New Pairs of Inks and Papers for Photolithography, Microcontact Printing, and Scanning Probe Nanolithography

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1 Mat. Res. Soc. Symp. Proc. Vol Materials Research Society F3.2.1 New Pairs of Inks and Papers for Photolithography, Microcontact Printing, and Scanning Probe Nanolithography Lon A. Porter, Jr., Hee Cheul Choi, J. M. Schmeltzer, Alexander E. Ribbe, and Jillian M. Buriak Department of Chemistry, 1393 Brown Laboratories, Purdue University, West Lafayette, IN , U.S.A. ABSTRACT Currently, there is considerable interest in producing patterned metallic structures with reduced dimensions for use in technologies such as ultra large scale integration (ULSI) device fabrication, nanoelectromechanical systems (NEMS), and arrayed nanosensors, without sacrificing throughput or cost effectiveness. Research in our laboratory has focused on the preparation of precious metal thin films on semiconductor substrates via electroless deposition. This method provides for the facile interfacing of metal nanoparticles with a group (IV) and III-IV compound semiconductor surfaces. Morphologically complex films composed of gold, platinum, and palladium nanoparticles have been prepared as a result of the immersion of germanium and gallium arsenide substrates into dilute, aqueous solutions of tetrachloraurate (III), tetrachloroplatinate (II), and tetrachloropalladate (II), respectively. Continuous metallic films form spontaneously under ambient conditions, in the absence of a fluoride source or an externally applied current. This facile electroless deposition methodology provides an alternative to complex and expensive vacuum methods of metallization, yet allows for the preparation of both thin and thick nanostructured films with control over surface morphology and deposition rate. Furthermore, precious metal films prepared in this way exhibit excellent adhesion to the underlying semiconductor substrate. The resultant films were characterized utilizing scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and scanning probe microscopy (SPM). In order to apply this novel metallization method toward the development of useful technologies, patterning utilizing photolithography, microcontact printing (µcp), and scanning probe nanolithography (SPN) has been demonstrated. INTRODUCTION Patterning metallic structures with micro- and nanometer resolution for both fundamental investigations and technological applications has recently attracted considerable interest. Developments toward this end, such as dip-pen nanolithography (DPN) [1] and micro-contact printing (µ-cp) [2], employ a liquid-phase ink to pattern a solid paper substrate. These are relatively straightforward methods to execute since they operate in air, using easily accessible equipment and simple procedures. Herein, we report the patterning of new pairs of metallic or semiconducting papers with inks of aqueous noble metal salts, chosen on the basis of mixed potential arguments [3], through photolithography, µ-cp, and DPN [4]. The resulting nanoparticle films deposited from these inks demonstrate complex morphological architectures [3], which are of importance

2 F3.2.2 for the interfacing of nanoparticles with metals and semiconductors, the development of high surface area catalysts, SERS, and other uses [5,6]. EXPERIMENTAL DETAILS Substrate preparation The Ge(100) and metal foil substrates were diced into 0.5 x 0.5 cm rectangles and degreased by immersion into 4 successive baths of each of the following solvents: acetone, methanol, and deionized water. An additional soak in the above solvents was carried out with sonication for 2 min. Finally, the Ge(100) samples were soaked in scintillation vials containing the aqueous metal salts for the designated time and temperature. The metal-deposited Ge(100) substrates were then removed from the metal solution and washed with copious amounts of 18 MΩ water, ethanol, and pentane. The surfaces were subsequently blown dry with a stream of nitrogen. Photopatterned UV induced hydrogermylation After a 10 min. immersion in an aqueous solution of HF (10%), the freshly prepared hydride-terminated Ge(100) substrate was transferred to an inert atmosphere [7]. Approximately 0.1 ml of 1-dodecene, filtered through alumina to remove peroxides, was dropped onto the Ge(100) surface. A nickel micromesh grid (Internet, Inc., 25 µm feature size), serving as a contact photomask, was placed onto the Ge(100) substrate. A brass washer was employed to secure the photomask, and the surface was then exposed to a mercury vapor lamp with 254 nm radiation from a penlight source (Jelight, 9 mw/cm 2 at 2 cm). UV photoinduced hydrogermylation was accomplished within 45 min and the sample subsequently washed with copious amounts of ethanol, pentane, and methylene chloride. Following UV induced hydrogermylation, the samples were immersed into aqueous solutions of either 2 x 10-3 MHAuCl 4,2x10-3 MNa 2 PdCl 4,or2x10-2 M Na 2 PtCl 4, depending on the metal species of interest. After soaking for 30 s, the substrates were removed from the metal solution and immediately washed with copious amounts of deionized water. The samples were finally blown dry utilizing a stream of nitrogen. Microcontact printing (µ-cp) on Ge(100) Approximately 1 ml of aqueous 2 x 10-3 MNa 2 PdCl 4 was dropped onto the surface of the Ge(100) wafer fragment. Immediately, the ozone-treated PDMS stamp was forced through the PdCl 4 - solution with moderate pressure, so that direct contact with the underlying substrate was achieved. The PDMS stamp served as a barrier to mask the Ge(100) surface from the aqueous metal solution, thereby preventing nanoparticle deposition in the regions where the stamp and substrate were in direct contact. After a period of 10 min, the excess metal solution was washed away with deionized water and the substrate transferred to a water bath while maintaining contact with the PDMS stamp. While immersed in deionized water, the stamp was removed and the substrate subsequently rinsed with additional water.

3 F3.2.3 Microcontact printing (µ-cp) on Zn foil A freshly prepared PDMS stamp, soaked in deionized (18 MΩ cm) water, was pressed upon a 2 x 10-2 M aqueous solution of NaPtCl 4 for15-30s. Afterdryingthe stamp at ambient conditions (30-50% relative humidity), both the stamp and a cut piece of Zn foil were stored in a humid ( 95%) jar for 5 min. The stamp was then lightly placed upon the foil for 15 s and removed; the foil was washed with deionized water and dried under dinitrogen flow. Dip-pen nanolithography (DPN) on Ge(100) A scanning probe microscope (Nanoscope III, Digital Instruments) was used as a lithography tool for writing and tapping mode atomic force microscopy (TM-AFM) for imaging. The Si tip was dipped into an ink solution [1:10 (v:v) mixture of aqueous solution of 20 mm AuCl 4 - and acetonitrile (99.8%, Aldrich)] for 5 min and dried under ambient conditions for 5 min. The humidity during writing was held constant at 50% using home-built humidifier. The writing speed for lithography was 0.2 µm/s for writing and 1.0 µm/s for imaging. The average height of the resulting gold line is about 4 nm and the diameter is in the range of 30 nm. While the height of a solid sample, such as gold, is usually accurate, the width of the observed line is widened due to convolution artifacts, a well known issue in STM and AFM, which causes the x,y-dimensions of sample features to appear larger as they are in reality when their dimensions are in the range of the tip-curvature. In our case the same tip used for writing and imaging and is, therefore, due to wear and pollution, likely to result in a tip with enlarged curvature and decreased accuracy of the line width. DISCUSSION In order to incorporate spontaneously and rapidly formed metallic nanoparticle films (see figure 1) into complex architectures, facile and efficient patterning of these assemblies is essential. Photolithography, negative µ-cp, and DPN, are demonstrated here employing Au, Pd, and Pt salt inks with a Ge(100) paper, and Pt salts with a Zn metal paper. Figure 1. Scanning electron micrographs of various noble metal nanoparticle films deposited onto germanium, copper, and zinc substrates [4].

4 F3.2.4 Figure 2. Schematic (a) and SEM micrographs of photolithography patterning of Pd on Ge(100) (b,c). The EDS spectrum of the bright gridlines (d) confirms nanoparticles composed of Pd, whereas the Pd signal is absent from the spectrum for the squares (e). Photolithography, as shown in figure 2, was accomplished through the use of an organic monolayer resist. A hydride-terminated germanium surface was exposed to 254 UV light (15 mw cm -2 intensity) through a metal contact mask in deoxygenated 1- dodecene. The illuminated regions undergo hydrogermylation in the presence of 1- dodecene [7]. A related spatially defined functionalization approach has been shown on silicon [8]. This results in 5-25 µm-sized features of dodecyl and hydride, respectively. Upon immersion of the hydride/alkyl-terminated surface, metal deposition occurs preferentially in the hydride areas since the alkyl monolayer functions as an effective dielectric barrier (see figure 2b,c). The hydride surface oxidizes in-situ and subsequently dissolves in the aqueous medium [9]. Metal salt reduction and deposition can then occur, leading to deposition between the alkylated domains. The germanium oxide dissolves in water, leading to intimate electrical contact between the semiconductor bulk and the metal salts, thus facilitating deposition. In the case of silicon, however, the native oxide has been shown to effectively prevent metal deposition due to its insolubility in water. Microcontact printing methods, as shown in figure 3, were also utilized to prepare micro- and nanosized features on surfaces. Negative patterning results in micron-scale deposition on Ge(100). Because PdCl 4 2- is reduced slowly on the germanium oxide interface, the solution was dropped upon the surface and was immediately pressed with an oxidized, hydrophilic PDMS stamp. Gentile pressure was applied to force excess aqueous PdCl 4 2- solution from the stamp/wafer interface. After approximately ten minutes, the stamp was removed and the germanium immediately rinsed with deionized water to yield negative patterning with a spatially defined resolution on the order of 40 µm (see figure 3c). Negative patterning on rough zinc foil with an aqueous PtCl 4 2- ink using a hydrophobic, untreated PDMS stamp further demonstrates the utility of this technique (see figure 3h). The zinc foil was not pretreated in any way to reduce or flatten the material. Micron-scale lines of the deposited platinum metal film can be clearly observed, in spite of the surface roughness. Consequently, this technique is not restricted to flat surfaces and can be extended to morphologically inhomogeneous interfaces.

5 F3.2.5 Figure 3. Negativeµ-CP patterning of Pd on Ge(100) (a-e) and Pt on Zn foil (f-j). Optical micrographs of the PDMS stamps employed in patterning (b,g) and SEM micrographs of patterning of Pd on Ge(100) (c) and Pt on Zn foil (h). The EDS spectrum of the bright regions (d) confirms nanoparticles composed of Pd, whereas the Pd signal is absent from the spectrum for the squares/rectangles (e). Likewise, the EDS spectrum of the bright gridlines (i) confirms nanoparticles composed of Pt, whereas the Pt signal is absent from the spectrum for the squares (j). Dip-pen nanolithography, as outlined in figure 4, was demonstrated through the writing of a 0.55 µm long gold line with a width of 30 nm and height of 10 nm through the spontaneous electroless deposition of AuCl 4 -, delivered via the AFM tip, upon an untreated Ge(100) wafer (see figure 4). Writing was accomplished at a rate of 0.2 µm/s in a constant humidity environment of 50%. Similar results were obtained with PdCl CONCLUSIONS In summary, electroless deposition of noble metal salts on semiconducting and metallic substrates leads to morphologically complex, nanostructured films that can be patterned via photolithography, µ-cp, and DPN. Current investigations in our group are focused on determining the extent of molecular contact between the metal particles and the underlying substrate as to explore their utility as nanoscale electrical contacts for

6 F3.2.6 Figure 4. Au line (550 nm long, 30 nm wide, 10 nm in height) drawn in air through DPN on a native oxide coated Ge(100) surface. interfacing a range of different organic and biomolecules, and for catalytic and sensor applications, among others. ACKNOWLDEGEMENTS Jillian M. Buriak gratefully acknowledges support from NSF for grants CHE and CHE and a predoctoral fellowship to LAP, the Purdue Research Foundation (fellowships to HCC and JMS), the Indiana Instrumentation Institute (fellowship to LAP), and the Sloan Foundation. JMB is a Cottrell Teacher-Scholar of Research Corporation ( ), and a Camille and Henry Dreyfus Teacher-Scholar ( ). The Purdue Laboratory of Chemical Nanotechnology is acknowledged for technical support and expert advice. Lindsay C. C. Elliott and Katie Jennings are thanked for help in preparing nanoparticle films. Profs. Ralph G. Nuzzo and Fred E. Lytle are thanked for providing samples for microcontact printing. Dr. Richard T. Haasch is acknowledged for the acquisition of XPS data, carried out at the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U. S. Department of Energy under grant DEFG02-96-ER REFERENCES 1. R.D.Piner,J.Zhu,F.Xu,S.Hong,andC.A.Mirkin,Science 283, 661 (1999). 2. Y.Xia,J.A.Rogers,K.E.Paul,andG.M.Whitesides,Chem. Rev. 99, 1823 (1999). 3. L. A. Porter, Jr., H. C. Choi, A. E. Ribbe, and J. M. Buriak, Nano Lett. 2,1067 (2002). 4. L.A.Porter,Jr.,H.C.Choi,J.M.Schmeltzer,A.E.Ribbe,L.C.C.Elliott,andJ.M. Buriak, Nano Lett. in press. 5. R. M. Penner, Acc. Chem. Res. 33, 78 (2000). 6. J. D. Aiken, and R. G. Finke, J. Mol. Cat. A. 145, 1 (1999). 7. K. Choi, and J. M. Buriak, Langmuir 16, 7737 (2000). 8. J. T. C. Wojtyk, M. Tomietto, R. Boukherroub, and D. D. M. Wayner, J. Am. Chem. Soc. 123, 1535 (2001). 9. F. Glockling, The Chemistry of Germanium, (Academic Press, 1969) p. 35.