanethiolpassivated gold nanoparticles as shown in Figure 1b.2,9 With the exception of some large aggregates, which deposit nonspecifically, the vast majority of the particles lie on the PS domains with the specificity of the patterning for single particles being estimated to be 1:1000 or greater. Previous studies by Whitesides and co-workers1e,10 have shown passivated palladium colloids to be effective catalysts for electroless copper plating, and they have used this reactivity to produce sub-micrometer features on surfaces by patterning the colloids with microcontact printing (µ-CP). In a similar vein, the substrates represented by Figure lb were immersed in a standard copper plating solution for 30 s or less, and as Figure 1c reveals, this served to limit copper deposition to only a few tens of nanometers around each particle, thereby, producing semicontinuous copper nanostructures, apparently over only the PS domains. Unfortunately, since the contrast of the metal features by TEM is much greater than that of (7) Hahm, J.; Lopes, W. A.; Jaeger, H. M.; Sibener, S. J. J. Chem. Phys. 1998, 109, 10111-10114. (8) Due to electron beam damage that occurs during TEM, imaged substrates were not used for further experiments. Accordingly, images presented in Figure 1 are representative of samples at each step in the metallization process and do not represent the history of a single substrate. (9) Tetraalkylammonium-passivated palladium nanomaterial (∼1 nm in diameter) was prepared according to published procedures.1e A solution of this material was produced by dissolving ∼1 mg in 100 µL of tetrahydrofuran and then diluting this solution to a total volume of 3 mL with absolute ethanol. Treatment of substrates with this solution consisted of placing one drop onto the polymer film, letting it sit for 20-30 s, and then wicking from the surface by placing a tissue at the edge of the drop. (10) Hidber, P. C.; Nealy, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209-5215.
10.1021/la9907782 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/10/1999
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Figure 1. (a) TEM of microphase-separated PS-b-PMMA. The inset shows a 2D FT spectrum of the image where the bright ring corresponds to a spacing of 41 ( 1 nm.7 (b) TEM of TOA@Pd deposited on PS-b-PMMA film. (c) TEM of TOA@Pd deposited on PS-b-PMMA after immersion in electroless copper plating solution. The upper inset shows an enlarged view of the copper nanostructures, the lower inset is a 2D FT spectrum of the large image where the bright ring corresponds to a spacing of 43 ( 1 nm.
the underlying polymer, it is difficult to directly determine if the copper nanostructures are in direct registry with the morphology of the film. However, a 2D FT spectrum of Figure 1c (see inset) allows a comparison of the spacings of this pattern with those of the original film of Figure 1a, and as can be seen, the single ring observed corresponds to a repeat length of 43 ( 1 nm, which agrees, within experimental error, with the polymer structure. Accordingly, it is concluded that the surface morphology of a phase-separated diblock copolymer film can indeed be used as a template to direct the formation of a patterned array of metallic nanostructures in a straightforward fashion. Further studies shed additional light on important factors that control the metallization process and the nature of the metallic material that is deposited. To begin, a comparison of plating results for tetraoctadecylammonium- and TOA-passivated particles revealed that the shorter chain passivant gave uniform metallization, while the longer alkylammonium passivant resulted in the rapid deposition of copper on some particles, while others showed no signs of growth. On the basis of this finding, it is presumed that particles with defects in the passivating layer activate rapidly, and those with thick shells take much longer to activate than those with thin shells. To confirm the chemical composition of the resulting nanostructures, X-ray photoelectron spectroscopy (XPS) was performed on samples for each stage of the process with the data being summarized in Figure 2. Thus, the polymer film of Figure 1a (trace a in Figure 2) showed only C(1s) and O(1s) signals from the PS and PMMA and a small Si(2s, 2p) signal for the underlying silicon nitride support, but no Cu(2p) or Pd(3d) signals. With the addition of a Pd colloid pattern, the expected Pd(3d) peaks now appeared as shown by trace b in Figure 2. Importantly, the presence of this Pd photoelectron signal, as well as the catalytic reactivity of the particles, confirms that the particles do, in fact, lie at the surface of the polymer film after patterning rather than being “dissolved” within the polymer domain (cf. the penetration depth for XPS is ∼4 nm11). After copper metallization, Auger and photoelectron lines for Cu are now present in the spectrum while the Pd
Figure 2. XPS spectra of the Pd(3d) and Cu(2p) regions of polymer substrates at different stages in the patterning process: trace a, untreated microphase-separated PS-b-PMMA; trace b, TOA@Pd patterned onto PS-b-PMMA; trace c, TOA@Pd patterned onto PS-b-PMMA and after electroless deposition of copper.
signals are no longer evident (see trace c in Figure 2). This last result suggests that the colloidal material is
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completely encapsulated within the copper. Finally, regarding stability, comparison of the Cu(2p) regions of freshly prepared and 2 week old samples showed that significant copper oxidation occurs with time in air. In conclusion, we have demonstrated a new strategy for the generation of arrays of 20-nm scale metallic nanostructures that utilizes microphase-separated diblock copolymer films as the template. While this proof-ofconcept study was performed using ‘off-the-shelf’ materials, it can be anticipated that a tighter control of patterning can be achieved by chemically tailoring, and enhancing, domain/particle interactions. Further, as a range of different morphologies for phase-separated di- and triblock polymer films are known, and as film thickness, substrate, and external forces can be used to control alignment of (11) Barr, T. L. Modern ESCA: the principles and practice of x-ray photoelectron spectroscopy; CRC Press: Boca Raton, FL, 1994.
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morphological features12, a variety of metallic structures ranging from ordered arrays of nanowires to arrays of nanodots are conceivable. Studies directed towards these targets are now in progress. Acknowledgment. This work was supported by the MRSEC program under the National Science Foundation (DMR-9400379) for which we are grateful. We wish to thank Mr. Ward Lopes and Professor Heinrich Jaeger, University of Chicago, for fruitful discussions. LA9907782 (12) See, for example: (a) Karim, A.; Douglas, J. F.; Lee, B. P.; Glotzer, S. C.; Rogers, J. A.; Jackman, R. J.; Amis, E. J.; Whitesides, G. M. Phys. Rev. E 1998, 57, R6273-R6276. (b) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrics, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Science 1996, 237, 931-933. (c) Rockford, L.; Liu, Y.; Mansky, P.; Russell, T. P.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1999, 82, 2602-2605.
