NANO LETTERS
Generation of Nanostructures by Scanning Near-Field Photolithography of Self-Assembled Monolayers and Wet Chemical Etching
2002 Vol. 2, No. 11 1223-1227
Shuqing Sun and Graham J. Leggett* Department of Chemistry, The UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, UK Received August 15, 2002; Revised Manuscript Received September 19, 2002
ABSTRACT Nanolithography of self-assembled monolayers (SAMs) has been performed by the new technique of scanning near-field photolithography (SNP). Patterns of parallel lines written by SNP have subsequently either been used to generate compositional chemical patterns or been transferred into the underlying substrate by wet etch techniques. Lateral force microscopy (LFM) and atomic force microscopy (AFM) analysis has indicated that the line width of both compositional chemical patterns in SAMs and structures in gold is limited only by the aperture of the fiber tip used to deliver UV light. Features only 55 ± 5 nm wide have been etched into gold films using this method.
The fabrication of nanostructures is vitally important to the development of nanoelectronics, molecular electronics, and other nanotechnological devices. For example, biological nanostructures are attracting interest for novel analytical systems and for biological computing.1,2 Therefore methodologies capable of creating nanoscale chemical patterns may offer many benefits. Self-assembled monolayers (SAMs) of alkanethiols on gold surfaces offer great flexibility for the control of surface structure and interfacial properties.3,4 The patterning of SAMs provides an attractive route to microand nanostructure fabrication and is the subject of much attention because of its potentially broad practical utility.5,6 Over the past decade, various techniques, such as photolithography,7-10 micromachining, and microcontact printing,11 have been used to generate micron-scale patterns in SAMs. Nanoscale patterning is more challenging, although recently there have been important advances in the use of techniques based on scanning probe microscopy (SPM),12-15 including the use of electron transport from tips in menisci and other controlled environments in order to generate oxidative behavior16,17 and electron beam lithography.18,19 UV photolithography of SAMs has proved to be a highly effective method for patterning SAMs, yielding clean, welldefined patterns with good edge definition.7-10 However, it has previously only been regarded as a means of micron scale patterning due to the well-known diffraction limit. Further to our recent discovery that SAMs on Au/Ag are subject to rapid oxidation when exposed to single line 254 nm UV * Corresponding author. E-mail:
[email protected]. 10.1021/nl025754l CCC: $22.00 Published on Web 10/02/2002
© 2002 American Chemical Society
light,20 we have explored the feasibility of nanoscale photolithography of SAMs by utilizing a frequency-doubled argon ion laser (λ ) 244 nm) coupled to a scanning nearfield optical microscope (SNOM), a technique we term scanning near-field photolithography (SNP). In the present study, we have investigated the possibility of using SNPpatterned SAMs as resists for wet etching of gold. Nanopatterned SAMs with ultimate feature sizes as small as the fiber aperture (40-50 nm) have been achieved routinely by SNP, although narrower lines can also be obtained sometimes. These materials have been utilized as masks for pattern transfer to the underlying gold substrate by wet chemical etching. Well-defined three-dimensional structures, with widths substantially less than 100 nm, have been etched into gold films in this way, illustrating the potential of this new approach to SAM patterning. Figure 1 summarizes the process for generating nanoscale chemical patterns in SAMs and nanostructures in gold films. In a typical experiment, a UV fiber with aperture size around 50 nm, coupled to a 244 nm laser, was scanned closely (5-10 nm) over the surface of gold film which was covered by solution-deposited SAMs (Figure 1a). Alkanethiolate molecules in the exposed region of the SAM were photooxidized to corresponding alkanesulfonate species that are weakly bonded to the gold surface (Figure 1b). The sample can either be dipped into a solution of a different thiol, resulting in the displacement of oxidized species and leading to a compositional chemical pattern (Figure 1c), or, alternatively, be immersed in a suitable etchant solution to cause removal of the gold film from the
Figure 1. A schematic illustration of the scanning near-field photolithography (SNP) process and procedures used to prepare chemical patterns on substrate surfaces and three-dimensional nanostructures in gold films. (a) A UV probe carrying 244 nm light is scanned close to the surface (i.e., operating in the near field regime); (b) SAMs in the exposed region are photochemically oxidized; (c) oxidized adsorbate molecules are displaced by immersion into an alcoholic thiol solution; (d) alternatively, selective etching of Au may be initiated in the regions where the SAMs have been oxidized.
exposed area while leaving it intact in the unexposed region protected by the well-ordered monolayer, which is chemically stable against the etchant21-23 (Figure 1d). By this means 3-dimensional nanostructures can be created in the underlying substrate. The instrumentation used in the present study comprised a ThermoMicroscopes Aurora II near-field scanning optical microscope (Veeco UK Ltd, Cambridge, UK) coupled to a Coherent Innova 300C frequency-doubled argon ion laser emitting at 244 nm with a peak output power of 100 mW. The Aurora utilizes shear-force feedback to control the fiber motions via a tuning fork attached to the fiber. The fibers used (Veeco UK Ltd) were manufactured from fused silica with Al-coated tips. The power loss along the optical fibers is estimated to be at least 106. In all cases reported here, the power quoted is the laser output power, and not the power at the fiber tip, which will be substantially less. Nominal apertures of 50 nm were confirmed in selected cases by electron microscopy. Patterns formed by SNP were imaged in contact mode with a ThermoMicroscopes Explorer AFM using silicon nitride probes (Nanoprobes, Veeco UK Ltd, Cambridge UK) with a spring constant of 0.12 N m-1. Figure 2 shows AFM and LFM images of SAMs patterned according to the procedure outlined in Figure 1a-c. A monolayer of the methyl-terminated thiol dodecanethiol (C11CH3) was prepared first, and lines were then generated by SNP and the oxidized material replaced by a carboxylic acid terminated thiol mercaptoundecanoic acid (C10COOH) by dipping the sample into an ethanolic solution of it. The topographic image (Figure 2a) shows no contrast due to the identical chain lengths of the two adsorbate molecules. However, LFM 1224
imaging can yield contrast that arises from local variations in the frictional interaction. Carboxylic acid terminated regions of the surface exhibit brighter contrast because the friction force measured on these polar regions is greater than that measured on the nonpolar C11CH3-functionalized regions.24,25 Hence the LFM is able to resolve the pattern while there is no contrast in topography, as shown in Figure 2b. The line width in Figure 2b is measured to be ca. 50 nm, corresponding to the diameter of the aperture in the SNOM probe. This result is promising because although, in principle, the ultimate spatial resolution of SNP is determined by the size of aperture at the end of the fiber, previous attempts to carry out lithography by using SNOM26,27 have generally failed to achieve this level of resolution. Previously reported feature sizes have been larger than the diameter of the aperture in the optical probe, attributed by the authors of those studies to energy migration from the spot of irradiation. Taking advantage of the photochemical process of SAM oxidation, which is confined to a monatomic layer of sulfur atoms at the gold surface which must lie in direct line-ofsight of the fiber aperture (compared to the thicker films of photoresist used in earlier studies), beneath a layer composed of discrete molecular units, enables the realization of optimal feature definition. In some cases, line widths narrower than 50 nm have been achieved. The morphology of the grains in the gold film has a strong effect on feature sizes, the patterned lines following the edges of gold grains with diameters in the range 25-35 nm, making the average line width to be around 30 nm (Figure 2c). Similar phenomena have also been observed for dip-pen nanolithography, and this may represent the resolution limit for lithography on this type of material. Another important factor is the detailed mechanism of photooxidation of SAMs. A previous STM study has suggested that SAM oxidation is strongly correlated with the domain structure,28 which in turn may be related to the grain morphology of the substrate. These phenomena clearly require further investigation. However, 30 nm corresponds to nearly λ/10, a level of performance superior to that of any other photolithographic method reported to date. Repetition of the experiment in the absence of UV light (by blanking off the laser prior to coupling with the SNOM fiber) resulted in a loss of writing capability, confirming the photochemical nature of the lithographic process and discounting other explanations for pattern formation (for example, mechanical scratching). If, instead of being immersed in a solution of a contrasting thiol, the sample was immersed in a ferri/ferrocyanide etch solution, three-dimensional nanostructures could be created in the gold film (Figure 1a, b, and d). Figure 3a shows an AFM topography image of such a structure with a set of parallel lines, etched into a SAM of hexadecanethiol (C15CH3). Methyl-terminated SAMs on gold with long alkyl chains have been demonstrated to be excellent resists against wet chemical etching due to their closely packed structures and high hydrophobicities. SAMs on gold composed of C15CH3 have been prepared and patterned by scanning nearfield photolithography at a scan speed of 0.2 µm s-1,29 and Nano Lett., Vol. 2, No. 11, 2002
Figure 2. (a) An AFM image of parallel lines of C10COOH written into a SAM of C11CH3 on gold by SNP shows no topographical contrast while (b) an LFM image acquired simultaneously reveals features with widths of 50 ( 5 nm. Lithography was carried out at writing rate of 0.2 µm s-1 and the laser power was 60 mW (before coupling to the fused silica UV fiber). (c) An LFM image of a 30 nm C11CH3 line written into C10COOH at a scan speed of 1 µm s-1 and laser power of 20 mW (before coupling to the fiber).
patterns have been subsequently transferred to the gold film. The width of the approximately 25 nm deep trenches has been measured to be 55 ( 5 nm (Figure 3b) full width halfmaximum (fwhm), similar to the width of the track oxidized by SNP. The grooves are slightly broadened due to the isotropic etch characteristic of the ferri/ferrocyanide etch in gold film. It should be noted that the etchant solution we used here is the same as reported by other workers.30,31 We observed an etching speed of bare gold film similar to that previously reported. However, the etching time here of 30 min for transferring the chemical patterns generated by SNP to the underlying substrate is longer than that required for patterns generated by microcontact printing and dip-pen nanolithography. This is in agreement with a previous study that suggested that oxidized SAMs do not collapse immediately due to strong molecular interactions.32 The high aspect ratio of the trenches that result from wet etching after SNP may lead to difficulties in measurement of their depth by AFM. In the present case, however, the trench depth is the same as the thickness of the metal film originally deposited (and measured using a quartz crystal film thickness monitor) so the value may be treated with Nano Lett., Vol. 2, No. 11, 2002
confidence. Our previous studies indicated that SAMs on gold and silver with carboxylic acid terminal groups oxidized much faster than those with methyl terminal groups upon exposure to 254 nm UV light.20 We have also been able to create methyl-terminated SAM nanopatterns in carboxylicacid terminated SAMs on either gold or silver at a faster scan speed. Efforts to transfer SNP-generated nanopatterns in carboxylic acid terminated SAMs to underlying substrates were less successful than those using nanopatterns in methylterminated SAMs. Due to their more hydrophilic surfaces and, possibly, less-tightly packed structures, carboxylic acid terminated SAMs were less resistant to attack by the etchant. Broadened features were observed, as shown in Figure 4, although the average line width here is still less than 100 nm and is 52 ( 5 nm for more than 50% of the length of the feature. It may also be observed in Figure 4 that erosion of Au has occurred in a small number of localized regions distant from the main feature, perhaps due to dissolution occurring at defects. Based on these studies, it appears that it is necessary to control the etch time strictly in order to transfer successfully a pattern in a carboxylic acid terminated SAM to the underlying substrate. 1225
structures of SAMs and the small size of the SNOM probe aperture. Such patterns have been transferred successfully to the underlying gold substrate by wet chemical etching, creating three-dimensional architectures with feature sizes well below 100 nm. Where methyl-terminated SAMs are used, feature sizes are effectively defined by the diameter of the aperture in the SNOM fiber. For future applications of scanning near-field photolithography of SAMs, we also envision the fabrication of compositionally patterned molecular surfaces where different molecules occupy distinct surface locations to perform specific tasks in molecular devices. From a chemical point of view, modification of the terminal group of patterned SAMs will enable them to be coupled to materials of interest and find applications in diverse areas of nanotechnology. Moreover, photochemical processes offer considerable utility and flexibility in surface functionalization, and this approach thus offers wide promise that extends beyond monolayers on gold and silver to a much broader class of materials. Acknowledgment. The authors thank the EPSRC (grant GR/N05390) for financial support and the CLRC Laser Loan Pool for provision of the UV laser. Figure 3. (a) An AFM topography image reveals a set of 55 ( 5 nm parallel trenches etched into a gold film following the procedure of (a), (b), and (d) outlined in Figure 1. To generate such features, a SAM of C15CH3 on gold was subjected to SNP (writing speed: 0.2 µm s-1, laser power: 60 mW) and wet chemical etching in an Fe(CN)62+/Fe(CN)63+ solution for 30 min. (b) Cross-sectional topography trace orthogonal to the lines.
Figure 4. An AFM topography image reveals a trench etched into gold film following the procedure of (a), (b), and (d) outlined in Figure 1. The etched line runs from the top to the bottom of the imaged area with a line width of approximately 52 ( 5 nm for more than 50% of its length. Etched holes have also been found in some areas between gold grains. In this example, a SAM of C10COOH on gold was subjected to SNP (writing speed: 1 µm s-1, laser power: 20mW) and wet chemical etching in Fe(CN)62+/Fe(CN)63+ for 30 min.
In conclusion, we have demonstrated the concept that nanoscale patterns can be generated in SAMs by photolithographic techniques, through the application of scanning nearfield photolithography. This is due to the chemical specificity of the photochemistry employed combined with the ultrathin 1226
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