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Fabrication of Metal Nanowires Using Microcontact Printing Matthias Geissler,† Heiko Wolf,† Richard Stutz,† Emmanuel Delamarche,† Ulrich-Walter Grummt,‡ Bruno Michel,† and Alexander Bietsch*,† IBM Research, Zurich Research Laboratory, Sa¨ umerstrasse 4, 8803 Ru¨ schlikon, Switzerland, and Friedrich-Schiller-Universita¨ t Jena, Institut fu¨ r Physikalische Chemie, Lessingstrasse 10, 07743 Jena, Germany Received March 18, 2003. In Final Form: May 2, 2003 Microcontact printing (µCP) is a versatile soft-lithographic technique to pattern substrates using an elastomeric stamp. We demonstrate the high-resolution capabilities of this technique for the fabrication of metal nanowires using either subtractive or additive patterning strategies. The subtractive method relies on printing a self-assembled monolayer (SAM) to protect a metal substrate selectively in a wetchemical etch process. We applied this approach to pattern Au, Ag, Cu, and Pd using eicosanethiol (ECT), and Al by printing hexadecanephosphonic acid (HDPA) as the resist-forming compound. As the etch process has to be selective and reliable, optimization of the etch chemistries is essential to obtain nanowires with excellent lithographic definition. The additive method involves the formation of wire template structures that can direct the electroless deposition (ELD) of a metal on a substrate. One variation of this approach entailed the patterning of a thin Au layer that was printed and etched to initiate ELD of Ag, Cu, and NiWP. Printing a colloidal Pd/Sn catalyst directly onto a substrate constitutes another variation of this patterning strategy. The use of a defined colloidal suspension as the ink, the derivatization of the stamp with poly(ethylene glycol) (PEG), and the pretreatment of the substrate with an amino-functionalized silane were the key elements of this approach, which was demonstrated for the fabrication of NiB and CoP nanowires. Devices with arrays consisting of 400-µm-long wires with 1 µm pitch were produced with these patterning strategies, and wire dimensions of 150-500 nm in width were achieved depending on the fabrication parameters. We have characterized the resulting nanowires using atomic force microscopy (AFM), determined their morphological properties, and addressed their electrical performance.
Introduction Metallic nanowires are present in many scientific and technological applications. They are used, for instance, in microelectronics, where the critical dimensions of interconnects shrink with the continuous trend to downscale the size of semiconductor devices such as integrated circuits or memory cells.1 They also serve in optics as diffractive gratings and polarizers,2,3 or as alignment features in liquid crystal displays.4 Furthermore, some metallic wires can act as sensor elements to detect H2, for example, with short response times.5 The standard technologies for the fabrication of wire devices in the submicrometer range are photolithography for high-throughput manufacturing and electron beam (e-beam) lithography for prototyping and high-resolution applications. These techniques rely on a radiation-sensitive layer of a polymeric resist, which is exposed and developed. The patterned resist is then used to mask the substrate selectively in subsequent etch or deposition steps. The * To whom correspondence should be addressed. Phone: 004117248469. Fax: 0041-17248966. E-mail:
[email protected]. † IBM Research, Zurich Research Laboratory. ‡ Friedrich-Schiller-Universita ¨ t Jena, Institut fu¨r Physikalische Chemie. (1) International Technology Roadmap for Semiconductors 2001, Interconnect. Online at http://public.itrs.net/Files/2001ITRS/Interconnect.pdf. (2) Schnabel, B.; Kley, E.-B.; Wyrowski, F. Opt. Eng. 1999, 38, 220226. (3) Schider, G.; Krenn, J. R.; Gotschy, W.; Lamprecht, B.; Ditlbacher, H.; Leitner, A.; Aussenegg, F. R. J. Appl. Phys. 2001, 90, 3825-3830. (4) Tanaka, M.; Nose, T.; Sato, S. Jpn. J. Appl. Phys. 2000, 39, 63936396. (5) Faver, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227-2231.
overall process can be positive or negative depending on the tone of the resist. The use of resist typically requires a number of specific processing sequences, starting with the application of adhesion promoters to the substrate and spin-coating the resist. Thermal annealing cycles are necessary later in the process. The patterns have to be developed in solvents after the exposure of the resist, which is postbaked before an etch or metallization step and then stripped by solvents or a plasma treatment. The fact that this type of lithography requires complex and costly facilities makes alternative concepts attractive, especially ones that simplify the manufacturing process or allow applications that are not compatible with resist or solvent chemistry. High-resolution microcontact printing (µCP) is one such promising alternative. It allows the simultaneous transfer of large-scale patterns with dimensions ranging from several millimeters down to less than 100 nm with good accuracy on a 4 in. wafer level, in addition to being time- and cost-efficient.6,7 Microcontact printing is a soft-lithographic technique that uses a micropatterned elastomeric stamp, which is inked, dried, and placed on a substrate to apply a reactant at the regions of contact.6,7,8 The material that forms the stamp has to be elastomeric to ensure an intimate (conformal) contact between the stamp and the substrate during printing.9 Thus far, poly(dimethylsiloxane) (PDMS) (6) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 551-575. (7) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. Dev. 2001, 45, 697719. (8) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (9) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310-4318.
10.1021/la034464x CCC: $25.00 © 2003 American Chemical Society Published on Web 06/17/2003
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Figure 1. There are two direct patterning strategies to fabricate arrays of metal nanowires using µCP. In the first one, (A) “print & etch”, a resist (usually a self-assembled monolayer) is printed on a metallic layer, and the pattern is transferred into the substrate by an etch process. The second concept (B) is additive, and relies in this case on the selective electroless deposition of a metal (or alloy) onto a catalytic seed pattern on a substrate. One variant of this approach, “print, etch & plate”, relies on patterning a seed layer using the concept shown in part A. The other variant, “print & plate”, entails the direct printing of a catalyst onto a substrate to activate those regions where ELD should occur.
is the material of choice for µCP because it is commercially available, resistant to many types of chemicals and pH environments, optically transparent, and nontoxic. PDMS stamps can be reused many times without noticeable degradation, and their hydrophobic surface can be hydrophilized using an O2 plasma treatment or by grafting silanes.10 Stamps are usually fabricated by curing the liquid prepolymers of PDMS on a patterned mold (master). Microcontact printing has been demonstrated for a variety of inks and substrates including self-assembled monolayers (SAMs) of alkanethiolates on metals,6,8 silanes on oxides,11,12 colloids13 and reactants14 printed onto organic or inorganic layers, and proteins transferred to silicon or glass.15,16 The printed patterns can drastically change the surface characteristics of a substrate and can be used as resists for selective wet etching,6,8,17,18 as templates for metallization13,14 or crystal growth,19 or to influence the wetting and adhesion properties of the substrate,20,21 for example. We exploit the high-resolution capabilities of µCP to fabricate nanowires using two patterning strategies (Figure 1). The first method, called “print & etch”, is illustrated in Figure 1A and corresponds to the most prominent application of µCP, which is the printing of alkanethiols onto Au.6-8 Alkanethiols rapidly chemisorb (10) Donzel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel, B.; Hilborn, J.; Delamarche, E. Adv. Mater. 2001, 13, 1164-1167. (11) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576-9577. (12) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382-3391. (13) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (14) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367-6373. (15) Bernard, A.; Delamarche, E.; Schmid, H.; Michel, B.; Bosshard, H. R.; Biebuyck, H. Langmuir 1998, 14, 2225-2229. (16) Renault, J. P.; Bernard, A.; Bietsch, A.; Michel, B.; Bosshard, H. R.; Delamarche, E.; Kreiter, M.; Hecht, B.; Wild, U. P. J. Phys. Chem. B 2003, 107, 703-711. (17) Geissler, M.; Schmid, H.; Bietsch, A.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2374-2377. (18) Xia, Y.; Zhao, X.-M.; Kim, E.; Whitesides, G. M. Chem. Mater. 1995, 7, 2332-2337. (19) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495-498. (20) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60-62. (21) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 27902793.
on Au surfaces via their thiol functionality, and the alkyl chains self-assemble into a dense and ordered monolayer.6,22,23 The chemisorbed monolayer, although only 2-3 nm thick, can provide a resist to fabricate thin Au structures using a wet-etch process.24 In addition to Au, this method can also be applied to other metals including Ag,25 Cu,17 and Pd.26,27 A similar process works with alkanephosphonic acids on metal oxide surfaces such as Al/Al2O3.28 In all cases, the development of etching systems that provide sufficient selectivity is essential. The second method uses the opposite strategy to “print & etch”; that is, the metal is deposited selectively onto the printed regions of a substrate (Figure 1B). This is achieved by patterning a catalytic seed layer using µCP, which then initiates the deposition of a metal from solution by means of electroless deposition (ELD). As an alternative to vacuum deposition processes, ELD is a method for obtaining films of a variety of metals or alloys on insulating substrates at low cost.29,30 This method uses metastable solutions containing complexed metal ions (as the source of the metal) and a reducing agent. When immersing a substrate that is activated with a proper catalyst into an ELD solution, the reducing agent supplies the electrons for converting the metal ions into the metal at the catalytic sites on the surface. The metal itself must also be catalytic so that the deposition can continue. We used two concepts for patterning a seed layer on a substrate using µCP. In one approach, called “print, etch & plate”, a layer catalytically active for ELD is present on the entire surface (22) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719-729. (23) Larsen, N. B.; Biebuyck, H. A.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017-3026. (24) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (25) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996, 143, 1070-1079. (26) Carvalho, A.; Geissler, M.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2406-2412. (27) Love, J. C.; Wolfe, D. B.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 1576-1577. (28) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182-1191. (29) Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (30) Paunovic, M.; Schlesinger, M. Fundamentals of Electrochemical Deposition; John Wiley & Sons: New York, 1998; pp 133-160.
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Figure 2. Optical microscope image and AFM inset of a nanowire test device. The 400 × 400-µm2-sized wire array consists of 400 individual wires (here ∼240 nm in width) spaced by 1 µm. The wires on the right-hand side of the array are interrupted in the middle by a small gap, thus forming openended, comblike structures. Void areas are filled with support posts, and guiding lines facilitate inspection of the sample. Large-scale electrodes and contact pads are designed as interconnect mesh. The Au structures have a thickness of ∼120 nm and were fabricated using the “print & etch” approach. A 0.2 mM solution of ECT provided the ink, and Fe3+/thiourea was used as the etch system to develop these patterns faithfully.
of the substrate and is subjected to a print-and-etch process. Alternatively, a catalyst is directly printed on the substrate, as has been demonstrated in previous studies13,14 and which is referred to as the “print & plate” approach. In this article, we discuss the various patterning strategies and demonstrate the fabrication of wires with sub-micrometer dimensions for a variety of metals and alloys. We discuss the morphological properties of these wires and address their electrical performance. Results and Discussion Design of the Pattern. The lithographic area was 40 × 40 mm2 with 16 design units; each design unit was 10 × 10 mm2 in size and included 16 high-resolution fields of 400 × 400 µm2, which contained dots, meshes, or wires with dimensions ranging from 100 to 600 nm.7 Here, we focus on the fabrication of a nanowire unit shown in Figure 2. The etch chemistry used to pattern this ∼120-nm-thick Au film will be described below. The central array contains wires with 100 nm design width and 1000 nm pitch. Half of the wires define an open-ended comblike arrangement, whereas the other half bridges the entire length of the array (400 µm). Support posts (2 × 2 µm2, 8 µm pitch) are required to ensure the mechanical stability of large void areas, thus preventing the stamp from collapsing in these regions during printing. Guiding lines, in addition, help locate the array during inspection of the sample. The largescale interconnect structures suitable as contact electrodes for electrical testing were designed as mesh patterns (i) to have a lower writing density for e-beam lithography during master fabrication, (ii) to ensure printing homo-
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geneity at lower ink concentrations without compromising the printed structures by geometrical effects,31 and (iii) to have a midresolution control pattern. A master was fabricated from a 4 in. silicon-on-insulator (SOI) wafer using e-beam lithography and reactive-ion etching to transfer the pattern into a 350-nm-thick Si top layer, for which the underlying oxide acted as an etch stop. All features were etched to the same depth with vertical side walls, as verified in control experiments using scanning electron microscopy (SEM). Stamp Fabrication and Metrology. We fabricated thin-film stamps by injection molding, and the stamps were supported on a thin flexible glass back-plane. The material used for molding had a Young’s modulus of 1012 MPa, which is beneficial for replicating high-resolution stamps (experimental details are provided in the section Materials and Methods).32 The topographies of the highresolution structures on the master and on the stamp have been characterized by atomic force microscopy (AFM) (Figure 3). The trenches in the master shown in Figure 3A reveal a width of 100 nm, a depth of 350 nm, and a pitch of 1000 nm. The width of the trenches could not be accurately verified because of tip convolution effects, which are always present in AFM imaging of high-aspect-ratio structures. The grooves in the master were replicated as ridges on the PDMS stamp (Figure 3B). Chemical and thermal shrinkage of the polymer slightly reduced the volume of the features, and surface tension led to the rounding of edges that were sharp in the mold.9,33 The convolution artifact of the AFM tip virtually broadened these ridges, but the image of the cap was quite accurate because we used a sharp tip. The ridges had a height of 335 nm, and the cross section in Figure 3B illustrates the rounded profile of the cap. On the basis of this profile, we evaluated the contact width during printing: we approximated the rounded ridge by a cylinder with a radius of r ) 95 nm having a Young’s modulus of E ) 12 MPa and a Poisson number of ν ) 0.3. The width d of the contact to a substrate during printing was derived from the Hertz theory:34
d)2
x
4(1 - ν2)(pλ + 2w)r πE
(1)
where λ is the pitch of 1 µm. With an applied external pressure of p ) 100 Pa and a typical work of adhesion of w ) 0.2 J m-2 to a hard surface, we calculated a contact width during printing of d ) 120 nm. “Print & Etch” Approach. Alkanethiols can, in principle, form SAMs of high quality on Au, Ag, Cu, and Pd either by adsorption from solution or by printing. Forming a well-ordered and dense monolayer by µCP requires a sufficient amount of molecules on the substrate as well as sufficient time for their delivery by the stamp and assembly on the surface. It is difficult to restrict the formation of a SAM to the regions of contact because ink molecules tend to diffuse away from these zones and alter the accuracy of the pattern. The diffusion front of the printed thiols, the morphology of the substrate (grain sizes, crystallinity, and impurities), and the etch chemistry all contribute to the final edge definition of the structures. We selected eicosanethiol (ECT) (MW ) 314.62 g mol-1) (31) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. A. J. Phys. Chem. B 1998, 102, 3324-3334. (32) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042-3049. (33) Huj, C. Y.; Jagota, A.; Lin, Y. Y.; Kramer, E. J. Langmuir 2002, 18, 1394-1407. (34) Johnson, K. L. Contact Mechanics; Cambridge University Press: Cambridge, U.K., 1985.
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Figure 3. AFM topographies of the SOI master and elastomeric stamp imaged with a sharp tip. (A) The high-resolution grooves are 100 nm wide and 350 nm deep and have a pitch of 1 µm. (B) The corresponding ridges on the stamp appear to be broadened and have a height of 335 nm and a rounded profile. From the cross-section images we estimated a contact width of 120 nm during printing according to the protocol described in the text.
because this thiol is less diffusive than its shorter analogues, such as hexadecanethiol or octadecanethiol. Previous studies showed that limiting the concentration of ECT generally resulted in more accurate patterns than higher concentrations did.31 For this reason, we used a 0.2 mM solution of ECT in ethanol as the ink to perform a set of printing experiments. Ethanol is commonly used as a solvent for alkanethiols because it is only slightly soluble in PDMS, which keeps the distortion of patterns due to swelling effects small, and it allows the stamp to be dried easily without leaving noticeable drying traces. The stamps were inked for 3 min, dried under a stream of N2, and subsequently placed on the various metal substrates for 2 min in all cases. An etch process is necessary to transfer the printed pattern into the underlying metal substrate. The selectiv-
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ity of this process largely determines the contrast of the final metal pattern. Wet-chemical etch systems for metals must contain an oxidizing agent to oxidize the metal and a complex-forming ligand to dissolve the oxidized metal from the surface. It is preferable to have a stable etch bath with a reasonable etch rate of a few tens of nanometers per min. Au is a challenging substrate because it is noble and the etch chemistries are limited in comparison to those of other metals and alloys. The etch system employed here is based on thiourea35 as the coordinating ligand and contains Fe3+ as the oxidant. The Fe3+/thiourea etch system is superior to CN-/O2 because the bath can be operated within a wide range of pH, the etch rate can be adjusted by the concentration of Fe3+ and thiourea in solution, and, most importantly, the selectivity is higher.36 We found empirically that a solution composed of 20 mM Fe(NO3)3 and 30 mM thiourea in deionized (DI) water, adjusted to pH 2.0 using HCl, proved selective for etching 100-nm-thick Au films that were microcontactprinted under the conditions listed above. The printed monolayer pattern was transferred into the Au with excellent contrast, as revealed by the pattern described in Figure 2. We imaged individual Au wires using AFM to determine their morphological properties (Figure 4A). The structures did not have visible defects, and the Au was removed from the nonprinted regions without leaving any noticeable background. Ag is chemically more reactive than Au and can be etched by combining a variety of oxidizers and ligands. Interestingly, the Fe3+/thiourea system again proved selective and reliable to obtain 100nm-thick Ag wires with excellent definition (Figure 4B). The composition of the solution was similar to the one used for etching Au except that it did not contain HCl and that it was employed at its initial pH of 2.25. In contrast to Au and Ag, the Fe3+/thiourea system showed insufficient selectivity when applied to microcontact-printed Cu substrates. We achieved selectivity by utilizing the sodium salt of 3-nitrobenzenesulfonic acid (NBSA) as the oxidizer and a branched polyethylenimine (PEI) as the coordinating ligand.17 Here, the size and the constitution of the ligand are the principal determinants for the selectivity that the system can offer (Figure 4C). Whereas SAMs formed on Au, Ag, and Cu surfaces serve as physical barriers for the etchants, SAMs on Pd behave more like a chemical barrier forming at the interface between the thiol molecules and the Pd.27 This difference accounts for the surprising etch resistance that a SAM can confer to a Pd substrate in a standard Fe3+/Cl- bath. The diffusion of thiols during printing has to be minimized to prevent undesired protection of the nonprinted regions of the substrate. The wire pattern, for example, printed using similar conditions to those for the other metals could not be developed with good contrast, because a noticeable background was present between the Pd structures. Attempts to dissolve or remove this background without damaging the wire array failed, but keeping the printing time short proved helpful in reducing it. The Pd nanowires shown in Figure 4D were fabricated using an ink solution with a slightly higher concentration of ECT, 0.3 mM, but printing the stamp for a shorter time, 20 s, onto the Pd substrate, followed by etching in an Fe3+/Cl- bath. Inspection of the Pd patterns using AFM revealed some background, which could be removed by cleaning the substrate first in a (35) Zhang, H.; Ritchi, I. M.; La Brooy, S. R. J. Electrochem. Soc. 2001, 148, D146-D153. (36) We observed that an etch bath with dissolved Fe3+ and thiourea remains reliable for a few hours but tends to decompose at least partially within 1 day when kept under ambient conditions.
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Figure 4. AFM images of (A) Au, (B) Ag, (C) Cu, and (D) Pd nanowires on oxidized Si substrates produced by the “print & etch” approach using ECT as the ink. The printed SAMs perfectly protected the underlying metals and preserved their initial thickness (at least 100 nm) during the etch process. The width of the wires varied due to surface diffusion of ECT molecules during printing and the duration of the etch process. The horizontal lines indicate the location of the cross-section profiles shown in the insets.
solution of 1% HF for 30 s and then with ethanol in an ultrasonic bath. We also envisaged fabricating Al nanowires by microcontact printing hexadecanephosphonic acid (HDPA), which forms the etch resist. There are several issues, however, that render this approach more difficult than printing alkanethiols on Au. Inking an alkanephosphonic acid onto a PDMS stamp is challenging because the polar acid group prevents the diffusion of molecules into the PDMS. Consequently, no reserve of ink molecules exists in the bulk of the stamp. We circumvented this problem by using a 10 mM solution of HDPA in ethanol to ink the stamp for 1 min. The stamp was then dried with a stream of N2 and printed onto an Al substrate for 3 min. Alkanephosphonic acids are known to readily adsorb on the native oxide that forms on the surface of Al substrates,37 but the interaction between the acid and the oxide is weak. The resulting monolayers are not stable enough to protect the underlying Al against etchants because the molecules can be easily displaced or removed from the substrate. A baking step at 120 °C for 1 min was necessary to form a more stable layer that can be used as a resist.28 The printed pattern became visible during baking. Inhomogeneities indicated the partial presence (37) Folkers, J. P.; Gorman, C. P.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813-824.
of multilayers in the printed regions of the substrate. Although it is believed that a P-O-Al bond is formed by subsequent dehydration and condensation reactions that occur during heating, the morphology of such films is not yet understood clearly. The substrate was allowed to cool to room temperature before being immersed into the etch solution. Interestingly, we could apply the NBSA/PEI system to etch Al substrates, and we were thus able to fabricate nanowires with good accuracy and reasonable contrast (Figure 5). Inspection of the printed devices using optical microscopy revealed a relatively low defect density for Au, Ag, Cu, and Pd wires. The yield of wires that were intact over their entire length usually exceeded 90% for all of these metals, which corresponds to an estimated density of one defect every 4 mm. The defect density for Al wires was much higher, and more than 50% of the wires had at least one defect. The predominant type of defects was in the range of a few hundred nanometers and was limited to individual wires without affecting adjacent wires. There is no indication that the stamps used for printing were damaged at these locations, and we believe that irregularities in the initial metal layer may be the reason for such defects. A second type of defects were circular voids (several micrometers in diameter) interrupting a set of wires. We attribute this finding to dust particles
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Table 1. Selected Properties of Nanowires Fabricated Using the “Print & Etch” Method
metal
printed resist
contact width (nm)
wire width (nm)
wire thickness (nm)
typical grain size, diameter (nm)
rms roughnessa (nm)
wire resistance (Ω µm-1)
specific resistivity, wire/film/bulkc (µΩ cm)
Au Ag Cu Pd Al
ECT ECT ECT ECT HDPA
120 120 120 120 120
237 ( 21 198 ( 18 226 ( 27 152 ( 14 210 ( 16
118 116 122 116 100
70 51 34 30 73
1.9 2.3 0.8b 1.2 2.4
2.2 2.7 1.0 11.8 1.5
6.2/3.3/2.2 6.2/ 2.7/1.6 2.8/2.2/1.7 20.7/19.4/10.5 3.1/3.0/2.6
a Values determined by AFM measurements on the blanket metallic film. b Roughness of the Cu after stripping the native oxide by immersion of the substrate into a solution of 3% HCl in DI water for ∼10 s. The roughness slightly increased during this procedure. c Specific resistivity at 20 °C. Source: Handbook of Chemistry and Physics, 78th ed.; Lide, D. R., Ed.; CRC Press, New York, 1997.
Figure 5. AFM image of Al nanowires on an oxidized Si wafer. The wires were fabricated by the “print & etch” approach using HDPA as the ink. Although this compound is not a favorable ink for µCP, the wires have good contrast and accuracy.
present on the stamp or on the substrate compromising the contact during printing in these regions. Working in a clean-room environment could reduce this type of defects. Properties of Nanowires Fabricated by “Print & Etch”. The geometrical dimensions and the electrical resistances of the nanowires fabricated by the “print & etch” approach are summarized in Table 1. All wires were printed using identical stamps having a contact line width of 120 nm, as discussed above. Wire width and thickness were extracted from the AFM data of the images shown in Figures 4 and 5. The widths of the wires have been measured and averaged at half the thickness and then reduced by the tip convolution effect. The variations of the wire width given in the table are root-mean-square (rms) values, which quantify the edge roughness. These values were of the order of 10% of the wire diameters. The characteristics of the wires do not depend only on the lateral extension of the printed SAM but also on the details of the etch process. The printed structures are attacked laterally with extended etching, which reduces the wire diameters and may also lead to an increased edge roughness. Although we used identical conditions for printing Au, Ag, and Cu, a direct comparison of the wire morphologies is barely possible because different etch systems have been employed. We found, however, that Au and Cu wires have almost the same width, whereas Ag wires are ∼10% narrower. This might be an indication that the oxidized silver surface suppresses lateral diffusion and consumes more ECT molecules than Au.31 In the case of Cu, the oxide was stripped off prior to printing by immersing the substrate into a diluted solution of HCl.17 Grain sizes and rms roughness were extracted from AFM data of the blanket metallic films assuming that the
morphology of the deposits was preserved in the process. The values for the grain sizes were determined by dividing the image area by the number of grains, which we obtained using a counting algorithm. We found, in agreement with previous studies, that the edge of the wires followed the granular structure of the metal film, suggesting that most etch systems are sensitive to grain boundaries.38 Highresolution µCP should therefore benefit from a relatively small grain diameter because the grain size of a substrate largely determines the achievable resolution. However, we did not optimize the various processing parameters during film fabrication to generally obtain small grains. When the film thickness and grain sizes are of the order of the electron mean free path, the effective resistivity of thin films is noticeably higher than the specific resistivity of bulk material and typically increases with the inverse of the thickness.39-41 However, resistivities of thin films are difficult to predict in general because they depend on the details of the film morphology, which are influenced by the deposition parameters. Additional size effects may show up for narrow wires.42 To measure the resistance of an individual wire, we reduced the wire array by cutting all wires except one using a focused ion beam (FIB) tool. We measured the resistance of a single 400-µm-long wire by two-point probing. Resistances of contacts and leads were measured by control experiments and subtracted from the data. The wire resistances normalized per micrometer are given in Table 1. For comparison, we determined the film resistances using a four-point probe setup and calculated the effective resistivities of the wires using the cross-sectional dimensions from the AFM data. We found that the values of the wire resistivities were consistently greater than those of the blanket metal films, which were in turn larger than the specific resistivities of the bulk materials taken from the literature. We attribute the discrepancy between wires and film to underetching and to a partial size effect due to the lateral constriction of the wires. The difference can be relatively small, however, and we conclude that film thickness and grain boundaries are the dominant reasons for the increase of resistivity, whereas lateral size effects are small for 150-250-nm-wide wires. “Print, Etch & Plate” Approach. The method described here relies on the selective deposition of a metal onto a substrate using ELD. As ELD necessitates the presence of a catalyst on the surface of the substrate, a localized deposition of a metal becomes possible by patterning a catalytic seed layer to define those regions where ELD should occur on the substrate. We verified that this is possible by using a 5-nm-thick Au film on an (38) Bietsch, A.; Michel, B. Appl. Phys. Lett. 2002, 80, 3346-3348. (39) Sondheimer, E. H. Adv. Phys. 1952, 1, 1-42. (40) Mayadas, A. F.; Shatzkes, M. Phys. Rev. B 1970, 1, 1382-1389. (41) Vancea, J.; Hoffmann, H.; Kastner, K. Thin Solid Films 1984, 121, 201-216. (42) Durkan, C.; Welland, M. E. Phys. Rev. B 2000, 61, 14215-14218.
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Figure 6. AFM images revealing Au seed lines (A) and the final electroless-deposited Ag, Cu, and NiWP wires on oxidized Si substrates, (B), (C), and (D), fabricated using the “print, etch & plate” process. The catalytic Au lines shown in part A were formed by microcontact-printing ECT (0.1 mM in ethanol) for 5 min onto a ∼5-nm-thick Au layer obtained from e-beam evaporation, followed by etching the printed substrate in a CN-/O2 bath (buffered at pH 12) for 4 min. The ECT was removed using an O2 plasma treatment (200 W, 2 min) prior to plating. ELD of the various metals onto the Au was achieved by immersing the substrate into an appropriate plating solution. The formation of Ag wires was challenged by background deposition, whereas plating of Cu and NiWP led to wires with excellent definition.
oxidized Si wafer (primed with 1 nm of Ti and prepared by e-beam evaporation) as the seed layer. Au was selected for this approach because (i) it is an excellent catalyst for many ELD systems, (ii) it can be patterned with high accuracy by µCP using print-and-etch schemes, and (iii) the printed monolayer resist can be removed from the patterned Au by an O2 plasma treatment without diminishing its catalytic activity. The Au seed layer was patterned by printing using a 0.1 mM solution of ECT in ethanol as the ink and etching it in a CN-/O2 bath buffered at pH 12.31 We inspected the printed patterns by AFM and found Au grains as large as 100 nm in width and 40 nm in height (Figure 6A). The initial Au layer had a uniform grain size of ∼10 nm. We attribute this finding to a dewetting of the Au, which was possibly promoted by a partial loss of the Ti adhesion layer due to the high pH of the CN-/O2 bath. Interestingly, the periphery of the lines remained accurate, and dewetting phenomena were more distinct inside the Au pattern. The substrate with the catalytic Au wires was exposed to an O2 plasma prior to plating to remove the ECT monolayer from the Au and to make its surface accessible to reactants in the ELD bath. The fabrication of nanowires using patterned Au templates was demonstrated for Ag, Cu, and NiWP (Figure
6B-D, respectively). ELD of these materials was achieved by immersing a freshly cleaned catalytic Au substrate into a plating solution. A commercial staining bath was used for the deposition of Ag. Like many other silvering solutions, this bath had a limited lifetime and became cloudy as soon as the plating process started. We consequently expected low selectivity of the metallization process due to the uncontrolled deposition of Ag colloids from solution onto the surface of the substrate. Although the Ag deposited preferentially on the Au, we indeed found that colloids were present on the entire substrate. It was possible to remove some of these colloids by cleaning the substrate with ethanol in an ultrasonic bath. The grains of the final Ag wires shown in Figure 6B have an irregular shape and did not form a very dense and uniform deposit. We achieved better results when plating Cu using a commercial alkaline bath, which contained formaldehyde as the reducing agent. This bath had a longer lifetime than the silver staining solution, thus providing a selective and reliable plating process. The Cu wires have high uniformity, and no background deposition was detected on the bare substrate (Figure 6C). Cu was deposited from this bath with high purity, and its properties can be similar to those obtained from e-beam evaporation or electroplating. In contrast to the cases of Ag and Cu, electroless Ni
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deposits are typically alloys, which can be divided into two groups, depending on the reducing agent used in the deposition process. Ni-P alloys are deposited using hypophosphite as the reducing component, whereas Ni-B codeposits are obtained using B-based reducing agents such as dimethylamine borane (DMAB). The chemical composition of the different Ni alloys depends strongly on the composition of the bath, its temperature, and pH. In this example, we used a hypophosphite-based Ni plating bath, which, in addition, contained sodium tungstate, to deposit a ternary alloy of NiWP.43 Figure 6D reveals that the deposition of NiWP was limited to the catalytic zones, and the image shows that high-quality wires were formed on the substrate. Optical inspection of the plated wire arrays showed that the practical yield of intact wires can exceed 90%, although the Au seed pattern consisted of individual grains that are separated from each other. As the patterns are amplified during the metal deposition, defects in the Au become invisible in the final structures, and small gaps interrupting an individual wire might even be closed during this process. Conversely, a minimum separation distance might be necessary to prevent individual structures from overplating. “Print & Plate” Approach. A catalyst can be applied directly to the substrate by printing, which has the advantage that an etch step is no longer necessary. We used a colloidal Pd/Sn catalyst to form the seed patterns on an oxidized Si wafer. Pd/Sn colloids can be obtained by mixing PdCl2 and SnCl2 in acidic solutions, and they are widely used as catalyst to activate nonconductive substrates for ELD.44-46 These colloids comprise a Pd-rich core and a shell of Sn2+ and Sn4+ species47 to protect the Pd against oxidation (oxidized Pd is not catalytically active) and agglomeration when kept in acidic solution. The size of the colloids and their chemical composition depend on the details of the preparation process. Ideally, the size of the particles should be in the range of only a few nanometers if high-resolution patterning is sought. We employed a commercial suspension of Pd/Sn colloids in HCl because (i) these colloids have a high catalytic activity for ELD, (ii) their inking and printing using PDMS stamps have been established in previous studies,48,49 and (iii) their relatively small size renders them attractive for highresolution µCP. We used dynamic light scattering experiments to determine the size distribution of the colloids. The suspension was diluted with 0.1 N HCl and filtrated using a 5 µm pore diameter. We detected multimodal size distributions with two major peaks at 1-2 nm and 100300 nm particle radius. Linear and logarithmic mass weighing both revealed that the majority of the colloids had a radius of 1-2 nm.50 Interestingly, the peak at the low nanometer range was independent of the colloid (43) Li, J.; Hu, X.; Wang, D. Plat. Surf. Finish. 1996, 83, 62-64. (44) Matijevic´, E.; Poskanzer, A. M.; Zuman, P. Plat. Surf. Finish. 1975, 62, 958-965. (45) Fujinami, T.; Watanabe, J.; Honma, H. Trans. Inst. Met. Finish. 1996, 74, 193-197. (46) Delamarche, E.; Geissler, M.; Vichiconti, J.; Graham, W. S.; Andry, P. A.; Flake, J. C.; Fryer, P. M.; Nunes, R. W.; Michel, B.; O’Sullivan, E. J.; Schmid, H.; Wolf, H.; Wisnieff, R. L. Langmuir, in press. (47) Cohen, R. L.; D’Amico, J. F.; West, K. W. J. Electrochem. Soc. 1971, 118, 2042-2046. (48) Geissler, M.; Kind, H.; Schmidt-Winkel, P.; Michel, B.; Delamarche, E. Langmuir, in press. (49) Delamarche, E.; Donzel, C.; Kamounah, F. S.; Wolf, H.; Geissler, M.; Stutz, R.; Schmidt-Winkel, P.; Michel, B.; Mathieu, H. J.; Schaumburg, K. Langmuir, submitted. (50) The dipole scattering intensity is proportional to r6 if based on the number of particles and to r3 if one refers to the particle mass, where r is the particle radius.
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concentration, whereas the other peak shifted continuously toward higher radii with increasing colloid concentration, which may indicate particle aggregation. After filtration of the colloidal solution using a filter with a 200 nm pore diameter, the peak in the region of 100-300 nm radius disappeared, suggesting that higher aggregates were completely removed from the suspension. Figure 7A shows the size distribution of a suspension of colloidal Pd/Sn diluted 1:25 with 0.1 N HCl after purification, revealing a maximum at a particle radius of ∼1 nm. The measurements were repeated after about 24 h to check the stability of the colloidal suspension, and the distributions were found to be reproducible within the experimental error. In addition, the experiments were independently reproduced after about 4 months, which provides evidence of the stability of the original colloidal solution stored at room temperature. The affinity of Pd/Sn colloids for a PDMS stamp is low, and the surface of the PDMS has to be modified in order to ink a stamp with these colloids. Hydrophilization is an important requirement, in particular when the colloids are suspended in an acidic aqueous solution. Previous studies suggest, however, that O2 plasma treatment alone is not sufficient.48 The presence of a positively charged polyelectrolyte48 or of a poly(ethylene glycol) (PEG) promotion layer49 on the hydrophilic PDMS proved helpful to increase the affinity between the stamp and the Pd/Sn particles, and it ensured inking of enough particles in a uniform manner. In this study, we relied on the latter strategy and derivatized a high-resolution stamp by (i) oxidizing the surface of the PDMS using an O2 plasma treatment and (ii) grafting a PEG-silane composed of 70 ethylene oxide units (on average) bearing two triethoxysilane anchor groups from an ethanolic solution to the hydrophilic surface of the stamp. Inking was achieved by immersing the PEG-derivatized stamp into a freshly prepared suspension of Pd/Sn colloids (1:25 in concentrated HCl, purified using a 220 nm filter) for 30 s. The stamp was then rinsed with DI water and dried under a stream of N2. A pretreatment of the substrate was necessary to promote a uniform transfer of the Pd/Sn particles from the stamp to the substrate and to ensure their immobilization on the surface. A thin layer of an aminofunctionalized silane, 3-(2-aminoethylamino)propyltrimethoxysilane (EDA-Si), served as a linker between the substrate and the Pd/Sn colloids.46,48,51 The substrate was derivatized by immersing the Si wafer into a 1% aqueous solution of EDA-Si for 10 min, rinsing it with DI water, and drying it under a stream of N2. The stamp was then placed on the pretreated substrate for ∼1 min. An acceleration step must be applied to the colloids after printing to remove part of the Sn shell, which makes the catalytic core more accessible to the reactants in the plating bath and ensures a better initiation (strike) of the ELD process.46 For this reason, the printed substrate was immersed into a 10% aqueous solution of an HBF4-based accelerator for 30 s and rinsed with water. Subsequent metal deposition was achieved by immersing a freshly printed and accelerated substrate into an appropriate plating solution. First, we inspected the outcome of plating NiB using a commercial DMAB-based bath, which yields Ni deposits that contain 0.25% B.46,48 Although the deposition of NiB initiated at the printed catalytic sites on the substrate, a background of NiB grains readily formed on the surface (Figure 7B). Changing the plating conditions and/or the composition of the bath can (51) Dobisz, E. A.; Bass, R.; Brandow, S. L.; Chen, M.-S.; Dressick, W. J. Appl. Phys. Lett. 2003, 82, 478-480.
Fabrication of Metal Nanowires Using µCP
Figure 7. High-resolution microcontact printing of a catalyst constitutes a plausible but challenging strategy to direct the ELD of a metal onto a substrate. The method inspected here starts by derivatizing the PDMS stamp with a PEG-silane, inking the modified stamp with an acidic suspension of Pd/Sn colloids, and finally printing the particles onto an EDA-Sitreated Si wafer. (A) The size distribution of the colloids (here diluted 1:25 in 0.1 N HCl) determined by dynamic light scattering reveals a maximum at ∼1 nm particle radius after appropriate filtration. The results of the plating process can vary as shown by the AFM images in parts (B) and (C). The formation of high-resolution NiB wires was compromised by the formation of background in the array, whereas wires of CoP were fabricated with good contrast and accuracy.
both be helpful to increase the selectivity of an ELD process in general. We verified this option by depositing Co from a bath that contained hypophosphite as the reducing agent. Figure 7C reveals that CoP wires were formed with good
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contrast and that only a few grains were present in the nonprinted regions of the substrate. The yield of intact wires in the array can be high, but there are several challenges that make this approach cumbersome. Since the stamp is hydrophilic, wetting phenomena can occur during printing, which can lead to artifacts in the wire array. The adhesion of the deposit to the substrate is another issue and can vary between different metals. NiB and CoP deposits were adhesive enough to grow on the EDA-Si-treated wafer, whereas Cu showed poor adhesion in comparative experiments, and the features easily peeled off during plating. The formation of background was an additional problem. We found that the intensity of the background was particularly strong for NiB and low for CoP, but some background appeared systematically in all wire arrays fabricated using this approach. It is unlikely that the background formed due to the diffusion of colloids during printing,48 and appropriate plating conditions might be helpful in reducing it. Properties of the Plated Nanowires. The characterization of individual electroless-deposited nanowires fabricated by both approaches was done in a manner similar to that for those obtained from “print & etch” (see above), and their properties are listed in Table 2. We did not verify the dimensions of the ridges on the stamp after the PEG-derivatization process, but we can assume that the contact line width might be slightly increased compared to the case of nonmodified stamps due to the thickness (in the range of a few nanometers)49 and the hydrophilic volume of the PEG layer. Plated wires show, in the ideal case, a symmetrically rounded (“mushroomlike”) profile due to the isotropic nature of ELD. For this reason, the wire diameters are larger than the initial seed patterns on the substrate. The wire widths were extracted from the AFM data at a height of 10 nm with respect to the surface of the substrate. The edge roughness of the wires was determined by the growth of individual grains and correlates with the typical grain size of the deposit. We did not intend to fabricate wires having a consistent thickness for all metals. In the case of NiB, the substrate was removed from the plating bath at an early stage of the deposition, but a strong background was already present between the wires. The background interfered with the analyzing algorithm and enhanced the variation of the wire width, although the grain size of the NiB deposit is small. Interestingly, the typical sizes of Ag and Cu grains were of the same order as those of films prepared by e-beam evaporation or sputtering. We found, however, that the rms roughness of the plated wires was generally higher than that for wires fabricated using the “print & plate” approach. We suspect that the roughness can be decreased in principle by having a more uniform seed pattern underneath and by optimizing the plating conditions. The wire cross-sectional areas were determined by integrating the AFM profiles in Figures 6 and 7, and these values were used to calculate the specific resistivity from the measured wire resistances. The resistance values presented in Table 2 were again determined from individual wires by two-point-probing (see above), and they show the trend that was expected for these deposits. Ag and Cu wires have a higher resistivity than their counterparts fabricated by the “print & etch” approach. This finding can be partially explained by the different morphology (shape and granular structure) of the wires that led to an increase of scattering effects when applying an electrical field. Another reason may be a higher content of impurities embedded during the deposition. We observed that the resistance of Cu nano-
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Table 2. Selected Properties of Metal and Alloy Nanowires Fabricated Using µCP and ELD
deposit
patterned seed material
contact width (nm)
wire width (nm)
average wire thickness (nm)
typical grain size, diameter (nm)
rms roughnessa (nm)
wire cross-sectional area (104 nm2)
wire resistance (Ω µm-1)
specific resistivity, wire/bulk (µΩ cm)
Ag Cu NiWP NiB CoP
Au Au Au Pd/Sn Pd/Sn
120 120 120 g120 g120
501 ( 37 485 ( 12 320 ( 15 356 ( 43 299 ( 35
106 156 71 23 76
64 54 58 30 85
15.9 7.2 4.5 1.0 10.5
4.8 6.0 1.9 0.8 1.9
2.1 1.1 23.8 98.2 6.8
10.1/1.6 6.6b/1.7 45.2/28-133c 78.6/18d 13.0/nae
a Values refer to a linear rms roughness parallel to the wire axes. b The measurement was performed using a one-year-old sample. Source: see ref 43. d Provided by the supplier of the bath. The amount of B in the deposit is 0.25%. The specific resistivity of pure Ni is 6.9 µΩ cm at 20 °C. e We do not know the amount of P in the deposit. The specific resistivity of pure Co is 6.2 µΩ cm at 20 °C.
c
wires fabricated with this approach increased with time. The specific resistivity of Cu provided in Table 2 was determined in this case using a sample that was one year old. We do not think that extensive oxidation reduced the conductivity of the Cu. Instead, we believe that Au atoms from the seed pattern diffused into the Cu deposit, thus forming an alloy-type of material, which has different electrical properties than the Cu had initially.52 The resistances of the Ni and Co deposits can vary depending on the composition of the alloy, but the values remain high compared to those of the pure metals. We did not focus on improving the conductivity of the plated NiWP, NiB, and CoP wires, but previous studies showed that the resistance of deposits decreases, for example, during an annealing step.46 Conclusion We demonstrated the fabrication of conductive nanowire devices using high-resolution µCP in combination with selective etching and ELD methods for a variety of metals and alloys. Reliable and selective etch chemistries were developed for “print & etch” strategies to achieve optimal contrast of the patterns. The metal wires obtained had excellent lithographic definition, and the level of residual background was low enough to exclude electrical shorts and “bridging” effects in the device. We did not intend to push the limits in resolution, but the edge roughness of wires seems to become a limiting factor for reasonable device fabrication. When higher resistances can be tolerated, thinner deposits and/or smaller grain sizes would be advantageous to extend this process to the fabrication of wires with dimensions below 100 nm. Complementary approaches involving ELD proved successful to enlarge the variety of materials accessible and to generalize highresolution µCP. The localized deposition of a metal or alloy onto sub-micrometer seed patterns may offer low-cost alternatives to vacuum deposition processes. The nonselective deposition of background might be an issue, however, and the enlargement of the printed pattern may demand appropriate precompensation on the design level. Although we focused our work on the fabrication of basic wire devices only, the achievements may encourage further efforts toward the integration of µCP into novel manufacturing schemes. Methods providing control over localized chemical processes will be useful when nanostructures should be assembled, amplified, connected, or otherwise tailored in top-down or bottom-up approaches. Materials and Methods Compounds. All commercial compounds used within this study were bought at the best grade available and were used without further purification. ECT was from Robinson Brothers Ltd. (West Bromwich, U.K.). HDPA was synthesized from sodium (52) Hartung, F.; Schmitz, G. Phys. Rev. B 2001, 64, 245418-1245418-13.
(Fluka), dibutyl phosphite (Fluka, Buchs, Switzerland), and 1-bromohexadecane (Aldrich, Buchs, Switzerland) via the Michaelis-Arbuzov reaction following a procedure described elsewhere.53 The compound was recrystallized from n-heptane (Fluka), separated by filtration as white crystals, and dried under vacuum conditions (∼2 × 10-3 mbar) for several hours. 1H NMR (300 MHz, DMSO-d6) δ (ppm): 0.83 (t, 3H, CH3), 1.21-1.53 (m, 30H, CH2), 3.86 (s, P(OH)2 + H2O). Mp ) 95 °C (reported: 94.5-95.5 °C).53 EDA-Si was purchased from Fluka. Pd/Sn colloids in acidic suspension (Crimson Activator 5300 B) and the accelerator (Accelerator 19H) were both from Shipley (Marlborough, MA). Preparation of Substrates. We consistently used Si wafers (Siltronix, Geneva, Switzerland) coated with an oxide layer (∼15 nm thick) as the substrates. The SiO2 layers were produced by chemical vapor deposition (Multiplex CVD, STS, Bristol, U.K.) using SiH4 in He/N2O (20 W, 1.2 mbar, 300 °C, 30 s). Layers of Au, Cu, Pd, and Al (at least 99.95% purity, all obtained from Balzers, Liechtenstein) on Si substrates were fabricated using e-beam evaporation (PLS 500, Pfeiffer Vacuum, Asslar, Germany). The oxidized wafers were first primed with ∼1 nm of Ti (except for Al) at a rate of ∼0.2 nm s-1 and subsequent evaporation of ∼100 nm of these metals at a rate of ∼0.5 nm s-1 and at a pressure between ∼0.5 and 5 × 10-6 mbar. Ag films on Si wafers (primed with 2 nm of Ti) were obtained from sputtering of Ag (99.97% purity, SenVac, Friedberg, Germany) using Ar (20 sccm, 113 W cm-2) at a rate of 1 nm min-1 W-1 and a pressure of 3 µbar (LA 440 S, von Ardenne, Dresden, Germany). The substrates, originally 4 in. in diameter, were usually split into 4-9 cm2 pieces to perform sets of printing experiments. Fabrication of Master and Stamps. The high-resolution master was prepared from an SOI wafer by (i) patterning a resist using e-beam lithography (Extreme Lithography, Illerrieden, Germany), (ii) transferring the pattern into the 350-nm-thick Si top layer using a deep-trench-etching tool (STS), (iii) stripping off the resist, and (iv) coating the patterned SOI wafer with a ∼10-nm-thick antiadhesive layer of (CFx) deposited using a C4F8based plasma treatment (Multiplex ICP, STS). Stamps (∼200 µm thick) supported on a thin, flexible glass back-plane (80 × 125 mm2, 146 µm thick, Schott/DESAG, Mainz, Germany) were prepared by injection molding using a home-built steel mold. Before molding, the glass was exposed to the vapor of hexenyltrichlorosilane (ABCR/Gelest, Karlsruhe, Germany) in a vessel evacuated to 300 mbar.54 Sandwiched between solid steel plates, the silanized glass sheet faced the patterned master, separated by a 200-µm-thick Teflon foil at its periphery, which served as a spacer element and a seal ring. The prepolymeric mixture was composed of 68 g of VDT 731 with Q-resin (custom-synthesized), 15 µL of SIP 6831.1 ()5 ppm of a Pt catalyst), and 22.4 mL of HMS 301 (all obtained from ABCR/Gelest). It was degassed prior to molding to avoid having air bubbles in the material, injected into the mold through filling ports, and cured at 60 °C for at least 48 h. After curing, stamps were peeled off the mold and usually cut into smaller pieces to perform sets of printing experiments. The stamps were derivatized with the PEG-silane as follows.49 The stamps were cleaned with ethanol in an ultrasonic bath, (53) Kosolapoff, G. M. J. Am. Chem. Soc. 1945, 67, 1180-1182. (54) The treatment of the glass with a vinyl-terminated silane improved the adhesion between the elastomeric material and the glass because the vinyl groups (at least a fraction of them) can be involved in cross-linking reactions that occur during the formation of the polymer network.
Fabrication of Metal Nanowires Using µCP dried, and oxidized using an O2 plasma treatment at 200 W for 30 s (Technics Plasma 100-E, Florence, KY). Either the oxidized stamps were stored in DI water to keep their surface hydrophilic or they were directly covered with a 10 mM solution of (EtO)3Si(CH2CH2O)70-Si(OEt)3 (MW 3400, Shearwater Polymers, Inc., Huntsville, AL) in ethanol for 10 min. The PEG-derivatized stamps were then rinsed with ethanol, dried with N2, and stored in a polystyrene box. Inking and Printing. All ink solutions were prepared fresh before their use. We inked the stamps by covering their patterned face entirely with the ink solution (see text for details), and then we dried them under a stream of N2. The stamps were placed on a substrate by hand and pressed slightly with a soft roller to ensure uniform printing conditions. Selective Etching. The solution for etching Au substrates was composed of 20 mM Fe(NO3)3‚9H2O (Fluka) and 30 mM thiourea (Fluka) in DI water, adjusted to pH 2.0 using HCl (Fluka). The bath was operated at 23-25 °C with moderate stirring and had an etch rate of ∼10 nm min-1. Ag substrates were etched in a solution of 20 mM Fe(NO3)3‚9H2O and 30 mM thiourea in DI water at pH 2.25. The etch rate was ∼30 nm min-1 at 23-25 °C with moderate stirring. Cu substrates were etched in a bath composed of 50 mM NBSA (Fluka) and 10 vol % PEI (branched type, Aldrich) in DI water. The bath was operated at 75 °C and a pH of 11.4, was stirred vigorously, and had an etch rate of ∼10 nm min-1. The solution for etching Pd substrates contained 20 mM Fe(NO3)3‚9H2O in DI water and had a pH of 2.0 adjusted with HCl. The etch rate was ∼10 nm min-1 when the bath was operated at 40 °C and stirred gently. Etching of Al substrates was done by using a solution of 10 mM NBSA and 10 vol % PEI in DI water. The etch rate was ∼30 nm min-1 at 23-25 °C and with moderate stirring. Electroless Metal Deposition. Ag was deposited from a commercial staining solution (#2013 LI Silver, Nanoprobes, Inc., Yaphank, NY), prepared according to the recommendations of the supplier and operated at 20-25 °C with gentle stirring. Cu was plated using a commercial bath (#1025, Fidelity Chemical Products, Newark, NJ), prepared according to the recommendations of the supplier and operated at 30 °C with gentle stirring. The bath for plating NiWP was prepared by dissolving 6.0 g (23 mmol) of NiSO4‚6H2O (Fluka), 6.0 g (57 mmol) of NaH2PO2‚H2O (Fluka), 10.5 g (36 mmol) of trisodium citrate dihydrate (Fluka), 1.2 mL (14 mmol) of L-(+)-lactic acid (88% in water, Fluka), 9.0 g (68 mmol) of (NH4)2SO4 (Fluka), and 12.0 g (36 mmol) of NaWO4‚ 2H2O (Merck, Darmstadt, Germany) in 200 mL of DI water.43 The pH of the solution was adjusted to 8.7 using aqueous ammonia (31%, Merck). The bath was diluted with DI water to an overall
Langmuir, Vol. 19, No. 15, 2003 6311 volume of 350 mL and operated with gentle stirring at ∼60 °C. NiB was deposited using a Niposit 468 plating bath (Shipley), prepared according to the recommendations of the supplier and operated at 56 °C and at a pH of 7.2 (adjusted with ammonia) with moderate stirring. The bath for plating CoP was prepared by dissolving 8.0 g (28 mmol) of CoSO4‚7H2O (Fluka), 20 g (68 mmol) of trisodium citrate dihydrate (Fluka), 16 g (299 mmol) of NH4Cl (Fluka), and 8.0 g (75 mmol) of NaH2PO2 (Fluka) in 400 mL of DI water.55 The bath was operated at pH 8.0 and a temperature of 90 °C with moderate stirring. Instrumentation. AFM investigations were performed with a commercial Nanoscope III (Digital Instruments, Santa Barbara, CA) operated under ambient conditions in tapping mode. We used Si cantilevers with conventional (NCH-W) and sharp tips (SuperSharpSilicon, tip radius < 5 nm) from Nanosensors GmbH & Co. KG (Wetzlar-Blankenfeld, Germany). NMR spectra were recorded using a Bruker Avance 300 MHz spectrometer (Bruker BioSpin AG, Fa¨llanden, Switzerland). Sheet resistance measurements were taken using a four-point probe station (Jandel Engineering Ltd., Bedfordshire, U.K.). Focused ion beam milling was performed using a FIB 200 instrument (Fei Deutschland GmbH, Kassel, Germany). The resistances of individual nanowires were measured using a home-built, two-point probe setup, and the values were recorded by a Semiconductor Parameter Analyzer 4145B (Hewlett-Packard, Palo Alto, CA). Optical inspection of substrates was carried out in reflection mode using a Nikon Optiphot 200 microscope equipped with a digital network camera (DN100, Nikon, Japan). Particle size distributions of the Pd/Sn colloids were determined by dynamic light scattering with a high performance particle size analyzer (ALV-NIBS/HPPS, Langen, Germany), which was operated in the noninvasive backscattering mode using single photon detection. The values were extracted from the autocorrelation function of the timedependent scattering intensity via a digital correlation technique.
Acknowledgment. We thank our colleagues Rene´ Beyeler, Michel Despont, Ute Drechsler, Hannes Kind, Urs Kloter, Hartmut Knoll, Heinz Schmid, and Meinrad Tschudy for technical assistance, and Paul F. Seidler for his continuous support. LA034464X (55) Berkenkotter, P.; Stephens, D. In Electroless Plating: Fundamentals and Applications; Mallory, G. O., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; pp 481-509.
