DPN-Generated Nanostructures as Positive Resists for Preparing

NANO LETTERS

DPN-Generated Nanostructures as Positive Resists for Preparing Lithographic Masters or Hole Arrays

2006 Vol. 6, No. 11 2493-2498

Khalid S. Salaita, Seung Woo Lee,† David S. Ginger,‡ and Chad A. Mirkin* Department of Chemistry, Department of Materials Science and Engineering, Department of Medicine, and International Institute for Nanotechnology, Northwestern UniVersity, EVanston, Illinois 60208 Received July 24, 2006; Revised Manuscript Received August 22, 2006

ABSTRACT Experiments that utilize structures generated by dip-pen nanolithography (DPN) as positive resists for fabricating nanohole arrays and lithographic masters are described. The technique takes advantage of the difference in desorption potentials for patterned structures made from 16mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT), respectively. In this approach, patterns of MHA on gold are generated by DPN, and surrounding areas are passivated by ODT. Electrochemistry is used to selectively remove the MHA nanofeatures made by DPN. The exposed gold can be used as an electrode to plate silver from solution, generating raised features and structures that can be transferred to PDMS to make a lithographic master, or alternatively, they can be etched to make arrays of nanoholes.

Nanostructured metal surfaces currently play a vital role in electronics, optics, biodiagnostics, and catalysis.1-3 In analogy to the role of a photoresist in conventional photolithography, self-assembled monolayers (SAMs) of alkanethiols have been utilized as masks to direct the patterning of underlying Au films. The use of SAMs to pattern thin Au films has relied on two approaches. The first is an indirect approach that locally excites or degrades an adsorbed monolayer with a focused ion4 or electron beam,5-8 photoradiation,9,10 or a scanning probe microscope tip.11 In most cases the monolayer is damaged or destroyed,12,13 but in some cases cross-linking is effected.14 SAMs patterned in this manner have been used as masks to control the electrodeposition of metal salts or substrate etching at the exposed regions of the Au surface.5,7,15-17 Furthermore, except for the resolution-limited photolithographic methods, indirect patterning of alkanethiol resists is a low-throughput process that typically requires serial scanning. The second approach uses direct deposition tools such as micro-contact printing (µCP),18 or dip-pen nanolithography (DPN)19,20 to directly deposit alkanethiol adsorbates onto a surface. µCP and variants of it are very popular and useful approaches for printing organic structures on surfaces because they are massively parallel and allow one to control feature size typically down to ∼200 nm. For example, patterns of hexadecanethiol (CH3(CH2)15SH) have been used as a mask * Corresponding author. E-mail: [email protected]. † Present address: School of Chemical Engineering & Technology, Yeungnam University, Gyeongsan 712-749, Korea. ‡ Present address: Department of Chemistry University of Washington, Box 351700 Seattle, WA 98195-1700. 10.1021/nl061719t CCC: $33.50 Published on Web 09/27/2006

© 2006 American Chemical Society

for the electroless deposition of Ni on Au.3 Others have shown that µCP patterned alkanethiols of various lengths can be used to direct the electrodeposition of Ag and Au salts depending on the applied potential.21 DPN has emerged as a powerful direct-write tool for tailoring the chemical composition of surfaces on the sub50 nm to many micrometer length scale.19,20,22 Indeed, small organic molecules,20,23-26 oligonucleotides,27 proteins,28 conducting polymers,29 and sol gels30 have been patterned on inorganic substrates such as Au, Ag, and SiOx. Importantly, DPN-patterned alkanethiols can be used as negative etch resists for generating a variety of inorganic nanostructures. For example, 16-mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT) were used as etch resists to generate 12 nm gaps and sub-50-nm metal disks composed of Au, Ag, and Pt on a silicon substrate.31-35 Utilizing alkanethiols in an inverted “positive” resist mode could be highly desirable because deposition of the etchprotecting layer could be performed by bulk SAM adsorption rather than printing, subsequently reducing the total patterning time (Scheme 1). This is especially the case when implementing low-density patterns. Approaches using alkanethiols as a positive resist have been proposed and demonstrated in the context of µCP using bulky or poorly ordered adsorbates that provide high adsorbate solution exchange resistance, but are poor etch resists.36,37 However, the synthesis of adsorbates that form poorly ordered monolayers is a challenge, and such molecules undergo solution exchange and still impede etching, leading to lower control over pattern formation and resolution. Ideally, one would

Scheme 1. Diagram Depicting How DPN-Generated Alkanethiol Adsorbates Can be Used to Generate Solid-State Features

like to place molecules as positive resists, passivate with a second molecule, and then completely remove the positive resist molecules. Herein we present a novel method for fabricating solidstate structures that couples the high-resolution of DPN38 with electrochemical selective desorption39,40 of alkanethiol adsorbates (Scheme 1). This approach was inspired from previous detailed studies into the selective electrochemical desorption of monolayers based upon adsorbate tailgroup.41 Specifically, MHA features generated in a DPN experiment desorb at 100 mV more positive potential than that for similar ODT structures under alkaline conditions.41-44 This was attributed by Kakiuchi et al. to the repulsive interactions between the negatively charged carboxylate tail groups at high pH, as well as the increased solubility of the desorbed species. It has been taken advantage of in the development of parallel miniaturization schemes developed by our group.42,43 In this work, patterned MHA features in an ODT matrix are selectively desorbed, leaving ODT intact and exposing a bare Au template that either directs the etching of an Au surface or directs the electrodeposition of metal salts. There are three key advantages of this approach. First, this is a maskless technique that does not require expensive highquality masters. As a result, rapid prototyping of combinatorial libraries of different feature sizes and spacings can be performed easily. Second, this is a relatively simple methodology (especially when compared with photolithographic techniques) that does not require the use of a clean room and instead relies on wet chemical approaches. Consequently, this wet-chemical-based technique may allow for precise immobilization of reagents site-specifically to the generated templates. Finally, the described approach is amenable to massive parallelization.45,46 As a proof of concept experiment, an array of 30 × 30 MHA structures consisting of rows of dots (4, 2, and 1 s hold time) with 430, 310, and 210 nm average dot diameters (( 20 nm) were generated using DPN on a 40-nm-thick Au film evaporated on a SiOx substrate (Figure 1A). The total time required to generate this array was less than 12 min. The Au substrate was then passivated in a 5 mM ODT 2494

Figure 1. (A) LFM image of part of an array of MHA dots generated on an Au substrate. The array consists of rows of dots prepared with 4, 2, and 1 s hold times, respectively, which correspond to 430, 310, and 210 ((20) nm dot diameters, respectively. (B) Tapping-mode AFM (TMAFM) image of an array of holes (380, 270, and 190 ((20) nm diameter) fabricated in a 40 nm Au film. (C) TMAFM image of the entire 900 hole array (30 × 30). (D) Height profile of two rows of holes as indicated in B. Note that wider templates generate deeper holes. The z scale is 60 nm for B and C.

solution for 15 min, and subsequently rinsed with ethanol and Nanopure water. To effect selective desorption of the MHA template, we applied a potential of -800 mV (vs Ag/ AgCl, 3M NaCl) to the substrate for 5 min. The exposed Au template was etched by holding the sample at open circuit potential (OCP) for 20 min in an alkaline 1 mM KCN solution (the OCP of the ODT protected Au was about -520 mV). Tapping-mode AFM images (TMAFM) indicate that the resulting hole structures are very uniform, as defined by the hole diameter and depth profile, and are consistent with the lateral dimensions and lattice spacings of the DPNgenerated MHA template, Figure 1B-D. Interestingly, the hole structures have a lateral diameter about ∼10% smaller than that of the original DPN-defined MHA patterns and the average dot diameters were 400 (( 21 nm), 270 (( 21 nm), and 190 (( 27 nm) for the dots prepared with 4, 2, and 1 s holding times, respectively (n ) 14). The difference between resist feature size and hole size is most likely the result of the exchange of adsorbed MHA with the ODT in solution at the periphery of the MHA features, which has been investigated previously in the context of DPN deposited structures.47 Notably, the depth of the holes is proportional to the diameter of the MHA-defined templates. For example, the 430-nm-diameter templates yielded holes with an average depth of 40 ( 3 nm, whereas the 310 nm diameter templates had an average depth of 37 ( 4 nm, and the 210 nm diameter templates had an average depth of 30 ( 3 nm (Figure 1D). For this example, gold remains at the bottom of each hole in all cases. Nano Lett., Vol. 6, No. 11, 2006

Figure 2. Cyclic voltammograms for the bulk reductive desorption of ODT and MHA monolayers on Au in 0.5 M KOH at a scan rate of 100 mV s-1. The vertical lines indicate the boundaries of electrochemical desorption for MHA and ODT, and the highlighted green region (-800 mV > Edes > -850 mV) indicates the potential where MHA is selectively desorbed and ODT adsorbates remain intact.

Crooks and co-workers examined the corrosion of Au(111) under potential control in CN- solution and found that the etching rate of bare Au is considerably faster than that of 1-hexadecanethiol monolayer passivated Au.48,49 Exclusive etching of the Au surface at the areas defined by the template created by MHA desorption indicates that the majority of MHA molecules are indeed reductively removed at the applied potential of -800 mV. This process is highly sensitive to the applied potential. For example, if the applied potential is -850 mV, then ODT desorption starts to occur, and as a result randomly dispersed pits across the Au samples can be observed (see the Supporting Information). Alternatively, holding an MHA patterned Au substrate at -750 mV for 5 min, and exposing it to a CN- etch solution for 20 min does not result in etching of the MHA defined regions (see the Supporting Information). Although MHA desorption is induced at all potentials more negative than -800 mV, only within a relatively small window (-800 mV > Edes > -850 mV) does selective desorption for MHA over ODT occur (green area, Figure 2). When the hole arrays are exposed to CN- solution for extended periods of time (e.g., 30 min), some of the fabricated holes reveal a highly faceted structure with 3-fold symmetry (Figure 3). This 3-fold symmetry most likely reflects the predominant Au(111) character of the evaporated Au films on Ti-coated silicon oxide and glass substrates.50 These results are in agreement with STM observations by Bard and McCarley of triangular etch pits formed during etching of single-crystal Au(111) in aqueous CN- solutions.51 Nano Lett., Vol. 6, No. 11, 2006

Figure 3. (A) TMAFM of etched hole in a 60 nm polycrystalline Au film. Note the faceted hole shape that reflects the predominant Au(111) character of the evaporated film. (B) Height profile of highlighted region in TMAFM image. The hole was etched down to the Ti/SiOx substrate base, which explains why the base of the hole is smoother than the polycrystalline top of the Au film.

Figure 4. (A) LFM image of part of an array of triangular MHA structures written using a 1 µm/s tip speed. The edge length is 740 nm, and the line width is 190 nm ((20 nm). (B) TMAFM image the same substrate in A after selective desorption (-800 mV, 5 min) of MHA and subsequent etching at open circuit potential (10 min). (C) Height profile of the highlighted region in B, showing the depth profile of the etched triangular structures. The average peak depth of all of the structures is 34 ( 3 nm.

Another striking feature of these etch pits is that once all of the Au is etched the base of each hole is extremely flat because the grain size of Ti/SiOx (∼5-10 nm) is smaller 2495

Figure 5. (A) Darkfield micrograph of Ag structures electroplated onto MHA defined dot templates with alternating diameters of 1.25 µm and 540 nm. The inset shows an AFM image of one set of structures. (B) AFM height profile of the region indicated in the optical micrograph. (C) Schematic illustration of the process of selective Ag electrodeposition onto the MHA-defined template.

than that of Au (∼30-50 nm) (compare Figure 3A and B). The chemical composition of the substrate underneath the DPN-templated nanoholes was confirmed by using energydispersive X-ray spectroscopy (EDS) experiments (see the Supporting Information). The EDS analysis indicates that the nanoholes exclusively exhibit the characteristic elemental signatures for SiOx, whereas ODT passivated Au exhibits the elemental signatures of both Au and SiOx. In principle, selective immobilization of reagents to the sidewall or the base of the holes is possible because they present different surfaces with varying reactivity toward thiol and silane groups, respectively. To demonstrate that this method can be used to generate nanoholes of almost any shape, DPN was used to pattern triangular MHA frames with an edge length of 740 ( 30 nm and a line width of 190 ( 20 nm (Figure 4A). This was achieved by using a relatively fast tip writing speed (1 µm/ s) to avoid filling in the centers of the triangular structures.35 The substrate was then passivated with ODT, and then a potential of -800 mV (vs Ag/AgCl, 3M NaCl) was applied for 5 min. After exposure to the CN- etch solution, the substrate was imaged by TMAFM (Figure 4B). The resulting triangular frame-shaped holes have an edge length of 730 ( 30 nm and a line width of 170 nm ( 25 nm (Figure 4C). Both the edge length and the line width of the resulting triangle-shaped holes are smaller than the original MHAdefined templates, which is consistent with the dot-shaped structures described above, and again is most likely a result of MHA exchange with the ODT in solution. The average peak depth of the pits was 34 ( 3 nm. Given the surge of interest in studying the optical transmission properties of nanoscale hole arrays, we also have demonstrated that optically transparent substrates such as quartz can be used in place of silicon to yield qualitatively similar results (see the Supporting Information).52 Bare Au templates can also be used to direct the selective electrodeposition of metal salts (Scheme 1). To demonstrate this capability, an array of 15 × 15 MHA dots with alternating diameters of 1 µm and 400 nm was generated on a polycrystalline Au substrate. The substrate was then passivated with ODT by immersing it in a 5 mM ODT 2496

solution for 15 min. After rinsing with ethanol and water, the MHA portion of the patterned substrate was then selectively desorbed at a potential of -800 mV (vs Ag/AgCl, 3M NaCl) for 5 min. Ag structures were then electrodeposited from a commercial Ag plating bath (Tetronics 1025 Ag plating solution, containing KAg(CN)2) by applying a potential of -800 mV for another 5 min. The resulting Ag structures had a hemispherical shape and were characterized using darkfield microscopy and AFM imaging (Figure 5). The height of the Ag features can be controlled by adjusting the total number of coulombs passed in the experiment, and the lateral dimension of the Ag structures is defined by the original MHA patterns. Interestingly, 400-nm-diameter dots resulted in Ag structures with a diameter of 550 ( 70 nm and a height of 150 ( 20 nm, whereas the 1-µm-diameter features yielded 1.27 ( 0.15-µm-diameter Ag structures with a height of 380 ( 20 nm. It is important to note that the resulting Ag structures have a hemispherical shape because it is equally likely that Ag deposits will grow from all directions after the Ag grows beyond the height of the ODT barrier layer (2.2 nm). The rate of growth shows a behavior similar to that observed with the nanohole arrays (Figure 1) where larger diameter templates generated deeper holes (Figure 5C). Although the reason for this is unclear, this phenomena is not the result of diffusion-controlled Ag particle growth because that would result in smaller templates growing faster than larger ones.53 The chemical composition of DPN-templated nanoscale Ag structures was confirmed by using EDS experiments. The EDS analysis of Ag electrodeposited structures exhibits the characteristic elemental signatures for both Au and Ag, whereas the background Au surface exhibits elemental signatures for Au exclusively. Poly(dimethylsiloxane) (PDMS) stamping is used widely as an inexpensive high-throughput technique to generate micrometer-scale features over large areas.18 However, expensive and precise photolithographic masks are typically used to make the micropatterned PDMS, and each pattern modification (i.e., shape, size, or spacing) requires the design of a new mask. An important application of solid-state features templated through DPN is to demonstrate that these structures can be used as a master from which a large number Nano Lett., Vol. 6, No. 11, 2006

generate structures over large areas at the nanometer length scale. Acknowledgment. C.A.M. acknowledges the Air Force Office of Scientific Research (AFOSR), the Defense Advanced Research Projects Agency (DARPA), and the NSF for support of this work. He is also grateful for a National Institute of Health (NIH) Director’s Pioneer Award. Supporting Information Available: EDS analysis of substrates, dependence of selective desorption on the applied potential and the duration of desorption, and a detailed experimental section. This material is available free of charge via the Internet at http://pubs.acs.org. References

Figure 6. Schematic representation of the process of molding PDMS “replicas” from DPN-defined “masters”. (A) AFM image of electrodeposited Ag “master” on DPN-defined templates. (B) Optical micrograph of a PDMS “replica” faithfully reproducing the master from A. (C) Optical micrograph of an array of Ag “masters”. The inset shows a representative AFM image of part of the array. (D) AFM image a of PDMS mold replicated from the Ag master shown in C.

of duplicate structures could be generated. Importantly, we chose to replicate these structures using PDMS because of its widespread applicability. The PDMS monomer and initiator were poured over an array of Ag structures (Figure 6), and the PDMS was allowed to cure overnight at 60 °C. The PDMS was then peeled, removed, and imaged using optical microscopy, and dot-shaped pits with geometry and dimensions identical to the original Ag master were replicated faithfully (Figure 6B). For example, 350-nm-wide Ag dots with alternating 1 and 2 µm spacings in an array were generated on Au (Figure 6C). AFM images of the PDMS replica indicate that the features were reproduced accurately, and 350 nm recessions were generated on the PDMS surface (Figure 6D). Although it may be possible to replicate sub50-nm features using this approach, conventional PDMS stamping fails when the feature size is below 500 nm.54 In summary, the combination of high-resolution alkanethiol templates with selective electrochemical control provides a simple and flexible approach for using alkanethiol resists in a positive mode on Au. Both the magnitude and the duration of the applied potential play a significant role in controlling the selective desorption of the alkanethiol adsorbates. This technique demonstrates that DPN templates can be used effectively to direct the selective etching of Au and the selective electrodeposition of Ag to nanopatterned regions of the substrates. Electrochemical control of patterned adsorbates is a promising strategy that can be applied to Nano Lett., Vol. 6, No. 11, 2006

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Nano Lett., Vol. 6, No. 11, 2006