Large-Area Patterning of Coinage-Metal Thin Films Using Decal

The current list of compatible materials includes silicon, silicon oxide, and oxidized PDMS ..... Loo, Y.-L.; Willett, R. L.; Baldwin, K. W.; Rogers, ...
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Langmuir 2005, 21, 195-202

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Large-Area Patterning of Coinage-Metal Thin Films Using Decal Transfer Lithography William R. Childs† and Ralph G. Nuzzo*,†,‡,# School of Chemical Sciences, Department of Materials Science and Engineering, and the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received August 24, 2004. In Final Form: September 28, 2004 We describe two new procedures that appear to hold significant promise as means for patterning thinfilm microstructures of the coinage metals (Cu, Ag, Au). A feature central to both is the modification of their surfaces to promote the adhesive transfer of PDMS thin-film microstructures, a material suitable for use as resist layers in large-area patterning, using Decal Transfer Lithography (DTL). The present work provides a significant extension of the capabilities of DTL patterning, providing general protocols that can be used to transfer decal resists to essentially any substrate surface. The first method involves the functionalization of a surface, specifically those of gold and silver films with a thiol-terminated silane coupling agent, (mercaptopropyl)trimethoxysilane. This self-assembled monolayer, when hydrolyzed to its silanol form, provides a robust adhesion-promoting layer suitable for use in DTL patterning. The second method exploits the surface chemistry provided by the deposition of a nanoscale silicon dioxide thin-film capping layer using e-beam evaporation. This procedure provides an exceptional method for patterning large-area, thin-film microstructures of Cusone compatible with micrometer-scale design rulessthat are essentially defect free. Both surface modification strategies enable high-quality poly(dimethylsiloxane) decal transfers, and as the current work shows, these structures are suitable for large-area micrometersized patterning of gold, silver, and copper thin films via both wet-etching and lift-off procedures.

Introduction A series of recent reports has described variations of soft-lithographic patterning that depend, in one form or another, on the use of engineered adhesion as an integral part of the fabrication process.1-6 Rogers and co-workers have described a very powerful method for patterning metal thin-films called Nanoscale Transfer Printing (nTP).7-11 This method uses a metal-coated, patterned poly(dimethylsiloxane) (PDMS) stamp to transfer a metal film in what might be described as an almost inverse model of lift-off lithography. The metal in this printing process serves as a “solid ink”, and the substrate to which this metal decal is transferred to is specially treated with an adhesion layer (a reactive mercaptosilane coupling agent for the case of Au thin-film patterns) to promote adhesive bonding of the metal, thus, enhancing the fidelity of pattern transfer. * Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: 217-244-0809. Fax: 217-244-2278. † School of Chemical Sciences. ‡ Department of Materials Science and Engineering. # Frederick Seitz Materials Research Laboratory. (1) Park, J.; Hammond, P. T. Adv. Mater. 2004, 16, 520-525. (2) Bernard, A.; Renault, J. P.; Michel, B.; Bosshard, H. R.; Delamarche, E. Adv. Mater. 2000, 12, 1067-1070. (3) Childs, W. R.; Nuzzo, R. G. J. Am. Chem. Soc. 2002, 124, 1358313596. (4) Schaper, C. D. J. Vac. Sci. Technol. B 2003, 21, 2961-2965. (5) Tan, J. L.; Tien, J.; Chen, C. S. Langmuir 2002, 18, 519-523. (6) Kim, Y. S.; Lee, H. H.; Hammond, P. T. Nanotechnology 2003, 14, 1140-1144. (7) Loo, Y.-L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. Appl. Phys. Lett. 2002, 81, 562-564. (8) Loo, Y.-L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654-7655. (9) Zaumseil, J.; Someya, T.; Bao, Z.; Loo, Y.-L.; Baldwin, K.; Cirelli, R.; Rogers, J. A. Mater. Res. Soc. Sym. Proc. 2003, 737, 589-594. (10) Loo, Y. L.; Lang, D. V.; Rogers, J. A.; Hsu, J. W. P. Nano Lett. 2003, 3, 913-917. (11) Menard, E.; Bilhaut, L.; Zaumseil, J.; Rogers, J. A. Langmuir 2004, 20, 6871-6878.

In a series of earlier communications,3,12 we have described methods for transferring patterned PDMS films onto silicon, silicon dioxide, and other polar surfaces from a PDMS support using a short-wavelength UV exposure in air (UVO) to induce strong adhesive bonding. In this process, PDMS pre-elastomer (depicted schematically in Figure 1) is cast onto a mold and cured to replicate the features of the mold in the film’s contours.13 This pattern is bonded and subsequently transferred to (through the use of a decal transfer layer) the surface of a substrate in the form of a thin, patterned PDMS decal. We have named this general form of soft lithography, one that uses UVO surface treatments to adhesively transfer PDMS films onto a substrate surface, Decal Transfer Lithography (DTL).3 The DTL process is widely variable but can be classified as operating according to two general modes of patterning. These processes are illustrated in Figure 1. Selective Pattern Release (SPaR)3 uses an engineered, multilayer stamp to release a PDMS decal and thus transfer it with minimal distortion onto a substrate surface. DTL patterning allows decals to be transferred as either open- or closed-form structures. The mechanical properties of the Sylgard 184 resin typically used in softlithographic patterning, and the bond strengths of the decal release interface, provide a set of constraints which tend to restrict the applicability of the SPaR form of DTL. SPaR is best used to pattern large feature sizes, ones in which long-ranging interconnections are present (e.g., long micrometer-scale lines, etc.).3 The second mode of DTL patterning is known as the cohesive mechanical failure method (CMF).3 This is the dominant mode of adhesive transfer in cases where a PDMS stamp that has not been engineered to release a PDMS film is used to transfer a DTL pattern. The PDMS is transferred by the mechanical (12) Childs, W. R.; Nuzzo, R. G. Adv. Mater. 2004, 16, 1323-1327. (13) Jackman, R. J.; Duffy, D. C.; Cherniavskaya, O.; Whitesides, G. M. Langmuir 1999, 15, 2973-2984.

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(5-20 nm) adhesion layers of silica. The latter process is especially useful for patterning copper, a material we found to be susceptible to extensive degradation by mercaptosilanes used in the first method. The silica modification appears to be an exceptionally useful strategy, one that holds significant promise for DTL-based patterning of thinfilm materials more generally. Experimental Section

Figure 1. A schematic illustration of the two fabrication processes that form the basis of DTL: (a) selective pattern release (SPaR) and (b) cohesive mechanical failure (CMF). Scheme 1. Schematic Representation of Chemical Modifications Needed To Provide a Mechanism for Promoting the Adhesion of PDMS to Metals Using the Thiol-Functionalized Silane Coupling Agent (Mercaptopropyl)trimethoxysilane (MPTMS).

rupture of each PDMS feature in contact with the substrate surface and, thus, is best suited for those cases where small, free-standing decal features are required (e.g., micrometer-width PDMS posts).3,12 Earlier reports have illustrated several model applications of DTL as a methodology for patterning important types of thin-film materials.3,12 The present report describes studies that extend the range of materials suitable for patterning through the adhesive transfer of a DTLPDMS resist. As in the original methods, a UVO surface treatment is used to modify the PDMS decal, which enables the formation of a strong adhesive bond between the polymer and the thin-film-bearing substrate.14-16 We specifically examine here surface treatments suitable for modifying coinage metals, materials of broad utility in technology but lacking the types of surface oxides needed to form strong adhesive bonds to a UVO-modified PDMS surface. In the first modification, a self-assembled monolayer (SAM)17,18 is used to generate an interface appropriate for bonding a PDMS decal. For metals such as gold and silver, we found that a SAM presenting the hydrolyzed form of the silane coupling agent, (mercaptopropyl)trimethoxysilane (MPTMS), allows the durable bonding and high-quality transfer of PDMS decals that are useful as resists for large-area patterning. This feature of the chemistry is depicted in Scheme 1. We also describe a method for decal bonding based on the deposition of thin (14) Anderson, J. R.; Chiu, D. T.; Jackman, R. J.; Cherniavskaya, O.; McDonald, J. C.; Wu, H.; Whitesides, S. H.; Whitesides, G. M. Anal. Chem. 2000, 72, 3158-3164. (15) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27-40. (16) Jo, B.-H.; Van Lerberghe, L. M.; Motsegood, K. M.; Beebe, D. J. J. Microelectr. Sys. 2000, 9, 76-81. (17) Bishop, A. R.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127-136. (18) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1-68.

Materials Used. Poly(dimethylsiloxane) (PDMS, Dow Corning Slygard 184), 〈100〉 boron-doped silicon wafers (Silicon Sense, Inc.), quartz slides (Chemglass, Inc.), glass slides (Gold Seal), (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (“No Stick”, Gelest), sulfuric acid (Fisher), hydrochloric acid (12.1 N) (Fisher), hydrogen peroxide (30%, Fisher), 1 M tetrabutylammonium fluoride (TBAF) in THF (Aldrich), buffered hydrofluoric acid (6:1, NH4F/HF) (Ashland Chemical), butanethiol (Aldrich), hydrofluoric acid (49%) (Fisher), TFA Gold Etch (Transcene), sodium hydroxide (Fisher), iron(III) chloride (Fisher), (mercaptopropyl) trimethoxysilane (MPTMS) (Aldrich), and common solvents were obtained from commercial sources. A home-built apparatus employing a low-pressure mercury lamp (BHK, 173 µW/cm2 from 240 to 260 nm at ∼1 mm) was used as a UV source for UVO treatments. Masters were produced using photolithography to pattern photoresists (AZ 5214, Shipley or SU-8-5, MicroChem) as described previously19 using 5080 dpi transparencies as exposure masks. All masters were cleaned with UVO or an oxygen plasma and treated with “No Stick” (as a mold-release agent) in a closed container at around ∼150 mTorr for 2 h.20 Solvents used in processing these samples were of analytical grade or higher and used without purification unless noted otherwise. Gold and Silver Film Deposition and Silanization. Gold and silver films were modified using a dilute solution of MPTMS prior to patterning, as illustrated in Figure 2a-d. Silicon wafers and glass slides were cleaned in a piranha solution (10 min, warning: strong acidic oxidant, very harmful to personal contact), rinsed with deionized water, and dried with a stream of nitrogen prior to metal deposition (Temescal FC-1800 electron beam evaporator). A 20 nm thick titanium film was added to promote adhesion between substrates and coinage metal layers. The 1 mM MPTMS solutions were prepared by measuring from a stock of MPTMS stored at 4 °C using a micropipet (1.8 µL/10 mL) and mixing it with freshly distilled toluene, which had been sparged with nitrogen for ∼20 min. This solution was spin-cast onto a freshly evaporated metal surface for 30 s at 4000 rpm. The silanefunctionalized gold surfaces were then washed with ethanol and deionized water, immersed in a 0.1 M HCl solution for 15 min, washed again with ethanol and water, and dried with a stream of nitrogen. An analogous procedure was used for silver with the substitution of a 0.01 M NaOH solution in place of the HCl solution for the hydrolysis step. In a typical procedure, the PDMS stamp was sealed to one of these two surfaces within 5 min of processing using the UVO adhesion technique described below. Copper Film Processing. A thin silicon oxide layer was evaporated on top of a copper thin-film layer to promote the adhesion necessary for DTL patterning, as illustrated in Figure 2g-m. Silicon wafers or glass slides were cleaned in a piranha solution (10 min), rinsed with deionized water, and dried with a stream of nitrogen prior to metal deposition. Metal films were deposited in the following manner: a 20-30 nm thick layer of titanium was evaporated onto a glass surface to promote adhesion, a copper film was deposited on the substrate to the thickness desired (50-100 nm), and a 5-20 nm thick capping layer of silicon oxide was deposited onto the copper film, as measured using a crystal monitor. All three depositions were carried out under vacuum during the same processing cycle by switching targets in a multi-hearth e-beam evaporator. A PDMS stamp was sealed to this surface within an hour of processing using the UVO adhesion techniques described below.3 CMF Resist Deposition. Cohesive Mechanical Failure (CMF) patterning was used to deposit a thin, patterned PDMS etch (19) Deng, T.; Wu, H.; Brittain, S. T.; Whitesides, G. M. Anal. Chem. 2000, 72, 3176-3180. (20) Deng, T.; Tien, J.; Xu, B.; Whitesides, G. M. Langmuir 1999, 15, 6575-6581.

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Figure 2. Schematic illustrations of two general processes used to promote adhesion between metals films and PDMS. The silane functionalization method proceeds as follows: (2a) a coinage metal film is deposited onto a surface, (2b) a thiol layer is coated onto the surface of the metal thin film and hydrolyzed, (2c) a PDMS stamp, modified by UVO, is placed in contact with the silane-covered surface and heated, (2d) a PDMS decal is transferred by peeling away the PDMS support, (2e) unprotected metal areas are etched away, and (2f) the PDMS resist layer is removed. The silicon dioxide capping layer method proceeds as follows: (2h) a copper film is deposited onto a substrate surface, (2i) a 5-20 nm thick silicon oxide layer is deposited onto the copper film, (2j) a PDMS stamp is modified by UVO, (2k) the UVO-modified stamp is placed into contact with the oxide surface and heated, and (2l) the PDMS decal is transferred by removing the PDMS support; (2m) the silicon oxide capping layer and copper film are etched, and the PDMS resist is removed from the patterned surface. resist upon a substrate as described in a previous publication.3,12 Briefly, PDMS (Slygard 184) was cast upon a silicon master to form an elastomeric replica. The resulting stamp was physically modified by a UVO treatment, which entailed holding the surface of the stamp ∼1 mm from a low-pressure mercury bulb for 150 s. The stamp was aligned optically within 1 min and placed into conformal contact with the surface of an appropriately modified substrate (see above). The PDMS stamp and substrate were heated, while still in contact, in an oven for 20 min at 70 °C. Tweezers were used to pull back any corner of the bulk stamp to deposit a CMF-patterned film based on areas of contact and the adhesion-induced fracture mechanics of the PDMS. SPaR Patterning of Surfaces. We used SPaR to deposit a patterned PDMS resist onto the metal films prior to etching and lift-off. As previously described,3 a PDMS stamp was engineered to support a PDMS film of defined thickness, and this film was deposited onto a substrate through UVO-induced adhesion. This procedure was performed as follows (Figure 1). A PDMS preelastomer and toluene were mixed in a 2:1 ratio and spin-cast onto a silanized master at 5 × 103 rpm for 120 s (master feature height of 13 µm). This film then was cured at 70 °C for at least 30 min, and the cured film was exposed to UVO for 3 min and sealed in a container with an open vial of (tridecafluoro-1,1,2,2tetrahydrooctyl)trichlorosilane for 20 min under ambient conditions. A thick layer of PDMS pre-elastomer was poured onto the treated film/master and allowed to cure. The surface of the composite stamp was exposed to UVO for 150 s, placed into contact with an appropriately treated substrate, as illustrated in Figure 2c, and heated to 70 °C for 20 min while contact was maintained. Tweezers were then used to peel away the bulk support layer, which deposited a PDMS decal. PDMS decals were removed by immersion and sonication for 90 s in 1 M TBAF in THF.21 General Analytical Methods. Film thicknesses were measured by surface profilometry (Sloan Dektak3 ST) and atomic force microscopy (AFM) (Digital Instruments Dimension 3100). AFM measurements were taken in contact mode with scan rates of 1 Hz or less. Optical micrographs were recorded using an Olympus BH-2 optical microscope interfaced with a Panasonic GP-KR222 digital color camera. Electron micrographs were recorded using either a Zeiss DSM 960 or Hitachi S-4700 scanning electron microscope (SEM). Prior to SEM studies, a 6 nm thick layer of a palladium/gold alloy was sputtered (EMI Tech K575) onto samples to facilitate imaging. Auger Electron Spectroscopy (AES). Auger spectra were obtained using a PHI 660 Scanning Auger multiprobe. Static elemental scans were acquired using an electron gun with a (21) Jackman, R. J.; Brittain, S. T.; Adams, A.; Prentiss, M. G.; Whitesides, G. M. Science 1998, 280, 2089-2091.

potential of 2 keV and a current of 50 nA to stimulate Auger electron emission while the sample was held at a tilt of 55° offnormal to reduce charging. For copper samples, static scans were acquired at 3 keV and 100 nA, and dynamic profiles required the use of a 1 keV Ar+ ion gun with a current density of 30.4 µA. X-Ray Photoelectron Spectroscopy (XPS). XPS data were acquired using a Kratos AXIS ULTRA XPS with a monochromatic Al KR source (225 W), a hemispherical analyzer positioned at a take-off angle of 54.7°, and a constant band-pass of 160 eV for survey spectra or 40 eV for high-resolution spectra. A sample spot size of roughly 200 µm2 was selected, and a charge neutralizer was used for all samples. Binding energy shifts were referenced to the Au 4f7/2 (84.0 eV) and Ag 3d5/2 (368.3 eV) core levels using literature values,22 and Gaussian-Lorentzian line shapes were employed in line fitting analyses. Samples used to probe the effects of the alkoxysilane hydrolysis on the adhesion process were introduced into vacuum within 5 min of modification to avoid hydrolysis by ambient humidity. Other samples were placed under vacuum within an hour of preparation. Ellipsometry. Ellpisometric measurements were made using a Gaertner Scientific L116C ellipsometer equipped with a HeNe laser set at a 70° angle of incidence with the sample surface.23 Measurements were taken and averaged at five random positions on each sample to establish pseudosubstrate constants for each clean substrate immediately after film deposition. This was performed for five identical samples. The unhydrolyzed samples were washed with fresh toluene after exposure and dried with a stream of nitrogen. After silianization and, again, after hydrolysis as described, the samples were measured for film thickness in five random spots by employing a two-layer transparent-film model with a refractive index fixed at 1.45. The thickness reported is the average thickness measured over all five samples for each set of conditions. Gold Etching. Gold films were etched by immersion in a 1:1 mixture of TFA gold etch and deionized water for 20 s with mild agitation. They were then washed with deionized water and dried under a stream of nitrogen. Silver Etching. Silver films were dry-etched in a parallelplate plasma chamber (Plasma Therm 790) using a carbon tetrafluoide plasma for 3.5 min (300 mTorr, 30 sccm, 30 W, -15 V). The samples were washed with isopropyl alcohol to remove fluoride side products as described in the literature.24,25 (22) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (23) Noble-Luginbuhl, A. R.; Nuzzo, R. G. Langmuir 2001, 17, 39373944. (24) Nguyen, P.; Zeng, Y.; Alford, T. L. J. Vac. Sci. Technol., B 2001, 19, 158-165.

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Copper Etching. The silicon oxide capping layer was etched by immersion with mild agitation in buffered oxide etch (BOE) (Caution: BOE is extremely hazardous to contact.) for 5 s and washed with deionized water prior to copper etching. The copper film itself was etched for 25 s in a 0.01 M FeCl3/0.4 M HCl aqueous solution.26 The sample was cleaned with deionized water and dried under a stream of nitrogen.

Results The objective of this study was to develop a general procedure for modifying surfaces to make them compatible with Decal Transfer Lithography (DTL);3 we especially sought to expand the scope of materials patternable by this technique to ones broadly used in microelectronics applications. Earlier reports3,12 have established DTM protocols that are especially useful for patterning silicon and silicon dioxide materials through the agency of adhesive bonds established between the silanol-terminated surface of silicon oxide and the UVO-modified surface of PDMS stamps. The strategy adopted here for expanding the scope of adhesion-compatible materials was to develop methods for terminating surfaces with the same type of silanol functionality. Silane coupling agents constitute one such method for doing this. The direct evaporation of a thin silicon dioxide capping layer provides another. As the results below show, these surface-modification techniques produce interfacial adhesion strengths sufficient to accommodate the mechanics of DTL processing. Further, the bonding is sufficiently robust so as to allow the PDMS features to serve as an effective etch resist. These aspects of the modification strategies outlined above are illustrated by the specific model examples of metal thin-film patterning discussed below. In each case, unless otherwise noted, the modified DTL processes were critically tested using decals bearing ∼1 cm2 of pattern area, a value sufficient to illustrate the low defect levels engendered by this process. Patterning Gold Films. We successfully extended DTL patterning techniques to gold films by using (mecaptopropyl)trimethoxysilane (MPTMS) SAMs to modify their surfaces. In a prototypical example, a SPaR-engineered PDMS stamp was used to transfer a PDMS decal from a support onto the surface of a 60 nm thick gold film (Figure 3a). This pattern consisted of lines of PDMS whose widths and spacing varied from 50 µm to well over 300 µm. The thickness of the decal was ∼10 µm. Unprotected areas of the gold film were etched by immersion in a TFA solution for 2-3 s with mild agitation, and the titanium adhesion layer was subsequently removed by immersion in a 0.1 M HCl solution for 5 s to yield the pattern shown in Figure 3b (the PDMS has not yet been removed). A solution of TBAF was then used to strip the PDMS resist (data not shown) using a protocol similar to that described in an earlier publication.3 In a second demonstration, a PDMS stamp with features appropriate for transferring CMF-style resist patternss a square-planar array of posts 2 µm in diameter with a 2.7 µm center-to-center separationswas modified by UVO and placed into contact with a silanol-functionalized surface. A PDMS pattern of circles 2 µm in diameter was deposited onto a 60 nm thick gold film as the stamp was peeled away from the surface (see below). Unprotected areas of the gold film were removed by immersion in a dilute TFA gold etch (3:1, DI:TFA) with mild agitation for (25) Alford, T. L.; Nguyen, P.; Zeng, Y.; Mayer, J. W. Microelectron. Eng. 2001, 55, 383-388. (26) Xia, Y. N.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601-603.

Figure 3. Optical and electron micrographs of PDMS resists transferred onto silzanized gold films using DTL. A SPaRengineered PDMS stamp was used to transfer a PDMS decal onto an ∼60 nm thick gold film with line widths and spacing ranging from 30 to 300 µm and a thickness of 5-10 µm (a). The CMF technique was used to transfer a square array of PDMS resist structures, circles 2.0 µm in diameter with a 2.7 µm centerto-center separation, onto a 60 nm thick gold film (c). The films transferred using DTL functioned as resists during wet etching (b and d).

Figure 4. AFM scans of the square-planar array of gold circles before (a and b) and after the PDMS resist was removed (c and d). This sample corresponds to the samples shown in Figure 3c and d.

a duration of 15 s (Figure 3c). The remaining titanium adhesion layer and PDMS resist debris were removed by sequential immersion in 0.1 M HCl for 5 s and 1 M TBAF in THF for 30 s to give the array of gold dots shown in Figure 3d. The resulting gold features were slightly less than 2 µm in diameter due to the isotropic etching conditions used (Figure 3d). However, the overall pattern shape was maintained and transferred with few evident defects across an area of more than 1 × 1.5 cm2. AFM scans were acquired to investigate the thickness of the PDMS resist after etching and to examine the top surface of the gold pixel array after the PDMS resist was removed. The base diameter of the cone-shaped PDMS decal was measured to be 2.0 µm in diameter and covered the entire surface of the circular gold (Figure 4a and b) (which corresponds to the SEM image shown in Figure 3c). A line scan revealed that the thickness of the resist was ∼250 nm (Figure 4b). After the titanium layer and PDMS resist were removed, the gold pixels that remained were determined to have a top diameter of 1.8 µm and a height of 56.3 nm (Figure 4c and d). The noted roughness of the gold circles is likely due to the polycrystalline nature of the evaporated metal film and (by way of a control experiment) was found to be comparable to the roughness of gold films deposited by e-beam evaporation that were not subject to further processing (data not shown).

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Figure 5. XPS data of the C1s (a) and O1s (b) core levels of MPTMS films on gold after the successive process steps of being washed with dry toluene (green), deionized water (black), and immersed in 0.1 M HCl for 15 min (red).

XPS Analysis of MPTMS Hydrolysis. The successful examples of Au thin-film patterning shown in Figures 3 and 4 depend sensitively on the hydrolysis step used to activate the MPTMS SAM. Without this process step, the adhesive bonding needed to transfer the decal resist was both failure prone and hard to control over large substrate areas. For this reason, the critical steps of the silane hydrolysis procedure were examined by XPS to establish some understanding of their nature. These data are shown in Figure 5. We found that simply washing gold films silanized with MPTMS with deionized water was not sufficient to promote uniform adhesion of a PDMS decal across a large surface area. More-stringent procedures were required to hydrolyze fully the methoxy groups of the silane. The presence of the S2p core peak at a binding energy of 162.0 eV on the MPTMS-modified surface (data not shown) indicated that only thiolates were present on the gold surface.22 A gold film silanized with MPTMS, after rinsing with toluene, gave an XPS spectrum that indicated a surface composition bearing a significant quantity of unhydrolyzed MPTMS, as evidenced by the intensity of the C1s core level peaks at 285.0 (methylene) and 286.3 eV (methoxy).27 The latter peak was absent, however, when a similarly processed film was also immersed in 0.1 M HCl for 15 min, washed with water for 15 s, and heated in an oven at 70 °C for 20 min (Figure 5a). A slight decrease in binding energy for the O1s peak from 532.0 to 531.3 eV was also observed (Figure 5b). Samples washed only with deionized water for 15 s also showed lesser contributions from methoxy species in the XPS data (data not shown), but this treatment was not sufficient to promote decal adhesion consistently across large surface areas. Ellipsometry Measurements. Ellipsometery was used as an indicator of MPTMS surface coverage. Ellipsometery measurements of the gold surface taken 15 min after MPTMS was spin-cast on the surface and washed with dry toluene indicated a film thickness of 12 ( 2 Å. After these samples were hydrolyzed using the procedure described above, the apparent film thickness measured was 15 ( 2 Å. Correlating MPTMS Coverage and PDMS Adhesion. A second XPS experiment was carried out to further explore the functioning of MPTMS as an adhesionpromoting agent by competitively adsorbing it with butanethiol from solution. PDMS samples were exposed to UVO for 150 s and placed directly into contact with mixed SAMs on gold to test the effects on decal adhesion. As a critical control, no adhesion of a UVO-modified PDMS material was observed for an untreated gold sample. A series of solutions was then prepared in which the mole ratio of MPTMS to butane thiol was varied while (27) Thompson, W. R.; Cai, M.; Ho, M.; Pemberton, J. E. Langmuir 1997, 13, 2291-2302.

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Figure 6. XPS data of the C1s (a) and O1s (b) core levels for films adsorbed on Au from the following mixture of thiols in solution: pure MPTMS (black), 6:1 MPTMS/butanethiol (red), 3:1 (green); 1:1 (yellow), 1:3 (dark blue), 1:6 (pink), and pure butanethiol (light blue).

Figure 7. Optical and electron micrographs of PDMS resists transferred onto silanized silver films using DTL. A SPaRengineered stamp was used to deposit a PDMS decal onto a 100 nm thick silver film with line widths and spacing ranging from 30 to 300 µm and a thickness of 5-10 µm (a). The CMF technique was used to transfer a square array of PDMS resist structures, circles ∼2.0 µm in diameter with a 2.7 µm center-to-center separation (c), onto a 100 nm thick silver film. Decals on silver films served as an etch resist for RIE processing (b and d).

maintaining the overall concentration of thiol present in solution. Solutions with MPTMS-to-butanethiol molar ratios (MPTMS:BT) of 6:1, 3:1, 1:1, 1:3, and 1:6 were examined. These solutions were all cast onto gold samples and processed under similar conditions. The ratio of MPTMS to butanethiol was characterized by XPS using as a metric the integration peak areas in the O1s and C1s core-level regions (Figure 6a and b). Specifically, the carbon-to-oxygen peak area ratio was 2:1 for the pure MPTMS sample (top of Figure 6a) and increased to more than 10:1 across the series. Adhesion trials using the corresponding mixed thiol solutions to promote adhesion resulted in cohesive mechanical failure in the bulk material of PDMS stamps (unpatterned) when solutions with butanethiol contents as high as 1:3 (MPTMS:BT) were employed. No adhesion was observed for surfaces functionalized using 1 mM butanethiol solution. Perhaps more interestingly, a seemingly random pattern of adhesion was observed for the gold surface treated with the 1:6 (MPTMS:BT) thiol solution. Taken together with the data shown in Figure 6, these results suggest that even a fractional coverage of MPTMS-derived silanols on the treated Au surface is sufficient to mediate an adhesive DTL transfer. The XPS data suggest that coverages as low as 25% of a monolayer are sufficient in this regard. Patterning Silver Films. The same general procedure developed to promote DTL patterning of gold films was successfully extended to silver films with essentially identical results. A SPaR-engineered stamp similar to that described above (Figure 3a) was used to transfer an ∼10 µm thick decal onto a 100 nm thick silver film (Figure 7a). Excellent pattern transfer was obtained across an area of

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Figure 9. XPS data of C1s (a) and O1s (b) core levels of MPTMS films on silver after being washed with toluene (green), deionized water (black), or immersed in 0.01 M NaOH for 15 min (red). Figure 8. Silver and sulfur Auger images acquired in static mode after key steps of the patterning process. A PDMS decal was transferred onto a 100 nm thick silanized silver film by DTL (a and c). These decals served as an etch resist during RIE processing using a CF4 plasma. The second set of images shows the surface after removal of the resist (b and d).

1.0 cm × 1.5 cm. A second SPaR-engineered stamp was used to transfer a PDMS decal with lines 120 µm wide, separated by 75 µm spaces (data not shown). The sample was then dry-etched using a CF4 plasma for ∼190 s (150 mTorr, 30 sccm, 100 W, -130 V), after which the PDMS resist was removed to reveal a regular pattern of silver lines (Figure 7b).24 In another demonstration, a PDMS stamp was used to deposit a CMF-style resist onto a 100 nm thick silver film. The stamp transferred a square array of PDMS resist structures similar to that shown in Figures 3d and 4a above. This pattern was etched with a CF4 plasma for 3 min (as described in the Experimental Section) to yield the pattern of PDMS-protected silver circles shown in Figure 7c, the titanium adhesion layer and other debris was removed by immersion in a 1% HF solution for 5 min, and the PDMS resist was stripped using TBAF in THF to give the planar array of silver features shown in Figure 7d. The edge of the silver circles appeared to be less irregular than that of the gold circles patterned above, a feature that arises as a consequence of both the grain sizes of gold versus silver and the effect of the anisotropic RIE processing method used for this example. MPTMS Coverage SAMs on Silver. Static-mode AES imaging was used to examine the general surface coverage of MPTMS on silver films between the various processing steps of the DTL procedure. An initial scan of a silver surface treated with hydrolyzed MPTMS and patterned with a PDMS decal revealed a correlated overlay of silver and sulfur Auger signals, corresponding to the pattern of uncovered areas between PDMS resist features (Figure 8a and c). After dry-etching the sample with a CF4 plasma, these signals are lost. Upon removal of the PDMS resist, the Auger maps detected consistent areas of overlapping silver and sulfur signals in areas where the surface had been protected by the PDMS resist (Figure 8b and d). These images are the exact inverse of the Auger image maps shown in Figure 8a and c. These latter results suggest that the modifications effected by MPTMS on Ag may involve the formation of a thin metal sulfide phase.28 XPS Analysis of MPTMS Hydrolysis. The hydrolysis of MPTMS bound to the surface of silver films was examined by XPS. As with gold, silver also required hydrolysis of the MPTMS coupling agents to promote consistent large-area adhesion. The hydrolysis procedure was examined by XPS at three points in the process to (28) Saber, T. M. H.; El Warraky, A. A. J. Mater. Sci. 1988, 23, 14961501.

Figure 10. Optical micrographs of copper surfaces coated with silicon dioxide to promote DTL patterning. A 100 nm thick copper film was coated with a 20 nm thick layer of silicon dioxide to promote adhesion, and a SPaR engineered stamp was then used to deposit a PDMS decal with features as small as 40 × 40 µm2 (a). The CMF technique was used to pattern a copper surface with a square array of PDMS structures, 2.0 µm in diameter with a 2.7 µm center-to-center spacing, which then in turn were used as a wet-etch resist (b).

monitor changes in the binding energy of the C1s and O1s core peaks.27 After a silver film was silanized and washed with toluene, the peak-area ratio of methylene (285 eV) to methoxy (287 eV) C1s peaks was 2:1. An O1s peak with a binding energy of 532.0 eV and a pronounced shoulder at 530.3 eV also indicated a surface dominated by methoxy oxygen (rather than silanol). The C1s peak ratio remained 2:1 after the silane films were washed with deionized water, but the O1s peak indicated some increase in silanol formation, as noted by an increase in the peak area of the core level appearing at 530.5 eV. Tests of adhesion made at this point of the process revealed it to be sporadic and irreproducible. The C1s ratio increased to 12:1 after the sample was immersed in base (0.01 NaOH) for 15 min and heated in a oven at 70°C for 20 min, while the O1s peak at 531.5 eV continued to broaden. Consistent adhesion was always observed for samples after this treatment. DTM Patterning of Copper. We found that MPTMS can rapidly and destructively corrode copper. This chemistry (in our hands) did not support high-quality DTL patterning. For this reason, we explored the utility of using an e-beam-deposited nanoscale silicon oxide layer as an adhesion layer to promote bonding to UVO-modified PDMS. The results demonstrated that the adhesion promoted by this oxide buffer layer was sufficient for the requirements of both the SPaR and CMF modes of decal pattern transfer. A SPaR-engineered stamp was used to transfer a complex PDMS decal onto a 100 nm thick copper film coated with a 20 nm thick silicon oxide layer across an area of approximately 2.0 × 2.0 cm2 (Figure 10). The PDMS decal features varied from 20 to 180 µm in width, separated by spaces ranging from 20 to 300 µm (Figure 10a). Discrete features of PDMS as small as 20 × 20 µm2 squares and lines 100 mm long were transferred together from this single stamp. Another stamp patterned with PDMS posts, identical to the ones used for gold and silver (Figures 3d and 8d), was used to transfer a square array

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Figure 11. A schematic depicting the proposed mechanism of bulk corrosion when PDMS is directly modified with MPTMS. The patterned face of the PDMS stamp is (a) modified using UVO and (b) exposed to MPTMS vapor in a closed container. The MPTMS molecules are (c) not only adsorbed onto the surface but also absorbed into the bulk interior of the PDMS stamp. (d) The MPTMS bleeds out of the stamp and cross-links to form a siloxane film when the stamp is placed into contact with a metal surface. The siloxane film formation leads to longer etch times for gold films and corrosion for copper films.

of PDMS resist structures onto a similar silicon dioxidecoated copper surface. For this latter sample, the surrounding regions of the 100 nm thick copper film were etched away and the resist removed to reveal the square array of copper features shown in Figure 10b. Direct Bonding of Mercapto-Silane-Modified PDMS Decals to Gold and Silver. We attempted to bind thiol-functionalized PDMS stamps directly to gold films using a variation of the procedure shown in Figure 11. To do so, the surface of a PDMS stamp was exposed to UVO for 150 s and placed in an ∼200 mTorr vacuum equilibrated with MPTMS vapor for 2-3 h, as illustrated schematically in Figure 11b. Little or no adhesion was observed when a stamp prepared in this way was subsequently placed into contact with a gold substrate. Excellent adhesion was obtained, however, when the stamp was heated to 70 °C for 20 min in contact with these gold substrates. Although PDMS decals were readily transferred to these gold surfaces, these decals were found to be exceptionally poor etch resists. Etching times slowed markedly (from 2-3 to 27 s for a 100 nm thick gold film), and premature delamination of the PDMS films during etching was frequently noted. This same bonding procedure also resulted in sporadic adhesion to copper surfaces. The treatment also led to marked corrosion of the copper; an optically observable pattern of reddish lines appeared on the copper surface when these stamps were brought into conformal contact with a copper film and heated at 70° for 1 h, a process that promoted (on occasion) durable adhesion. Using a stamp similar to that used to pattern the decal shown in Figure 10a, we found the reddish features whose shape and overlay matched the relief pattern of the PDMS stamp (Figure 12a). An AES copper Auger image detected larger copper signals (brighter) from areas that matched the contact pattern (Figure 12b). A point in each area was analyzed by AES depth profiling to verify a difference in surface film thickness between the two patterned areas. Carbon, oxygen, silicon, and sulfur signals declined nearly twice as quickly during sputtering in areas of contact than in areas of noncontact (Figure 12c and d). As expected, the copper signal increased at a correspondingly higher rate for each area. These latter results demonstrate that significant bleeding of MPTMS from the stamp occurs in this processing. This excess evidently deposits thick multilayers in a nonpatterned form on gold and promotes significant corrosion of the copper films. For the latter case, thick metal sulfide corrosion products are obtained, albeit here in a patterned form.

Figure 12. Optical micrographs and Auger analyses for copper surfaces placed in contact with PDMS stamps that were treated with UVO and subsequently exposed to MPTMS. Optical micrographs recorded a reddish hue (a) on the copper surface in a pattern that matched the relief structure of the PDMS stamp used for patterning. A static Auger copper map (LMM) of a reddish pattern on a copper surface with a point marked in each patterned area, contact with PDMS (cross) and noncontact (triangle). Dynamic Auger depth profiles were acquired at each point (c and d) (Cu, blue (right scale); S, black; C, red; O, yellow; and Si, green).

Discussion DTL relies on adhesion to transfer molded PDMS films from stamps onto compatible substrates for lithographic patterning. Adhesion between the PDMS decal and substrate is crucial and is one of the fundamental challenges encountered in extending the scope of this technique. The current list of compatible materials includes silicon, silicon oxide, and oxidized PDMS surfaces,3 which are important materials for microelectronics,29-31 sensors,29,32,33 and lab-on-a-chip applications.29,34,35 The highest-resolution features transferable by DTL serve as etch resists, but usefully so, only if PDMS adheres strongly to the substrate surface. The utility of DTL, then, follows directly from the capacities that exist for promoting the adhesive bonding of PDMS decals, ones that can serve as resist structures in lithographic procedures for patterning materials. For this reason, new techniques to reproducibly promote that adhesion to PDMS surfaces were examined in this work. The present research describes a useful set of new procedures for patterning coinage metals, a highly desir(29) Madou, M. Fundamentals of Microfabrication; CRC Press: Boca Raton, Florida, 1997. (30) Jeon, N. L.; Hu, J.; Whitesides, G. M.; Erhardt, M. K.; Nuzzo, R. G. Adv. Mater. 1998, 10, 1466-1469. (31) Erhardt, M. K.; Nuzzo, R. G. Langmuir 1999, 15, 2188-2193. (32) Street, R. A. Phys. Stat. Sol. A 1998, 166, 695-705. (33) Middelhoek, S.; Bellekom, A. A.; Dauderstadt, U.; French, P. J.; Thout, S. R. I.; Kindt, W.; Riedijk, F.; Vellekoop, M. J. Meas. Sci. Technol. 1995, 6, 1641-1658. (34) Unger, M. A.; Chou, H. P.; Thorsen, T.; Scherer, A.; Quake, S. R. Science 2000, 288, 113-116. (35) Nehilla, B. J.; Popat, K. C.; Vu, T. Q.; Chowdhury, S.; Standaert, R. F.; Pepperberg, D. R.; Desai, T. A. Biotechnol. Bioeng. 2004, 87, 669-674.

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able class of materials enjoying wide use in microelectronics. These methods extend DTL, a form of softlithography, to surfaces modified for adhesive bonding either through the use of silanol-forming SAMs or thin silicon dioxide buffer layers. These procedures obviate several significant challenges to adhesive transfer patterning. It is known that PDMS can be made to adhere to a wide variety of plasma-oxidizable materials. Notable examples include materials such as polyethylene,36 polystyrene,36 silicon nitride,8 and titanium.8 However, coinage metals, such as gold and platinum, present a significant challenge because they are not easily oxidized.37 Other coinage metals, such as copper, do form oxides under ambient conditions, but these latter phases are known for the poor qualities of the adhesive interfaces they form.38 The modification of coinage metals with thiols to form SAMs of thiolates is well documented,22 and provides an exceptionally useful way to tailor many surface properties,39-42 including adhesion.8,39 Thiol SAMs provide a seemingly general adhesion platform since they react with a wide variety of materials. The assembly of thiolate SAMs on Au and Ag has been extensively studied, and the character of these structures in now understood in some detail.22,43 The SAMs formed on Cu, however, remain poorly characterized and are difficult to prepare reproducibly.22 It is known that some form of assembly can be obtained on copper surfaces. Such SAMs have been exploited as a means through which to impede oxidation44 and corrosion.45 In addition, tricholorosilanes have been used to modify the surface of oxidized PDMS,46-48 and silane coupling agents (SCA) have a long history of use as reagents to enable the adhesion of polymers to a wide range of metals and oxide-bearing materials,49 including silicon oxides. By using a thiol-functionalized silane, the advantages of both methods are combined to provide a method for extending DTL patterning to thiol-compatible (i.e., SAM-modifiable) surfaces. The present data demonstrate that mercapto-silane-based SAMs are an exceptionally useful addition to DTL-patterning protocols. The bonds they form with UVO-modified PDMS are quite strong, being sufficient to induce cohesive mechanical failures in a fully cured Sylgard 184 resin. However, it is not a uniformly applicable strategy. Our attempts to (36) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153184. (37) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry; 3rd ed.; John Wiley and Sons: New York, 1995. (38) DeLollis, N. J. Adhesives for Metals: Theory and Technology; Industrial Press: New York, 1970. (39) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (40) Qian, X.; Metallo, S. J.; Choi, I. S.; Wu, H.; Liang, M. N.; Whitesides, G. M. Anal. Chem. 2002, 74, 1805-1810. (41) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 12301232. (42) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550-575. (43) Sandhyarani, N.; Pradeep, T. Int. Rev. Phys. Chem. 2003, 22, 221-262. (44) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022-9028. (45) Jennings, G. K.; Laibinis, P. E. Colloids Surf., A 1996, 116, 105114. (46) Efimenko, K.; Wallace, W. E.; Genzer, J. J. Colloid Interface Sci. 2002, 254, 306-315. (47) Genzer, J.; Efimenko, K. Science 2000, 290, 2130-2133. (48) Xiao, D.; Zhang, H.; Wirth, M. Langmuir 2002, 18, 9971-9976. (49) Plueddemann, E., P. Silane Coupling Agents; Plenum Press: New York, 1982.

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promote adhesion between copper and PDMS using silane coupling agents were occasionally successful but more typically failed. Freshly evaporated copper films coated with pure MPTMS or immersed into dilute MPTMS solutions immediately after copper deposition occasionally produced adhesion suitable for decal transfer. More generally, however, the reaction with MPTMS led to corrosion of the copper surface, the formation of sulfide phases, and gross uncontrolled deposition of the silane. The deposition of silicon oxide capping layers onto copper films proved to be a viable alternative method for promoting the adhesion necessary for DTL patterning in this instance. Although we have not explored this chemistry more broadly, by its character, we suspect it will be a method of quite general applicability. It is interesting to note that, at least for the case of Cu, exceptionally thin silica adhesion layers can be used. Our results suggest films as thin as several nanometers are fully adequate for effecting DTL pattern transfer. These thin oxides are too thin to complicate subsequent patterning and are easily stripped. Their presence and the method used to deposit them had secondary benefits as well. As noted above, the silicon oxide layers were evaporated onto the copper substrate surfaces during the same evaporation process cycle. Most notable in this regard, this procedure was found to slow the subsequent oxidization of the copper markedly. As the capping layer used was typically 5-20 nm thick, this coverage of the Cu was sufficient to provide a protective barrier layer for the copper while also serving as a mechanically durable bonding layer. The PDMS adhesion effected in this way was not indefinitely durable, being found to degrade measurably a few hours after the deposition for samples left exposed to air. This window was long enough, however, to allow DTM patterning, and no delamination was observed later as a result of oxidation once the pattern was transferred. The PDMS-encapsulated structures are therefore ones possessed of considerable corrosion resistance and might in some elaborated form serve as the basis of a useful package scheme for Cu interconnects.50,51 Future work will explore this possibility more completely. Acknowledgment. This work was supported by the Department of Energy through the Seitz Materials Research Laboratory (DEFG02-91ER45439) and the National Science Foundation (Grant No. CHE 0097096). This material is based upon work supported by the U.S. Department of Energy, Division of Materials Sciences under Award No. DEFG02-91ER45439 through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign. Ellipsometry data were obtained in the Laser and Spectroscopy Facility at the Frederick Seitz Materials Research Laboratory. We would also like to thank Prof. George M. Whitesides and Kateri Paul for their generous gift of several masters and Ernie Sammann, Vania Petrova, Dr. Rick Haasch, Dr. Jeff White, and Nancy Finnegan for their technical expertise and many useful conversations. LA047884A (50) Rosenberg, R.; Edelstein, D. C.; Hu, C. K.; Rodbell, K. P. Annu. Rev. Mater. Sci. 2000, 30, 229-262. (51) Yang, M. X.; Mao, D.; Yu, C.; Dukovic, J.; Xi, M. Solid State Technol. 2003, 46, 37-38.