纳米电子封装20

Chapter 20 Nanowires in Electronics Packaging Stefan Fielder(*ü ), Michael Zwanzig , Ralf Schmidt, and Wolfgang Scheel 20.1 Introduction In the light of continuous miniaturization of traditional microelectronic components, the demand for decreasing wire diameters becomes immediately evident. The observa- tion of metallic conductor properties for certain configurations of carbon nanotubes (CNT) and their current carrying capability [1] set the minimal diameter of a true wire to about 3 nm (compare Chap. 15). Investigations are in progress even below that diameter on nanocontacts, formed by single metal atoms, i.e., quantum wires. Quantum wires can be produced by mechanical wire breaking [2] , its combination with etching and deposition [3] , or other techniques. The properties of quantum wires are only about to be understood theoretically [4] . Doubtless, they are worth considering for packaging solutions in molecular electronics to come [5] . In this chapter, we focus on metal wires and rods in the size range above 10 nm up to submicron diameters, evalu- ated already to be attractive for microelectronic packaging purposes. Techniques to generate, to characterize, and to handle them, as well as their interaction with electro- magnetic fields will be useful for packaging applications in the age of nanotechnology. With the wealth of information available, this review focuses on general trends and starting points for deeper study. Although the cited references are representative, they cannot be complete, since numerous activities are ongoing to produce and to character- ize new kinds of wire-like geometries from different materials. Packaging-specific applications of nanowires (NWs) lie mainly in the fields of interconnect formation, sensor development, and photonics. Given the common understanding of a wire, one would expect NWs to be usually cylindrical conductive strands with diameters below 100 nm, ideally of infinite length, but at least elon- gated. Whereas common wires are drawn from metal rods, NWs cannot be produced by wiredrawing and do not necessarily consist of a metal or one single material. But approaches to draw electrically conductive polymer NWs in electronic circuitry by initiating chain polymerization with a STM cantilever do exist [6] . S. Fielder Department of Module Integration and Board Interconnection Technologies, Fraunhofer Institute for Reliability and Microintegration, Berlin, Germany J.E. Morris (ed.) Nanopackaging: Nanotechnologies and Electronics Packaging, 441 DOI: 10.1007/978-0-387-47326-0_20, © Springer Science + Business Media, LLC 2008 Morris_Ch20.indd 441Morris_Ch20.indd 441 9/29/2008 8:09:33 PM9/29/2008 8:09:33 PM 442 S. Fielder et al. Although rod-like colloidal structures are often mentioned as NWs in the litera- ture, they should rather be regarded as rods or crystal needles. If they consist of metals, we include at least typical publications. Nevertheless, we exclusively focus on metallic wires and wire-like structures, even if they are fixed to a solid substrate as pillars or come as brushes or lawn-like structures. Supramolecular wire-like geometries are also depicted as molecular NWs , such as in the case of tropomyosin fibers, whose length and diameter can be directed by Na + or Mg 2+ concentration [7] . Deoxyribonucleic acid (DNA) can form molecular NWs [8] , which in turn can be used as templates to produce true metal NWs (see later). Especially alien to tradi- tional electronic engineering are charge-transfer complexes of wire-like geometries. Such supramolecular NWs, e.g., porphyrin NWs generated by ionic self-assembly, perhaps can be used in microelectronic devices – thanks to their photocatalytic activity [9] and hence switchability. Those NWs fall beyond the scope of the present work. If molecular wires are largely short structures, seldom extending the micrometer scale, CNTs can reach even millimeter length scales and are therefore just as interesting for microelectronic packaging. They have been proposed, e.g., as transistor elements in logic circuits, field emitting structures, or vias [10 – 15] . NWs and nanotubes can be produced from semiconductor materials such as silicon, gal- lium nitride, or others. Because of the familiarity of microelectronics with those materials they could become even more important for sophisticated future microe- lectronic applications [16 , 17] . Their synthesis and integration into classical planar technology, e.g., by the superlattice NW pattern transfer (SNAP) [18] and resulting application perspectives have been reviewed recently [19 – 22] . Excluded from this compilation are all sorts of oxide and multicomponent oxide NWs, e.g., ZnO. We consider them to be more important for sensors due to well-measurable conductivity changes with analyte adsorption [23] . Our own results in production, characterization, and application of gold submi- cron wires in the shape of nanolawn have been included to share the excitement of NW packaging research, connecting usually separated fields like low-temperature joining and interfacing electronics with biological cells. 20.2 Nanowires and Packaging Research Reliability issues arising from contemporary microelectronic applications have wid- ened the scope of packaging over the last decades remarkably. Modern packaging research for the development of sustainable technologies covers photonics, optical waveguide and fiber integration, (bio)microfluidics, joining, thermal management, wire-, wafer-, and flip-chip bonding, soldering and encapsulation, foil batteries and energy harvesting, and includes also solid mathematical modeling and simulation. This is shown in too many publications and annual reports to be reviewed here. Metal NWs can be attractive for packaging in nearly every field mentioned [24] due to unique properties in comparison with mesoscaled and bulk materials. Their functional role as interfaces has been envisioned for future microelectronic applica- tions toward 3D nanostructure integration [25] . Characteristics, production methods, Morris_Ch20.indd 442Morris_Ch20.indd 442 9/29/2008 8:09:34 PM9/29/2008 8:09:34 PM 20 Nanowires in Electronics Packaging 443 and proposed applications of metallic NWs have been reviewed in depth previously [26 – 35] . Recent international research activities, evaluated and ranked by citation, indicate US leadership until 2005 [36] . Main international research institutes, engaged in microelectronic packaging in alphabetical order of the country, are as follows: • Inter-University Micro Electronics Centre (IMEC), Belgium • VTT Technical Research Centre of Finland/VTT Electronics, Finland, and Oulu University Electronics Materials, Packaging and Reliability Techniques, Finland • Laboratoire d’Electronique de Technologie de l’Information (LETI), France • Fraunhofer Institute Reliability and Microintegration (IZM), Germany • Central Electronics Engineering Research Institute (CEERI), India • Tyndall National Institute, Ireland • Korea Advanced Institute of Science and Technology/Center for Electronic Packaging Materials (CEPM) and Samsung Advanced Institute of Technology (SAIT), Korea • Philips Research Laboratories, Eindhoven, The Netherlands • Institute of Microelectronics (IME), Singapore • Industrial Technology Research Institute (ITRI), Taiwan • Packaging Research Centre at Georgia Institute of Technology, USA 20.3 Nanowires: Fabrication In the plethora of production principles and approaches, nevertheless typical ones can be distinguished and will be presented later. The reproducible generation of metal NWs with identical diameters can be dated back until 1970, when Possin described metal deposition inside etched tracks of high-energy charged particles in mica and proposed to use this method to form NWs in track-etched polymers as well [37] . The technological importance of such tracks had been foreseen even earlier [38] . Many more applications for swift ions in nanoscale microelectronics have been designed independently [39] . Unilaterally etched pores in flex substrates have been proposed for improved copper adhesion [40] . For wire diameters above some tens of nanometers, the use of exotemplates is still the most important production technique so far [41] . Depending on the application, such templates can serve as the scaffold remaining after metal filling by the formation of composites, e.g., dipole storage devices [42] . But the exotemplate can also be dissolved yielding a lost form approach to produce suspended single wires or more complex metal nanostructures. Exotemplates are also suited to produce wires consisting of conductive polymers [43 , 44] . The most important (hard) exotemplates are anodic aluminum oxide (AAO) and track-etched polymer membranes (TEM). AAO [45] offers electrochemically tunable nanopores in a rigid matrix and there- fore finds wide application for single wire and wire array production [46 – 49] even at a very large scale [50] . Dispersions of high aspect ratio wires can be produced [51] , or layers of anisotropically conductive or magnetically polarizable materials in dielectric matrices can be prepared. Beside the standard aqueous metallization Morris_Ch20.indd 443Morris_Ch20.indd 443 9/29/2008 8:09:34 PM9/29/2008 8:09:34 PM 444 S. Fielder et al. baths used, AAO is especially suited for plating from aprotic media, due to its high stability in organic solvents [52] , opening a way toward electrochemical deposition of NWs consisting of metals with low redox potential (below hydrogen), like Al or Ti [53] . Because of its high stability, AAO has been also used for NW production by high-pressure filling with molten metals; for a compilation, see [35] . Etched ion track polymer membranes (TEM) are other practically important exotemplates. Polyethylene terephthalate, polycarbonate [54 , 55] , or even polyimide [56] are typically used for their reproducible etchability [57] . Isodiametric and nearly monodisperse shape distributions can be generated following standard etching protocols [58] . Pore diameters in those materials reach about 0.002–1 µm for AAO and 0.010–20 µm for TEM. The density of the stochastically distributed pores in TEM can be chosen from a single pore [59 , 60] up to ~109 cm −2 depending on the desired pore diameter [61] . The percolation-based electrochemical pore etching in aluminum allows pore densities of AAO templates up to ~1,011 cm −2 (e.g., com- mercially available ANOPORE™ and ANODISK™ inorganic aluminum oxide membrane filters). Whereas the distribution of pores in TEM follows statistics ful- filling Poissonian distribution criteria [62 , 63] , pores in AAO are always densely arranged. They can even be hexagonally packed over small domains, and if com- bined with imprinting [64] even over the whole area of a wafer [65] . The typical distances between single pores (i.e., insulating material around neighboring wires) reach the same dimension as the pore diameter for wires generated in AAO. The typical distances of pores in TEM without special precautions (e.g., mask or shutter) will always vary due to the inherent statistics of high-energy particles used. Therefore the distances between metal wires in ensembles generated with TEM, like the nanolawn introduced by us [66] , are varying too. With commercially available TEM (Nucleopore™, SPI-pore™, Cyclopore™ – to name a few brands only) pore-to-pore distances vary between 0 and 2 µm at a pore density of 106 cm −2 on track-etched foils. Supramolecular assemblies can work as exotemplates as well: Self-assembling calix [4] hydroquinones form in aqueous photochemical solutions chessboard-like arrays of very narrow rectangular pores. Such pores have been used as silver-ion reduc- ing templates in a process resembling photochemical development. Stable NW arrays consisting of 0.4-nm wires grown up to micrometer length have been prepared [67] . Molecular endotemplates [68] , characterized by inner (bio)molecular scaffolds (especially proteins, lipids, and DNA) in combination with a toning approach [69 – 76] , or bioparticles, e.g., tobacco mosaic virus, are suitable for metal wire production [77] and should be mentioned as alternatives. Such a nanobiotechnological approach in combination with microelectronic technologies offers another additional advantage: the localized maneuverability of inorganic (conducting) structures, as shown for gold wires, driven by highly specific biochemical molecular machines (e.g., actin–myosin interaction) and a molecular fuel [78] . The metallization of (bio)polymer endotemplates offers additional advantages for complex 3D arrangements of wires and wire networks at the microscale, because of inherent self-assembly principles and specifically directed labeling (addressing). Complex nanotubular networks as generated naturally by living cells on artificial substrates [79 , 80] or Morris_Ch20.indd 444Morris_Ch20.indd 444 9/29/2008 8:09:34 PM9/29/2008 8:09:34 PM 20 Nanowires in Electronics Packaging 445 produced artificially by manipulation of liposomes [81 – 83] can be transformed into hard-wired circuitry by gentle metallization. This approach has been demonstrated for DNA [69] and lipid tubules [84 , 85] . NWs can be grown also epitaxially at high temperatures by diffusion in grain boundaries [86] . The similar whisker growth is a notorious failure mechanism causing microelectronic reliability issues, worth mentioning in this context. The colloid-chemical approach , effectively producing homogeneous – regarding their composition – one-dimensional nanomaterials from salt solutions is usually diminished to the lower nanoscale and low aspect ratios. However, even several micrometer-long gold wires measuring only 15 nm in diameter have been produced by chemical reduction [87] . Seed-mediated growth has been described for gold rods when the seeding nanoparticles have been attached to a solid substrate [88] . The introduction of rod-like micellar templates, e.g., the cationic detergent CTAB, allows the production of suspended cylindrical gold NWs [89] . Comparably long silver wires have been produced in a diameter controlled (20–500 nm) manner by a modified polyol process [90] using different growth-controlling modifi- ers. A somewhat similar molecular shielding (templating) strategy, i.e., soft exo- templates, comprises the use of temporarily arranged supramolecular ensembles in block-copolymer solutions [91] . For a review of nanostructure-producing tech- niques with block copolymers, see [92] . Classical photolithography and successors like deep UV lithography [93 , 94] , colloid mask/nanosphere lithography [95] , and competing technologies, with nanoimprint (cold) lithography being the most ripened one among them [96] , have been used to generate metal NWs directly on planar substrates. Typically, the NWs and NW grid arrays produced are oriented parallel to the substrate plane. Hence, they can be useful for applications as subwavelength metal gratings or directly as plasmonic waveguides and photonic crystals [97 , 98] depending on a guiding medium in close proximity. In a top–down approach, photolithographically generated trenches in a resist layer can be filled forming stretched wires on a planar substrate [99] . A maskless alternative based on substrate steps to fabricate wires by deposition has been introduced as step-edge lithography (SEL) [100] , the principles of which have been used to produce molybdenum NWs (15 nm–1 µm diameter and length up to 500 µm). NW composites have been formed starting with electrodeposited molyb- denum oxide wires and their reformation in hydrogen and subsequent liftoff in poly- styrene layers [101] . Palladium wires embedded in a cyanoacrylate film sensitive toward hydrogen have been prepared as well [102] . A similar growth of metal wires occurs along material cracks [103] . Among maskless NW production techniques the direct writing approach, either by direct atomic metal deposition [104] , e-beam induced CVD [105] , or by indirect structuring of metal substrates with small molecules via dip-pen technique [106] should be mentioned. With a sweeping AFM- cantilever, copper NWs have been assembled from deposited nanoparticles on a polymer substrate and cut afterward at will [107] . Direct writing techniques have been under intense investigation. To overcome seriality drawbacks, e-beam arrays [108] and cantilever arrays [109] have been proposed. Near-field laser nanofabrication has been scaled down to 80-nm resolution [110] . Two- dimensional photonic crystal Morris_Ch20.indd 445Morris_Ch20.indd 445 9/29/2008 8:09:34 PM9/29/2008 8:09:34 PM 446 S. Fielder et al. structures in polymer have been produced by a combination of interference and e- beam lithography techniques [111] . Meanwhile, similar structures are becoming attractive for wire fabrication via nanoimprint techniques. Other techniques to generate metal wires include assembly from suspended metal particles by dielectrophoresis [112 , 113] . Plasma-enhanced growth tech- niques, e.g., by vapor–liquid–solid growth or vapor–solid transitions, cannot be covered here but could become more important for packaging, if applied at reduced temperatures. Single crystalline Ni NWs with diameters of 40 nm confined inside multiwall CNTs have been grown by a CVD process with lengths of some tens of micrometers [114] . 20.4 Metal Nanowires: Materials Nearly every electrochemically reducible cation has been deposited inside the pores of different exotemplates already as a wire. The galvanic deposition of metals inside nanopores has been thoroughly investi- gated from aqueous electrolytes [115] and from nonaqueous ionic liquids [116] extending the range of available wire materials. Therefore, beside the colloid-chemical approach suitable for mass fabrication of nanoscale metal rods and wires [117] , exotemplate methods can be used as well to prepare single wire contacts [118] or special polymer composites, requiring gram amounts of monodisperse wires [119] . Envisioned applications of periodic arrays of magnetizable wires [120] embed- ded in a dielectric matrix (e.g., AAO) are information storage via perpendicular data recording, characterized, e.g., for arrays of ferromagnetic Ni and Co NWs by high remanence and coercivities [121 , 122] . The preparation of similar e-beam written pillar arrays proposed for high-density data storage [123] is much more time consuming and hence expensive. Based on their giant magnetoresistance prop- erties, magnetic multistack layers have been considered as high-density storage elements [124] . Magnetic polymers are another application of polymer composites, e.g., with Ni wires [125] . The integration of oriented wires into polymer films can be used for anisotropically conductive interposer fabrication useful for chip inter- connection [126] . 20.5 Segmented Metal Nanowires Such stacks, representing multilayered wires, can be prepared with exotemplates. The common practice of sequential layer plating of different metals or crystal mor- phologies yielding functional multilayers works well for exotemplate wires, too. Segmented NWs may possess different spectral characteristics, depending on the orientation of polarized light in relation to the axis of the wires [127 , 128] . They therefore can serve as embedded identification tags (barcodes) or labels, not disturbing Morris_Ch20.