Langmuir 2003, 19, 8065-8068
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Electroless Deposition of Copper onto 4-Mercaptobenzoic Acid Self-Assembled on Gold Christopher D. Zangmeister* and Roger D. van Zee Chemical Science and Technology Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899 Received November 5, 2002. In Final Form: June 4, 2003 This paper reports the electroless deposition of copper, from a basic solution of copper sulfate, sodium hydrogentartrate, and formaldehyde, onto 4-mercaptobenzoic acid self-assembled on gold. The copper was found to be ≈180 nm thick after a ≈30-min immersion. Deposition did not occur on bare gold or selfassembled layers of 1-octadecanethiol or 3-mercaptobenzoic acid. This latter observation suggests that the carboxylic acid functional group and its position play a role in the deposition process. Infrared absorption spectroscopy was used to evaluate these layers. On surfaces that were microcontact printed with 4-mercaptobenzoic acid, copper deposited only on the stamped areas.
Introduction The interaction of metals deposited on top of organic layers is an area of active investigation. Much of this work has involved evaporative deposition of metals onto solution-grown monolayers.1-3 Another approach to forming a metal overlayer is electroless deposition, a method most commonly used to plate metal films onto plastics and polymer-covered surfaces.4 This process occurs through chemically promoted reduction of metal ions without an externally applied potential. Electroless deposition usually requires a catalyst, often palladium or tin. A chemical treatment is usually required to fix the catalyst to the surface before the deposition process. Predeposition steps are not usually spatially selective, though schemes using optical patterning and microcontact printing have been developed to allow selective deposition.5,6 This paper reports the electroless deposition of copper on 4-mercaptobenzoic acid that has been self-assembled on gold. This technique does not require the pretreatment or fixation of the surface with a catalyst. Selective patterning of these copper deposits has been demonstrated using microcontact printing. Experimental Section Self-assembled layers were grown on ≈180-nm-thick gold (99.999% stated purity) films that were vacuum evaporated, over a 2-nm-thick chromium adhesion layer, onto silicon wafers. These wafers were cut into ≈1 cm2 squares, washed with ethanol and water, and dried with gaseous nitrogen. The diced samples were then placed in an ultraviolet light/ozone cleaner for ≈15 min, rinsed for ≈30 s with water, and dried with nitrogen. The cleaned substrates were then immersed in 1.0 mM ethanolic solutions for 1 day. Previous studies have shown that similar procedures * Corresponding author. E-mail: christopher.zangmeister@nist. gov. (1) Jung, D. R.; Czanderna, A. W.; Herdt, G. C. J. Vac. Sci. Technol., A 1996, 14, 1779. (2) Herdt, G. C.; King, D. E.; Czanderna, A. W. Z. Phys. Chem. 1997, 202, 197. (3) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. J. Adhes. 1997, 60, 197. (4) Goldie, W. Metallic Coating of Plastics; Electrochemical Publications: Middlesex, England, 1968. (5) Chen, M.; Brandow, S. L.; Dulcey, C. S.; Dressick, W. J.; Taylor, G. N.; Bohland, J. F.; Georger, J. H.; Pavelchek, E. K.; Calvert, J. M. J. Electrochem. Soc. 1999, 146, 1421. (6) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375.
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grow mercaptobenzoic acid monolayers.7,8 After rinsing with ethanol, the 4-mercaptobenzoic acid samples had a sessile waterdrop contact angle between 4° and 6° in room air. Comparable contact angles have been reported for self-assembled monolayers with terminal carboxylic acid groups.9 Absolute ethanol was distilled under nitrogen with magnesium shavings. Water was treated to remove ionic, organic, and biological impurities and had a resistivity of ≈18.2 MΩ/cm and a pH between 5.7 and 5.9. The 3- and 4-mercaptobenzoic acids (99% and 97%, respectively) and 1-octadecanethiol (98%) were used as received from the supplier. Vacuum sublimation of the 4-mercaptobenzoic acid before use did not affect any observations. Patterned substrates were prepared by microcontact printing with a poly(dimethylsiloxane) stamp. The 1 cm2 square stamp consisted of a series of parallel, raised 10-µm-wide lines on 25 µm pitch. Thiol patterns on gold were produced by coating the stamp with 1.0 mM ethanolic thiol solution for about 1 min and drying with nitrogen. The stamp was then placed on a cleaned gold surface and hand pressed lightly for about 1 min. The stamp was removed, and the sample was rinsed with ethanol and dried with nitrogen. The electroless deposition solution used in this study consisted of 40 mM CuSO4‚5H2O (99.995%), 0.27 M sodium hydrogentartrate (99%) for chelation, and 0.13 mM formaldehyde (37% in water) as a reducing agent. The solution pH was adjusted between 12.7 and 12.9 using sodium hydroxide (99%). Solutions were used immediately after preparation. Deposition took place at room temperature for 30 min, except where otherwise noted. The solution’s composition was optimized for slow, spatially controlled copper deposition. All containers contacting the electroless deposition solution were rinsed with nitric acid and water before use. Infrared absorption spectra were recorded using a Fourier transform spectrometer and a specular reflectance accessory at a sampling angle of 75° relative to the surface normal. Spectra shown here were referenced to gold-coated substrates prepared and cleaned as described. Topographical images were acquired in air using a commercial force microscope operated in contact mode with sharpened silicon tips. Commercial scanning-electron and optical microscopes were also used to image the copper deposits.
Results Infrared Spectra. Infrared spectra of the mercaptobenzoic acid layers are shown in Figure 1. These spectra (7) Creager, S. E.; Steiger, C. M. Langmuir 1995, 11, 1852. (8) Wells, M.; Dermody, D. L.; Yang, H. C.; Kim, T.; Crooks, R. M.; Ricco, A. J. Langmuir 1996, 12, 1989. (9) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370.
This article not subject to U.S. Copyright. Published 2003 by the American Chemical Society Published on Web 08/08/2003
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Figure 1. Infrared spectra of (a) 4- and (b) 3-mercaptobenzoic acid layers on gold.
are representative of fully protonated layers and are consistent with previously published spectra.7,8 The spectrum of 4-mercaptobenzoic acid is dominated by ν(CdO) at 1747 cm-1. A shoulder at 1710 cm-1 is also seen. Creager and Steiger7 and Wells et al.8 have argued that these two transitions correspond to molecules that are unassociated (1747 cm-1) and associated (1710 cm-1) through hydrogen-bonding interactions within the layer. Wells et al.8 based their assignment on the observation that the ratio of the former to the latter changed from ≈6 to ≈2.5 after storage in dry air.10 In our experiments the ratio was ≈5 after growth and rinsing, which appears to be similar to that observed by Creager and Steiger,7,10 and did not dramatically change after storage in a desiccator. At least two explanations can be offered for the small differences in the ratios observed in these studies: differing packing configurations and differing hydrogen bonding with adsorbed water. Our observation of a broad ν(O-H) feature between 3000 and 3600 cm-1 (not shown) bolsters the second alternative. An infrared spectrum of 3-mercaptobenzoic acid selfassembled on gold is shown in Figure 1b. Unlike the 4-mercaptobenzoic acid, the 1730 cm-1 ν(CdO) in 3-mercaptobenzoic acid consists of several peaks. This observation has been explained8 by the position of the carboxylic acid group in 3-mercaptobenzoic acid, which allows for neighboring molecules to hydrogen bond in a variety of configurations. Because the pH of the electroless deposition solution (≈12.8) is higher than the pKa of benzoic acid11 (≈4.2), one anticipates that the mercaptobenzoic acid layers will be completely deprotonated when in solution. To confirm this, infrared spectra of 4-mercaptobenzoic acid were recorded after rinsing with an aqueous pH ≈12.8 sodium hydroxide solution (Figure 2a). A transition at 1421 cm-1, assigned to the νs(COO-), dominates this spectrum. A water rinse following pH ≈12.8 rinse reprotonates the layer (Figure 2b). This assignment of the asymmetric (10) Ratios estimated from Figure 2 of ref 7 and Figure 1 of ref 8. (11) Martell, A. E. In Critical Stability Constants; Plenum Press: New York, 1974.
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Figure 2. Infrared spectra of gold-coated substrate after growth of 4-mercaptobenzoic acid layer and (a) rinsing with aqueous pH ≈12.8 solution, (b) rinsing with pH ≈12.8 aqueous solution and then water, and (c) immersion in electroless deposition solution for ≈120 s and then rinsing with water.
carboxylate stretch follow that of Wells et al.,8 who observed a broad peak centered around 1410 cm-1 following a potassium hydroxide rinse and a sharper transition at 1397 cm-1 following gaseous dosing with decylamine in a dry environment. A spectrum was also measured after immersing the 4-mercaptobenzoic acid covered substrate in the electroless solution for 120 s (Figure 2c) and rinsing with water. Both -COOH and -COO- vibrational modes are observed. The protonated molecules are most likely located on portions of the substrate where copper deposits have not yet formed. The deprotonated molecules are presumably located on those portions of the surface where water cannot reach, most likely under or near the edge of the copper deposits. These molecular anions might be coordinated with the atomic cations present in the plating solution. Topographic Images. Topographic images were measured to evaluate the substrate surface before and after immersion in the electroless deposition solution. The rootmean-square roughness of cleaned gold-coated substrates was found to be 3.4 nm. Figure 3a shows an image of the surface after formation of the 4-mercaptobenzoic acid selfassembled layer. This represents the condition of the surface after metal deposition, cleaning, and growth of the organic layer. The out-of-plane gray scale encompasses 30 nm. A topographic image collected after exposure of 3-mercaptobenzoic acid to a copper electroless deposition solution for 30 min is shown in Figure 3b. There are no apparent changes to the surface (3.5 nm root-mean-square roughness). No difference was measured for 3-mercaptobenzoic acid covered surfaces immersed for several hours. In contrast, significant copper deposition is observed on 4-mercaptobenzoic acid layers exposed to the electroless deposition solution for 30 min (Figure 3c). The root-meansquare roughness of this image is 56 nm, and the outof-plane gray scale encompasses 220 nm. Deposition on Patterned Surfaces. Microcontact printing is a method that uses an elastomeric stamp with micrometer-dimension features to transfer organic thiols
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Figure 3. Topographic images of the gold-coated substrate obtained after (a) formation of the 4-mercaptobenzoic acid layer (z-axis scale ) 30 nm), (b) formation of the 3-mercaptobenzoic acid layer followed by exposure to copper electroless deposition bath for ≈30 min (z-axis scale ) 30 nm), and (c) formation of the 4-mercaptobenzoic acid layer followed by exposure to copper electroless deposition bath for ≈30 min (z-axis scale ) 220 nm).
Figure 5. Optical microscope image of gold-coated substrate microcontact printed with 4-mercaptobenzoic acid and immersed in copper electroless deposition solution for ≈30 min. Copper deposits appear darker, and gold appears lighter. Inset is an electron micrograph of the interface of electrolessly deposited copper at the edge of a stamped line.
Figure 4. Schematic of microcontact-printing process used to create patterned 4-mercaptobenzoic acid surfaces and deposit copper.
onto surfaces.13 To investigate whether this electroless deposition process could be used in conjunction with microcontact printing to create small copper features, goldcoated substrates were patterned using the stamping protocol described above. These substrates were then immersed into the electroless deposition solution (cf. Figure 4). An optical image of a typical substrate after this process is shown in Figure 5. The darker regions are where the stamp contacted the surface to form a 4-mercaptobenzoic acid layer and copper then deposited. The lighter areas correspond to regions where the stamp did not contact and where no copper was deposited. A comparison of the size of the two features to those of the stamp confirms this intuitive assignment. Selective, patterned copper deposition was observed over nearly the entire microcontactprinted area. There were small patches where no copper grew, corresponding to defects on the stamp. Line profiles from topographic images were used to assess the average dimension of these particles. The average height of these deposits was 179 nm (standard (12) Hsu, H.; Lin, K.; Lin, S.; Yeh, J. J. Electrochem. Soc. 2001, 148, C47. (13) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550.
deviation of 37 nm), corresponding to a deposition rate of ≈6 nm of copper per minute. The average width was 581 nm (standard deviation of 119 nm). Information on the grain structure of the copper deposits was acquired using a scanning-electron microscope (Figure 5, inset). This electron micrograph shows that the copper film is composed of particles that appear crystalline. Many show a 6-fold symmetry, suggestive of 〈111〉 growth. Discussion While a definitive mechanism has not been established for this process, we propose the following. The 4-mercaptobenzoic acid forms a monolayer with the carboxylic acid group pointing upward, away from the gold-coated substrate. Upon immersion in the basic deposition solution, this monolayer becomes deprotonated, and a carboxylate/copper(II) complex is formed. Possibly, a tartarte ion is still bound to the copper cation. Formation of a similar complex is sterically hindered in the case of 3-mercaptobenzoic acid because of the position of the functional group and the bulk of the large-sized water/ tartrate/copper(II) complex.13 This accounts for the observed selectivity. Even on the 4-mercaptobenzoic acid layer the formation of a carboxylate/copper(II) complex may be sparsesthe formation constant between copper(II) and 4-mercaptobenzoic acid’s closest free-solution analogue, benzoic acid,11 is ≈32 compared to ≈105 for tartrate/copper(II)15sand this may account for the formation of discrete copper crystallites on the surface. Once the surface-bound carboxyl/copper complex is formed, it can be reduced by formaldehyde. From this seed, the
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crystallites grow. Based on the work of Czanderna et al.,16 one might anticipate that a unidentate C-O-Cu bond is formed. As King and Czanderna have previously noted,17 however, care must be taken in the interpretation of carboxyl/copper monolayer chemistry whenever oxygen is present. This paper has demonstrated the selective, electroless deposition of copper onto a layer of 4-mercaptobenzoic acid self-assembled on gold. To the best of our knowledge, this is the first demonstration of electroless deposition onto a self-assembled organic layer without a catalyst. This process may be useful in the formation of small-scale copper wires using microcontact printing. Another application would be the formation of top contacts of molecular electronic devices that use aromatic thiol compounds as the active component. Present device (14) Kirschner, S.; Kiesling, R. J. Am. Chem. Soc. 1960, 82, 4174. (15) Meites, L. In Handbook of Analytical Chemistry; McGraw-Hill: New York, 1963. (16) Czanderna, A. W.; King, D. E.; Spaulding, D. J. Vac. Sci. Technol., A 1991, 9, 2607. (17) King, D. E.; Czanderna, A. W. Langmuir 1994, 10, 1630.
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prototypes use evaporative deposition to form a top electrode over pores (20-80 nm in diameter) in which self-assembled monolayers have been grown on a bottom electrode.18 Evaporatively deposited metals atoms often penetrate organic monolayers.19,20 The process reported here offers a possible strategy for circumventing this problem. Acknowledgment. This research was funded by the NIST Molecular Electronic Competence-Building Project. Thanks to M. J. Tarlov for help using the scanning-electron microscope, S. W. Robey for help with the force microscope, and K. Kieltyka and N. R. Armstrong (University of Arizona) for the PDMS stamp. C.D.Z. is the recipient of a National Research Council Postdoctoral Associateship. LA026801S (18) Chen, J.; Reed, M. A. Chem. Phys. 2002, 281, 127. (19) Haynie, B. C.; Walker, A. V.; Tighe, T. B.; Allara, D. L.; Winograd, N. Appl. Surf. Sci. 2003, 203, 433. (20) Dake, L. S.; King, D. E.; Czanderna, A. W. Solid State Sci. 2000, 2, 781.
