New Pairs of Inks and Papers for Photolithography, Microcontact Printing, andScanning 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 47907-1393, U.S.A.

ABSTRACT

Currently, there is considerable interest in producing patterned metallic structureswith reduced dimensions for use in technologies such as ultra large scale integration(ULSI) device fabrication, nanoelectromechanical systems (NEMS), and arrayednanosensors, without sacrificing throughput or cost effectiveness. Research in ourlaboratory has focused on the preparation of precious metal thin films on semiconductorsubstrates via electroless deposition. This method provides for the facile interfacing ofmetal nanoparticles with a group (IV) and III-IV compound semiconductor surfaces.Morphologically complex films composed of gold, platinum, and palladium nanoparticleshave been prepared as a result of the immersion of germanium and gallium arsenidesubstrates into dilute, aqueous solutions of tetrachloraurate (III), tetrachloroplatinate (II),and tetrachloropalladate (II), respectively. Continuous metallic films form spontaneouslyunder ambient conditions, in the absence of a fluoride source or an externally appliedcurrent. This facile electroless deposition methodology provides an alternative tocomplex and expensive vacuum methods of metallization, yet allows for the preparationof both thin and thick nanostructured films with control over surface morphology anddeposition rate. Furthermore, precious metal films prepared in this way exhibit excellentadhesion to the underlying semiconductor substrate. The resultant films werecharacterized utilizing scanning electron microscopy (SEM), X-ray photoelectronspectroscopy (XPS), and scanning probe microscopy (SPM). In order to apply this novelmetallization method toward the development of useful technologies, patterning utilizingphotolithography, microcontact printing (µCP), and scanning probe nanolithography(SPN) has been demonstrated.

INTRODUCTION

Patterning metallic structures with micro- and nanometer resolution for bothfundamental investigations and technological applications has recently attractedconsiderable 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 patterna solid “paper” substrate. These are relatively straightforward methods to execute sincethey operate in air, using easily accessible equipment and simple procedures. Herein, wereport the patterning of new pairs of metallic or semiconducting papers with inks ofaqueous noble metal salts, chosen on the basis of mixed potential arguments [3], throughphotolithography, µ-CP, and DPN [4]. The resulting nanoparticle films deposited fromthese inks demonstrate complex morphological architectures [3], which are of importance

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for the interfacing of nanoparticles with metals and semiconductors, the development ofhigh 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 anddegreased by immersion into 4 successive baths of each of the following solvents:acetone, methanol, and deionized water. An additional soak in the above solvents wascarried out with sonication for 2 min. Finally, the Ge(100) samples were soaked inscintillation vials containing the aqueous metal salts for the designated time andtemperature. The metal-deposited Ge(100) substrates were then removed from the metalsolution and washed with copious amounts of 18 MΩ water, ethanol, and pentane. Thesurfaces 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 preparedhydride-terminated Ge(100) substrate was transferred to an inert atmosphere [7].Approximately 0.1 mL of 1-dodecene, filtered through alumina to remove peroxides, wasdropped onto the Ge(100) surface. A nickel “micromesh” grid (Internet, Inc., 25 µmfeature size), serving as a contact photomask, was placed onto the Ge(100) substrate. Abrass washer was employed to secure the photomask, and the surface was then exposed toa mercury vapor lamp with 254 nm radiation from a penlight source (Jelight, 9 mW/cm2

at 2 cm). UV photoinduced hydrogermylation was accomplished within 45 min and thesample subsequently washed with copious amounts of ethanol, pentane, and methylenechloride. Following UV induced hydrogermylation, the samples were immersed intoaqueous solutions of either 2 x 10-3 M HAuCl4, 2 x 10-3 M Na2PdCl4, or 2 x 10-2 MNa2PtCl4, depending on the metal species of interest. After soaking for 30 s, thesubstrates were removed from the metal solution and immediately washed with copiousamounts of deionized water. The samples were finally blown dry utilizing a stream ofnitrogen.

Microcontact printing (µ-CP) on Ge(100)

Approximately 1 mL of aqueous 2 x 10-3 M Na2PdCl4 was dropped onto the surfaceof the Ge(100) wafer fragment. Immediately, the ozone-treated PDMS stamp was forcedthrough the PdCl4

- solution with moderate pressure, so that direct contact with theunderlying substrate was achieved. The PDMS stamp served as a barrier to “mask” theGe(100) surface from the aqueous metal solution, thereby preventing nanoparticledeposition in the regions where the stamp and substrate were in direct contact. After aperiod of 10 min, the excess metal solution was washed away with deionized water andthe substrate transferred to a water bath while maintaining contact with the PDMS stamp.While immersed in deionized water, the stamp was removed and the substratesubsequently rinsed with additional water.

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Microcontact printing (µ-CP) on Zn foil

A freshly prepared PDMS stamp, soaked in deionized (18 MΩ⋅cm) water, waspressed upon a 2 x 10-2 M aqueous solution of NaPtCl4 for 15-30 s. After drying thestamp at ambient conditions (30-50% relative humidity), both the stamp and a cut pieceof Zn foil were stored in a humid (≥95%) jar for 5 min. The stamp was then lightlyplaced upon the foil for 15 s and removed; the foil was washed with deionized water anddried under dinitrogen flow.

Dip-pen nanolithography (DPN) on Ge(100)

A scanning probe microscope (Nanoscope III, Digital Instruments) was used as alithography tool for writing and tapping mode atomic force microscopy (TM-AFM) forimaging. The Si tip was dipped into an ink solution [1:10 (v:v) mixture of aqueoussolution of 20 mM AuCl4

- and acetonitrile (99.8%, Aldrich)] for 5 min and dried underambient 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 writingand 1.0 µm/s for imaging. The average height of the resulting gold line is about 4 nmand the diameter is in the range of 30 nm. While the height of a solid sample, such asgold, is usually accurate, the width of the observed line is widened due to convolutionartifacts, a well known issue in STM and AFM, which causes the x,y-dimensions ofsample features to appear larger as they are in reality when their dimensions are in therange 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 anddecreased 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 theseassemblies is essential. Photolithography, negative µ-CP, and DPN, are demonstratedhere employing Au, Pd, and Pt salt inks with a Ge(100) paper, and Pt salts with a Znmetal paper.

Figure 1. Scanning electron micrographs of various noble metal nanoparticle filmsdeposited onto germanium, copper, and zinc substrates [4].

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Figure 2. Schematic (a) and SEM micrographs of photolithography patterning of Pd onGe(100) (b,c). The EDS spectrum of the bright gridlines (d) confirms nanoparticlescomposed 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 anorganic monolayer resist. A hydride-terminated germanium surface was exposed to 254UV 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 onsilicon [8]. This results in 5-25 µm-sized features of dodecyl and hydride, respectively.Upon immersion of the hydride/alkyl-terminated surface, metal deposition occurspreferentially in the hydride areas since the alkyl monolayer functions as an effectivedielectric barrier (see figure 2b,c). The hydride surface oxidizes in-situ and subsequentlydissolves in the aqueous medium [9]. Metal salt reduction and deposition can then occur,leading to deposition between the alkylated domains. The germanium oxide dissolves inwater, leading to intimate electrical contact between the semiconductor bulk and themetal salts, thus facilitating deposition. In the case of silicon, however, the native oxidehas 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 preparemicro- and nanosized features on surfaces. Negative patterning results in micron-scaledeposition on Ge(100). Because PdCl4

2- is reduced slowly on the germanium oxideinterface, the solution was dropped upon the surface and was immediately pressed withan oxidized, hydrophilic PDMS stamp. Gentile pressure was applied to force excessaqueous PdCl4

2- solution from the stamp/wafer interface. After approximately tenminutes, the stamp was removed and the germanium immediately rinsed with deionizedwater to yield negative patterning with a spatially defined resolution on the order of 40µm (see figure 3c). Negative patterning on rough zinc foil with an aqueous PtCl4

2- inkusing a hydrophobic, untreated PDMS stamp further demonstrates the utility of thistechnique (see figure 3h). The zinc foil was not pretreated in any way to reduce or flattenthe material. Micron-scale lines of the deposited platinum metal film can be clearlyobserved, in spite of the surface roughness. Consequently, this technique is not restrictedto flat surfaces and can be extended to morphologically inhomogeneous interfaces.

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Figure 3. Negative µ-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 SEMmicrographs of patterning of Pd on Ge(100) (c) and Pt on Zn foil (h). The EDS spectrumof the bright regions (d) confirms nanoparticles composed of Pd, whereas the Pd signal isabsent from the spectrum for the squares/rectangles (e). Likewise, the EDS spectrum ofthe bright gridlines (i) confirms nanoparticles composed of Pt, whereas the Pt signal isabsent from the spectrum for the squares (j).

Dip-pen nanolithography, as outlined in figure 4, was demonstrated through thewriting of a 0.55 µm long gold line with a width of 30 nm and height of 10 nm throughthe spontaneous electroless deposition of AuCl4

-, delivered via the AFM tip, upon anuntreated Ge(100) wafer (see figure 4). Writing was accomplished at a rate of 0.2 µm/sin a constant humidity environment of 50%. Similar results were obtained with PdCl4

2-.

CONCLUSIONS

In summary, electroless deposition of noble metal salts on semiconducting andmetallic substrates leads to morphologically complex, nanostructured films that can bepatterned via photolithography, µ-CP, and DPN. Current investigations in our group arefocused on determining the extent of molecular contact between the metal particles andthe underlying substrate as to explore their utility as nanoscale electrical contacts for

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Figure 4. Au line (550 nm long, 30 nm wide, 10 nm in height) drawn in air throughDPN on a native oxide coated Ge(100) surface.

interfacing a range of different organic and biomolecules, and for catalytic and sensorapplications, among others.

ACKNOWLDEGEMENTS

Jillian M. Buriak gratefully acknowledges support from NSF for grants CHE-9875150 and CHE-0110846 and a predoctoral fellowship to LAP, the Purdue ResearchFoundation (fellowships to HCC and JMS), the Indiana Instrumentation Institute(fellowship to LAP), and the Sloan Foundation. JMB is a Cottrell Teacher-Scholar ofResearch Corporation (2000-2002), and a Camille and Henry Dreyfus Teacher-Scholar(2002-2004). The Purdue Laboratory of Chemical Nanotechnology is acknowledged fortechnical support and expert advice. Lindsay C. C. Elliott and Katie Jennings are thankedfor help in preparing nanoparticle films. Profs. Ralph G. Nuzzo and Fred E. Lytle arethanked for providing samples for microcontact printing. Dr. Richard T. Haasch isacknowledged for the acquisition of XPS data, carried out at the Center for Microanalysisof Materials, University of Illinois, which is partially supported by the U. S. Departmentof Energy under grant DEFG02-96-ER45439.

REFERENCES

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