J. Phys. Chem. B 1997, 101, 9263-9269
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Making Gold Nanostructures Using Self-Assembled Monolayers and a Scanning Tunneling Microscope E. Delamarche,† A. C. F. Hoole,‡ B. Michel,† S. Wilkes,§ M. Despont,† M. E. Welland,‡ and H. Biebuyck*,† IBM Research DiVision, Zurich Research Laboratory, CH-8803 Ru¨ schlikon, Switzerland; Department of Engineering, UniVersity of Cambridge, Cambridge CB2 1PZ, England; and RISØ, Department of Solid State Physics, DK-4000 Roskilde, Denmark ReceiVed: March 28, 1997; In Final Form: May 30, 1997X
We explored the applicability of a system of self-assembled monolayer (SAM) resists on gold, recently developed by Tam-Chang et al. [Langmuir 1995, 11, 4371-4382], to electron-beam lithography carried out at high (>1000 eV) and low (30 µC/cm2, whereas contamination from the chamber in moderate vacuum (10-6 Torr) interfered with the process and provided equally useful resist layers against a cyanide etch of the gold in the absence of monolayers. Low-energy electron lithography of the same monolayer using a scanning tunneling microscope (STM) as the source proved more reliable and allowed the formation of 30-40 nm structures wherever the STM tip passed over the surface with sufficient voltage and current. Our data highlight some of the difficulties encountered when using self-assembled monolayer resists as components in “positive” electron-beam lithography on gold and suggests constraints on using SAMs as ultimate resists.
1. Introduction High-energy electron-beam lithography is the most practical high-resolution lithography technique known. Electron beams with energies >30 keV can be highly focused by electron lenses and easily deflected using positioning coils that permit nanometer-scale writing over large fixed areas, typically ≈0.2 mm2. Patterns result by writing into polymeric resists, like poly(methyl methacrylate) (PMMA), causing localized chain modification under moderate fluxes of electrons (≈100 µC/cm2) with consequent changes in dissolution of the material by solvent.1 PMMA resists are useful barriers to a variety of liquid or gaseous etchants, making pattern transfer into the underlying base material practical. Resists based on polymers are less obviously useful for low-energy forms of electron-beam lithography, particularly when a scanning tunneling microscope (STM) is the source. The mean free path of electrons becomes increasingly small with decreasing electron energy so that, for energies 30 µC/cm2 (≈3 electrons per molecules of MMEA in the monolayer), and immersed in a solution of HDT. We repeated these experiments at energies of 1, 10, and 35 keV without perceptible differences in their outcomes. A control experiment showed that regions of a gold substrate subjected to currents of 10 keV electrons under HV, but not subsequently exposed to a solution of HDT (i.e., the absence of step 2 in Figure 2) also formed patterns of protected gold, albeit at much higher doses of electrons. Two explanations were possible. First, at very high doses of exposure (>1000 µC/ cm2), the monolayer of MMEA might itself be converted to a form that resisted the cyanide etch. As the resistance to this etch is known to be strongly dependent on the thickness and properties of the monolayer,18 we do not favor this hypothesis. A second possibility was the accumulation of a “contamination” layer on top of the monolayer that provided the requisite local barrier to etching.19,20 Such adsorbate films, whose presence clearly depended on the dose of electrons, might result from interaction between ambient species in the chamber of the SEM (largely hydrocarbons typical of the oils used in pumping our system), the electron beam, and the surface. An experiment that provided evidence for this second possibility was the observation that gold, otherwise untreated with a monolayerforming thiol, was also protected from a cyanide etch at comparatively low doses of electrons. Our gold substrates initially had ≈1 nm of carbonaceous material on their surface, the result of exposure to the laboratory environment.21 Intriguingly, there was a significant difference between MMEA-treated gold (MMEA+/HDT-) and gold otherwise left underivatized (MMEA-/HDT-): the latter formed protected regions on its surface at much lower doses. This result showed that SAMs
J. Phys. Chem. B, Vol. 101, No. 45, 1997 9265 can amplify the processes of electron-beam-induced modification of interfaces, probably by their control over interfacial energy and adsorbate density even at high energies of irradiation although their role remained uninvestigated by us and is likely to be complicated in its detail. We repeated the same set of experiments described above under UHV conditions (p ≈10-9 Torr) but were unable to form patterns of gold in any of the three cases we had investigated previously at the highest doses available (105 µC/cm2) in this system. These observations further supported the role of ambient contaminants in the chamber as agents in the chemistry between high-energy electron beams and the gold substrate and as a possible source of protective coatings for the substrate. We were able to form patterns in monolayers of HDT,22 however, under these same conditions, reinforcing our previous statements that negative lithography schemes that simply rely on partial destruction of a monolayer allow pattern formation at much lower doses of electrons than do strategies involving the exchange of monolayers as shown in Figure 1. Monolayers of HDT nevertheless failed to demonstrate any obvious benefit over the use of more conventional, polymeric electron-beam resists for pattern transfer. The dose necessary to expose the monolayer was evidently much higher than that needed to expose relatively thick films (50 nm) of PMMA, already well-known in electron-beam lithography. The insensitivity of the monolayer resist, given its thickness, was disappointing and suggested that the majority of electrons did not contribute to the productive removal of MMEA molecules in the monolayer. The relatively high energy of the primary beam (100 keV) probably allowed little direct interaction with the monolayer because the mean free path of these electrons is many times (>100×)23 the distance of the characteristic thickness of the SAM. Secondary electrons generated in the top surface of the gold, already known to be the primary damaging agent in the exposure of monolayer resists,14 were evidently not sufficient to effect a high yield (low dose) of the desired transformation of the monolayer following its immersion in a solution of HDT. This result parallels the observation that relatively high doses of UV photons (compared to Novalac and other conventional photoresists) are also needed for useful transformation of gold coated by MMEA.12,15 Together these data highlight a general weakness of the use of monolayer resists with high-energy sources of electrons or light: their transparency and the independence of molecules within self-assembled monolayers compared to that typical of polymer systems tend to require high doses of radiation to allow useful formation of features. STM data on the order and type of structures in SAMs on gold reinforce this point, demonstrating that defects and disruptions of molecules in SAMs are highly localized so that control over interfacial properties of the underlying substrate by molecules in the monolayer occurs only over distances corresponding to a few times their widths in the SAM.24 Self-assembled monolayers are “ultimate” resists, and the price for their extreme resolution is dose. Replacement schemes that protect an underlying substrate by exchanging one type of monolayer for another exacerbate these problems because the yield of transformation must be better than 99.99% to allow the formation of relatively defect-free structures. The development used in negative and positive polymeric resists amplifies the chemical transformations induced by the irradiation due to the considerable nonlinearity which exists between the solubility of the polymer and its molecular chain length, lowering the unit doses required to create patterns. In summary, lithography using replacement of monolayers of MMEA by HDT does not offer a significant advantage in
9266 J. Phys. Chem. B, Vol. 101, No. 45, 1997 dose or resolution compared to conventional resists based on cross-linked polymers such as PMMA or, more simply, compared to contamination resists built up directly on the surface by interaction between the electron beam and substrate in moderate vacuum. The general sensitivity of monolayer resists to ambient contaminants also makes their selection as resists for high-energy lithography somewhat dubious, particularly because such contamination is difficult to identify and control. As monolayer resists are also generally suited only to specialized agents for pattern transfer, their utility in high-energy electronbeam lithography seems confined, at best, to a few applications. 3.2. Low-Energy Electron-Based Lithography. We moved our investigation of electron-beam lithography using monolayers of MMEA on gold to the much lower energies characteristic of STMs. Here, the monolayer resist is a possibly useful alternative to polymers because the thinness of the SAM permits tunneling through their bulk at technically accessible currents and convenient speeds (>1 pA in a 10 kHz bandwidth). SAMs on gold also tend to be well defined structurally, having crystalline order at room temperature in the best cases. STM offers the convenience of working in air, is relatively simple, and enjoys widespread availability, so we decided to focus on this instrument as a lithographic tool. In an STM experiment, a metallic tip scans in close proximity (5 µm without suffering apparent interruption. Such continuous lines could only be formed by STM using the HDT replacement step (Figure 1). Where an MMEA monolayer was used without HDT replacement we could not make these fine structures without observing a large number of gaps along the span of the written area. A test grid of lines written using the MMEA replacement scheme (Figure 5f) showed the same average line width as the individual lines
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Delamarche et al. with no apparent broadening at their junction, indicating that proximity effects during writing on the MMEA monolayer were not a limitation at this scale. An important additional result from these data was that the “imaging” conditions were well separated from the “writing” conditions on the MMEA monolayer on gold because the movements of the STM tip, controlled using imaging parameters, that connected written features did not appear in the etched patterns. We do not know whether these lines represent ultimate examples of the ability of an STM to form independent features in a monolayer because our experiments were strongly affected by the isotropic characteristic of the development step (i.e., the cyanide etch). We think that the line width observed for a single pass of an STM tip is inconsistent with a highly localized flow of tunneling electrons and is more indicative of a broader region of emission of electrons from the tip, as we inferred from Figure 3a. Why so many electrons per molecule (≈50 000) are required to effect a productive transformation of MMEA remains a puzzle but itself may reflect the broad range of chemistries carried out by low-energy electrons, a minority of which lead to a state that lets the MMEA leave the gold.
Figure 5. Examples of the use of STM and the positive lithography system outlined in Figure 1 to form a series of features in gold. After processing the MMEA/Au substrate by STM (I ) 50 pA, V ) 10 V, V ) 15 µm s-1) and immersing the sample in a solution of HDT (30 s), etching the gold revealed the patterns shown. (a) The square resulted from writing 1024 lines consecutively with a spacing of 4.4 nm. (be) One hundred, 50, 25, and finally one pass with the tip yielded progressively smaller rectangles. The structures appearing in the latter are crystallites characteristic of the gold substrate. (f) This test grid demonstrates the possible fabrication of single line patterns over several microns in length and the absence of proximity effects at their cross junctions. The background texture represents underetched gold; we did not attempt to optimize the etching time in forming these structures.
4. Conclusion Self-assembled monolayers of MMEA on gold have some value as an agent in positive lithography on gold using electron beams. Their principal utility in this study occurred in combination with local transformation of this SAM by lowenergy (4 eV per molecule of MMEA in air. The large number of electrons per molecule used in this positive lithography scheme compared well with the dose (∼200 per molecule) we found necessary for negative lithography on gold using SAMs of HDT, considering the former required a much higher yield of transformation (probably >99.9% per unit area) because of the isotropic characteristic of the cyanide etch. Whether a more productive electron-beam lithography system in air using SAMs and STM as the source is possible remains open, although we note that the exposures used to affect MMEA are already close to the limits of maximum scan rates and minimum allowable currents set by conventional STM-based instrumentation. The much higher energies (>1000 eV) of electron beams representative of standard scanning-beam instrumentation that also allow much faster movement of the source than is possible by STM or foreseeable in AFM-based instruments proved nonadvantageous for the SAM-based positive lithography scheme on gold. Here the monolayer system required exposures to these electrons at least similar to those needed to carry out lithography in polymer-based systems, and the lithography process was interfered with by ambient contaminants typical of the moderate (10-6) vacuum conditions in our systems. As a consequence of these observations, it is difficult to imagine a practical role for the use of SAMs on gold as part of a high-energy lithography system to form metallic gold features.
Gold Nanostructures Acknowledgment. E.D., H.B., and B.M. acknowledge support from the Swiss Federal Office for Education and Science within the ESPRIT basic research PRONANO (8523) and NANOWIRES (23238). S.W. thanks the PRONANO project for financial support. A.H. and M.W. thank the PRONANO and NANOWIRES projects for financial support. We thank S. Alvarado for his help with the interpretation of the STM experiments, H. Schmid for technical assistance, and H. Rothuizen for providing the lithographic test templates. References and Notes (1) Broers, A. N. J. Electrochem. Soc. 1981, 128, 166-170. (2) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (3) Dagata, J. A.; Schneir, J.; Harary, H. H.; Evans, C. J.; Postek, M. T.; Bennett, J. Appl. Phys. Lett. 1990, 56, 2001-2003. (4) Sugimura, H.; Uchida, T.; Kitamura, N.; Masuhara, H. J. Phys. Chem. 1994, 98, 4352-4357. (5) Kramer, N.; Jorritsma, J.; Birk, H.; Scho¨nenberger, C. Microelectron. Eng. 1995, 27, 47-50. (6) Snow, E. S.; Campbell, P. M. Science 1995, 270, 16391641. (7) Scho¨nenberger, C.; Kramer, N. Microelectron. Eng. 1996, 32, 203217. (8) Schoer, J. K.; Crooks, R. M. Langmuir 1997, 13, 2323-2332. (9) Lercel, M. J.; Tiberio, R. C.; Chapman, P. F.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1993, 11, 2823-2828. (10) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1994, 12, 3663-3667. (11) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1995, 13, 11391143. (12) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371-4382.
J. Phys. Chem. B, Vol. 101, No. 45, 1997 9269 (13) The amide function (-NHCO-) increases the lateral interactions between molecules in the film by forming H bonds, hence improving the thermal and chemical stability of the SAM. (14) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1993, 97, 9456-9464. (15) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 33423343. (16) Michel, B.; Travaglini, G. J. Microsc. 1988, 152, 681-685. (17) Danger: CN- dissolved in water has HCN gas above its surface. Keeping the pH of the solution high with KOH minimized the production of HCN. Dilution of the bath with water or addition of acids can be dangerous. (18) Zhao, X.-M.; Wilbur, J. L.; Whitesides, G. M. Langmuir 1996, 12, 3257-3264. (19) Broers, A. N.; Hoole, A. C. F.; Ryan, J. M. Microelectron. Eng. 1996, 32, 131-142. (20) Johnson, K. S.; Berggren, K. K.; Black, A.; Black, C. T.; Chu, A. P.; Dekker, N. H.; Ralph, D. C.; Thywissen, J. H.; Younkin, R.; Tinkham, M.; Prentiss, M.; Whitesides, G. M. Appl. Phys. Lett. 1996, 69, 27732775. (21) The thickness and composition of the typical contamination film that forms in our laboratory were verified by ellipsometry and X-ray photoelectron spectroscopy and support a model of random adsorption of saturated hydrocarbons onto the high-energy gold surface. (22) Hoole, A. C. F.; et al. Unpublished results. (23) Seah, M. P. In Practical Surface Analysis; Briggs, D., Sheah, M. P., Eds.; Wiley: Chichester, 1983. (24) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, Ch. AdV. Mater. 1996, 8, 719-729. (25) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611-614. (26) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853-2856. (27) Bucher, J. P.; Santesson, L.; Kern, K. Appl. Phys. A 1994, 59, 135138. (28) Mesa, G.; Sa´enz, J. J.; Garcı´a, N. J. Vac. Sci. Technol. B 1996, 14, 2403-2406. (29) Biebuyck, H.; Larsen, N.; Delamarche, E.; Michel, B. IBM J. Res. DeV. 1997, 41, 159-170.
