NANO LETTERS
Deterministic Positioning of Three-Dimensional Structures on a Substrate by Film Growth
2006 Vol. 6, No. 4 858-861
Kevin F. McCarty* Sandia National Laboratories, LiVermore, California 94550 Received January 6, 2006; Revised Manuscript Received February 27, 2006
ABSTRACT A process to fabricate three-dimensional crystalline structures at controlled locations on a substrate during film growth and annealing is demonstrated. Low-energy electron microscopy reveals that silver is transported to regions on a tungsten surface with closely spaced atomic steps. By controlling the substrate topography using a focused ion beam to machine small holes, this general mechanism produces an array of cylinders as a silver film dewets the substrate.
Simple schemes to fabricate nanometer-scale structures of controlled shape at predetermined locations on surfaces are desirable for nanoscience applications.1-4 In principle, threedimensional (3D) structures can be formed by depositing onto a surface a material that does not want to uniformly cover (wet) the substrate. Controlling the shape or location of the 3D structures through this approach, however, is challenging.5 Some methods of positioning 3D islands rely on controlling where islands nucleate,6 for example, by accommodating the strain in the substrate.7 In this Letter, a different method is presentedsislands nucleate on the substrate and then move toward surface regions with high step density. This motion occurs not because islands prefer to be on top of atomic steps but because the steps allow the islands to become thicker without having to nucleate new layers. The mechanism controlling island motion is remarkably simple and generalsthe rate at which the film dewets to form 3D structures is controlled by the separation of atomic substrate steps.8 Here, low-energy electron microscopy (LEEM)9 is used to show that 3D Ag structures are formed at predetermined locations on a W(110) surface by the dewetting mechanism during both film deposition and annealing. A general approach to making controlled 3D structures is described and demonstrated by fabricating an array of Ag cylinders. The W(110) surface was cleaned by annealing in 10-8 Torr of oxygen at 1000 °C and flashing the crystal to about 1600 °C to remove the adsorbed oxygen. Ag was evaporated from a molybdenum crucible heated by electron-beam bombardment. LEEM images were formed during the deposition and annealing of the Ag films from 20-eV electrons specularly * Corresponding author. E-mail:
[email protected]. 10.1021/nl060030l CCC: $33.50 Published on Web 03/11/2006
© 2006 American Chemical Society
reflected from the surface. Because this is the energy of Bragg diffraction from Ag with (111) planes oriented parallel to the substrate, regions thicker than the Ag wetting layer appear relatively bright. Figure 1 illustrates how a pit in a W surface can be controllably filled to make a 3D cylinder of Ag. The 0-min LEEM image shows the clean substrate before Ag deposition. The circular pit is bounded by a dark band, which consists of closely spaced substrate steps (i.e., a step “bunch”). Monatomic W steps, the curved dark lines, wrap around the top of the pit. Ag on W(110) is a well-documented StranskiKrastanov system, where the stable configuration is 3D Ag islands on top of a thin Ag wetting layer that uniformly covers the substrate.10-14 During the initial Ag deposition, the wetting layer forms. Analysis of film-growth videos and characterization using the quantum-size effect15 establishes that the wetting layer is 3 monolayers (ML) thick. Thicker Ag islands, which appear as bright, irregularly shaped patches in the 6.7-min image, form once this wetting layer is complete. 3D Ag islands also emerge during deposition on the step bunch bounding the pit. After 7.5 min (average coverage of 4.7 ML), the Ag flux was halted and the substrate was warmed. The 3D island marked by the arrows moves across monatomic substrate steps toward the pit. With time (see the 10- and 10.25-min images in Figure 1 and the Supporting Information), the Ag island breaches the pit’s step bunch and rapidly “flows” into the pit. During the later stages of annealing, the thinner 3D islands surrounding the pit lose mass to the thicker island that migrated to the pit. That is, the thicker island in the pit grows (“ripens”) at the expense of the thinner islands. In the end, within a diameter of at least 5 µm, the only Ag that is thicker than the wetting layer is in the pit.
Figure 1. Low-energy electron microscopy (LEEM) images showing a pit in a W(110) surface being filled with Ag during annealing. The clean W surface (0-minute image) contains monatomic steps, the curved dark lines, wrapping around a bunch of steps (the wide, dark band) that defines a circular, flat-bottomed pit. 4.7 ML of Ag was deposited on the surface over 7.5 min. 3D Ag islands, which image bright, form when the wetting layer of 3 ML is complete. During annealing, the 3D island marked by the arrows moves down the staircase of substrate steps. Eventually, the only 3D island left within the field of view is that which completely fills the pit. A movie version is available in the Supporting Information.
The fact that annealing produced a thick Ag structure is not surprisingsby thickening, the bulklike Ag islands reduce the area of the energetically costly interface with the wetting layer.8 What is surprising is how the Ag thickens into 3D structures. The Ag island moves directly to the pit because this is the direction of descending substrate steps. Analysis by LEEM and selected-area electron diffraction establishes that the tops of the 3D Ag islands are flat (111) facets, consistent with previous observations.9,11,16 Since the island maintains a flat (111) facet on its top as it extends across the descending substrate steps,8 the island’s leading edge gets an atomic layer thicker with each step it crosses. This thickening, which is illustrated schematically in Figure 2, lowers the Ag’s free energy. This mechanism of “downhill migration” was first elucidated by examining Ag and Cu films on a Ru substrate.8 There are two important points to consider. First, film dewetting occurs by the downhill-migration process because it permits thickening without having to nucleate new atomic Nano Lett., Vol. 6, No. 4, 2006
Figure 2. Schematic illustration of how film mass is transported into a substrate pit by the “downhill migration” of a crystalline island. The leading edge of the flat-topped island thickens by an atomic layer each time it extends itself across a descending substrate step. Because of the high step density in the pit, the island rapidly thickens and captures mass from surrounding 3D islands that are thinner and, therefore, less stable. A 3D cylinder of film is formed in the pit.
layers of the film.17 That is, downhill migration provides a low-barrier pathway for the 3D islands to lower their free energy by thickening. (In Stranski-Krastanov systems, when the leading edge of a 3D island reaches a descending substrate step, the wetting layer at the step just has to advance away from the substrate step. The step array comprising the 3D island’s leading edge can then advance across the lower terrace.) Second, the thickening process is deterministics the islands extend themselves in the direction that enables them to thicken as quickly as possible. This attribute allows 3D structures to form at predictable locations. That is, the migration direction and rate are determined by the local height gradient (i.e., the spacing of atomic substrate steps). Even though the motion of the Ag island appears to be liquidlike as it fills the pit, the substrate temperature is well below Ag’s melting point. That is, the mobile Ag islands are crystalline. Thus, the 3D Ag islands that form during deposition and annealing end up at deterministic positions on the substrate. For example, a pit’s boundary has a high density of substrate steps. As a result, Ag preferentially goes to the pit and makes a Ag cylinder. With this knowledge, nanostructures can be fabricated by controlling the surface topography before film deposition. 3D islands will then form at and move to regions 859
Figure 3. Fabrication of an array of Ag cylinders on a W(110) surface: (a) An array of 200-nm-diameter holes in the W substrate machined by the FIB technique and imaged by scanning electron microscopy (SEM). (b-f) LEEM images of the same hole array after: flashing the crystal twice to about 1550 °C (b); depositing 19 ML of Ag at 110 °C (c); annealing to 375 °C (d), 430 °C (e), and 540 °C (f). A movie version is available in the Supporting Information.
engineered to have high step density. Indeed, the ability to control the local density of atomic steps on a surface is well demonstrated.18 An example of making metal nanostructures of controlled shape at chosen locations on a surface is given next. Figure 3a shows an array of 200-nm-diameter holes that were machined into the W substrate using the focused ion beam (FIB) technique.19 As the LEEM image in Figure 3b shows, during the flashing of the crystal, the holes developed into flat-bottomed cones whose bottoms are about 200 nm in diameter. Ag was then deposited at low temperature uniformly over the surface. The regions between the holes, which are relatively flat, are covered with 3D Ag (the large bright areas in Figure 3c). 3D Ag islands, the bright dots in Figure 3c, also nucleated on the cone side walls and bottoms. During annealing, the 3D Ag that surrounds the holes is transported to and fills in each of the 24 holes. By Figure 3e, only a small amount of Ag thicker than the wetting layer remains between the holes (next to the arrow, for example). 860
In image Figure 3f, the only regions that image bright, that is, contain 3D Ag, are the holes. Thus, the holes have been filled with Ag cylinders. As the video in the Supporting Information documents, the mass transfer that fills the holes involves 3D islands migrating downhill (Figure 2) and “flowing” into the holes and the ripening (thickening) of Ag islands in the holes as Ag adatoms diffuse from thinner (less stable) regions. Considering the tight packing of the hole array, the ability to fill each hole with Ag demonstrates a degree of process robustness. As an additional example of the approach, Ag “wires” have been fabricated by filling trenches machined by FIB. In Figure 3’s demonstration, the bottoms of the Ag cones are about 200 nm in diameter. This approach of using gradients in the spacing of atomic steps should be capable of fabricating much smaller 3D features. For example, the bottom diameter of a pit is only limited by its tendency to fill in due to curvature (i.e., the Gibbs-Thomsen effect). Yet even for a relatively low-melting material such as Cu, closed steps with radii of tens of nanometers are stable for hours near room temperature.20 A variety of techniques besides FIB exist to control the local density of atomic steps, which in turn controls the location of 3D structures. Lithography21 based on photons, electrons, and imprint stamps can vary a substrate’s topography, typically by subtracting material. Importantly, simple, nonlithographic approaches can also be used to control the step density at nanometer-dimension lengths. For example, the surface topography can be varied between reasonably uniform arrays of “ripples” (i.e., periodic-height modulations) and pyramids simply by blanket sputtering the surface with an ion beam.22 Another nonlithographic approach to controlling the surface step structure is to utilize the common tendency of atomic steps to bunch together during annealing or crystal growth.23 A film dewetting on a surface composed of alternating flat terraces and step bunches would produce wires along the sides and bases the step bunches. Since these structures have exposed side surfaces, fabricating them by lithography alone would require an additional step of material removal. Overall, a variety of lithographic and nonlithographic techniques are available to combine with the downhill-migration process to fabricate simple through complex structures. In the examples of Figures 1 and 3, the substrate holes are largely empty when the film deposition ends. The holes are mainly filled during annealing, with the Ag coming from the surrounding 3D islands that formed during deposition. However, at higher temperature, pits can be filled with Ag directly during film growth, i.e., without annealing. Figure 4 shows an example of six adjacent pits being filled. During the first portion of film growth, the W surface becomes uniformly covered by the wetting layer, with each atomic layer nearly finishing before the next starts. As soon as the wetting layer is complete, a single 3D island, marked by the arrows in the 25-s image of Figure 4, forms on the step bunch of each pit. These islands thicken quickly as they extend themselves across the closely spaced descending steps that define the pit. Being thick, these islands are then extremely effective sinks for the deposition flux. In fact, no other 3D Nano Lett., Vol. 6, No. 4, 2006
Sciences of the U.S. DOE under Contract No. DE-AC0494AL85000. The author thanks the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory and Andrew Minor for use of and technical assistance with the FIB and N. C. Bartelt and K. Thu¨rmer for valuable discussions. Supporting Information Available: Movie versions of Figures 1, 3, and 4. This material is available free of charge via the Internet at http://pubs.acs.org. References
Figure 4. LEEM images showing six adjacent pits in a W(110) surface directly being filled with Ag (bright regions) during film growth at 290 °C. As soon as the wetting layer completed (just before 25 s), 3D islands (marked by arrows) form exclusively on the step bunches that bound the pits. These 3D islands thicken and expand until they fill the pits. A movie version is available in the Supporting Information.
islands form in the immediate vicinity of the pits. After 255 s of film deposition (bottom image of Figure 4), all six pits are filled with Ag cylinders. Thus, 3D structures can be fabricated directly either during film growth (Figure 4) or during annealing (Figures 1 and 3). I close by noting that a large number of film/substrate systems should undergo downhill migration. StranskiKrastanov film growth is common, occurring in metal, semiconducting, and insulating materials.24 Downhill migration has been observed in the Stranski-Krastanov systems of: Ag on Ru(0001)8 and W(110), Au on Ru(0001)25 and W(110),26 Co on Ru(0001),27 and Cu on Ru(0001).8 Furthermore, when the literature is analyzed with knowledge of the downhill-migration process, there are clear indications of other systems where the phenomenon occurs. Examples include Cu on Mo(110),9 Ag on low-index Si(100)28 and vicinal Si(557),29 Pb on Ge(111)30 and Si(111),31 and Au electrochemically deposited on Pt(111).32 (Since the wetting layers are extremely thin, typically one to three atomic layers, it can be easily removed by blanket sputtering if undesired for an application.) Downhill migration may also occur in nonwetting systems devoid of wetting layers (i.e., VolmerWeber systems). Thus, fabrication of 3D nanostructures using the downhill-migration mechanism should be possible in a wide variety of material systems. Acknowledgment. This work was supported by the Office of Basic Energy Sciences, Division of Materials
Nano Lett., Vol. 6, No. 4, 2006
(1) Rosei, F. J. Phys.: Condens. Matter 2004, 16, S1373. (2) Plass, R.; Last, J. A.; Bartelt, N. C.; Kellogg, G. L. Nature 2001, 412, 875. (3) Shin, K.; Leach, K. A.; Goldbach, J. T.; Kim, D. H.; Jho, J. Y.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Nano Lett. 2002, 2, 933. (4) Tian, Z. R. R.; Liu, J.; Xu, H. F.; Voigt, J. A.; McKenzie, B.; Matzke, C. M. Nano Lett. 2003, 3, 179. (5) Shchukin, V. A.; Bimberg, D. ReV. Mod. Phys. 1999, 71, 1125. (6) Chambliss, D. D.; Wilson, R. J.; Chiang, S. Phys. ReV. Lett. 1991, 66, 1721. (7) Tersoff, J.; Teichert, C.; Lagally, M. G. Phys. ReV. Lett. 1996, 76, 1675. (8) Ling, W. L.; Giessel, T.; Thu¨rmer, K.; Hwang, R. Q.; Bartelt, N. C.; McCarty, K. F. Surf. Sci. 2004, 570, L297. (9) Bauer, E. Rep. Prog. Phys. 1994, 57, 895. (10) Bauer, E.; Poppa, H.; Todd, G.; Davis, P. R. J. Appl. Phys. 1977, 48, 3773. (11) Harland, C. J.; Akhter, P.; Venables, J. A. J. Phys. E: Sci. Instrum. 1981, 14, 175. (12) Jones, G. W.; Marcano, J. M.; Norskov, J. K.; Venables, J. A. Phys. ReV. Lett. 1990, 65, 3317. (13) Feydt, J.; Elbe, A.; Engelhard, H.; Meister, G.; Goldmann, A. Surf. Sci. 2000, 452, 33. (14) Vyalykh, D. V.; Shikin, A. M.; Prudnikova, G. V.; Grigor’ev, A. Y.; Starodubov, A. G.; Adamchuk, V. K. Fiz. TVerd. Tela 2002, 44, 157. (15) Altman, M. S.; Chung, W. F.; Liu, C. H. Surf. ReV. Lett. 1998, 5, 1129. (16) Melmed, A. J.; McCarthy, R. F. J. Chem. Phys. 1965, 42, 1466. (17) Mullins, W. W.; Rohrer, G. S. J. Am. Ceram. Soc. 2000, 83, 214. (18) Tanaka, S.; Umbach, C. C.; Blakely, J. M.; Tromp, R. M.; Mankos, M. Appl. Phys. Lett. 1996, 69, 1235. (19) Kammler, M.; Hull, R.; Reuter, M. C.; Ross, F. M. Appl. Phys. Lett. 2003, 82, 1093. (20) Giesen, M. Prog. Surf. Sci. 2001, 68, 1. (21) Marrian, C. R. K.; Tennant, D. M. J. Vac. Sci. Technol., A 2003, 21, S207. (22) Valbusa, U.; Boragno, C.; de Mongeot, F. B. J. Phys.: Condens. Matter 2002, 14, 8153. (23) Teichert, C. Phys. Rep. 2002, 365, 335. (24) Venables, J. A.; Spiller, G. D. T.; Hanbucken, M. Rep. Prog. Phys. 1984, 47, 399. (25) McCarty, K. F. Unpublished. (26) de la Figuera, J.; McCarty, K. F. Unpublished. (27) El Gabaly, F.; de la Figuera, J. To be submitted for publication. (28) Li, B. Q.; Swiech, W.; Venables, J. A.; Zuo, J. M. Surf. Sci. 2004, 569, 142. (29) Zhachuk, R. A.; Teys, S. A.; Dolbak, A. E.; Olshanetskii, B. Z. Surf. Sci. 2004, 565, 37. (30) Metois, J. J.; Lelay, G. Surf. Sci. 1983, 133, 422. (31) Jiang, C. S.; Li, S. C.; Yu, H. B.; Eom, D.; Wang, X. D.; Ebert, P.; Jia, J. F.; Xue, Q. K.; Shih, C. K. Phys. ReV. Lett. 2004, 92, 106104. (32) Sibert, E.; Ozanam, F.; Maroun, F.; Behm, R. J.; Magnussen, O. M. Surf. Sci. 2004, 572, 115.
NL060030L
861
