Atomic force microscope based nanofabrication of

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force microscope with the parallel patterning abilities of soft lithography. Master pattern ... The feasibility of using dip-pen nanolithog- raphy to generate master ...
APPLIED PHYSICS LETTERS 91, 123111 共2007兲

Atomic force microscope based nanofabrication of master pattern molds for use in soft lithography Matthew S. Johannesa兲 Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA and Center for Biologically Inspired Materials and Material Systems (CBIMMS), Duke University, Durham, North Carolina 27708, USA

Daniel G. Cole Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

Robert L. Clarkb兲 Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA and Center for Biologically Inspired Materials and Material Systems (CBIMMS), Duke University, Durham, North Carolina 27708, USA

共Received 26 July 2007; accepted 30 August 2007; published online 20 September 2007兲 The authors have developed a technique that couples nanolithographic patterning using an atomic force microscope with the parallel patterning abilities of soft lithography. Master pattern generation is accomplished using local anodic oxidation as a mask pattern for anisotropic wet etching of Si共110兲. The resulting nanostructures are then used as master patterns for the molding of polymeric stamps to be used for microcontact printing of alkanethiols. Analysis of the resulting patterns demonstrates the validity of this method as a simple, effective, and low cost alternative to conduct and prototype nanoscale patterning in a parallel fashion. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2787965兴 Nanoscale patterning has emerged as an important research area due to its direct impact in areas of nanotechnology with applications in biosensing, immunological detection assay generation, and DNA templating. Soft lithography is a useful research prototyping technique to pattern organic and biological molecules on surfaces at the nanoscale.1,2 In soft lithography, polymeric materials are cast against rigid master patterns with relief structures on the order of 30 nm– 500 ␮m. The initial development of soft lithography focused on a technique, now termed microcontact printing 共␮CP兲, in which a polydimethylsiloxane 共PDMS兲 stamp with micron-scale protrusions was linked with an alkanethiol and used to pattern self-assembled monolayers 共SAMs兲 on gold.3 Typically photolithography or e-beam lithography is employed to generate master patterns. We outline a technique through which master patterns for soft lithography are generated using atomic force microscope 共AFM兲 based patterning followed by wet chemical etching to form high aspect ratio nanostructures that are then used as molds for the formation of polymeric stamps to pattern nanoscale alkanethiol SAMs on gold using ␮CP. AFM based anodization nanolithography is a patterning technique in which a voltage bias between a conducting AFM tip and sample in the presence of a water meniscus generates a localized oxide layer. Current research focuses on determining the very nature through which this process occurs, including modeling and predicting the resulting oxide aspect ratio due to experimental parameters such as voltage, humidity, tip dwell time, and gaseous ambient environment.4 The result of such investigations demonstrates that anodization nanolithography on Si substrates can be used to control oxide features with spacing and width typia兲

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cally in the 10– 150 nm range, which is attractive for nanoscale patterning. The feasibility of using dip-pen nanolithography to generate master patterns for soft lithography has been mentioned previously; however, the effectiveness and execution of such a method have yet to be demonstrated.5,6 Thorough research on the anisotropic etching characteristics of Si共110兲 exists in the literature using wet chemical etching schemes of strong alkaline bases such as potassium hydroxide 共KOH兲.7,8 This characteristic has been combined with AFM based anodization nanolithography to create nanostructures 50 nm wide and 300 nm tall.9 The crystal lattice structure of Si共110兲 enables the formation of high aspect ratio nanostructures, with etch rate selectivity S between 兵110其 and 兵111其 as high as 650:1.8 If the oxide mask is oriented along the 关112兴 direction, KOH etching results in nanostructures with horizontal top planar 共110兲 faces and vertical 共111兲 sidewalls. Substrates used in the experiments were cleaved from a 3 in. n-type phosphorous doped Si共110兲 wafer with a resistivity of 5 – 10 ⍀ cm 共Montco Silicon Technologies兲. The samples were cleaned in a solution of 70% H2SO4 and 30% H2O2 at 80 ° C for 10 min to remove contamination. Samples were then dipped in buffered oxide etch 共estimated to be 40% NH4F, 5% HF, and 55% H2O兲 for 30 s to remove the native oxide. The AFM cantilevers used are Si3N4 cantilevers that have a nominal force constant of 0.58 N / m 共Veeco Metrology兲 and are tipside coated with 30 nm of evaporated Ti at 2 Å / s. In this work, anodization nanolithography is conducted in contact-mode AFM 共CM-AFM兲 at 5 nN force setpoint using a custom AFM in a humidity controlled environment 关5%–90% relative humidity 共RH兲兴.10 Image measurements were made using IMAGEJ 共NIH兲. Figure 1 outlines the observed effects that humidity has on oxide linewidth and measured current during AFM based anodization nanolithography at 9 V tip-sample bias and

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FIG. 1. 共Color online兲 AFM based anodization nanolithography generated oxide FWHM and tip-sample faradaic current as influenced by changes in relative humidity. Each data point for width 共diamond兲 and current 共star兲 is an average over the length of a 2 ␮m oxide line created at 9 V bias and 0.5 ␮m / s. Linear fitting for width 共dotted兲 and current 共solid兲 is displayed.

0.5 ␮m / s translational velocity. The figure indicates that humidity regulation can be used to control resulting oxide linewidth as features vary from 91 to 175 nm over changes in relative humidity from 10% to 75%. Faradaic current is also monitored 共6485 picoammeter, Keithley Instruments兲 and can be used as a means for quality control during lithography.11,12 Each data point for current and linewidth 关full width at half maximum 共FWHM兲兴 is the average over the length of a 2 ␮m oxide line. Linear data trends are displayed to guide the eyes. A minimum oxide mask thickness of 0.4– 0.7 nm has been shown to be effective for anisotropic Si etching.13 Master pattern generation using anodization nanolithography was conducted at 10 V bias 共oxide height ⬃2 nm兲, 0.5 ␮m / s, and 30% RH; these parameters were chosen as they are typical deposition parameters used for CM-AFM anodization nanolithography. Two mask patterns were generated that have lines spaced 500 and 350 nm, center to center, that are oriented in the 关112兴 direction. After oxide mask deposition the samples were immersed in 40 wt % aqueous KOH at 45 ° C for 30 s under ultrasonic agitation. Ultrasonic agitation during etching in KOH lowers roughness on the micron and millimeter scales.14 We have observed roughness on the order of 5 nm over 40⫻ 40 ␮m2 with this method measured using AFM. Under these conditions, the etch rate of 44 wt % KOH on an unobstructed Si共110兲 surface is estimated to be 3.8 nm/ s, which yields an expected feature height of 113 nm.15 The two oxide mask patterns cover an area of 5 ⫻ 15 ␮m2. To deposit the oxide mask over a larger area, counteract tip wear issues, or minimize oxide linewidth faster scan speeds 共up to 0.13– 0.5 mm/ s for CM-AFM兲, alternative AFM modes of operation 共intermittent contactmode AFM兲 and high performance tips 共carbon nanotube terminated兲 can be employed.16,17 Figure 2 depicts scanning electron microscope 共SEM兲 images of the resulting nanostructures and AFM measurement yields an average feature height of 120 nm, which agrees well with predicted values. The prescribed pitches of 350 and 500 nm and resulting pitches of 356 and 503 nm 共50 measurements, ␴ = 5 nm兲 agree well and amount to a lateral pitch error of 1.7% for the 350 nm master and of 0.6% for the 500 nm master. Commonly used PDMS material 共Sylgard 184兲 for soft lithography applications is incapable of reliably patterning

Appl. Phys. Lett. 91, 123111 共2007兲

FIG. 2. Si master patterns generated using AFM based anodization nanolithography followed by anisotropic wet etching for use as molds for polymeric stamps in soft lithography. The left image is of a 500 nm pitch master 共width, 157± 5 nm; spacing, 350± 6 nm; pitch, 503± 5 nm兲 and the right is a 350 nm pitch master 共width, 158± 3 nm; spacing, 197± 4 nm; pitch, 356± 5 nm兲. Areas of lighter contrast correspond to taller height.

features below 1 ␮m primarily due to the low modulus of the cured PDMS 共1 – 3 MPa depending on preparation兲.18 h-PDMS 共⬃9 MPa兲 was developed to solve stability issues and composite stamps help alleviate the tendency towards stamp cracking.19 The master patterns of Fig. 2 form stamps with feature aspect ratios of 0.4 共500 nm master兲 and 0.6 共350 nm master兲, which are ideal for stamp stability during ␮CP.18 h-PDMS stamps were molded to the master patterns of Fig. 2 following procedures outlined in the literature.20 Au samples were prepared by coating a Si共100兲 wafer with 3 nm of Cr followed by 22 nm of Au. The stamp was used for ␮CP of 1 mM octadecanethiol 共ODT兲 onto the Au substrates 共30 s兲. Figure 3 depicts SEM images of ODT SAM patterns formed using h-PDMS stamps cast against the master patterns of Fig. 2. SEM imaging has been shown to be a viable method for imaging alkanethiolate SAMs on gold.21 Patterning was executed on a number of repeated printing attempts with no discernible degradation in pattern quality. Measurements were taken on the two printed SAM patterns shown in Fig. 3. Each measurement of width 共black contrast, 109± 18 nm for the 500 nm pattern and 96± 12 nm for the 350 nm pattern兲, spacing 共white contrast, 375± 11 nm for the 500 nm pattern and 237± 10 nm for the 350 nm pattern兲, and pitch 共485± 11 nm for the 500 nm pattern and 335± 9 nm for the 350 nm pattern兲 is the result of 50 random measurements. There exists a noticeable 18 and 21 nm of pitch loss of the pattern 共stamp兲 compared to the master.

FIG. 3. SEM images of nanoscale SAM patterns of ODT created using ␮CP with an h-PDMS stamp molded from AFM created masters. The left image shows a pattern formed using a stamp molded to the 500 nm master pattern of Fig. 2 while the right pattern was generated using a stamp molded to the 350 nm master. Lighter areas correspond to the methyl terminated ODT SAM while the darker areas represent the Au substrate. Downloaded 11 Dec 2009 to 128.151.110.245. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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FIG. 4. SEM images of nanoscale Si master with decreasing pitch of 5 nm per instance from top to bottom. Areas of lighter contrast correspond to taller features. The spacing shown at the bottom of the image is 25± 3 nm.

For h-PDMS, the thermal expansion coefficient is reported as 0.450 nm/ ␮m ° C.19 The h-PDMS was cured at 80 ° C, 56 ° C above room temperature, and corresponds to an expected pitch contraction of 13 nm for the 500 nm stamp and of 9 nm for the 350 nm stamp. There also exists a measured decrease in feature width and corresponding increase in feature spacing noticed in both the 350 and 500 nm patterns. This is directly due to alkanethiol spreading during ␮CP.22,23 One report found that feature edge spread was 30 nm using ODT printed for 10 s during ␮CP.24 In our work the average edge spread is determined to be 25 nm over 30 s, which shows good agreement. What are the minimum feature dimensions for master patterns that can be fabricated in this manner? We explored this by generating a varying pitch master pattern 共Fig. 4兲 using the nanofabrication process outlined above. In this experiment, the cantilever is Si with 3 nm Cr followed by 20 nm PtIr coated on the tipside with a stiffness of 0.2 N / m 共Veeco Metrology兲 and previously defined lithography parameters are used. Pitch variation of the oxide mask was generated at a decrease of 5 nm per instance. The substrate was then etched in 40 wt % aqueous KOH and analyzed using SEM. The depth of the relief structures is assumed to mirror the measured height as the SEM contrast outside the patterned area is identical to that between the raised features. Measurement analysis performed in Fig. 4 demonstrates good agreement between the planned and the measured pitch 共maximum difference of 3 nm兲 and highlights a minimum feature spacing of 25± 3 nm. Below this spacing level it was found that the etch depth was uneven across the length and could indicate a critical limit for nanofabrication in this manner. Recent work focusing on decreasing the minimum feature dimensions of h-PDMS stamps molded to rigid master patterns demonstrated the generation of 40 nm feature widths with an aspect ratio 共h / w兲 of 1.5.25 The authors hypothesize that smaller stamp features could possibly be created using their designed methods as they were hindered by the inability to obtain master patterns with smaller feature sizes. We have developed a method by which master patterns created using AFM based anodization nanolithography and

wet chemical etching are used for soft lithographic patterning via ␮CP. We demonstrate that master patterns with pitches of 500 and 350 nm are used to form h-PDMS stamps that are subsequently used in ␮CP of alkanethiol SAMs down to feature dimensions of 96 nm. The fabrication of master pattern relief structures spaced 25 nm is highlighted, which is below the demonstrated limit for the critical dimension of polymeric stamp formation for soft lithography 共40 nm兲. Master patterns generated using AFM based anodization nanolithography followed by wet chemical etching can aid in the exploration of the resolution limitations of stamp formation and patterning using soft lithographic techniques. This method provides for a simple, fast, costeffective, and accessible alternative for master fabrication for soft lithographic stamp molding compared to more common methods such as electron beam lithography, ion beam milling, and photolithography. Due to the fast and simple generation of these master patterns, prototyping of new materials and process concerning polymeric stamp formation for soft lithography can easily be investigated. The fabrication of such master patterns covering larger areas will be facilitated as parallel cantilever arrays become more commonplace in the research environment. This work was supported by the NSF under NIRT DMI-0609265. Y. N. Xia and G. M. Whitesides, Annu. Rev. Mater. Sci. 28, 153 共1998兲. B. D. Gates, Q. Xu, M. Stewart, D. Ryan, C. G. Willson, and G. M. Whitesides, Chem. Rev. 共Washington, D.C.兲 105, 1171 共2005兲. 3 A. Kumar and G. M. Whitesides, Appl. Phys. Lett. 63, 2002 共1993兲. 4 D. Stievenard and B. Legrand, Prog. Surf. Sci. 81, 112 共2006兲. 5 M. S. Johannes, D. G. Cole, and R. L. Clark, Proceedings of the ASME International Mechanical Engineering Congress, Anaheim, 2004 共unpublished兲. 6 K. S. Salaita, S. W. Lee, D. S. Ginger, and C. A. Mirkin, Nano Lett. 6, 2493 共2006兲. 7 D. L. Kendall, Appl. Phys. Lett. 26, 195 共1975兲. 8 K. E. Bean, IEEE Trans. Electron Devices 25, 1185 共1978兲. 9 F. S.-S. Chien, C.-L. Wu, Y.-C. Chou, T. T. Chen, S. Gwo, and W.-F. Hsieh, Appl. Phys. Lett. 75, 2429 共1999兲. 10 M. S. Johannes, J. F. Kuniholm, D. G. Cole, and R. L. Clark, IEEE. Trans. Autom. Sci. Eng. 3, 236 共2006兲. 11 M. S. Johannes, D. G. Cole, and R. L. Clark, Appl. Phys. Lett. 90, 103106 共2007兲. 12 W. C. Moon, T. Yoshinobu, and H. Iwasaki, Jpn. J. Appl. Phys., Part 1 41, 4754 共2002兲. 13 B. Legrand, D. Deresmes, and D. Stievenard, J. Vac. Sci. Technol. B 20, 862 共2002兲. 14 T. Baum and D. J. Schiffrin, J. Micromech. Microeng. 7, 338 共1997兲. 15 H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgartel, J. Electrochem. Soc. 137, 3612 共1990兲. 16 P. A. Fontaine, E. Dubois, and D. Stievenard, J. Appl. Phys. 84, 1776 共1998兲. 17 H. J. Dai, N. Franklin, and J. Han, Appl. Phys. Lett. 73, 1508 共1998兲. 18 E. Delamarche, H. Schmid, B. Michel, and H. Biebuyck, Adv. Mater. 共Weinheim, Ger.兲 9, 741 共1997兲. 19 H. Schmid and B. Michel, Macromolecules 33, 3042 共2000兲. 20 T. W. Odom, J. C. Love, D. B. Wolfe, K. E. Paul, and G. M. Whitesides, Langmuir 18, 5314 共2002兲. 21 G. P. Lopez, H. A. Biebuyck, and G. M. Whitesides, Langmuir 9, 1513 共1993兲. 22 E. Delamarche, H. Schmid, A. Bietsch, N. B. Larsen, H. Rothuizen, B. Michel, and H. Biebuyck, J. Phys. Chem. B 102, 3324 共1998兲. 23 R. B. A. Sharpe, D. Burdinski, J. Huskens, H. J. W. Zandvliet, D. N. Reinhoudt, and B. Poelsema, Langmuir 20, 8646 共2004兲. 24 R. B. Bass and A. W. Lichtenberger, Appl. Surf. Sci. 226, 335 共2004兲. 25 H. Kang, J. Lee, J. Park, and H. H. Lee, Nanotechnology 17, 197 共2006兲. 1 2

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