Block Copolymer Templated Chemistry for the ... - ACS Publications

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Chem. Mater. 2007, 19, 5090-5101

Block Copolymer Templated Chemistry for the Formation of Metallic Nanoparticle Arrays on Semiconductor Surfaces Masato Aizawa*,† and Jillian M. Buriak*,†,‡ National Institute for Nanotechnology and Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2 ReceiVed May 22, 2007. ReVised Manuscript ReceiVed July 30, 2007

Precise positioning of metallic nanostructures on semiconductor surfaces is important for applications such as photovoltaics, photoelectrocatalysis, metal interconnects, sensing platforms, and many others. In this paper, we demonstrate the utilization of self-assembling diblock copolymer monolayer films, made up of polystyrene-block-poly(2- or 4-vinylpyridine) (PS-b-P2VP or PS-b-P4VP), to spatially direct an aqueous metal reduction reaction on semiconductor surfaces, a process we call galvanic displacement. The diblock copolymer forms hexagonal arrays of spherical micelles consisting of a P2VP or P4VP core surrounded by a PS corona. Two approaches were developed, termed method 1 and method 2, to deliver metal ions to the semiconductor interface in a spatially defined manner utilizing the diblock template. In method 1, a metal complex preloaded into the P4VP cores is spontaneously reduced on the surface to form hexagonally ordered metallic nanoparticles whose structures mirror the parent polymer templates. This approach was employed to produce ordered Ag nanoparticles on Ge(100), InP(100), and GaAs(100) surfaces. Method 2, on the other hand, involves coating the semiconductor surface with an unloaded self-assembled block copolymer monolayer, followed by immersion in a solution of the metal ions and additional reagents, if required. Method 2 is particularly useful to pattern semiconductor surfaces that require the presence of hydrofluoric acid (HF) as an etchant to initiate the galvanic displacement, including Si(100). Using method 2, Cu, Au, Pt, and Pd nanoparticles were patterned on the semiconductor surfaces. In addition, the apparent order of the self-assembled monolayers is better as compared to that of the preloaded block copolymers (prepared via method 1). Since the self-assembling nanostructures of the PS-b-P2VP or PS-b-P4P diblock copolymers can be inverted to a PS core surrounded by a P2VP or P4VP corona (the so-called core-corona inversion) in the presence of HF, patterns of the resulting metallic structures are influenced by this morphological shift. The effects of polymer morphology on the galvanic displacement is described, and as an alternative approach, metal ion reduction and polymer removal with hydrogen/argon plasma is outlined.

Introduction One of key ingredients for many future applications is the ability to precisely pattern nanoscale features on technologically relevant semiconductor surfaces such as silicon and germanium, as well as compound semiconductors such as gallium arsenide and indium phosphide. While photolithography continues to make seemingly unrelenting progress in the realm of patterning of sub-100 nm surface features using, for instance, shorter wavelength light sources such as extreme ultraviolet (EUV),1 excimer lasers,2 and synchrotron sources,3 the processes are challenging both technologically and economically. Alternative methods such as electron beam lithography,4 focused ion beam lithography,5 and scanning probe tip-mediated lithography such as dip-pen nanolithography (DPN)6 can be used to pattern sub-20 nm surface * Corresponding authors. E-mail: [email protected] (M.A.) and [email protected] (J.M.B.). † National Institute for Nanotechnology. ‡ Department of Chemistry.

(1) Gwyn, C. W.; Stulen, R.; Sweeney, D.; Attwood, D. J. Vac. Sci. Technol. B 1998, 16, 3142. (2) Chou, Stephen, Y.; Keimel, C.; Gu, J. Nature 2002, 417, 835. (3) Miyatake, T.; Li, X.; Hirose, S.; Monzen, T.; Fujii, K.; Suzuki, K. J. Vac. Sci. Technol. B 2001, 19, 2444.

features but are not necessarily suitable for large surface area fabrication due to their serial nature. To satisfy the need to prepare increasingly small metallic features on semiconductor surfaces, new synthetic strategies are required for patterning in parallel, with nanoscale accuracy, at minimal expense. One possibility that has been receiving a great deal of attention is the use of self-assembled (4) (a) Marrian, C. R. K.; Tennant, D. M. J. Vac. Sci. Technol. A 2003, 21, S207. (b) Pashkin, Y.; Nakamura, Y.; Tsai, J. S. Appl. Phys. Lett. 2000, 76, 2256. (c) Liu, K.; Avouris, P.; Bucchignano, J.; Martel, R.; Sun, S.; Michl, J. Appl. Phys. Lett. 2002, 80, 865. (d) Craighead, H. G. Science 2000, 290, 1532. (e) Ito, T.; Okazaki, S. Nature 2000, 406, 1027. (5) Matsui, S.; Kaito, T.; Fujita, J.-i.; Komuro, M.; Kanda, K.; Haruyama, Y. J. Vac. Sci. Technol. B 2000, 18, 3181. (6) (a) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30. (b) Hong, S.; Mirkin, C. A. Science 2000, 288, 1808. (c) Lenhert, S.; Sun, P.; Wang, Y.; Fuchs, H.; Mirkin, C. A. Small 2007, 3, 71. (d) Li, Y.; Maynor, B. W.; Liu, J. J. Am. Chem. Soc. 2001, 123, 2105. (e) Nyamjav, D.; Ivanisevic, A. AdV. Mater. 2003, 15, 1805. (f) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (g) Porter, L. A.; Jr.; Choi, H. C.; Schmeltzer, J. M.; Ribble, A. E.; Elliott, L. C. C.; Buriak, J. M. Nano Lett. 2002, 2, 1369. (h) Salaita, K. S.; Lee, S. W.; Ginger, D. S.; Mirkin, C. A. Nano Lett. 2006, 6, 2493. (i) Su, M.; Dravid, V. P. Appl. Phys. Lett. 2002, 80, 4434. (j) Zhang, H.; Amro, N. A.; Disawal, S.; Elghanian, R.; Shile, R.; Fragala, J. Small 2007, 3, 81. (k) Zhang, H.; Elghanian, R.; Amro, N. A.; Disawal, S.; Eby, R. Nano Lett. 2004, 4, 1649.

10.1021/cm071382b CCC: $37.00 © 2007 American Chemical Society Published on Web 09/25/2007

Metallic Nanostructures on Semiconductor Surfaces

block copolymer nanostructures as templates.7 Block copolymers can self-assemble into a broad range of homogeneous structures on surfaces (arrays of dots, horizontal cylinders, lamellae, and so on)7c,h,j-l,o,u,8 over large areas without specialized apparatus. Due to the ubiquity of polymers in silicon processing (photoresists, for instance), IBM has been pursuing the usage of self-assembled polymeric nanostructures due to their compatibility with standard semiconductor fabrication technology.7w,9 The IBM group has demonstrated the use of polystyrene-block-poly[methyl(methacrylate)] (PS-b-PMMA) self-assembled structures on silicon as a template for dry plasma etching (with O2 and with SF6 + O2) of the underlying silicon; the PMMA block is removed with glacial acetic acid, and the remaining PS nanostructure serves as an etch mask. Lines, dots, and pillars of silicon are achieved by this approach, and the silicon dot nanostructure may see use in flash memory devices.10 In addition to the regularity of their self-assembled nanostructure, multiblock copolymer constructs are intrinsically chemically selective due to the differing solubility and coordination properties of the blocks.7a,b,h,11 For instance, with the diblock copolymer poly(butadiene)-block-poly(ethylene oxide) polymer (PB-b-PEO), the late transition metal complexes KPt(C2H4)Cl3 and (CH3CN)2PdCl2 form π-complexes with the alkene groups in the PB group; no interaction is (7) (a) Antonietti, M.; Foerster, S.; Hartmann, J.; Oestreich, S. Macromolecules 1996, 29, 3800. (b) Antonietti, M.; Wenz, E.; Bronstein, L.; Seregina, M. AdV. Mater. 1995, 7, 1000. (c) Balsara, N. P.; Hahn, H. Chem. Nanostruct. Mater. 2003, 317. (d) Boeker, A.; Mueller, A. H. E.; Krausch, G. Macromolecules 2001, 34, 7477. (e) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. AdV. Mater. 2006, 18, 2505. (f) Cox, J. K.; Eisenberg, A.; Lennox, R. B. Curr. Opin. Colloid Interface Sci. 1999, 4, 52. (g) Cuenya, B. R.; Baeck, S.-H.; Jaramillo, T. F.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 12928. (h) Foerster, S. Top. Curr. Chem. 2003, 226, 1. (i) Hamley Nanotechnology 2003, 14, R39. (j) Hamley, I. W. Angew. Chem., Int. Ed. 2003, 42, 1692. (k) Castelletto, V.; Hamley, I. W. Curr. Opin. Solid State Mater. Sci. 2005, 8, 426. (l) Hamley, I. W., Introduction to block copolymers. In DeVelopments in Block Copolymer Sceince and Technology; Hamley, I. W., Ed.; John Wiley & Sons: West Sussex, England, 2004; p 1. (m) Haupt, M.; Miller, S.; Bitzer, K.; Thonke, K.; Sauer, R.; Spatz, J. P.; Mossmer, S.; Hartmann, C.; Moller, M. Phys. Status Solidi B 2001, 224, 867. (n) Jaramillo, T. F.; Baeck, S.H.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148. (o) Krausch, G.; Magerle, R. AdV. Mater. 2002, 14, 1579. (p) Lazzari, M.; Lopez-Quintela, M. A. AdV. Mater. 2003, 15, 1583. (q) Lin, Y.; Boeker, A.; He, J.; Sill, K.; Xiang, H.; Abetz, C.; Li, X.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Nature 2005, 434, 55. (r) Malenfant, P. R. L.; Wan, J.; Taylor, S. T.; Manoharan, M. Nature Nanotechnol. 2007, 2, 43. (s) Meli, M.-V.; Badia, A.; Gruetter, P.; Lennox, R. B. Nano Lett. 2002, 2, 131. (t) Raez, J.; Tomba, J. P.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 2003, 125, 9546. (u) Segalman, R. A. Mater. Sci. Eng., R 2005, 48, 191. (v) Spontak, R. J.; Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1999, 4, 140. (w) Thurn-Albrecht, T.; Schotter, J.; Kastle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (x) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2000, 12, 787. (y) Webber, S. E. J. Phys. Chem. B 1998, 102, 2618. (8) Ruzette, A.-V.; Leibler, L. Nature Mater. 2005, 4, 19. (9) (a) Guarini, K. W.; Black, C. T.; Zhang, Y.; Kim, H.; Sikorski, E. M.; Babich, I. V. J. Vac. Sci. Technol. B 2002, 20, 2788. (b) Black, C. T.; Bezencenet, O. IEEE Trans. Nanotechnol. 2004, 3, 412. (c) Black, C. T.; Guarini, K. W.; Breyta, G.; Colburn, M. C.; Ruiz, R.; Sandstrom, R. L.; Sikorski, E. M.; Zhang, Y. J. Vac. Sci. Technol. B 2006, 24, 3188. (d) Zeng, H.; Black, C. T.; Sandstrom, R. L.; Rice, P. M.; Murray, C. B.; Sun, S. Phys. ReV. B 2006, 73, 020402/1. (10) IBM, Demos New Nanotechnology Method to Build Chip Components. http://demino.watson.ibm.com/comm/pr.nsf/pages/ news.20031208_selfassembly.html, part of IBM Research Press Release. http://www.almaden.ibm.com/laborday/index.html

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noted with the PEO block.12 Anionic complexes such as AuCl4-, PtCl42-, and PdCl42- will selectively bind to a protonated poly(vinyl-2 (or 4)-pyridine) block (P2VP or P4VP) of different di- or triblock copolymers due to strong electrostatic interactions.7m,11a,b,g-i,13 Metal cations are also sequestered in the P2VP and P4VP blocks, presumably due to direct py-Mn+ binding interactions.7m,13 Therefore, in addition to the incredible array of structural diversity of selfassembled block copolymers, spatially defined chemical selectivity and sequestration is possible. In this paper, we demonstrate that self-assembled block copolymers on surfaces can template surface chemical (11) (a) Bronstein, L. M.; Sidorov, S. N.; Valetshy, P. M.; Hartmann, J.; Colfen, H.; Antonietti, M. Langmuir 1999, 15, 6256. (b) Bronstein, L. M.; Sidorov, S. N.; Zhirov, V.; Zhirov, D.; Kabachii, Y. A.; Kochev, S. Y.; Valetsky, P. M.; Stein, B.; Kiseleva, O. I.; Polyakov, S. N.; Shtykova, E. V.; Nikulina, E. V.; Svergun, D. I.; Khokhlov, A. R. J. Phys. Chem. B 2005, 109, 18786. (c) Foerster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195. (d) Sakai, T.; Alexandridis, P. Langmuir 2004, 20, 8426. (e) Sakai, T.; Alexandridis, P. Langmuir 2005, 21, 8019. (f) Sakai, T.; Alexandridis, P. Nanotechnology 2005, 16, 344. (g) Semagina, N. V.; Bykov, A. V.; Sulman, E. M.; Matveeva, V. G.; Sidorov, S. N.; Dubrovina, L. V.; Valetsky, P. M.; Kiselyova, O. I.; Khokhlov, A. R.; Stein, B.; Bronstein, L. M. J. Mol. Catal. A: Chem. 2004, 208, 273. (h) Sidorov, S. N.; Bronstein, L. M.; Kabachii, Y. A.; Valetshy, P. M.; Soo, P. L.; Maysinger, D.; Eisenberg, A. Langmuir 2004, 20, 3543. (i) Vamvakaki, M.; Papoutsakis, L.; Katsamanis, V.; Afchoudia, T.; Fragouli, P. G.; Iatrou, H.; Hadjichristidis, N.; Armes, S. P.; Sidorov, S.; Zhirov, D.; Zhirov, V.; Kostylev, M.; Bronstein, L. M.; Anastasiadis, S. H. Faraday Discuss. 2004, 128, 129. (12) Bronstein, L.; Kramer, E.; Berton, B.; C, B.; Forster, S.; Antonietti, M. Chem. Mater. 1999, 11, 1402. (13) (a) Aizawa, M.; Buriak, J. M. J. Am. Chem. Soc. 2005, 127, 8932. (b) Aizawa, M.; Buriak, J. M. J. Am. Chem. Soc. 2006, 128, 5877. (c) Boyen, H.-G.; Kastle, G.; Zurn, K.; Herzog, T.; Weigl, F.; Ziemann, P.; Mayer, O.; Jerome, C.; Moller, M.; Spatz, J. P.; Garnier, M. G.; Oelhafen, P. AdV. Funct. Mater. 2003, 13, 359. (d) Glass, R.; Arnold, M.; Bluemmel, J.; Kueller, A.; Moeller, M.; Spatz, J. P. AdV. Funct. Mater. 2003, 13, 569. (e) Glass, R.; Moeller, M.; Spatz, J. P. Nanotechnology 2003, 14, 1153. (f) Haupt, M.; Miller, S.; Glass, R.; Arnold, M.; Sauer, R.; Thonke, K.; Moller, M.; Spatz, J. P. AdV. Mater. 2003, 15, 829. (g) Haupt, M.; Miller, S.; Ladenburger, A.; Sauer, R.; Thonke, K.; Spatz, J. P.; Riethmuller, S.; Moller, M.; Banhart, F. J. Appl. Phys. 2002, 91, 6057. (h) Kaestle, G.; Boyen, H. G.; Weigl, F.; Ziemann, P.; Riethmueller, S.; Hartmann, C. H.; Spatz, J. P.; Moeller, M.; Garnier, M. G.; Oelhafen, P. Phase Transitions 2003, 76, 307. (i) Kaestle, G.; Boyen, H.-g.; Weigl, F.; Lengl, G.; Herzog, T.; Ziemann, P.; Riethmueller, S.; Mayer, O.; Hartmann, C.; Spatz, J. P.; Moeller, M.; Ozawa, M.; Banhart, F.; Garnier, M. G.; Oelhafen, P. AdV. Funct. Mater. 2003, 13, 853. (j) Koslowski, B.; Strobel, S.; Herzog, T.; Heinz, B.; Boyen, H. G.; Notz, R.; Ziemann, P.; Spatz, J. P.; Moller, M. J. Appl. Phys. 2000, 87, 7533. (k) Kuo, S.-W.; Wu, C.-H.; Chang, F.-C. Macromolecules 2004, 37, 192. (l) Moeller, M.; Hartmann, C. S.; Sihler, J.; Fricker, S.; Chan, V. Z. H.; Spatz, J. P. Polym. Mater. Sci. Eng. 2004, 90, 255. (m) Moeller, M.; Spatz, J. P.; Moessmer, S.; Eibeck, P.; Ziemann, P.; Kabius, B. Polym. Mater. Sci. Eng. 1999, 80, 3. (n) Moessmer, S.; Spatz, J. P.; Moeller, M.; Aberle, T.; Schmidt, J.; Burchard, W. Macromolecules 2000, 33, 4791. (o) Potemkin, I. I.; Kramarenko, E. Y.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P.; Eibeck, P.; Spatz, J. P.; Moeller, M. Langmuir 1999, 15, 7290. (p) Sohn, B.; Choi, J.; Yoo, S.; Yun, S.; Zin, W.; Jung, J.; Kanehara, M.; Hirata, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 6368. (q) Sohn, B.; Yoo, S.; Seo, B.; Yun, S.; Park, S. J. Am. Chem. Soc. 2001, 123, 12734. (r) Spatz, J. P. Angew. Chem., Int. Ed. 2002, 41, 3359. (s) Spatz, J. P.; Chan, V. Z. H.; Mossmer, S.; Kamm, F.-M.; Plettl, A.; Ziemann, P.; Moller, M. AdV. Mater. 2002, 14, 1827. (t) Spatz, J. P.; Eibeck, P.; Moessmer, S.; Moeller, M.; Kramarenko, E. Y.; Khalatur, P. G.; Potemkin, I. I.; Khokhlov, A. R.; Winkler, R. G.; Reineker, P. Macromolecules 2000, 33, 150. (u) Spatz, J. P.; Herzog, T.; Moessmer, S.; Ziemann, P.; Moeller, M. AdV. Mater. 1999, 11, 149. (v) Spatz, J. P.; Moessmer, S.; Hartmann, C.; Moeller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407. (w) Spatz, J. P.; Moller, M.; Ziemann, P. Phys. Blaetter 1999, 55, 49. (x) Spatz, J. P.; Roescher, A.; Moeller, M. AdV. Mater. 1996, 8, 337. (y) Spatz, J. P.; Roescher, A.; Sheiko, S.; Krausch, G.; Moeller, M. AdV. Mater. 1995, 7, 731. (z) Spatz, J. P.; Sheiko, S.; Moeller, M. Macromolecules 1996, 29, 3220.

5092 Chem. Mater., Vol. 19, No. 21, 2007 Scheme 1. Method 1 for Block Copolymer Templated Chemistry on Semiconductor Surfacesa

a

The diblock copolymer template, PS-b-P4VP, is shown in blue (core, P4VP) and red (corona, PS). The P4VP cores of the diblock copolymer preferentially coordinate metal precursor ions, Mn+, which are then reduced by the semiconductor surface via a galvanic displacement reaction (Scheme 3a). The pattern of the resulting metallic nanoparticles resembles that of the polymer template.

reactions by harnessing the specificity of one block over the other to bind and localize reagents in close proximity to the interface, as outlined in Schemes 1 and 2. The block copolymers, polystyrene-b-poly(2- or 4-vinylpyridine) (PSb-P2VP or PS-b-P4VP), adopt a quasihexagonal close-packed array of vertical P2VP or P4VP cylinders, surrounded by a PS corona. These arrays of cylinders template surface reactivity exclusively underneath the P2VP or P4VP block; following removal of the polymer, the result is a chemically functionalized interface with a pattern identical to that of theparenttemplate.Thechemistry,previouslycommunicated,13a is based upon a simple electrochemical reaction, termed galvanic displacement, in which sufficiently oxidizing metal ions such as Ag+ and AuIII (in the form of AuCl4-) are reduced by a semiconductor with which they are in direct contact.6q,14 According to mixed potential theory, a spontaneous reduction occurs, resulting in deposition of metallic nanoparticles bound to the interface, possibly via an intermetallic layer.15 If the oxidizing potential of the metal is insufficient or the galvanic displacement is slow, the metal ions encapsulated in a block may be reduced via brief plasma treatment; the polymer is also removed simultaneously in the plasma approach. Because the spacing is controlled by the block copolymer template, the average center-to-center (14) (a) Aizawa, M.; Cooper, A. M.; Malac, M.; Buriak, J. M. Nano Lett. 2005, 5, 815. (b) Nezhad, M. R. H.; Aizawa, M.; Porter, L. A., Jr.; Ribbe, A. E.; Buriak, J. M. Small 2005, 1, 1076. (c) Porter, L. A., Jr.; Choi, H. C.; Ribbe, A. E.; Buriak, J. M. Nano Lett. 2002, 2, 1067. (15) Magagnin, L.; Maboudian, R.; Carraro, C. J. Phys. Chem. B 2002, 106, 401.

Aizawa and Buriak Scheme 2. Method 2 for Block Copolymer Templated Chemistry on Semiconductor Surfacesa

a A semiconductor surface coated with PS-b-(P2VP or P4VP) is immersed in an aqueous solution of metal ions, Mn+, with or without hydrofluoric acid, HF (aq). (a) For the Au and Ag depositions in the presence of HF, galvanic displacement (Scheme 3b) occurs faster than the inversion of the diblock copolymer, termed core-corona inversion, to form hexagonal close packed arrays on the surface. (b) For the Cu, Pt, and Pd depositions in absence of HF (aq), the reduction of these metal precursor ions proceeds only within the original positions of the P2VP or P4VP cores, forming hexagonal arrays on the surface. (c) For the Cu, Pt, and Pd depositions in the presence of HF (aq), the metal ions associate with the inverted structure and are reduced by a hydrogen/argon plasma.

distances of the nanparticles can be controlled via modulation of the molecular weight of the polymer. The sizes of the metallic nanoparticles themselves can be modulated through variations in the deposition conditions, such as immersion time, metal ion concentration, and other factors. While preliminary data concerning this approach was communicated earlier, in this full paper we extend the approach beyond gold and silver to other metals, including copper, platinum, and palladium, and provide insight into the role of the block copolymer; observed morphological changes of the polymer in response to the aqueous solution have a critical effect on the shape and size of the metal deposition process. Experimental Section Generalities. Unless otherwise noted, all the experiments were performed under ambient laboratory conditions. Si(100) (p-type, B-doped, F ) 0.01-0.02 Ω‚cm, 500 µm thickness) wafers were purchased from Addison Engineering. Ge(100) (p-type, Ga-doped, F ) 0.018-0.036 Ω‚cm, 350 µm thickness) wafers were purchased from Umicore. InP(100) (p-type, S-doped, carrier density ) 4.6 × 1018 cm-3, F ) 0.01 Ω‚cm, 500 µm thickness) wafers were purchased from Crystacomm. GaAs(100) (p-type, Zn-doped, carrier density ) 1.5 × 1018 cm-3, F ) 0.027 Ω‚cm, 400 µm thickness) wafers were purchased from El-cat. AgNO3 (99.99995%), AgClO4‚ H2O (99%), KAuCl4‚xH2O, Na2PtCl4‚xH2O, Na2PdCl4‚3H2O (99%), and CuSO4‚5H2O (99.999%) were purchased from Strem Chemicals, whereas HAuCl4 (99.9995%) was purchased from Sigma-

Metallic Nanostructures on Semiconductor Surfaces

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Figure 1. AFM images and line profiles for diblock copolymers, PS-b-P2VP, with different molecular weights on Si(100). The scale bars correspond to 500 nm.

Aldrich. Diblock copolymers used in this study were polystyreneblock-poly(4-vinylpyridine) [PS(x)-b-P4VP(y)], with molecular weights of (x, y) ) (20 000, 19 000), (57 300, 24 700), and (128 400, 33 500), and polystyrene-block-poly(2-vinylpyridine) [PS(x)-bP2VP(y)], with molecular weights of (x, y) ) (48 500, 70 000), (91 500, 105 000), and (190 000, 190 000). All the diblock copolymers were purchased from Polymer Source (www.polymersource.com). Water was obtained from a Millipore system (resistivity ∼18.2 MΩ). Toluene used in this study was HPLC grade (SigmaAldrich), and the methanol was Optima grade (Fisher). HF (aq) (49%), H2O2 (aq) (30%), HCl (aq) (36.5-38.0%), and NH4OH (aq) (10-35%) were semiconductor grade, obtained from J. T. Baker. NH4F (40%) was purchased as an aqueous solution from RiedeldeHaen. All reagents listed above were used without further purification. Teflon beakers and tweezers were used exclusively during the cleaning and preparation of the Si wafers and for all metal deposition procedures. Pretreatment of Wafers. All wafers were diced into 7 mm2 pieces prior to use with a diamond scriber. Si(100) shards were degreased in a methanol ultrasonic bath for 15 min and dried with a nitrogen stream. The wafers were then cleaned via standard RCA cleaning procedures.16 The wafers were first immersed in a hot solution of H2O:NH4OH:H2O2 (5:1:1) for 15 min, and after rinsing of the wafers with excess water, they were immersed in a hot solution of H2O:HCl:H2O2 (6:1:1) for 15 min. The wafers were again rinsed with an excess amount of water. Following this cleaning procedure, water on the surface was immediately removed with a stream of nitrogen before polymer spin-coating. Ge(100) shards were degreased in a methanol ultrasonic bath for 15 min, in boiling dichloromethane for 10 min, and then a methanol ultrasound bath for 10 min. The oxide layer was removed with a solution of NH4OH:H2O (1:4) for 5 min17 After etching, the wafers were thoroughly rinsed with DI water and dried with a stream of nitrogen. GaAs(100) and InP(100) were degreased in hot acetone for 10 min and a methanol ultrasonic bath for 10 min. They were then thoroughly rinsed with methanol and dried under a nitrogen stream. Polymer Template Preparation. Polystyrene-block-poly(4vinylpyridine) (PS-b-P4VP) was dissolved in toluene at 70 °C, (16) Kern, W. Overview and evolution of semiconductor wafer contamination and cleaning technology. In Handbook of Semiconductor Wafer Cleaning Technology; Kern, W., Ed.; Noyes Publications: Park Ridge, NJ, 1993; p 3. (17) (a) Akane, T.; Tanaka, J.; Okumura, H.; Matsumoto, S. Appl. Surf. Sci. 1997, 108, 303. (b) Okumura, H.; Akane, T.; Matsumoto, S. Appl. Surf. Sci. 1998, 125, 125.

the selective solvent for the PS block, to make a 4-6 mg/mL solution, and then allowed to cool to room temperature. Polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) was dissolved in o-xylene with stirring for 1 h to make a 5 mg/mL solution. The solution was allowed to equilibrate for at least 1 day prior to use.18 Deposition Method 1. AgNO3 or HAuCl4 was dissolved in a toluene solution of PS-b-P4VP by stirring at least for 48 h. The molar ratio of AgI or AuIII to the pyridine groups in the P4VP block is 0.3:1. Ten microliters of the metal-loaded polymer solutions were spin-coated (Laurell, WS-400B-6NPP-Lite) on a 7 mm2 wafer shard at 4000 rpm for 40 s under an inert atmosphere nitrogen environment (located within a Vacuum Atmospheres glove box). After allowing the reaction to proceed for 10 min, the polymer templates were dissolved in a toluene ultrasound bath (Cole Palmer, 08891021) for 5 min and the shards dried under a nitrogen stream. Deposition Method 2. For PS-b-P4VP, 10 µL of the polymer solution was dropped on a 7 mm2 wafer and spin-coated at 2000 rpm for 40 s in an inert environment (same set up as above). For PS-b-P2VP, spin-coating was performed at 6000 rpm for 60 s in ambient conditions. The polymer-coated wafer was then immersed in a mixture of 1 mL of 1 mM metal salt (aq) and 9 mL of 1% or 0.25% HF (aq) for a given time [final solution concentration is therefore 0.1 mM metal salt (aq)/0.9% or 0.225% HF (aq)]. After metal deposition, the sample was thoroughly rinsed with water and dried under a nitrogen stream. For Au and Ag deposition, the PSb-P4VP (or PS-b-P2VP) templates were dissolved in a toluene (or o-xylene) ultrasound bath for 5 min, and the sample dried under a nitrogen stream. For the Cu, Pt, and Pd deposition, however, a H2/ Ar plasma (Harrick Plasma, PDC 32G, 18W) was employed to remove the polymers. Surface Characterization. The metal nanostructures on the semiconductor surfaces were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), scanning Auger microcopy (SAM), and X-ray photoelectron spectroscopy (XPS). SEM, SAM, and XPS were performed under high vacuum conditions (