NANO LETTERS
Edge-Spreading Lithography: Use of Patterned Photoresist Structures to Direct the Spreading of Alkanethiols on Gold
2005 Vol. 5, No. 1 31-36
Matthias Geissler, Joseph M. McLellan, and Younan Xia* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 Received September 14, 2004; Revised Manuscript Received November 8, 2004
ABSTRACT Edge-spreading lithography (ESL) has been extended to fabricate gold structures with different geometries and feature sizes on silicon substrates. In the present variant of ESL, we used photoresist structures patterned on a thin film of gold to transport alkanethiol molecules from an elastomeric stamp to the gold surface where they formed a self-assembled monolayer (SAM) along the edge of each resist feature. The emerging SAM could expand laterally on the gold via reactive spreading, during which the footprints of the resist structures were enlarged in the same fashion. Upon removal of the photoresist, selective etching of gold revealed those regions that were protected by the SAM, yielding accurate outlines of the resist features on the substrate. The width of resultant gold structures was determined by the distance over which the monolayer expanded during spreading, and could be conveniently controlled well below 200 nm by varying the contact time and/or the concentration of alkanethiol in the ink.
The development of experimentally simple and inexpensive techniques for patterning surfaces with micro- and nanometer-scale features is an exciting and rapidly evolving field with potential impacts on many research areas that include chemistry, materials science, condensed matter physics, and biotechnology.1 A wealth of unconventional techniques that rely on various forms of molding,2 embossing,3 writing,4 and printing5 have been devised over the past decade. These techniques offer some immediate advantages in applications where conventional fabrication techniques such as photolithography and e-beam writing are usually ineffective despite their impressively high levels of maturity and sophistication. Some of these applications may include the patterning of curved or contoured substrates,6 the use of materials that are incompatible with photoresist or development chemistries,4c,5c and large-area fabrication where cost is often a major concern.1b,7 The resolution of a patterning technique is another important criterion since the ever-increasing complexity of surface architectures in many areas demands continuous reduction of the critical feature sizes.1a Among the various strategies that are currently being pursued to achieve smaller structures, edge lithography is both particularly interesting and versatile. This type of lithography comprises an ensemble of techniques that commonly use the edges of a pattern to determine the resultant structures. Well-established examples include near-field phase-shifting photolithography,8 controlled undercutting,9 topographically directed etching,10 and * Corresponding author. E-mail:
[email protected]. 10.1021/nl048497o CCC: $30.25 Published on Web 11/25/2004
© 2005 American Chemical Society
several other methods.11 All these techniques use macro- or mesoscopic features to produce high-resolution dots, wires, rings, trenches, or other related structures rapidly and inexpensively. We recently demonstrated a new variant of edge lithography that we called edge-spreading lithography (ESL).12 ESL exploits the reactive spreading of alkanethiol molecules on coinage metals13,14 and is carried out by delivering an alkanethiol ink from a poly(dimethyl siloxane) (PDMS) stamp onto the substrate via the surface of a relief structure that serves as the guide. In our initial demonstration, we employed a two-dimensional (2D) array of silica colloids as the guide, which was supported on a thin film of gold or silver. By using the resultant monolayer as a resist for wetchemical etching, we have successfully fabricated 2D arrays of gold and silver rings on silicon substrates. Here, we extend the capabilities of ESL by using patterned relief structures of a photoresist to mediate the transport of alkanethiol molecules to a gold surface. The use of photoresist structures offers an important advantage in that they are not limited to any specific test pattern, as is the case for 2D arrays of colloidal spheres. In the present approach, photolithography (beyond its capability to fabricate master patterns) complements processing techniques based on soft contact in a useful and convenient manner. Figure 1 outlines a typical procedure for ESL; it begins with the patterning of a photoresist film (spin-coated on top of a thin layer of gold) using conventional photolithography.
Figure 1. Schematic illustration of a new variant of ESL where photoresist structures are used to guide the transport of ODT molecules from a planar PDMS stamp to the surface of a thin layer of gold supported on a silicon substrate. Upon reaching the gold surface, the ODT molecules self-assemble into a monolayer that can expand laterally from the edge of each photoresist structure. Once the stamp has been removed and the photoresist has been stripped off, the regions covered by the ODT monolayer can be developed by selectively etching the gold in the bare regions of the substrate. The width (w) of resultant gold features should be equivalent to the distance of lateral spreading.
We have examined both negative- and positive-tone resists, polyimide (PI) and AZ 1512, respectively, which can be conveniently patterned using standard procedures (see Experimental Section for details). A planar PDMS stamp was then inked with a 2.0 mM solution of octadecanethiol (ODT) in ethanol for 1 min, dried with a stream of nitrogen gas, and placed on the photoresist pattern for different durations ranging from 2 to 15 min. During contact, ODT molecules were released from the stamp and guided along the surface of the photoresist structures to the gold surface, where they formed a self-assembled monolayer (SAM) along the edges of the guides. As long as a continuous supply of molecules was maintained, the area of the SAM would expand laterally due to reactive spreading. The footprint of each photoresist structure thereby dictates the shape of the emerging pattern: it defines the region inaccessible to thiol molecules, and the outline of its geometry propagates through lateral extension of the SAM. After the stamp had been separated, the photoresist was stripped off the gold substrate and the sample was etched in an aqueous solution containing Fe(NO3)3 and thiourea.5e Upon removal from the etch bath, the samples were rinsed with deionized (DI) water, dried with a stream of nitrogen gas, and characterized using scanning electron microscopy (SEM). 32
Figure 2 shows a number of gold patterns that were fabricated by ESL. The gold wires in Figure 2A were generated using a test pattern (∼4 mm2 in area) comprising lines of PI resist. We were able to achieve an accurate outline of these features with good uniformity over the entire array. No photoresist remained on the substrate after the stripping step, as suggested by the high contrast of the gold wires. We noticed, nevertheless, that the wires were occasionally interrupted by small defects, which we attribute to contaminants and dust particles that were present on the gold surface, thus preventing the formation of dense and ordered SAMs in these regions. We obtained similar results when we changed the feature size and shape of the photoresist structures, although a geometric effect was observed for some test patterns. For example, when PI posts of triangular geometry were used (Figure 2B), the corners of each gold triangle were consistently less developed compared to the other portions of the structure resulting in a slightly rounded appearance.15 Replacing PI with another type of resist (e.g., AZ 1512) did not lead to adequate results instantaneously because it was difficult to completely remove the photoresist with the common stripping procedures.16 As a result, background in the form of gold grains was present in those regions of the substrate that were initially covered by the resist for the vast majority of our samples. We solved this problem by introducing a buffer layer (∼400 nm in thickness) of poly(methyl methacrylate) (PMMA) between the gold and the AZ resist. The PMMA layer could be readily removed by dissolution in acetone, and the quality of resultant gold patterns became comparable to those obtained with PI structures as the guide. The SEM images in Figure 2C and 2D show two typical examples. While rings (Figure 2C) could be readily produced with a high degree of uniformity, a geometric effect again came into play for other, more complex patterns, such as the stars depicted in Figure 2D. Also, the size and the aspect ratio of the photoresist features (beyond their importance in preventing sagging of the elastomeric PDMS stamp in the recessed regions of the surface17) both seemed to have a direct influence on the width of resultant gold structures (see Figure 2A and 2B, for example). While a systematic investigation of this relationship may be warranted, our initial studies have revealed some trends in a more quantitative (but purely empirical) manner. The graph in Figure 3 shows the evolution of rings that were generated by ESL using two complementary test patterns: cylindrical holes in a PI film and cylindrical posts of PI separated from each other. Although their geometric dimensions were comparable, the scenarios for the spreading of alkanethiols from these structures were somewhat different for the following reasons (see the illustrations in Figure 3). When a hole is used, the thiol spreads toward the center of an area that is continuously shrinking. In contrast, the area available to the SAM is less confined when it expands from a post. For both samples, our measurements indicate that the evolution of the ring width followed the same linear trend when printing times shorter than 10 min were involved. For printing times longer than 10 min, a continuation of this Nano Lett., Vol. 5, No. 1, 2005
Figure 2. Scanning electron micrographs of gold test patterns prepared by ESL and selective wet etching. All patterns represent accurate outlines of the corresponding photoresist features that are shown by the optical microscope images in the insets. Planar PDMS stamps were inked with a 2.0 mM solution of ODT in ethanol, and placed on the 4.0-µm-thick PI resist features for 15 and 4 min to achieve the patterns depicted in (A) and (B), respectively. For (C) and (D), the total thickness of the AZ 1512/PMMA layer was 1.6 µm, and a contact time of 15 min was used for both samples.
Figure 3. Plots showing the dependence of ring width on the contact time. These data sets were obtained by using a 2.0 mM solution of ODT in ethanol to ink a PDMS stamp, which was placed on PI arrays comprising (1) cylindrical holes or (2) cylindrical posts that were 4.0 µm in height. Each data point was calculated from more than 300 individual rings at different locations of the sample. The SEM image shows gold rings of ∼300 nm in width illustrating the smallest feature size obtained within this series. The scale bar in the inset corresponds to 10 µm.
linear trend was observed only for rings that were derived from holes, whereas rings resulting from the posts did not follow this trend. The onset time of ∼3.5 min corresponds to an average velocity of ∼1.1 µm/min for ODT molecules to transport across the surface of PI resist. We found it challenging, however, to fabricate high-resolution structures by just reducing the printing time. The gold rings shown in the inset of Figure 3 probably represent the smallest features Nano Lett., Vol. 5, No. 1, 2005
(∼300 nm in width) that could be achieved (in a reproducible manner) under these conditions. Decreasing the concentration of ODT in the ink (e.g., to 0.5 mM) was the key to limiting the spreading of thiols to shorter (
