pubs.acs.org/NanoLett
Programmable Growth of Branched Silicon Nanowires Using a Focused Ion Beam Kimin Jun,†,‡ and Joseph M. Jacobson*,† †
The Center for Bits and Atoms, Media Laboratory, and ‡ Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ABSTRACT Although significant progress has been made in being able to spatially define the position of material layers in vapor-liquid-solid (VLS) grown nanowires, less work has been carried out in deterministically defining the positions of nanowire branching points to facilitate more complicated structures beyond simple 1D wires. Work to date has focused on the growth of randomly branched nanowire structures. Here we develop a means for programmably designating nanowire branching points by means of focused ion beam-defined VLS catalytic points. This technique is repeatable without losing fidelity allowing multiple rounds of branching point definition followed by branch growth resulting in complex structures. The single crystal nature of this approach allows us to describe resulting structures with linear combinations of base vectors in three-dimensional (3D) space. Finally, by etching the resulting 3D defined wire structures branched nanotubes were fabricated with interconnected nanochannels inside. We believe that the techniques developed here should comprise a useful tool for extending linear VLS nanowire growth to generalized 3D wire structures. KEYWORDS Focused ion beam, silicon, nanowire, galvanic displacement, vapor-liquid-solid, branch
A
wire structures has been carried out using random deposition of catalyst nanoparticles resulting in randomly branched nanowires. In this research, we demonstrate that a focused ion beam (FIB) may be used for the precise definition of branching points in silicon nanowires. Here we develop a FIB-mediated galvanic displacement technique to enable the selective deposition of gold particles on predefined positions which subsequently serve as catalytic growth points for VLS nanowire growth. In general, FIB-induced chemical vapor deposition such as that used to deposit metallic wires for circuit repair is a low throughput process since the entire new wire or circuit element must be deposited by FIB.11,12 Here the FIB is used only to define the catalytic points with the wires themselves grown in parallel after all points have been defined. This approach should allow reasonable processing speeds even for complex structures. Using this technique, various structures including multiple branches and several generations of nanowires were fabricated. In addition, oxidation followed by selective etching was explored to fabricate branched silica nanotube. Branched silicon nanowire synthesis started from a (111) directional silicon wafer diced into 1 cm by 1 cm pieces (i.e., the substrate). This silicon wafer substrate was initially cleaned by acetone, isopropyl alcohol (IPA), and deionized water (DIW) in sequence. Afterward, the substrate was etched in 1:10 diluted hydrofluoric acid (HF) solution for 2 min to remove native oxide. This clean substrate was then transferred to the center of a tube furnace, followed by dry oxidation at 900 C with 200 standard cubic centimeters per minute (sccm) oxygen flow for 5 min. This formed about 2-3 nm of thermal oxide on the substrate.
s the development of nanowire fabrication advances, there is considerable interest in extending 1D nanowire growth to more complex structures of higher dimensionality such as may be used to form interconnects for electronic circuits. An ultimate goal in this direction would be the development of a technology for fabricating 3D circuitry with a size and complexity equal to that of biological neural logic.1 An immediate motivation is current work on the fabrication of three terminal devices based on branched nanowires.2 To date such branch points are randomly located. The development of a technology for programmatically directing the position of nanowire branchpoints takes a first step toward the general creation of higher dimensional structures. Similarly, in the area of axial nanowire heterostructures, comprising single or multiple devices such as p-i-n structures3 the ability to deterministically define nanowire connection points, as opposed to photolithographically defined metal contacts, to the various parts of the device brings into parity the device density with the wiring density. Previously, branched nanowire structures have been demonstrated using both chemical synthesis4 and vaporliquid-solid (VLS) synthesis methods.5-10 Of these approaches, the VLS synthesis method should in principle provide a means for directing nanowire growth from arbitrary branch locations. To date, research in 3D branched
* To whom correspondence should be addressed. E-mail:
[email protected]. Address: Massachusetts Institute of Technology, 20 Ames St. E15-413, Cambridge, MA 02139. Phone: 617-253-7209. Received for review: 02/23/2010 Published on Web: 07/12/2010 © 2010 American Chemical Society
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FIGURE 1. FIB etched silicon substrate with thin oxide mask (a-d), and corresponding gold film deposition by galvanic displacement (e-h). Tilted view. Pattern size (a) 70 nm, (b) 100 nm, (c) 300 nm, and (d) 1 µm. Scale bars: (a,e) 500 nm; (b-d,f-h) 1 µm.
A focused ion beam (dual beam machine, Strata DB-235, FEI) was used to define the positions for gold catalyst deposition as follows. Using a focused gallium ion beam, square etching patterns were drawn on an oxidized substrate to remove the oxide and expose the silicon. The pattern dose was about 5.6 mC/cm2 with beam conditions of 30 kV acceleration voltage, 10 pA aperture, 1 us dwell time, and 50% overlap. This dose was determined to be the minimum value which can expose the underlying silicon layer. We tested hole size range from small (∼70 nm) to large (∼5 µm) area. The smallest feature size was limited by our galliumbased FIB machine. Higher resolution, down to perhaps 10 nm should be achievable with other advanced focused ion beam technologies such as helium ion beam.13 On these FIB-defined patterns, gold catalysts were deposited by galvanic displacement. The solution was prepared with a concentration of 0.01 M of potassium gold(III) chloride (KAuCl4, Sigma-Aldrich) in DIW. This solution was dropped on the patterned substrate by pipet and maintained for 1 min. The substrate was then cleaned with IPA and quickly dried on a hot plate at 200 C. This treatment formed gold nanoparticles selectively on the exposed silicon while the oxide layer served as a mask. Using these gold nanoparticles, silicon nanowires were grown in a home-built atmospheric chemical vapor deposition (CVD) chamber with conditions similar to those reported elsewhere.14 The substrate was put in the center of the tube furnace and heated in a hydrogen environment. When the temperature reached 890 °C, precursor gas (50 sccm of hydrogen bubbled through silicon tetrachloride (SiCl4, SigmaAldrich) with an additional 190 sccm of hydrogen for dilution) was fed into the quartz tube. After about 1 min of nanowire growth, the reaction was quenched by evacuating the tube by vacuum pump and backfilling hydrogen. This produced vertically grown silicon nanowires. For secondary nanowire growth, the primary gold catalyst was removed before further processing. The substrate © 2010 American Chemical Society
was dipped in 1:10 HF solution for 2 min, followed by gold etchant (TFA, Transene) for 2 min and 1:10 HF solution for 10 s. This clean substrate was again oxidized in the same oxidization condition for the substrate preparation. After oxidation, the sample was loaded in the FIB chamber where the stage was tilted to align the nanowire sidewall toward the ion beam irradiation direction. Square patterns of 0.2-0.3 µm were drawn on the nanowire. Another galvanic displacement was conducted on this patterned substrate, followed by secondary nanowire growth. These overall processes were repeated several times to synthesize the desired number of branch generations. Scanning electron microscope (SEM) images were taken by the same dual beam machine, and transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were obtained by TEM (JEM-200CX, JEOL). For nanotube fabrication, the branched nanowires were oxidized for a longer time (60 min) at 900 C in oxygen environment to form a thick oxide on the nanowires surface, making silicon-silicon dioxide core-shell structure. After oxidation, small holes (0.1 µm by 0.1 µm square) were patterned near the tip of backbone nanowires to expose silicon core. Then, this substrate was placed in xenon difluoride (XeF2) isotropic silicon etcher (ES-2000XM, SE Tech), and etched for 60 s and 3 cycles (total 180 s). Figure 1 describes galvanic displacement result on flat silicon substrate with thin oxide mask. Conventionally, gold galvanic displacement was conducted using solution containing gold salts along with HF. The overall reaction has been described by15
Si + 6HF f SiF62- + 6H+ + 4e-
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FIGURE 2. Nanowire branching procedure. (a) Vertically grown silicon nanowire with thin oxide, (b) side etching by FIB (inset, magnified view of etched position), (c) secondary catalyst deposition by galvanic displacement (inset, magnified view of deposited gold nanoparticle), (d) nanowire growth from the secondary catalyst, (e) oxide and catalyst removal by wet etching. Tilted view. Scale bars: 5 µm for all.
Au3+ + 3e- f Au
primary catalyst and oxide etching. Afterward, a thin oxide was formed by thermal oxidation (Figure 2a). We tried to make this thermal oxide as thin as possible to minimize the sacrificial nanowire volume and the FIB etching dose. Figure 2b shows an etched pattern by FIB. Not only a single pattern, but multiple patterns were drawn from various directions when multiple branches were intended. A second galvanic displacement was conducted in this substrate to deposit gold clusters on the holes (Figure 2c). Special care was given to the cleaning process, since it was found that gold particles were easily detached when the nanowire diameter was small due to the large surface tension of the cleaning solution during the drying process. To resolve these problems, we made the nanowire diameter thicker than the minimum we can achieve. This also helps to maintain branched shape during followed HF cleaning and gold etching, which also easily break branches. Additionally, instead of water, IPA was adopted as a cleaning solution if applicable, which has a lower surface tension. This resulted in well-defined catalyst particles from which secondary nanowires would be grown (Figure 2d,e). Again, we believe that the size of nanowires could be shrunken by applying gentle cleaning techniques such as supercritical drying. Figure 3 shows fabricated nanowire structures with multiple branches. The first group (Figure 3a) is the structure in which multiple branches were derived from a single backbone nanowire. Any directions accessible by ion beam can have catalyst deposition, which allows the arbitrary definition of secondary branching points. In the second group, repeated branching is shown in Figure 3b,c. Several generations of a “parent/child” pair could be fabricated by repeating
Because of HF, thin silicon dioxide (SiO2) is not a good masking material. To compensate for the partial etching of oxide, one needs a thicker SiO2 layer16,17 or another inert material in the HF environment. In our initial trial with thin oxide, HF solution (0.2 or 0.01 M of 49% HF and 0.01 M of KAuCl4 in DIW) deposited gold particles with low selectivity between FIB-etched and nonetched places. As an alternative, we removed HF and used this non-HF solution to induce galvanic displacement. As seen in Figure 1, gold clusters filled all the sizes of patterns with high selectivity. Since our solution does not contain a reducing agent and the substrate was placed in an insulating plate, it is thought that the galvanic cell is formed between the substrate and solution. To clarify whether implanted gallium contributes the gold deposition or not, the same patterning and deposition was tried on thick (200 nm) SiO2 surface, which resulted in very small or negligible amount of gold deposition. Therefore, we believe that the displacement is predominantly attributed to underlying silicon. Although ionic analysis is beyond our scope in this research, we assume that possible chemical reaction is hydroxide-induced silicon etching18 and gold ion reduction. Regarding this result, it is notable that similar HFfree galvanic displacement was observed in germanium (100) substrate.19 This allowed a simple approach to make a selective catalyst deposition on the sidewall of the nanowires. The overall procedure is described in Figure 2. First, a backbone nanowire was grown from the flat substrate, followed by a © 2010 American Chemical Society
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When more than three generations of branches were fabricated, some of them have in-plane configuration (Figure 4a), that is, all of the branches lie in the same plane. At the same time, there also exist 3D configurations (Figure 4b,c) in which part of the component branches have out-of-the plane direction. In our nanowire size and similar experimental conditions, it is well-known that most of the nanowires have a silicon (111) direction.20,21 Therefore, the backbone nanowires which are grown epitaxially from (111) surface are mostly vertical. However, these vertical nanowires usually have (110) or (112) termination on the sidewall.22 In this respect, growth direction uncertainty may derive from small misalignment of the FIB hole or randomness of nucleation during the growth process. Nucleation randomness could be affected by FIB-etched surface conditions, among which gallium implantation and damaged surface crystal might be possible causes. From SRIM23 simulation, the implanted gallium and corresponding damages lie near the surface. This surface disruption may be similar to that of heterojunction, which probably increases the uncertainty of nucleation. From this standpoint, it may be desirable to adopt a manipulation method that results in lower surface disruption. Instead of gallium ion beam etching, such methods as focused electron beam-assisted oxide etching24 or direct FIB gold deposition25 could be alternatives. In addition to lattice mismatch, it was also suggested that catalyst size uniformity and nanowire growth conditions also affect the branch type.26 Therefore, fine-tuning of the overall processes should decreases the uncertainty. Although nucleation may result in variation of initiation process, the body of nanowire is mostly (111) directed, maintaining epitaxial relationship between “substrate” nanowire and branches.27 In Figure 5, the first generation nanowire (bottom thinnest one) is vertical from the substrate, that is (111) directed. During the second growth, the interface
FIGURE 3. Complex nanowire branching structures. (a) Multiple branches from single backbone. (b,c) Repeated branching events for multiple generations of parent/child pair. (d) Multiple branches junction after sequential growth. Tilted view. Scale bars: 5 µm for all.
the processes described in Figure 2. Creative sequential growth could make more complex structures such as multiple branches junction in Figure 3d. Branch 1 and 2 were grown, followed by focused ion beam irradiation in the arrowed direction. This ion beam made aligned holes both on the substrate and branch 2. Branch 3 began from substrate and met branch 2. At the same time, another branch, 4, was grown from branch 2. As a result, three branches formed a one junction/four terminal structure. These basic manipulations establish a basis set for the construction of a large number of 3D structure classes. Although branching points could be exactly defined in our experiment, there were variations in the growth directions.
FIGURE 4. (a-c) Multigeneration nanowire structures, and (d-f) corresponding (111) vector space with dark-colored arms meaning occupied by branches. Tilted view. Scale bars: 10 µm for all. © 2010 American Chemical Society
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FIGURE 5. TEM image of three branches structure. (Left insets, SAED patterns on each branch; right insets, magnified junction images). Scale bar: 5 µm for main image, 500 nm for right insets.
(circle in the Figure 5) indicates the second generation branch experiences fluctuation during nucleation. However, the first-second and the second-third branches show 70.5° angle respectively, which corresponds to adjacent (111) direction angle (Figure 5 main image, and right insets). The electron diffraction images (Figure 5, left insets) were taken from the same focal plane. They show that all the three branches have identical crystal structure. From this observation, the complexity of branched structure can be expressed in linear combinations of (111) vector components. Figure 4d-f describes vector space that consists of eight (111) vectors, and dark-colored vectors are occupied ones by Figure 4a-c, respectively. This allows us to predict possible final structures, and final structure complexity is on the order of 8n, in which “n” stands for number of generations. This nanowire branching method was further extended to branched nanotube fabrication in the similar manner as reported before.28 Global oxidation formed an oxide shell while the inner silicon core was connected throughout the branched structure. After making a small hole at the end of the branched structure to expose the silicon core (Figure 6a,b), selective isotropic silicon etching was conducted to remove this core (Figure 6c,d). Etching gas penetrated through the silicon core up to the substrate. In Figure 6c, dark circular mark on the base shows etched silicon substrate, which makes it a free-standing nanotube on a silica membrane. To see the hollow inside, each end of the backbone and branch were cut by FIB as shown in Figure 6e,f. Since the shell thickness can be readily controlled by oxidation time and temperature,29 it would be possible to synthesize branched nanochannel with predetermined diameter or encapsulated hollow nanostructure by filling the etch hole with FIB-CVD or ion-beam sculpting.30 © 2010 American Chemical Society
FIGURE 6. Branched nanotube fabrication. (a) Branched nanowire with thick oxide. (b) Magnified view of (a) near the backbone tip showing FIB etched hole. (c) after XeF2 etching. (d) Magnified view of (c) near the tip. (e,f) Magnified views of (c) after FIB cutting. Tilt view. Scale bars: (a,c) 5 µm, (b,d-f) 1um.
In summary, we have successfully fabricated branched silicon nanowire structure by FIB etching, catalyst deposition, and nanowire growth. Unlike other random catalyst deposition based processes, we could precisely define branching points. By multiple points definition and repeated processes, complex and multigeneration branches were also demonstrated. In our growth conditions, most of the nano2781
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wire grew in the direction of (111) plane. This allows us to expect the complex structure with the combination of (111) vectors. Finally, by oxidation and selective etching, branched silica nanotubes were fabricated with interconnected hollow channel. We believe that this research should the exploration of further 3D branched nanowire structures. Acknowledgment. This work was supported by MIT’s Center for Bits and Atoms and the U.S. Army Research Office under Grant W911NF-08-1-0254. K.J. is on the partial fellowship support of the Samsung Scholarship. REFERENCES AND NOTES (1) (2)
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