Synthesis, Characterization, and Thermal Properties of Nanoscale

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J. Phys. Chem. C 2009, 113, 9546–9552

Synthesis, Characterization, and Thermal Properties of Nanoscale Lead-Free Solders on Multisegmented Metal Nanowires Fan Gao, Subhadeep Mukherjee, Qingzhou Cui, and Zhiyong Gu* Department of Chemical Engineering and Nanomanufacturing Center, UniVersity of Massachusetts Lowell, One UniVersity AVenue, Lowell, Massachusetts 01854 ReceiVed: December 19, 2008; ReVised Manuscript ReceiVed: April 7, 2009

Nanoscale lead-free solders (“nanosolders”) have been synthesized directly onto multisegmented metal nanowires using an electrodeposition method in nanoporous templates. The nanosolders fabricated include tin (Sn), tin/silver (Sn/Ag), and indium (In), and the diameter of the nanosolder nanowires ranges from 30 nm to 200 nm and the length from 1 to 10 µm. The microstructures of the lead-free nanosolders on nanowires have been characterized using optical microscopy and electron microscopy including a field-emission scanning electron microscope (FESEM) and a transmission electron microscope (TEM), along with energy-dispersive X-ray spectroscopy (EDS). Thermal properties of lead-free nanosolders on nanowires were characterized using a temperature-programmable furnace tube under a controlled atmosphere. It was found that nitrogen plays an important role in the nanosolder reflow process. The effect of base layer, barrier layer, and wetting layer on nanosolder reflow was studied, and an optimal nanowire nanosolder system with effective barrier and wetting layers was obtained. A liquid phase-based solder reflow process was developed, in which the nanosolder nanowires were assembled in a liquid medium and solder joints were formed between nanowires. 1. Introduction Significant development has occurred in the past two decades in the synthesis and characterization of nanomaterials, and many novel nanostructures have been obtained, such as carbon nanotubes, nanowires/nanorods, quantum dots, and nanocomposite materials, which have shown great promise in many new applications.1-8 Both the traditional top-down approach, based on photolithography and thin film processing (deposition or etching), and the emerging bottom-up approach have shown the capability to assemble nanobuilding blocks into ordered and, in some occasions, large-scale structures. Molecular or atomiclevel manipulation has been enabled by atomic force microscopy (AFM) 9,10 or optical tweezers,11,12 which was used to accomplish such tasks as moving, pushing, pulling, or cutting nanoscale objects. However, these types of methods are slow, expensive, and difficult to scale-up into manufacturing processes. Many new assembling methods based on self-assembly strategy, mimicking biological phenomena, have showed promise in integrating nanocomponents into ordered one-dimensional (1D), 2-D, and 3-D ordered structures.13-20 However, in many cases the self-assembled structures are fragile, not permanent, and difficult to be transferred or moved to another surface, which are crucial to manufacture functional electronics or devices. Effective joining and interconnect formation methods between these self-assembled nanostructures are much less developed and little knowledge is known at the current stage, and significant improvement is needed to achieve effective nanoscale interconnection with thermal and electrical functionality. Some effort has been made in joining nanocomponents into functional circuit or prototype nanoelectronic devices. Two main joining techniques that have been studied so far are (1) hightemperature annealing or sintering21-23 and (2) focused electronbeam (FEB) or focused ion-beam (FIB) irradiation.24-29 An* Corresponding author. Phone: (978) 934-3540; fax: (978) 934-3047; e-mail: [email protected].

nealing at high temperature is an effective way to lower contact resistance between nanocomponents, especially between metals. However, the annealing or sintering temperature is normally very high (200-300 °C or higher) and the time duration is long, which may affect or damage many nanocomponents. For organic and polymer-based nanomaterials or nanoelectronic devices with hybrid structures that are composed of both organic and inorganic materials, the chance to survive high temperature is low. FEB/FIB irradiation has been successfully used to bond carbon nanotubes to substrates, nanotubes to nanotubes, and gold nanowires to nanowires;24-29 however, this technique suffers from contamination of uncompleted precursor decomposition. For both methods above, slow and harsh processing conditions and high cost make them almost impossible for manufacturingscale production other than laboratory-scale integration. Besides these two widely used methods, some new techniques such as atomiclayerdeposition(ALD),30,31 welding,32,33 inkjetprinting,21,34,35 dip-pen nanolithography,36,37 and electroless plating38 are also being developed. Solders and soldering technique are widely used in microelectronics assembly and packaging.39,40 Solders are metals or metal alloys with low-melting points, and they can form functional (electrical and mechanical) interconnects with high thermal stability. One major application of soldering is assembling electronic components to printed circuit boards (PCBs) or printed wire boards (PWBs). The most widely used solders are eutectic 63/37 tin/lead (Sn/Pb) solder with a melting point of 183 °C. Due to environmental concerns, health and safety issues of lead (Pb), “no-lead” or “lead-free” (Pb-free) solders are being implemented and becoming more widely used.41-44 Unfortunately, many Pb-free solders are not eutectic formulations, which normally require higher temperature processing, making it difficult to create reliable joints. Thus, there is a tremendous need to develop Pb-free solders with higher quality than (or comparable quality to) the traditional Sn/Pb solders for electronics and other industries. Nanotechnology offers good

10.1021/jp8112396 CCC: $40.75  2009 American Chemical Society Published on Web 05/06/2009

Nanoscale Lead-Free Solders opportunities to overcome the existing problems and meet the challenges. For example, the phenomena of melting point depression for nanoparticles have the potential to lower the leadfree solder reflow temperature and thus decrease the thermal stress correspondingly. On the other hand, further device miniaturization into nanoelectronics means limited usage of microsized solder particles in the solder paste. Nanoscale solders, preferably lead-free ones, have to be developed in order to meet the demand to assemble and integrate nanobuilding blocks such as carbon nanotubes, nanowires, nanocomposite, or hybrid materials. In this paper, we show that lead-free nanosolders were synthesized onto multisegmented metal nanowires. The nanowire fabrication process involves sequential electrodeposition in nanoporous templates. The nanosolders fabricated have been characterized with optical microscopy and electron microscopy (FESEM and TEM) along with energy dispersive X-ray spectroscopy (EDS). Differential scanning calorimetry (DSC) was used to measure the melting point of nanosolder nanowires. Nanosolder reflow properties have been studied using a temperature-programmable furnace tube under controlled atmosphere and the effect of base layer, barrier layer, and wetting layer on solder reflow was investigated. Nanosolder joints were formed between nanowires through the solder reflow process when the nanowires were assembled in a liquid medium of high boiling point. The results showed in this paper will be important information for future assembly of functional nanowires or nanowire-based nanoelectronics devices and will also be relevant to nanoelectromechanical system (NEMS)-based applications or biomedical devices. 2. Experimental Section 2.1. Materials. Commercial plating solutions of silver (Ag) (Techni Silver E-2), gold (Au) (Techni Gold 25E), nickel (Ni) (Techni Nickel Sulfamate Bath RTU), Copper (CU) (Copper U Bath RTU) and tin (Sn) (Techni Tin) were purchased from Technic, Inc. Tin plating solution was prepared by using the Techni tin solution together with corresponding makeup solution and antioxidant (obtained from Techni too). Tin/silver (Sn/Ag) plating solution was prepared in accordance with a recipe in the reference.45 Indium (In) plating solution (indium sulfamate) was purchased from Indium Corp. Sodium hydroxide (NaOH), dichloromethane, and ethanol were purchased from Fisher Scientific. 1H-Benzotriazole (99+%) and tri(ethylene glycol) (99%) were purchased from Acros Organics. All the chemicals were used as received. Nitrogen gas (Industrial, g99.8%) was purchased from Airgas East. Deionized water was made by a Barnstead E-pure system (model no. D4541) at 18.2 MΩ · cm. Alumina and polycarbonate membranes were purchased from Whatman (now part of GE Healthcare). Silver (99.99% purity) was purchased from Alfa Aesar, and silver thin film deposition was conducted in a NTE-3000 thermal evaporator (Nano-Master, Inc.). Nanowire fabrication was performed on a Princeton Applied Research (PAR) Model 362 pontentiostat. Nanosolder reflow was carried out in a Thermo Scientific Lindberg Blue M tube furnace with programmable temperature control. 2.2. Synthesis and Fabrication of Lead-Free Nanosolders onto Nanowires. Nanoscale lead-free solders (“nanosolders”) have been synthesized directly onto nanowires using electrodeposition in nanoporous templates. A typical nanowire fabrication process is described as below.46-48 First a thin layer of Ag was evaporated on one face of a commercial anodized alumina (AAO) or track-etched polycarbonate membrane. The membrane was placed in contact with a copper plate and

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9547 TABLE 1: Melting Points of Several Lead-Free Solder Candidates (bulk materials) melting points (°C)

Sn

In

Sn/3.5Ag

Sn/Pb

232

157

221

183

restrained by a glass joint with Viton O-ring seal. The membrane was soaked in water for 5 min and then filled with the electrolytic solution of choice. Plating solutions for silver, gold, nickel, copper, tin, indium, and tin/silver were utilized. Typical plating conditions were 0.8 to 1.2 mA/cm2 for Au, Ag, Cu, and Ni; a higher current was used for tin, indium, and tin/silver to ensure good plating quality. The length of nanowires was controlled by the duration of the applied current. The Ag on the other side of the AAO membrane was dissolved in 6 M HNO3 after electrodeposition. After being rinsed several times in water, the membrane was dissolved in NaOH (for AAO membrane) or dichloromethane (for polycarbonate membrane) to release the nanowires. Benzotriazole was used in NaOH solution during template dissolution to protect tin, tin/silver, and indium solder segments from corrosion. The nanowires were rinsed repeatedly with water and ethanol (using sonication and centrifugation cycles) and then stored in ethanol for further utilization. The AAO template was used for nanowires with a nominal diameter of 200 nm while polycarbonate membranes were used for smaller diameter nanowires such as 30 nm and 50 nm. 2.3. Structural and Compositional Characterization of Nanosolders on Nanowires. Optical images of nanowires fabricated were taken using an Olympus CX-41 microscope equipped with a DP-71 CCD camera. A JEOL JSM-7401F fieldemission scanning electron microscope (FE-SEM) equipped with an EDAX Genesis V4.61 X-ray detector and Philips EM400 transmission electron microscope (TEM) were used to characterize the size, structure, and composition of the nanosolders and nanowires. TA Instruments Q100 differential scanning calorimetry (DSC) was used to measure the melting point of nanosolder nanowires. 2.4. Thermal Property Characterization. Nanosolder Reflow. 2.4.1. Nanosolder Reflow on a Si Wafter. Most of the nanosolder reflow experiments were carried out in a programmable high-temperature tube furnace with controlled atmosphere. The nanosolders or solder-containing nanowires in ethanol were drop-cast on a piece of 0.5 cm × 0.5 cm silicon substrate and introduced into the furnace for nanosolder reflow after solvent evaporation. The actual temperature inside the tube was monitored with a thermometer. The dwell time (the period above the solder melting point) was minimized to reduce interdiffusion of metals, normally within 5 min. Two atmospheric environments were used: air and nitrogen. In this project, the reflow process for nanosolders followed a reflow profile similar to that of the conventional eutectic solder reflow processes that are adopted by the microelectronics industry. Table 1 lists several lead-free nanosolders that have been fabricated onto nanowires and the corresponding melting points of the bulk solder materials. The melting point of the legacy tin/lead solders was also provided as a reference to lead-free nanosolders that are fabricated in this work. 2.4.2. Liquid Phase-Based Nanosolder Reflow. Solder reflow in a liquid medium is an effective method to melt solder and provides an excellent platform for self-assembly into large-scale 3D structures for many potential applications. In this work we have developed a liquid phase-based solder reflow technique. The principle of liquid reflow is similar to solder reflow on a silicon wafer which is heating up the solder material over the

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Figure 1. (A) Nanosolders on different locations of nanowires. (B) Optimized nanosolder structure on nanowires. (C) Solder joining and interconnect formation between nanowires.

melting point and soldering the discrete components together. Tri(ethylene glycol) (TEG) is the solvent that we have used in liquid reflow because of its high boiling point (285 °C). A small glass vial with 0.5 mL of solvent was preheated to 120 °C on a hot-plate for 5 min, and 10 drops of nanosolder nanowire suspensions were put into the glass vial. The temperature was then increased to ∼250 °C for 10 min. After the liquid reflow process was finished, the sample was cooled to room temperature and ethanol was used to clean the sample several times. 3. Results and Discussion Many nanosolder works so far focused on nanosolder particle synthesis and characterization,49-51 which would potentially be used in the ball grid array (BGA) related applications. Nanowires have been investigated intensively in many applications including nanoelectronics, optical devices, sensors, information storage, and drug delivery.52-56 Direct fabrication of nanoscale solders onto nanowires has several advantages: precise placement of solders at any location of nanowires, less solder volume being used, excellent size control, wide selection of base and barrier layers, and easy to combine with self-assembly or directed assembly techniques. In Figure 1A, three different scenarios including nanowire-solder, nanowire-soldernanowire, and solder-nanowire-solder were demonstrated, which shows the versatility and potential in using nanosolders for nanowire assembly and packaging. In Figure 1B, it is illustrated that an “ideal” nanowire system includes the base, barrier, wetting, and solder segments, which is incited by the soldering process in microelectronics assembly and packing industry, for example, existing Ni/Au or other effective surface finishing. The barrier segment is to avoid the intermetallic formation and/or base layer dissolution between the functional nanowire segment and the solder-joining segment. The wetting segment is used to protect the metal surface and to ensure good solder reflow properties (solderability). Figure 1C demonstrates that nanocomponents such as nanowires could be joined together after solder reflow. 3.1. Single-Segment Lead-Free Nanosolder Nanowires. As a first step to examine the feasibility of various lead-free nanosolders on nanowires, single segment solder nanowires were fabricated using the electrodeposition method in nanoporous templates. Figure 2 shows the SEM images of lead-free nanosolder nanowires, including tin, tin/silver, and indium nanosolder nanowires in the diameter range of 30-50 nm, and DSC and EDS characterization. The end of the nanowires are normally close to the nominal 30 nm or 50 nm diameter size; however, the diameter of the middle section is larger than the nominal size (about 2-2.5 times of the nominal size). This is

Figure 2. (A-C, E) SEM images of single segment nanosolder nanowires: (A) 30 nm Sn nanowires; (B) 50 nm In nanowires; (C) 50 nm Sn nanowires; (E) 50 nm Sn/Ag nanowires. (D) DSC result of 50 nm diameter Sn nanowires. (F) EDS spectrum of Sn/Ag nanowires dispersed on silicon wafer. Scale bar: 500 nm.

because of the nonuniformity of the templates that were used. The EDS element analysis of the Sn/Ag alloy nanowires in Figure 2F demonstrates the coexistence of Sn and Ag elements. The DSC measurement of tin nanowires with 50 nm diameter is shown in Figure 2D. In the spectrum, there was an endothermic peak at 231.2 °C which corresponded to the melting point of the tin solder nanowires. This suggests that the melting point of pure tin nanowires in the diameter around 50 nm (and above) is almost the same as that of the bulk tin material. Different kinds of solder nanoparticles based on tin such as Sn/Ag, Sn/Ag/Cu, and Sn/Ag/Zn alloys have been synthesized and characterized.49-51 The melting point depression of nanoparticles was calculated theoretically and also experimentally observed as a function of nanoparticle size. It was found that the melting point depression increases rapidly for nanoparticle size below 20 nm.50,57,58 However, for the lead-free nanosolders that have been fabricated in this work, the smallest diameter is about 30 nm, it is not surprising that the melting temperature of tin nanosolder nanowires is almost the same as that of bulk tin material. Also, considering that the length of nanosolder nanowires are much larger than that of the diameter (one to a few micrometers), this may also contribute to the melting point. On the basis of the above information, the melting points of bulk tin, indium, and tin/silver alloys are used in the thermal reflow study of lead-free nanosolders on nanowires fabricated in this work. 3.2. Lead-Free Nanosolders on Multisegmented Metal Nanowires. In order to realize nanoscale joining or interconnection between nanocomponents, multisegmented nanowires with soldering segments are promising starting structures because the solder segments on the nanowires could be controlled precisely and joined efficiently without other timeconsuming and expensive tools such as e-beam, FIB, etc. Several types of nanowires have been fabricated using sequential

Nanoscale Lead-Free Solders

Figure 3. Solder (Sn)-Au-solder (Sn) nanowires: (A) optical image; (B) TEM image (inset is an SEM image; Scale bar: 500 nm).

Figure 4. SEM images of (A) Au-Sn/Ag nanowires; (B) Au-Cu-Sn nanowires; (C) Au-Ni-In nanowires; (D) Sn-Ni-Au-Ni-Sn nanowires. Scale bar: (A and B) 500 nm; (C and D) 1 µm.

Figure 5. Au-Ni-Au-In nanowires with EDS point analysis at different segments.

electrodeposition technique, including Au-Sn nanowires, Au-Ni-Sn nanowires, Au-Ni-Au-Sn nanowires, and SnAu-Sn nanowires. Because of the very high aspect ratio (length/ diameter) of nanowires, characterizations based on both optical microscopy and electron microscopy are effective tools to characterize their morphology. Figure 3 shows tin-gold-tin three-segment nanowires using optical and electron microscopy. Sn/3.5Ag alloy-solders and low melting point solder-indium (In) were also incorporated onto nanowires using a similar method. Figure 4 shows SEM images of Au-Sn/Ag, Au-Cu-Sn, Au-Ni-In, and Sn-Ni-Au-Ni-Sn-nanowires, respectively. These nanowires include various base layers, barrier layers, and wetting layers, which will be ideal candidates for thermal reflow studies. Figure 5 shows the SEM image of a single four-segment nanowire (Au-Ni-Au-In nanowire) and related EDS element analysis (point analysis for each segment). Since the Au, Ni, and In segments are relatively long, EDS point analysis is sufficient to distinguish the element within each segment, and the EDS spectra at positions 1, 3, and 4 clearly show that Au, Ni, and In elements are only present within respective segments (Si peaks are from the Si substrate). For segment 2, both Au and In peaks were detected in the spectrum, which is probably due to the short length of the middle Au segment. However,

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9549 the possibility of intermetallic formation between Au and In cannot be excluded and will need further investigation using more advanced tools such as high-resolution transmission electron microscopy (HRTEM) together with selected area electron diffraction (SAED). The thermal property of these nanosolders in single-segmented and multisegmented structure is discussed in the following sections. 3.3. Thermal Properties. Nanosolder Reflow on Nanowires. Solder reflow is a process used in electronics industry to form joints by reflowing solders. There are normally four stages (“zones”) in a typical solder reflow process: preheat, thermal soak, reflow and cooling. Solder manufacturers normally supply the reflow profiles to industrial customers. In the present study, we have initiated the study of thermal reflow properties of lead-free nanosolders on nanowires, which will provide insight into new applications involving nanowires as nanoelectronic components or functional devices. This will also be useful for hybrid systems that need a lower solder reflow temperature (indium has a lower melting temperature than tin; see Table 1). Since the nanosolders that have been fabricated in this study are in the range of 30-200 nm in diameter (length significantly larger), the melting points of these nanosolders are assumed to be close to these of the bulk materials, as evidenced by the DSC result shown previously in Figure 2. In this research, the standard solder reflow profiles were adopted with consideration of the melting points of bulk solder materials (for both tin- and indium-based lead-free nanosolders). The melting points of pure Sn, Sn/3.5Ag, and In bulk material are 232 °C, 221 °C, and 157 °C, respectively, which lead to a different peak temperature set point during the reflow process. A reflow process with precise temperature control and peak temperature dwell time is needed to achieve optimum reflow results. In this study, the rate of temperature increase and cooling in the tube furnace are controlled at a speed of 0.5 °C/s and 0.25 °C/s, respectively. The maximum reflow temperature is about 30-50 °C higher than the nominal melting temperature of respective solder materials, which is consistent with the peak temperature range that is used in microelectronics industry. 3.3.1. One-Segment Nanosolder Nanowire Reflow and Effect of Nitrogen. In solder reflow process, prevention and/or elimination of solder surface oxide is critical for optimal solder reflow. There are several parameters that may affect solder reflow process, e.g., flux, environment, etc. In the nanosolder reflow experiments in this study, two atmospheres were used: air and nitrogen. It has been found that using nitrogen can appreciably improve solder reflow, even though partial (incomplete) reflow can still be achieved in the air environment. The tube furnace was first purged with nitrogen for 5 min and then was kept at certain flow rate until the finish of the whole process (cooling completely). In Figure 6, we show that reflow results are achieved for pure Sn and Sn/Ag alloy single-segment nanosolder nanowires. Comparing the nanowires before and after the reflow process, the nanosolder reflow in air is not as dramatic as that in the presence of nitrogen. In the presence of N2, original nanosolder wire structures changed into almost spherical structures, indicating good reflow, even though some tails still existed and were kept close to the reflowed nanosolders. This result is consistent with the EDS measurement in Figure 2, which shows a small peak of oxygen in the spectrum, indicating there is slight surface oxidation on nanosolder nanowires. However, when the reflow was conducted in the air, significant surface oxidation may have occurred at the relatively high reflow temperature, which could prevent further change of the solder morphology. Sn nanowires (30 nm diameter) were observed to

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Figure 6. Reflow of Sn and Sn/Ag nanowires in (A) air and (B) nitrogen. Scale bar: 1 µm.

have more tails (incomplete reflow), probably because of the fact that the ratio of the thickness of surface oxidation layer to the nanowire diameter for 30 nm diameter solder nanowires is larger than that of the 50-nm diameter solder nanowires. Another possible reason for incomplete nanosolder reflow may be due to the interaction of nanowires with substrate (in this case silicon), which may prevent complete solder reflow in a limited dwell time above the solder melting point. Flux is widely used in the solder reflow process in the electronics industry to protect the solder surface and improve solderability. Some effort has been made to investigate its effect in the nanosolder reflow experiments; however, it is still challenging to fully understand its effect due to the much smaller size of nanosolders and nanowires (compared with solder powders in the size of the tens of micrometers in the solder paste used in electronics manufacturing). The cleaning of flux after nanosolder reflow for imaging and characterization is also a major concern. More experiments are being conducted to investigate the flux effect on nanosolder reflow. In addition, Sn-based ternary alloy system seems to be necessary in order to understand thoroughly the thermal behavior of widely used tin-silver-copper Sn/Ag/Cu (SAC) solder system in microelectronics industry and also very low melting temperature solder alloys such as tin/indium solders. 3.3.2. Effect of Base Layer on Nanosolder Reflow on Nanowires. Gold nanorods/nanowires have received great attention in many applications such as sensors, optics, catalysts, and biomedical engineering. For multisegmented nanowires with nanosolders, we used Au as a base layer to demonstrate nanosolder reflow property. When the gold layer was connected to the lead-free nanosolder segment, the reflow result is significantly different from that of single-segmented solder nanowires as shown in Figure 7. The nanowires became waved structures and the diameters become much less uniform compared to the nanowires before solder reflow. At some locations, some nanowires were even broken. Gu et al. had explained the phenomena for the tin/lead and pure tin solder systems in a previous work that during the solder reflow process, the intermetallic diffusion between gold and tin was very dramatic as gold diffused rapidly into tin.59 On the other hand, tin was also observed to wet/diffuse on the surface of gold nanowires. For lead-free Sn/Ag alloy, similar phenomena exist (Figure 7B). In addition to gold, copper was also fabricated onto nanowires as a base layer for solder reflow (gold-copper-tin nanowires). There was also significant wetting/diffusion between copper segment and solder segment (Figure 8A). This problem could be minimized by adding a spacer (barrier) segment

Figure 7. Reflow of nanosolders on gold nanowires. (A) Au-Sn; (B) Au-Sn/Ag; (C) Sn-Au-Sn. Scale bar: 500 nm.

Figure 8. Reflow of nanosolders on multisegmented nanowires: (A) Au-Cu-Sn; (B) Au-Ni-Sn; (C) Sn-Ni-Au-Ni-Sn; (D) Au-Ni-AuSn. The images on the left side are original nanowires with nanosolders; the images on the right side are reflowed results.

between the gold segment and the tin solder.59 Hence, the study of barrier layer effect of lead-free nanosolders on nanowires is very important. 3.3.3. Effect of Barrier Layer and Wetting Layer on Nanosolder Reflow on Nanowires. Inspired by the successful industrial nickel/gold surface finish (and other effective surface finishing), a nickel barrier layer was inserted into gold-solder nanowires (Figure 8B,C, left-side images). The barrier layer nickel did prevent the diffusion during reflow as shown in Figure 8B and 8C (right-side images). However, direct use of nickel as a barrier layer may not be good enough for solder wetting, which is also crucial for soldering process. A solution is to add one thin wetting layer such as gold between solder and barrier layers. As demonstrated in Figure 8D, a gold wetting layer assisted in forming spherical solder balls on nanowires. 3.4. Liquid Reflow for Nanosolder Joint Formation. Fluidic assembly based on solder materials has shown promise in enabling micrometer-scale assembly into 3D functional electronics or devices,17,60,61 and has the potential to scale down to the nanometer scale.48,59,62 In this work, we show that many

Nanoscale Lead-Free Solders

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9551 Massachusetts Toxics Use Reduction Institute (TURI) and the NSF Center for High-rate Nanomanufacturing (CHN). We thank Pamela Civie and Gregory Morose from TURI and Robert Farrell from Benchmark Electronics for valuable suggestions and input. References and Notes

Figure 9. (A) Schematic of nanosolder nanowire reflow in a liquid medium. (B) Sn-Au-Ni-Au-Sn nanowires before solder reflow. (C) Sn-Au-Ni-Au-Sn nanowires after solder reflow in a liquid medium. Solder joints formed between multisegmented nanowires after solder reflow. Inset is a solder joint between two nanowires.

nanosolder joints have been formed successfully between multisegmented nanowires in a liquid reflow process. Figure 9A shows the schematic of nanowires with nanosolders, and agitation of fluidic force helped nanowire assembly and solder joint formation. Figure 9B shows the SEM images of SnAu-Ni-Au-Sn five segment nanowires before liquid reflow, and Figure 9C shows the result of reflowed nanosolder nanowires. The Sn solder segments melted and bonded nanowires together to form spherical solder joints, which indicates the technical feasibility of interconnect formation between nanowires using lead-free nanosolders. Solder reflow in a liquid phase may provide more potential for nanowire assembly and further work is currently underway in order to obtain 3D large-scale integrated structures. 4. Conclusion Nanoscale lead-free solders have been synthesized onto multisegmented metal nanowires using an electrodeposition method in nanoporous templates. The diameter of nanosolder nanowires ranges from 30 nm to 200 nm and length from 1 to 10 µm. The microstructures of nanosolder nanowires are studied using optical microscopy and electron microscopy (FESEM and TEM) along with energy-dispersive X-ray spectroscopy (EDS). Thermal reflow property of nanosolders was characterized using a temperature-programmable furnace tube. After thermal treatment, soldering through nanosolders on nanowires is a powerful tool to modify desired nanostructures for technological applications and to form joints and interconnections between nanowires. It was found that nitrogen plays an important role in the nanosolder reflow process. Base layer, diffusion barrier layer, and wetting layer effect on solder reflow were studied. Nickel/ gold surface finishing was found to be effective for the nanosolder reflow on nanowires. Solder joints were formed when the nanosolder nanowires were reflowed in a liquid medium, which shows the potential in using fluidic based self-assembly methods to integrate nanowires into 2D or 3D functional structures or electronic devices. Acknowledgment. We are grateful for the financial support from the Sustainability Research Fellows Program of the

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