Interconnect formation is critical for the assembly and integration of nanocomponents to enable nanoelectronics- and nanosystems-related applications. Recent progress on joining and interconnect formation of key nanomaterials, especially nanowires and carbon nanotubes, into functional circuits and/or prototype devices is reviewed. The nanosoldering technique through nanoscale lead-free solders is discussed in more detail in this Review. Various strategies of fabricating lead-free nanosolders and the utilization of the nanosoldering technique to form functional solder joints are reviewed, and related challenges facing the nanosoldering technique are discussed. A perspective is given for using lead-free nanosolders and the nanosoldering technique for the construction of complex and/or hybrid nanoelectronics and nanosystems.
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.
We report an efficient silver etchant in the fabrication of active nanowire structures by the electroplating technique using an anodized aluminum oxide template. These active nanowires include solder materials ͑low melting point metals or metal alloys͒ and magnetic materials. Single-or multisegmented nanowires with these active materials have been prepared using the new etching solution. A conducting polymer such as polypyrrole can also be fabricated onto these nanowires to enable hybrid structures and interfaces. These multisegmented active nanowires open a new platform in the fabrication and integration of nanowire-based electronics and hybrid devices.Anodized aluminum oxide ͑AAO͒ membranes have been widely used in many emerging applications, e.g., synthesis of nanowires and nanotubes, biosensing, drug delivery, and tissue engineering. 1-4 Nanowire/nanotube fabrication is probably the most studied and exploited application to date among these applications. Several methods, including electroplating, sol-gel synthesis, and pressure-driven filling, have been utilized to fabricate nanowires. 5-13 Among these methods, electroplating is the most versatile in the fabrication of metal, metal oxide, semiconducting and polymeric nanowires.An important and necessary step in the electroplating of nanowires/nanotubes using AAO is to sputter or thermally evaporate a thin layer of metal onto one side of the template. This metal layer serves as the working electrode through a connection to a power supply. Besides, this layer serves as a sealing layer, preventing the electrolytic solution from leaking, and also as a seed layer for growing nanowires inside the AAO channels. A counter electrode on the other side of the electrolyte completes a circuit for electroplating to occur. The nanowire diameter is restricted by the pore size of the template and the length is controlled by the plating duration. After the nanowire fabrication, AAO can be dissolved by a sodium hydroxide solution or an acidic solution ͑e.g., a mixture of phosphoric and chromic acids͒. In certain applications, e.g., fabrication of electrode materials, solar cell integration, and enhanced heat transfer surfaces, nanowires are preferably kept vertical and, thus, the sacrificial metal layer can be kept as it is to support the nanowire arrays. 14,15 However, in most other applications, the sacrificial layer is etched by an appropriate etchant. The nanowires are then released from the AAO and dispersed in a solvent for further processing or assembly for nanoelectronic device applications. [16][17][18][19][20][21][22][23] Among the sacrificial layers used, including gold ͑Au͒, platinum ͑Pt͒, silver ͑Ag͒, and copper ͑Cu͒, silver is probably the most widely used due to its relative inexpensiveness ͑compared to Au and Pt͒ and inertness to environments ͑compared to Cu͒. Traditionally, the etchant solution for silver is concentrated acid ͑e.g., HNO 3 solution͒; however, many active materials that need to be fabricated using AAO may be dissolved or damaged by nitric acid an...
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