Bottom-up Nanoconstruction by the Welding of Individual Metallic

NANO LETTERS

Bottom-up Nanoconstruction by the Welding of Individual Metallic Nanoobjects Using Nanoscale Solder

2009 Vol. 9, No. 1 91-96

Yong Peng,†,‡ Tony Cullis,‡ and Beverley Inkson*,† Department of Engineering Materials, Department of Electronic and Electrical Engineering, Sheffield UniVersity, Sheffield S1 3JD, U.K. Received August 19, 2008; Revised Manuscript Received November 17, 2008

ABSTRACT We report that individual metallic nanowires and nanoobjects can be assembled and welded together into complex nanostructures and conductive circuits by a new nanoscale electrical welding technique using nanovolumes of metal solder. At the weld sites, nanoscale volumes of a chosen metal are deposited using a sacrificial nanowire, which ensures that the nanoobjects to be bonded retain their structural integrity. We demonstrate by welding both similar and dissimilar materials that the use of nanoscale solder is clean, controllable, and reliable and ensures both mechanically strong and electrically conductive contacts. Nanoscale weld resistances of just 20Ω are achieved by using Sn solder. Precise engineering of nanowelds by this technique, including the chemical flexibility of the nanowire solder, and high spatial resolution of the nanowelding method, should result in research applications including fabrication of nanosensors and nanoelectronics constructed from a small number of nanoobjects, and repair of interconnects and failed nanoscale electronics.

One of the central challenges for the bottom-up construction, integration, and repair of nanoscale systems incorporating single nanoobject building blocks is to develop reliable methods of joining individual nanoobjects together and to substrates.1-5 Individual nanoobjects such as nanoparticles, nanowires, and nanotubes can now be routinely fabricated and manipulated at the nanoscale using a range of scanning probe-based methods.5-15 However, simply bringing nanoobjects into contact cannot be relied upon to generate secure, functional, and long-lasting bonds that are a necessity for devices with a long lifetime. A key challenge for joining nanoobjects is the formation of mechanically strong bonds whose size and chemistry can be tailored to generate the desired functional properties such as electrical conductivity. A number of localized joining methods suitable for individual nanoscale objects have recently been proposed including thermal heating,16,17 ion beam deposition of material,18 laser heating,19 ultrasonic irradiation,20 high-energy electron beam bombardment,21 and Joule heating from electrical currents.13,22 Of these the method of Joule heating may hold the most potential because of its formation of electrically conducting junctions, inherent simplicity, cleanliness, and reliability. The other methods suffer from a number of problems for practical industrial application. For example, high-energy electron beam systems are expensive and are * To whom correspondence should be addressed. [email protected]. † Department of Engineering Materials. ‡ Department of Electronic and Electrical Engineering. 10.1021/nl8025339 CCC: $40.75 Published on Web 12/10/2008

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 2009 American Chemical Society

not easily integrated into industrial production lines. Focused ion beam-based deposition currently only has limited chemistries with poor control of functionality and contamination, and thermal heating methods have very poor spatial control of heated zones and contamination. At the nanoscale, structurally resilient carbon nanotubes (CNTs)23 and CNTs filled with copper13 have been successfully welded together by current-induced flow of carbon and metal to the junction. These CNT methods require that a very high current density (typically >1 × 1011 A·m-2 for melting metal) has to flow through the nanotubes into the adjacent CNT or nanostructure. This means that nanostructures bonded by this method have to be both conductive and resistant to current-induced damage. Currently, the successful bonding of metal nanostructures by Joule heating has been limited to the welding of Pt wires with 650 nm diameter.22 It is well known that high current flow through nanostructured metals face a major problem in that the nanoobjects can be severely structurally and functionally damaged due to atom migration from Joule heating and electromigration.2,24 Here we describe a new nanoscale electrical welding technique that radically improves the spatial resolution, flexibility, and controllability of welds between individual nanowires and nanoobjects. The key advance is to avoid detrimental current flow though the nanoobjects to be joined and instead to locally deposit nanoscale volumes of a chosen metal at the weld site by Joule heating a sacrificial nanowire. The principle of the process is illustrated in Figure 1.

Figure 1. Schematic of nanoconstruction by the welding of individual nanoobjects (nanowires) using nanoscale solder. (a) Assembly of individual nanoobjects into a desired pattern using a nanomanipulator probe. (b) Placement of a sacrificial nanowire in contact with the nanostructure to be welded. (c) Nanowelding the nanoobjects together by an electrical signal. (d) Completed nanoweld.

Importantly, the use of nanoscale “solder” to bond the nanoobjects together also offers the opportunity to tailor the weld’s mechanical and functional properties by controlling the chemistry, structure, and volume of solder material used. Furthermore the entire process can be carried out within a standard SEM system. The soldering of nanoobjects into complex networks is demonstrated here by two examples which demonstrate the wide applicability and performance of the technique. First 55 nm Au nanowires are welded with gold solder into complex patterns, including triple junctions, which cannot be achieved by direct Joule heating of the nanowires due to morphological instabilities induced by current flow. The ability to weld Au nanowires is important since, due to their excellent electrical conductivity and stability in oxidizing environments, they are widely used for electrodes and interconnects in the semiconductor industry and nanoelectronics research. In the second example, we demonstrate that dissimilar nanowires can be welded together, using a third material as solder. SnAu alloy solder, widely used in macroscopic welding technique due to its excellent conductivity, low melting point, and large corrosion resistance,25 is specifically chosen to result in an ultralow electrical resistance join between nanowires. The ability to integrate functional nanowires into circuits via high-performance welded joins represents a significant advance for nanoscale sensor development. A range of nanowires were fabricated by electrodeposition into porous templates, and then dispersed onto Si/SiO2(100 nm) wafers. To construct nanowire networks, individual nanowires were assembled into complex patterns by mechanical manipulation using nanomanipulators within an SEM. SEM nanomanipulators are particularly suitable for industrial applications of localized bottom-up integration and repair nanotechnology due to their nanoscale spatial resolution, intrinsic long manipulating distance,26 no-specialrequirement-of-sample,27 large working areas,27 speed effi92

ciency of manipulation and observation,26,27 and proven assimilation into production lines.27 First the nanowires were released from the strong bonding to the substrate wafer by a mild mechanical push from a SEM nanomanipulator (Figure 2a). The chosen nanowire is then lifted from the substrate using Van der Waals force (Figure 2b), followed by precise release at the desired location adjacent to another nanowire (Figure 2c). The whole SEM mechanical manipulation process is clean, simple, quick, and reliable. It also avoids the increased time, expense, and contamination of methods involving nanoobject pick-up via e-beam and ionbeam gas deposition.19 By means of this clean manipulation method, complex structures, like “NANO” shown in Figure 2d, can be easily assembled. Once nanowires have been positioned into a required design, the goal is to securely join them together ensuring mechanically strong and electrically conducting junctions. Electrical currents can be directly and controllably applied to individual nanowires and through sections of nanowire circuits using two SEM nanomanipulators (Figure 3a). Using SEM nanomanipulators, the 55 nm Au nanowires here exhibited a maximum transport current of 11.73 mA at applied voltage 0.72 V (Figure 3b), giving a calculated maximum current density Jmax of 4.94 × 108 A·cm-2.28 Figure 3 illustrates the reason why successful bonding of ,100 nm metallic structures by direct Joule heating is technologically exceptionally difficult. During current flow, Joule heating and electromigration causes diffusion of atoms along the length of nanowires. If not carefully controlled, high and persistent current flow causes morphological instabilities to develop along the length of the nanowires, which eventually causes their catastrophic failure (Figure 3c,d). Failure of nanowires due to electromigration is a well-documented problem in electronic interconnects.2,24 Attempts to electrically weld overlapping metal nanowires together by directly running a current through their junction, using up to 30% of maximum current density Jmax, were found to be unpredictable, uncontrollable, and mostly unsuccessful. Although the local increase in resistance at the nanowire junction enhances Joule heating and thus material from the individual wires diffuses to the junction to form a weld, the unpredictable development of morphological instabilities close to the weld and along the lengths of the nanowires (particularly at grain boundaries) frequently causes thinning, break-up, and failure of the wires (Figure 3e, inset, before nanowelding). Consistent with this, successful bonding of touching metal nanowires by electrical Joule heating has only been reported for thick Pt nanowires 650 nm in diameter, which are more resistant to morphological failure by virtue of their increased size.22 Dramatic advances in electrical nanowelding of nanowires are achieved here by an approach that avoids significant current flow and associated morphology changes along the nanowires to be joined. Joule heating of a sacrificial nanowire positioned at the required weld point enables nanovolumes of conductive solder to be applied directly to the junction, forming a controllable, secure, and conductive weld (Figures 1and 4). Nano Lett., Vol. 9, No. 1, 2009

Figure 2. Manipulation and assembly of individual gold nanowires. (a) Release of a gold nanowire from a Si/SiO2(100 nm) wafer by using a nanomanipulator probe to mechanically push the chosen nanowire. (b) Lift-up of the released individual gold nanowire by Van der Waals force. (c) Assembly of the picked-up nanowire adjacent to another nanowire. (d) Nanoscale word NANO written by individual gold nanowires.

Figure 3. Electrical characterization of individual gold nanowires. Au nanowire (55 nm) before (a) and after (b) 2.5 mA current for 2 min, showing current-induced shape change and failure. Inset: magnified view of failure. (c) Overlap of two nanowires N1 and N2 (arrowed) prior to welding. (d) Nanowire failure (N2, arrowed) after directly running a current through two overlapping nanowires.

Figure 4 shows representative experimental images of the nanoweld process, showing how a sacrificial Au nanowire was used to deposit solder material onto an assembled Au nanowire pattern resulting in a welded nanowire configuration in the form of the Chinese character meaning human being. Prior to welding, a 0.7-0.85 mA current is run through the Au solder nanowire for 1∼2 min to improve the controllability and reliability of the nanowelding procedure. The moderate current density causes a softening of the solder nanowire due to Joule-heating and limited diffusion of the solder material onto the nanostructures at their contact point induces the first join to the patterned nanowires (Figure 4a). A rectangular pulse signal of 0.7-1.0 V and 100 ms width for Au is then applied to the solder nanowire to form the weld. The short voltage pulse causes significant Joule Nano Lett., Vol. 9, No. 1, 2009

heating and associated rapid material diffusion of the solder nanowire onto the chosen junction (Figure 4b). The pulse magnitude and time is controlled to provide just adequate heating and diffusion of the nanosolder, and to minimize heat dissipation into the adjacent nanostructure. For the 55 nm Au solder wires the optimum current density typically ranges from about 1.8 × 108 to 2.5 × 108 A·cm-2 to obtain well-controlled, reproducible, and reliable nanowelds. The nanowelding technique can be used to weld complex crossed structures with high angles between the connecting nanowires, for example, the Chinese character and the letter “N” in Figure 4. Once formed the welds joining two nanowires are strong and very conductive. If the solder nanowire remains attached to the junction after welding (Figure 4b), mechanical force can be applied to separate them by pulling the probe away (Figure 4c). Furthermore, if any unwanted residues of solder nanoparticles and rods exist, which are conductive and therefore dangerous and potential short-circuit sites for real nanoelectronics devices, the nanomanipulators can easily and precisely mechanically remove the welding residues. The welded letter N of Figure 4f is the third letter of NANO in Figure 2d, which was cleaned by nanomanipulators after the whole word NANO had been accomplished. An immediate check of nanoweld quality is essential for practical nanoconstruction, and can be done in situ directly after welding. Figure 4e shows the quantification of resistances across a “T” shaped weld by directly touching probes to the ends of the welded nanowires (see inserts of Figure 4e, with the tip-tip circuit resistance subtracted). The electrical characterization of a range of welded nanowires confirms the formation of electrically conducting ohmic junctions. Although the resistances across the Au welds are typically higher than along single integral nanowires (Figure 4e), the welded junctions still exhibit low contact resistances in the range ∼10-1000 Ω. This phenomenon is similar to traditional macroscopic welds using the solder technique. 93

Figure 4. Nanowelding complex nanowire structures using nanovolumes of metal solder, using 55nm Au nanowires as an example. (a) Preweld softening of the solder nanowire and contact to the two nanowires to be welded. (b) Nanowelding two nanowires together by a rectangular voltage pulse through the solder nanowire, causing significant material diffusion onto the junction. (c) Mechanically removal of the solder nanowire, leaving miniaturized Chinese character. (d) High magnification SEM image of the nanoweld. (e) Immediate quality check of the welded nanojunction in panel d, blue curve showing the resistance of the left nanowire (left insert), and red curve resistance through the welded nanojunction (right insert). (f) Fabrication of miniaturized letter “N” welded using nanoscale solder.

The ability to weld similar nanowires together with matching solder material will have many applications, particularly in the repair of failed interconnects and electronic circuitry. However the presented welding technique has far wider applicability in the area of integrating individual nanoobjects into circuits where their functional properties can be exploited. To do this, dissimilar nanoobjects have to be joined together with welds with the desirable characteristics such as strength and low electrical resistance. Here we demonstrate that individual 55 nm diameter (CoPt/Pt)n multilayer nanowires, which have potential uses as nanosensors and nanoelectronics due to their GMR effect29,30 can be successfully integrated with 55 nm Au interconnect nanowires. The two dissimilar nanowires are welded together using Sn99Au1 solder. The use of a solder with a melting point significantly below that of the materials to be joined minimizes the likelihood of any structural damage due to heat dissipation during the welding process. The welding technique used here, carried out in vacuum with an extremely short electrical-heat pulse, also inhibits oxidation effects which can affect welds formed by prolonged heating in air.25 Figure 5 illustrates the successful welding of a 55 nm (Co78Pt22/Pt)n multilayer nanowire to a 55 nm Au nanowire using a 200 nm Sn99Au1 solder nanowire. Before assembly, the resistances of the circuit components (consistently reproducible for the given SEM tip-pair) were determined to be the following: SEM manipulators tip-to-tip circuit ) 74 Ω, the chosen individual (Co78Pt22/Pt)n multilayer nanowire ) 4153 Ω, and the gold nanowire ) 79 Ω. After the chosen (Co78Pt22/Pt)n multilayer nanowire and gold nanowire were assembled together (Figure 5a) and prior to welding, the resistance ATD was measured to be isolating due to a poor contact. A sacrificial 200 nm Sn99Au1 solder nanowire was then placed at the weld point (Figure 5b), and a weld formed by using a 0.5∼1.0 mA current for 1∼2 min, followed by a 50 mA rectangular pulse signal with 150 ms width (Figure 5c). It was found that to obtain well-controlled, reproducible, and reliable nanowelds, the optimum current density for the sacrificial 200 nm Sn99Au1 nanowires typically 94

Figure 5. Nanowelding an individual 55nm (Co78Pt22/Pt)n multilayer nanowire to a 55 nm gold nanowire using a sacrificial 200 nm Sn99Au1 solder nanowire. (a) Assembly of a gold nanowire (CTD) adjacent to a (Co78Pt22/Pt)n multilayered nanowire (ATB). (b) Placement of a sacrificial 200 nm Sn99Au1 nanowire in contact with the nanostructure to be welded. (c) Nanowelding the nanowires together by a rectangular voltage pulse through the solder nanowire, causing significant material diffusion onto the junction. (d) Immediate quality check of the welded nanojunction in panel c: I-V curves showing the resistance of SEM tip-tip circuit (left y-axis, black curve), resistance of gold nanowire CTD (left y-axis, red curve), resistance through (Co78Pt22/Pt)n multilayer nanowire ATB (right y-axis, green curve), resistance through whole structure ATD (right y-axis, blue curve).

ranged from about 0.9 × 108 to 1.6 × 108 A·cm-2. The volume of solder in Figure 5c is larger than the nanowire thickness; however, if required the volume of solder can be controlled and reduced by altering the sacrificial nanowire diameter. The resistances across a range of nanoscale welded heterojunctions were measured in situ directly after welding. The resistances of the individual nanowires and SEM tipto-tip circuits were found to be the same, within error, before and after the weld process. For the dissimilar nanowire weld Nano Lett., Vol. 9, No. 1, 2009

Figure 6. Characterization of Au weld microstructure. (a) Weld P formed between two Au nanowires (I and II) using Au solder. The welded nanostructure was picked up on a SEM nanomanipulator tip. (b) TEM characterization of the weld confirms a fully integrated join, with a θ fcc diffraction pattern of the solder material shown.

in Figure 5, the I-V curves for the SEM tip-to-tip circuit, the individual (Co78Pt22/Pt)n multilayer nanowire ATB, the gold nanowire CTD, and the whole welded structure ATD are shown in Figure 5d. The difference between the sum of the nanowire resistances (ATB plus CTD), and the resistance of the whole welded structure (ATD), gives a nanoweld resistance of just 21.6 Ω.This extremely low weld resistance shows that an excellent quality of weld has been achieved using the Sn99Au1 alloy as solder material. Experimental data from further nanowelding experiments shows that the resistances of Sn99Au1 nanowelds were reliably in the range ∼10-100 Ω, indicating that the tin alloy is a reliable and reproducible nanosolder material. The quality and structure of the welded nanojunctions have also been checked by transmission electron microscopy (TEM). The mechanical strengths of the welds were easily strong enough to enable welded nanowires to be directly lifted up from the Si/SiO2 (100 nm) wafers by a SEM nanomanipulator, and then transferred to TEM on the nanomanipulator tip using a modified Gatan TEM tomography sample holder. Figure 6 shows an example of two Au nanowires (I and II) welded together at a junction P using Au solder (Figure 6a). The weld P is confirmed by TEM to be a completely integrated Au join with polycrystalline fcc structure (Figure 6b). In summary, the high spatial resolution nanowelding method presented here provides a controllable and reliable approach for manipulating, assembling and welding