Nanomanufacturing of random branching material architectures

Microelectronic Engineering 86 (2009) 467–478

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Nanomanufacturing of random branching material architectures Charalabos C. Doumanidis * Marie Curie Chair, Hephaistos Nanotechnology Research Center, University of Cyprus, 75 Kallipoleos, Nicosia 1678, Cyprus

a r t i c l e

i n f o

Article history: Received 4 November 2008 Received in revised form 12 February 2009 Accepted 13 February 2009 Available online 25 February 2009 Keywords: Nanomanufacturing Fractals Random branching materials Ultrasonic corrosion texturing Ultrasonic powder consolidation Block copolymer self-assembly Plasma processing Fiber electrospining Anodized aluminum oxide Carbon nanotubes Carbon nanofoams Nanoheaters Nanocomposite foils Tissue scaffolds

a b s t r a c t Research in vital fields such as micro/opto-electronics, fuel cells and tissue engineering calls for fabrication of functional structures with optimal harvesting or perfusion of matter, energy and information, via permeation and transport through random branched conduit networks. This article reviews research aimed at establishing and investigating universal, scalable manufacturing techniques for materials and structures with stochastic tree architectures, and custom-designed, controlled probabilistic leafage, branch and trunk features. These are achieved by combination of complementary, multiscale fabrication processes for robust, affordable, productive manufacture, such as ultrasonic corrosion texturing, ultrasonic powder consolidation, block copolymer self-assembly, plasma processing of polymers, fiber electrospinning, anodized alumina templating and carbon nano-network deposition. Their process–structure–property relations are studied by microscopic, spectroscopic and other experimentation, coupled with computation via atomistic and continuum simulations. Such modelling will enable design and optimization, as well as real-time identification and process control. Manufacturing synthesis of such processes is illustrated in research related to nanoheaters, nanocomposite foils and tissue scaffolds. The platform illustrates and exemplifies some of the key salient features of nanomanufacturing for scalable production. (This paper was presented as an invited talk in the MNE 2008 Conference, www.mne08.org, www.mne-conf.org). Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction and objectives Modern fabrication technology at the micro- and nanoscale has introduced novel geometrical and material structuring capabilities, enabling unprecedented realization and integration of devices and systems for a constantly broadening spectrum of transformative applications. Among these, its long-standing implementation in the mature microelectronics industry must be chiefly credited for securing the resources, as well as intellectual drive, that permitted the field to grow to its current state of the art. On the other hand, this umbilical connection has also inherited the area with certain emerging limitations to its future development. Fabrication typically shapes well-defined, deterministic geometry patterns and scale-specific features. Material choices are often expensive and process-limited by CMOS compatibility, and include toxic chemicals. Processing is based on planar lithographic patterning and 3D layered structuring, with stringent interlayer alignment and registration requirements. As a result, the requisite research labgrade instrumentation is generally costly and necessitates cleanroom environment conditioning. Production yield is often defectlimited, while rates and throughputs are hampered by single-wafer * Tel.: +357 22892265; fax: +357 22892254. E-mail address: [email protected] 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.02.024

or batch processing. In deviation, certain present research targets functional material structures with stochastic forms and branched geometries across multiple (hierarchical) scales. These would allow broader, lower-cost material selections with environmentally benign options. Non-lithographic processing could eliminate alignment and registration steps in layered architectures, and thus equipment could be of ordinary industrial, affordable quality. Non-cleanroom, industry-floor environments would be sufficient for robust processes and products achieving functionality through parallelism and redundancy, while high throughputs would be enabled by continuous processing. Therefore, although product market domains are still nascent, there is a growing need for technologies complementary to traditional fabrication, addressing the manufacturing scalability issues. Looking into nature (as well as art) for creative paradigms of such random branching structures, one realizes that their beauty and harmony is attributable to mathematically intelligible forms and patterns, tantalizing human mind. The elegance of multidimensional, self-similar structures, such as trees, corals, rivers and snow flakes, has inspired the vibrant field of fractals [1,2], with revolutionizing impact in science and engineering, such as crystallography, biochemistry, physiology and imaging. Particularly intriguing are architectures with statistical self-similarity, i.e. developed by multiscale replication of a generator pattern with

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probabilistic features, such as Brownian trees and dendrites, described as random iterative function systems (IFS) [3]. These are ubiquitous in animate and inanimate nature, generated by a plethora of diffusive and reactive material aggregation, erosion and transformation processes, with admirable self-organization characteristics across multiple spatial and temporal scales. Reaching down to the nanoworld, self-assembly of atomic and molecular complexes yields a large variety of random branching trees, such as grain structures in metals and alloys, crystalline dendrites of oxide nanorods, block copolymer, star and fractal polymer networks, biomolecular constructs of proteins, DNA etc. [4]. What often evades attention is the functionality dictating such wonderful structural order. What is the purpose behind the striking geometric resemblance of lightings, thunderbolts, floral branching in trees and shrubbery, vasculatures in animals (circulatory, lymphatic, alveolar, neural etc.), but also networks of roadways, pipelines, and telecommunications? It so appears that in all cases mass, energy or information must be harvested from and/or dissipated to a spatial domain (e.g. a 3D volume), often by interfacial permeation (through a 2D surface), through linear transport (via an assembly of 1D conduits), in an optimally efficient way. To illustrate, Fig. 1a shows transport of generalized charge Q from two points A and B to a location C. If the individual resistances RAC and RBC for separate (dotted) radial flows are higher than those for a merging (solid) flow through resistances RAD, RBD and RDC, then there is a topological merge point D minimizing the potential differences V required. The resulting branched (Y) pattern, with the branch lengths and angles determined by such optimization, is the generator for an IFS pathway (Fig. 1b). The same optimal field transport functionality of natural branching structures is also important to numerous vital engineering technologies, such as: electromagnetic antennae and shielding; photovoltaic and thermophotovoltaic cells; fuel cell membranes; hydrogen storage and batteries; photoelectrolysis; electro-osmosis in desalination of water; substrates for catalysis and environmental remediation; biocompatible materials and coatings in biomedical implants; vascularized scaffolds for cell perfusion in tissue engineering, etc. [5–10]. However, such materials can only be microfabricated by ad hoc techniques, producing a limited class of shapes. For example, an electrostatic technique for material ablation mimicking a lightning discharge, has been known to produce Lichtenberg figures [11]. Such discharge structures (Fig. 2) are generated in insulators, first irradiated by a high-energy electron beam (e.g. a linear accelerator) and then discharged to a local ground, while the breakdown potential of the material is exceeded. Despite the spectacular process and patterns generated, high-voltage processing is challenging to scale up for industry in terms of cost and safety. Unlike stochastic fractals in nature, universal production of custom hierarchical, random branched architectures in engineering encroaches upon fundamental manufacturing difficulties. At the macro- to microscale, as implied, top-down fabrication is primarily oriented towards deterministic Euclidean designs rather than multiscale probabilistic patterns. Making of branched whole structures (i.e. without assembly) meets with challenges stemming from the non-differentiable nature of fractals [1,2]. For 3D geometries, inter-

B

A D

Fig. 2. Discharge structure.

ference (visibility) constraints arise between the shaping tool or beam and the solid part morphed to a recursive, often self-intersecting topology [12]. Microfabrication of multiply-connected 3D shapes by layered manufacturing (e.g. solid freeform fabrication, SFF), i.e. by stacked contoured 2D sections, may also be limited by removal of overhang-supporting material. At the micro- to nano-scale, on the other hand, the morphology of bottom-up, self-assembled structures is inexorably determined by the physicochemical connectivity of their building blocks. Thus, their fractal architecture is coupled with material selection, and allows for little flexibility in geometry control. In both cases, the multi-scale complexity of self-similar forms challenges the workspace and resolution range of any fabrication process, additive or subtractive, or involves the tedium and cost of scaled lithographic masks and other patterned tools. Clearly, new basic micro-/nanomanufacturing research is needed to address such limitations, and bring forth the powerful functionality of stochastic branched structures to energy, health and environmental applications. This article overviews related ongoing research by the team and coworkers of the author, towards a versatile manufacturing framework for multi-scale, random branching structures and functional materials, suitable for scale-up to massive production. Such research requires original, nature-inspired designs and processes, i.e. emulating natural fractal (leafage–branch–trunk) tree structures and random phenomena. Its objectives include: (1) Introducing a complementary set of mutually compatible, scalable manufacturing processes for continuous production of a broad variety of controlled random branched architectures in diverse materials, combining novel techniques with established fabrication methods. (2) Understanding the manufacturing process-material structure– functional properties relationships of random branching architectures, by studying the influence of salient process conditions on structural features and to the resulting transport behaviour, by experimental material analysis coupled with computational process simulation. (3) Demonstrating manufacturing synthesis into original integrated technological platforms, such as nanoheaters, nanocomposite foils and tissue scaffolds, for microelectronics, construction, biomedical and other applications. In this paper, seven such complementary micro/nanomanufacturing processes are described first, followed by their analysis via laboratory experimentation, computational simulation and process control, and by their synthesis into nanoheaters, nanocomposite foils and tissue scaffolds. Conclusions and further research are summarized at the end. 2. Micro/nanomanufacturing processes

C Fig. 1. (a) Y-shaped generator, (b) IFS tree.

As discussed above, a number of disparate engineering fields share similar operational and manufacturing needs for functional

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materials: Efficient volume field perfusion or harvesting, through interfacial permeation at large internal surfaces, and directed linear transport via percolated, interpenetrating networks. Together with their material science, research attempts to break new grounds in the geometric architecture of active materials, through versatile micro/nanomanufacturing of random branching structures. A comprehensive variety of such architectures requires the combination of specific manufacturing processes for their scalable production. These processes should be suitable for continuous, energy-efficient, non-cleanroom, economical manufacture of controlled branching material structures. The description below introduces certain such techniques to produce the leafage, branches and trunks of universal stochastic trees. 1. Ultrasonic corrosion texturing is a new fabrication process, originating from the familiar (and often catastrophic) corrosion fatigue in metals exposed to an electrochemically active environment under dynamic stresses. Through a combination of chemical solvation and mechanical fracture, cracks initiate and propagate transgranularly or mostly intergranularly, i.e. along grain boundaries, because of their increased free energy and electrochemical potential, due to lattice discontinuity, segregated impurities etc. (Fig. 3). In ultrasonic loading, cracks propagate during the tensile part of each cycle, while during the compression part crushing, attrition and dissolution of surface oxide films widen the conduits of the corrosive medium to the crack. The thermodynamics and kinetics of the simultaneous (reaction/diffusion-limited) formation and rupture of the oxide films have been established through empirical models of anodic dissolution and mechanical fracture for most metals [13]. For example titanium, in the presence of oxygen or water traces at lower temperatures, instantly forms a protective TiO2 film (typically < 10 nm thick), with a n-type semiconductive anatase open structure. This passivating film is attacked only in highly reducing acid environments, such as hot concentrated HF, HCl, H2SO4, fuming HNO3, and anhydrous N2O4 gas or methanolhalide solutions. Zinc, on the other hand, promptly corrodes and forms ZnO in soft water with high CO2 content, or bubbled oxygen at 60 °C, and in dilute acidic conditions. Moreover, tin forms stannic oxide (SnO2) readily in HNO3 and hot HBr or HI acids, especially when in electrical contact with a nobler cathode, e.g. Cu or Ni. In all cases, essential process parameters affecting cracking patterns include: temperature, pressure, corrosive species and concentration, stirring or mixing, applied current density (for concurrent anodization), stress amplitude and frequency, residual stresses, grain size and composition in the bulk and grain boundaries. Intergranular ultrasonic corrosion texturing is thus investigated for controlled erosion of random fractal forms, because of the ability to precisely regulate its progress (at rates 1 lm/s) through the number of applied cycles (Fig. 4) [14]. Crack propagation and anodic oxidation can be localized over a predefined branching network of grain boundaries, forming a dense fractal leafage structure down

Fig. 3. Intergranular cracking of Ti alloy.

stirrer

thermometer

ultrasonic vibrator

test foil acid bath

heater Fig. 4. Ultrasonic corrosion texturing setup.

to the nano-scale. Such nanocrystalline metal films are conveniently deposited by physical vapor deposition (PVD, e.g. by evaporation) especially when electroplating is difficult, e.g. for Ti. Alternatively, nanogranular metals can be produced through grain refinement by severe plastic deformation (SPD) processes [15], such as hydrostatic extrusion, high pressure torsion etc. Variable-density leafage in functionally graded material (FGM) architectures are produced through grain distributions of varying size in nanocrystalline films, transiently heat-treated in a proper temperature gradient, causing controlled, variable grain growth. Moreover, custom-designed leafage structures can be fabricated by localized and anisotropic corrosion, through directed perfusion by the corrosive medium via a conduit network embedded in the metal film. This porous network can be generated by SPD of microporous foams [16] rather than solid metals, where pores collapse and form surface pathways, or by metal coating over a sacrificial fiber mesh, pre-deposited e.g. by electrospinning as below. 2. Ultrasonic powder consolidation is another recent development in solid-state compaction of fractal granular networks (porous or dense), bonding polverized materials by high-frequency vibration, combined with compression and mild or no preheat [17]. The powder materials are confined in a stationary enclosure (anvil), and pressed vertically by an ultrasonic probe (sonotrode), vibrated at ultrasonic frequencies (20 to >100 kHz) via a coupling to piezoelectric transducers (Figs. 5 and 6). The direction of vibration is parallel to the consolidation plane (shear) for metals, and normal (pressure) in plastics, and the resulting interface scrubbing produces their local welding into a random branched grain network. During processing of metal powders, the highly localized interfacial slip causes disruption and dispersion of surface films (oxides), breakdown of asperities and interpenetration of adjacent boundaries, permitting direct metallic contact and formation of local lattice bonds [18]. Further processing results in friction at the surface, elastic hysteresis, plastic flow and grain distortion in the

Fig. 5. Ultrasonic powder consolidation.

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Fig. 6. Ultrasonic powder consolidation laboratory setup.

material volume, generating modest heating. Analogous phenomena occur in compaction of thermoplastic powders, including internal viscous friction, molecular chain entanglement, crosspolymerization and potential local melting. Early research has investigated ultrasonic welding mechanics by analytical and experimental methods, including photoelasticity and microscopy, and heat transfer by calorimetric techniques, while newer work addressed precision joining, high-frequency technologies, and ultrasonic process monitoring, control and optimization [19,20]. Ultrasonic bonding is flexible in consolidating dissimilar and composite material powders, such as mechanically alloyed Al–Ni micro-nanoparticulates (by planetary ball milling of Al and Ni powders, Fig. 7), exhibiting percolated fractal structures [21]. Most metal powders, including Al, Cu, precious metals, Fe and steel, Ni, Ti and many of their alloys can be readily compacted autogenously or with other metals [18]. Thermoplastics can also be compacted with other polymers (ABS, PVC, acrylics etc.), while some dissimilar material types of powders and substrates, i.e. metals bonding to certain polymers and ceramics such as glass, alumina, silicon and quartz, are also weldable. The process permits simultaneous consolidation of discrete multiple layers and continuous FGM fractal networks from various powder sizes and materials, e.g. via centrifugal segregation. Its crucial parameters include: powder material composition and geometry distribution; powder and substrate surface condition; normal pressure, ultrasonic frequency and vibration amplitude; processing time or speed; preheat temperature and environmental conditions. Ultrasonic consolidation requires only short bonding times and limited pressure and heat, preventing damage to plastics, semiconductors and particulates, as well as deformation and residual stresses. Also, bonding quality is insensitive to ordinary surface films and coatings, and normally requires no protective atmosphere. There is no need for special

Fig. 7. Ball-milled Al–Ni particulate (1:3 mol. ratio).

health and safety precautions, and no environmental hazards, and the process has very good energy efficiency. Fig. 8a shows a random composite Ni-Al network, compacted from Al (50 lm average size) and Ni (110 lm) powders (1:3 mol. ratio) at 300 °C and 115 MPa, with a 10 lm vibration amplitude at 20 kHz ultrasonic frequency. Its high interface surface/volume ratio yields a highly reactive bimetallic structure, which upon spark ignition produces the homogeneous nickel aluminide material (Fig. 8b) by a heat-releasing self-propagating exothermic reaction (SPER) [22]. Research addresses the relation between ultrasonic processing conditions and material and geometry distribution of the particulate structures, as well as their 3D percolation features ensuring functional leafages with thermal and other transport properties [23]. 3. Block copolymer self-assembly employs macromolecules consisting of at least two blocks of different monomeric units (usually thermodynamically incompatible [24]) connected via covalent bonds. Their immiscibility causes a microphase separation, whereby similar blocks of different block copolymer molecules selfassemble and form discrete microdomains in bulk and in solution, with sizes usually ranging between 10 lm–100 nm. The selfassembled structures of amphiphilic block copolymers depend on parameters such as: chemical composition and relative volume fraction of the constituent blocks; temperature, concentration, and interfacial energy. A number of different architectures can be obtained, such as micelles of different shapes (spherical, cylindrical), ordered continuous morphologies (lamellae, ordered cylinders) or bicontinuous structures. Furthermore, complex controllable micro/nano-structured fractal patterns were produced in block copolymer thin films and in solution [25]. Such complex templates are explored for manufacturing of functional devices, i.e. nanopatterned magnetic storage media, semi-conductor capacitors, quantum dots etc. Research focuses on novel block copolymer systems with binding functionalities onto metal ions, metals, metal oxides and nanoparticles, to develop organic–inorganic hybrid materials with special properties. For synthesis, a controlled radical polymerization method is used, namely reversible addition-fragmentation chain transfer (RAFT) polymerization. This is a versatile technique allowing for preparation of well-defined polymers with narrow molecular weight distributions, linear molecular weight conversion profile and control over the molecular weight [26], from a wide range of monomers and reaction conditions. For example, hybrid micellar nano-moprhologies stabilized in selective organic solvents (e.g. n-hexane), consisting of a new type of diblock copolymers, poly[laurylmethacrylate]-block-poly[2-(acetoacetoxy)ethyl methacrylate] (pLauMA-b-pAEMA) [27], and Pd nanoparticles are prepared following a simple, two-step synthetic methodology (Fig. 9 – Dr. T. Krasia, Personal Communication). The amphiphilic character of the pLauMA-b-pAEMAs leads to self-organization, creating nanospherical micelles ideal for complexation and solubilization of Pd(II) ions, further reduced into Pd(0) upon addition of a reduc-

Fig. 8. SPE reactive Al–Ni network (a) before and (b) after ignition.

C.C. Doumanidis / Microelectronic Engineering 86 (2009) 467–478

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Fig. 9. Step-wise process and AFM of aggregated micelles with Pd nanoparticles on mica.

ing agent (e.g. hydrazine monohydrate). Visualization by atomic force microscopy (AFM) after spin coating of the solution on a mica surface reveals the presence of closely-packed spherical micellar nanodomains, exhibiting strong non-linear optical (NLO) response and optical limiting action, of interest in photonic and optoelectronic devices. In addition, diblock copolymer micelles were used to prepare magnetic utrahigh-density data-storage media. New magnetic macromolecular aggregates (i.e. micelles) were recently developed via self-assembly of a new class of diblock copolymers consisting of AEMA and poly(ethyleneglycol) methyl ether methacrylate (PEGMA) in water, that were used to stabilize iron oxide magnetic nanoparticles [28]. These hybrid materials were found to exhibit superparamagnetic behavior, with tunable saturation magnetization depending on magnetic loading, and providing magnetizable leafages. 4. Plasma processing of polymers offers a new method for random branched nanotexturing of plastic surfaces. Optically transparent plates of commercial polymers, including poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA) and poly(ether etherketone) (PEEK) were exposed to low-pressure, high-density plasmas in SF6 or O2 gas, yielding a columnar high-aspect-ratio (HAR) surface topography, as evidenced by AFM and scanning electron microscopy (SEM) analysis (Fig. 10) [29–32]. Plasma processing ensures low-temperature, dry conditions, and definitive control of the surface chemistry and height of the branching morphology via the etching time. Tree-like nano-columns (37–98 nm high) appear

Fig. 10. O2 plasma-treated PMMA surfaces for (a) 20 min, (b) 60 min, and time– height relation.

within short process times (1–2 min) preserving the original transparency, and growing into the microscale upon further treatment, with transparency reduction. Oxygen-treated polymer surfaces exhibit super-hydrophilicity, which is transformed into super-hydrophobicity upon brief (1– 5 min) plasma deposition of a Teflon-like fluorocarbon film (20 nm thick), reducing also surface reflectivity but preserving transparency. Such plasma treatment through the large HAR surface area allows for highly increased electroosmotic flow (EOF) mobilities and closely controlled wetting angles of the superhydrophobic surfaces. These properties are explored for applications in microfluidic channel devices and flexible PDMS stamps with controlled surface tension (Fig. 11). In addition, branch-nanotextured surfaces (both fresh and aged) were tested in protein adsorption, by coating with biotinylated bovine serum albumin (BSA), and fluorescence testing after reaction with AF548-labelled streptavidin. The tests showed increased adsorption (but reduced homogeneity) with plasma treatment time and ageing of the surface, enabling applications of treated branching surfaces in bioanalytics. 5. Fiber electrospinning, a long-known process, has attracted new attention mainly in filter and textile applications, because of its continuous production of long fibers down to nanoscale diameters (10 lm–10 nm), in non-woven mats or oriented ensembles of high surface/volume ratios at low cost [33]. Typically, polymer solutions or melts are syringed through spinneret needles or pores against a conductor target or mandrel, under a potent electrostatic field (Fig. 12). Charged droplets ejected into a Taylor cone break up into jets, as electrostatic attraction overcomes surface tension and viscous forces. Jets undergo a hydrodynamic bending instability on the flight to the collector, spiralling within a conical envelope, thus accelerating, stretching, thinning and solidifying by cooling or solvent evaporation, until deposited as fibers on the target. Electrospun nanofibers have been coated by sol–gel, electrodeposition and PVD processes with metals and oxides into core-shell and hollow forms by pyrolysis or evaporation [34].

Fig. 11. Plasma-treated PDMS branched-structure surface.

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Fig. 12. Fiber electrospinning setup.

Electrohydrodynamic modelling of electrospinning has elucidated the radial perturbation and non-Newtonian flow conditions of the process [35]. The essential process parameters include: molecular weight and concentration of the solute; volatility, viscosity and conductivity of the solvent; temperature, pressure, and flow rate; spinneret diameter, electric voltage and target distance, as well as process atmosphere. Fig. 13 shows a mesh of cellulose acetate (CA) fibers, electrospun from a 15% w/v solution in acetone, fed at 100 ll/min into a 10 kV potential across a 75 mm tip-collector distance [36]. Electrospinning research investigates manufacture of stochastic patterns with controlled fiber diameters and interstices, as well as fused fiber networks with density gradients and directional transport in layered structures. This is achieved by in-process control of the fiber size, in-plane stretching and normal compression of the membrane layers, and by combining random nanofiber branches with aligned nanorod trunks, e.g. via the anodized alumina process below. Such branched biopolymer fiber membranes are tested as reinforcements in all-bioconsumable polymer matrix composites and as tissue engineering scaffolds [37]. 6. Anodized alumina templating offers an affordable, non-cleanroom, high-rate patterning technique for deposition of directional nanorods in dense arrays across the layer thickness. A high purity aluminum layer (99.99%) deposited by evaporation (PVD) is anodized in an acidic bath (sulphuric, oxalic, phosphoric etc.), yielding a porous alumina film (Al2O3) (Fig. 14). Such anodized aluminum oxide (AAO) membranes [38] typically display uniform, densely packed, hexagonally ordered arrays of parallel cylindrical nanopores, perpendicular to the AAO surface and penetrating through its thickness (up to 500 lm), with varying diameters (7–250 nm) and pore densities (109–1011 cm 2). Such nanopore arrays in AAO films have been used as templates for fabrication of various metal-

Fig. 13. Electrospun CA fused fiber mesh.

Fig. 14. (a) AAO anodization cell, (b) AAO nanopore array (top view).

lic and semiconductor nanorod arrays by electrodeposition or PVD [39]. Interestingly in polymer rods templated in AAO nanopores, quantum confinement (Stark effect, [40]) due to strong electric field polarization, as well as alignment of macromolecular chains along the nanopores, improve directional conductivity of polymer nanorods. The salient AAO process parameters include Al film thickness, acid bath type and concentration, temperature, stirring, anodization voltage and time [41]. Fig. 15 illustrates branched AAO nanotubes resulting from dissociation of AAO films, produced by anodization of an Al film in a mixture of phosphoric and sulphuric acid (96 ml H3PO4 (85%) + 24 ml H2SO4 (96%) + 80 ml H2O DI), for 14 min under a constant voltage of 20 V. The AAO cells of Fig. 14 were detached and each one formed a tube with a pore in the middle (Fig. 15). Such AAO nanotubes can be filled and sealed to provide self-standing nanocapsules for a variety of materials. In this research, the anodized alumina templating process is investigated for fabrication of nanorod trunks with custom-designed length, diameter and density, also in bifurcating and branching patterns [42,43], connecting with branch and leafage structures made e.g. by fiber electrospinning and ultrasonic corrosion texturing. 7. Carbon nano-network deposition was shown to produce fractal tree structures with trunks, branches and leafages made of various C allotropic forms [44,45]. Branching carbon nanotubes (CNT) were fabricated by catalytic decomposition of ethylene (C2H2 1.12% in He), using Ni nanoparticles (12 nm size, 0.5% wt) as the active catalyst, laid on multi-wall carbon nanotubes (50–60% MWNTs,

Fig. 15. Branched AAO patterns.

C.C. Doumanidis / Microelectronic Engineering 86 (2009) 467–478

Fig. 16. CNT network Polychronopoulou].

(a)

before,

(b)

after

regeneration

[courtesy

K.

diameter 8 nm and length 225 nm) as the substrate, for 14 hours at 400 °C (gas hours space velocity GHSV = 200,000/h). The MWNT substrate appears to play a significant role in this decomposition, via modification of the electronic state properties of the Ni nanoparticles towards higher reaction rates, and via increasing the catalyst lifetime. Fig. 16a shows the resulting tree-structured C network, including various amorphous allotropes (about 8.13 g of C per gram of catalyst) and also new secondary CNTs (diameter 55 nm and length of several microns) grown on the original substrate of MWNTs. Fig. 16b shows the primary and secondary branching nanotube network, after removal of amorphous C via catalyst regeneration by 20% O2 in He at 400 °C. In contrast to plain flat substrates for Ni, known in the literature to grow deterministically aligned CNT forests, the random orientation and local topology of the original MWNT mesh at the catalyst sites produces a stochastic branch-trunk tree structure [46]. More branching generator levels of the IFS fractal architecture can be further developed by using the network of Fig. 16b as the substrate for re-decoration with Ni catalyst and repetition of the process in extra stages. The stochastic CNT networks bear similarity with deterministically controlled ones by AAO templating [47]. In addition a new form of carbon, carbon nanofoam, was synthesized by a unique technique [48,49]. This new material was produced by laser ablation of an ultra pure glassy carbon target in an argon-filled chamber using a high-repetition-rate, high-power laser. Electron microscopy techniques revealed that the nanofoam possesses a fractal-like structure, consisting of carbon clusters with average diameter of 4–9 nm, randomly interconnected into a weblike foam. The nanofoam exhibits some remarkable physical properties, including ultra-low density (2 mg/cm3) and a large surface area (comparable to zeolites) of 300–400 m2/g. Most importantly, the low-density cluster-assembled carbon nanofoam shows strong positive magnetization, some of which is lost in the first few hours after synthesis, but much of which is persistent at low temperatures [50]. The origin of the unusual magnetism in the carbon nanofoam has been investigated theoretically using a geometry which contains hyperbolic, negatively curved surfaces, known as ‘‘schwarzite” structures [51]. Careful selection of laser ablation synthesis conditions may allow controlled adjustment of the properties in these complex nanocluster leafages.

attributes and probability distributions, are established. This is done by exploring the input/output spaces, nominal conditions and differential sensitivities through a fractional factorial design of experiments. Such process–structure–property mapping guides the design of the integrated architectures below, and defines their processing conditions. i. Laboratory experimentation in all fabrication tests characterizes intermediate and final material structures by microscopic, spectroscopic and other analytical methods. Scanning (SEM) and transmission (TEM) electron microscopy are used to assess micro-/ nano-features such as component shapes, sizes, distances, distributions of grains, particles, fibers, rods etc. (Fig. 17). Interfacial morphology and states are studied by atomic force (AFM) and scanning tunnelling (STM) probe microscopy, and specific surface area is measured via the isotherm of nitrogen adsorption (Brunauer–Emmet– Teller, BET). Elemental speciation of surfaces are determined by energy dispersive X-ray spectroscopy (EDS), while bulk phases and crystallinity are studied by X-ray diffraction (XRD). Rotational and vibrational modes of molecular surface complexes are assessed by Fourier transform infrared (FTIR) and Raman spectroscopy. Layer surface uniformity is checked by white-light profilometry, and internal density, thickness and elastic modulus by laser acoustics. Composite mechanical properties are also measured by dynamic mechanical analysis (DMA) and 4-probe electric testing, and thermal stability over environmental temperatures is evaluated by simultaneous thermal analysis (thermogravimetric analysis-TGA, differential scanning calorimetry-DSC). Aside from off-line analysis, real-time process monitoring is also essential to establish process dynamics and control schemes. Electrochemical impedance spectroscopy / tomography (EIS) across the film thickness images structure changes as its layers are processed, affecting their electrical impedance. An electrochemical AFM (ECAFM) unit [52] is used to study the reaction and diffusion thermodynamics and kinetics on local areas (10  10 nm) of interface fronts (corrosion surface, oxide layer). Micro-features in assembled architectures are non-destructively monitored for defects by X-ray microtomography. ii. Computational simulation is coupled to experimentation for deeper causality insights to nanomanufacturing process modelling, based on different numerical formulations. At the atomistic level, molecular dynamics (MD) descriptions are adopted (Fig. 18) to model chemical reactions based on density functional theory (DFT) [53]. Hybrid classical/quantum potentials in the bulk and on interfaces, respectively, and implicit solvation (i.e. continuum modelling of liquid solvents) are used for computational efficiency over a 128-processor cluster. Simulation over limited spatial (e.g. 5  5  5 nm) and time domains (few ns) assesses the dynamics and kinetics of reaction/diffusion interfaces (corrosion surfaces, oxide films etc.) which are validated against the respective ECAFM tests. At the nanoscale, level set methods [54] use dynamic potentials and kinetic rates from the MD model, along with initial interface geometries imaged by electron microscopy, to simulate

3. Process analysis and manufacturing synthesis The fundamental influence of nanomanufacturing processes to generated material structures and functional properties of random branched architectures, is researched in the individual basic processes above. For each separate process, the relationship of its essential parameters (inputs) to the resulting branched structure features (outputs), such as the stochastic IFS generator geometric

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Fig. 17. TEM of AAO branching.

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Fig. 18. MD setting of polymer between nanoscale interfaces.

advancing corrosion, oxidation and deposition surface fronts in a layer, as isosurface contours. For design and optimization of the processes, i.e. to obtain specified depths and propagation rates of interfaces via proper electrochemical actuation on the surfaces, a modified diffusive Green–Galerkin method for distributed-parameter system control is employed [55,56]. At the microscale, percolation theory techniques use the level surface topology to define percolation clusters of linked points, on which random walks in arbitrary directions are simulated by Monte-Carlo methods [57]. The connectivity properties (or percolation probability) of the multiscale random branched structures are computed this way, along with equivalent continuum properties of the materials for input to standard finite element analysis (FEA), to evaluate the macromechanical, electrical and thermal properties, validated against respective laboratory data (DMA, STA, EIS etc.) above. iii. Process control, using real-time sensing and feedback of structure–property outputs to modulate process inputs, is essential for quality in each processing step during continuous material manufacture in film/strip form. Fourier-transform capacitive or electrochemical impedance spectroscopy/tomography (FT–EIS) is employed for non-invasive, in-process sensing across the layered strip thickness. This is implemented via a transverse AC field between a metal substrate (Al) and a grid electrode (Pt) sandwiching the moving strip layers as they are built. Material features in the layers are monitored as they are processed, through respective changes in their electrical impedance (complex resistance). Real-time impedance spectra permit in-process identification of salient process parameters, through complex nonlinear least squares (CNLS) or simplex polytope parameter fitting [58] to an extended Randles electrochemical cell model. This includes representative parameters for the solution, pore and charge transfer/polarization resistances; the interfacial (double layer) and oxide coating capacitances; and the diffusion (Warburg) impedances. The processed film outputs, such as layer thickness, porosity, permeability etc., provide feedback to a closed-loop control strategy, designed on the basis of adaptive multi-input, multi-output (MIMO) algorithms [59] (Fig. 19). This is based on dynamic process models, with their

Specs

MIMO adaptive control

Process parameters

Actuators inputs Power Supplies Identification model Output feedback

parameters updated by the identification scheme above. The controller modulates the essential process inputs, such as voltage, temperature, concentration, flow etc., so that the cell material outputs comply to the desired quality specifications. This work is currently in progress. Manufacturing synthesis of the previous complementary, multiscale processes is essential for production of controlled random branched structures, such as interpenetrating percolated fractal networks, from diversified material types. The inherent scalability and compatibility of such techniques is key for their integration, in serial or parallel processing procedures, into continuous manufacture of functional transport architectures. These are further combined with traditional micro/nanofabrication patterning and layering methods, such as lithography and SFF. Three examples below illustrate such process synthesis. a. Nanoheaters consist of nanoscale heterostructures made of reactive multi-material systems (e.g. bimetallic Ni–Al networks, Fig. 8), which upon external ignition (e.g. via electrical discharge or induction) release instantaneous, locally concentrated exothermic heat, conducted to a substrate or surrounding material (Fig. 20) [60]. Such thermally active, random-structured elements were fabricated across various scales by recursive roll bonding of alternating metal foil stacks; by sequential multi-target sputtering; by mechanical alloying of Al–Ni powders [21] and by preheated ultrasonic micropowder consolidation as above [17]. Their exothermic (SPER) thermodynamics and formation of e.g. nickel aluminides have been studied by high-speed and infrared pyrometry, differential scanning calorimetry and electron microscopy, coupled with finite-difference simulations of the resulting temperature and concentration distributions upon ignition [22,23]. Spatio-temporal thermal processing control and observation of nanoheater sources has also been addressed by distributed-parameter Green–Galerkin methods for thermal nanomanufacturing applications [61], such as rapid thermal processing of semiconductors, self-heating and repairing materials, metrological compensators, thermal nanobatteries, heated bioMEMS devices etc. In particular at the nanoscale, AAO templating was combined with PVD deposition of Al and Ni thin films to produce nanorods as nanoheaters (Fig. 21) [62]. An Al thin film, sputtered onto a Si substrate and electropolished, was anodized first to yield a porous alumina template film (200 nm thick). Then, successive films of Al and Ni were deposited by aligned electron beam evaporation into the AAO pores. Progressive blocking of the deposition pathway in each pore during the process permits deposition of a bilayer Al– Ni nanorod (diameter and height 60–70 nm) at the Si end. Eventually the pore closure effect seals the free end also with a continuous Al–Ni bilayer, while the rest of the pore remains empty. After removal of the Si substrate and ignition of the nanoheaters and continuous layer in the AAO template, the intense heat release melts

layer EIS Sensor

Ignition

Al Ni

Process

HF

Al Ni Al Si etc

~

Heat

Substrate

Heat pad conductor Heated spot

Spectrum Analyzer Substrate

Fig. 19. Closed-loop adaptive control and in-process identification strategy.

Fig. 20. Schematic layout of a nanoheater island.

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475

Fig. 21. Al–Ni nanorods.

the nickel aluminide products, which exit the pores and coagulate into larger molten spheroidal aggregates on the AAO film surface (Fig. 22), revealing the underlying pore apertures [62]. It is postulated that the pores open because of the poor wetting of the high aspect ratio AAO surface topology by the metallic melt, attempting to reduce the free energy of contact. Current research addresses this actuated opening effect in nanoheater-sealed individual AAO nanotubes (Figs. 15 and 17) in both random branching aggregates and free-standing nanotubes, as a mechanism for controlled flow transport or fluid containment and release by sealed nanocapsules, with applications in targeted drug delivery etc. b. Nanocomposite foils are manufactured via a new technique, from stacked metal or plastic foils with intermediate sandwiched layers of nanoparticulates (nano-powders, nanotubes, nanoclays), by solid-state surface ultrasonic consolidation [63]. Fig. 23a illustrates the continuous process configuration with two (or multiple) material foils fed from supply rolls, then coated on their inner surface(s) with the nanoparticles by uniform deposition (e.g. electrostatically similar to xerography), in order to be consolidated through seam ultrasonic welding between a rotating (multi-) disk sonotrode and an anvil drum, before it is collected as finished nanocomposite sheet in a takeup roll [64]. Such universal foil precursor materials can be integrated into final products, such as cast or molded (e.g. by semi-solid injection) objects, layered rapid prototypes by particle, wire or foil-based SFF, nano-composite-coated or laminated parts, joined frame structures etc. Fig. 23b details how ultrasonic consolidation breaks up the clusters of nanoparticles, and leads to their controlled disaggregation, dispersion and penetration into the matrix foils before the metal or polymer materials come in contact and bond.

Fig. 22. Opening AAO pores with Ni–Al nanoheaters.

Fig. 23. Schematic of ultrasonic consolidation of nanocomposite foils.

Ultrasonic consolidation of nanocomposite foils employed nanomagnetic ferrofluids, i.e. hexanol and aqueous suspensions of magnetite nanoparticles (Fe3O4) spin-coated between poly(vinyl chloride) (PVC) foils (0.5 mm thick) [63,64]. For the polymer matrix composites (PMC) and after drying of the reinforcement, an ultrasonic seam welder (40 kHz) was used at room temperature, normal pressure of 20 MPa, vibration amplitude of 10 lm and roll speed of 1 cm/s for foil compaction. Ultrasonic welding controls the dissociation of Fe3O4 micellar aggregates and their distribution in random networks branching into the PVC bond zone (TEM of cross-section in Fig. 24), preserving the super-paramagnetic properties of the magnetoresponsive hybrid micelles, and avoiding intra-cluster antiferromagnetic interactions reducing magnetic saturation. Fig. 25 shows the magnetization curves of the resulting PVCFe3O4 composites, corresponding to increasing nanomagnetic particulate loadings. Nanocomposite foils exhibit a strong ferromagnetic behavior, showing decreasing saturation magnetization with increasing magnetite loading, probably because of intra-cluster antiferromagnetic interactions. A continuum mechanical model for ultrasonic levitation of the micellar particulates from the interface zone into the polymer was developed for ultrasonic processing, with particle-PVC interfacial slip velocities determined via molecular dynamics (Fig. 18). Nanomagnetic PMC foils are tested for magnetic actuation and data storage with controlled magnetization properties, and similar metal matrix composites (MMC) with Al foils were also implemented. In addition, ultrasonically consolidated PMC foils with a CNT/carbon mesh were also synthesized, with an electrically percolated interlayer network at minimal CNT loading, for structural fortification, electromagnetic interference shielding and as substrates for flexible electronics

Fig. 24. Magnetic particle clusters (cross-section).

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the cellular layer and enhance the expression of differentiated cell function. In addition, hybrid electrospun fiber membranes of CA containing nanoaggregates of artificial hydroxyapatite (HA), i.e. the hydrated calcium phosphate mineral of natural bone [67], cultured with human osteoblasts (SaOS2), displayed significant cell attachment, spreading, growth and proliferation without phenotype changes [68]. Current research addresses the microvascularization of such tissue engineering scaffolds with fractal branching capillary networks [69], via ultrasonic corrosion and AAO templating methods etc.

100 80 60

M (emu/gr)

40 20 0 -20 -40 -60

0.1 wt % 0.5 wt % 2 wt %

-80 -100 -40

0.3 wt % 1 wt %

4. Epilogue and further research -30

-20

-10

0

10

20

30

40

H (K Oe) Fig. 25. Magnetization curves of PVC nanocomposites with increasing Fe3O4 particle loading.

[65]. Finally, all-biodegradable PMC foils were produced by ultrasonic bonding of electrospun CA fiber mats sandwiched between CA foils [36], providing autogenous strenghtening due to the fiber scale effects. c. Tissue scaffolds mimicking the branching random structure of natural extracellular matrix (ECM) are essential for growth, proliferation and differentiation of seeded cells in engineered tissue for regenerative medicine. In particular, the 3D topology of ECM has been reported to modulate the polarity of attached cells, thus affecting development e.g. of thyroid cells, muscle cells and hepatocytes. The configuration of ECM, e.g. from the porcine urinary bladder (which was proven suitable as a scaffold for regeneration of tissue for arterial grafts, vena cava and myocardium, Fig. 26a), exhibits layers with diversified morphology at multiple length scales. ECM-like scaffolds from synthetic biopolymers such as CA were prepared by templated fiber electrospinning in successive layers (Fig. 26b) [37]. The fibers (average diameter 1.7–2 lm) were deposited in a dense mat (bottom layer) and a fibrous membrane (top layer, average pore size 36.6 lm), connected by a cellular interlayer at regularly spaced intervals (1 mm) separated by large pores (357 lm), which were templated via a copper wire mesh (300 lm diameter) [66]. Such sandwiched ECM configurations were shown to reduce cell spreading of hepatocytes seeded in

In conclusion, fundamental research contributions stem from discovery of the principal nanomanufacturing process-material structure-functional properties relationships in processing of random branched architectures across multiple spatial and temporal scales. In particular, key issues lie in understanding the causality between probabilistic processing mechanisms or parameters at the atomic/nano-level, and the stochastic geometric features of branching patterns generated at the nano-/microscale, as well as their resulting bulk absorption/dissipation, surface permeation and linear transport properties in the micro-/macroworld. Establishing this relation among probability distributions of connected inputs and outputs entails a hierarchical interplay between discrete and continuum descriptions of matter and phenomena, crossing multiple energetic domains (mechanical, fluidic, thermal, electrical, chemical etc.) and disciplinary boundaries. A dual effort consists in inverting these causal mappings to define, from the desired functional properties of the random materials and architectures, their requisite branched structure features, and therefore the proper fabrication processes and conditions needed. This inversion of both manufacturing and operational process topology and dynamics, on the basis of their previous formulations, is essential for off-line design and optimization, along with real-time identification and process control. Finally, the previous research on tree-structured materials illustrates several subtle but salient diversions from current micro/ nano-fabrication state of the art to improve industrial production. The issues of process robustness, flexibility, throughput, yield, efficiency, affordability, health, safety and environmental sustainability are central in the repertory of technologies and platforms proposed for nanomanufacturing. Although product market domains are often distinct, it is clear that such manufacturing attempts to ameliorate the scalability issues of fabrication for massive production. Regarding scalable industrial realization, the non-cleanroom, robust, affordable, continuous manufacturing processes are designed for low infrastructure needs, and for scaleup to multiple parallel strip production lines, processed concurrently through the reactors, at continuous throughputs yielding low production costs. Therefore, manufacturing platforms of random branching material architectures could have enabling impacts to a variety of fields involving mass, energy and information processing, such as microelectronics, photovoltaics, electromagnetics, chemical catalysis, hydrogen energy, biomedicine and many other prominent scientific areas. Acknowledgments

Fig. 26. ECM scaffolds: (a) natural porcine E, (b) by fiber electrospinning [70] (different scales).

The author wishes to gratefully acknowledge the contribution of his colleagues Profs. Teiichi Ando, John Giapintzakis, Evangelos Gogolides, Perena Gouma, Maria Kokonou, Theodora Krasia, Theodora Kyratsi, Kyriaki Polychronopoulou and Claus Rebholz to the research material of this article. Research support by the European Commission FP6 EXC-006680 (UltraNanoMan), EXT-023899 (NanoHeaters),

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IP 026467 (ManuDirect), Interreg IIIA (NanoSpin), FP7 ICT-0224594 (NanoMA); the Research Promotion Foundation (NanoCyprus, NanoPlasi); and National Science Foundation (USA) Grants NSF DMI-0531127 and EEC-0738253, is also thankfully acknowledged. References [1] B.B. Mandelbrot, The Fractal Geometry of Nature, W.H. Freeman & Co., New York, 1982. [2] K. Falconer, Fractal Geometry: Mathematical Foundations and Applications, John Wiley & Sons, 2003. [3] M.F. Barnsley, S. Demko, Iterated function systems and the global construction of fractals, Proc. Roy. Soc. London A399 (1985) 243. [4] M. Boncheva, G.M. Whitesides, Making things by self-assembly, MRD Bull. 30 (2005) 736. [5] R. Hohlfeld, N. Cohen, Self-similarity and the geometric requirements for frequency independence in antennae, Fractals 7 (1) (1999) 79. [6] M. Grätzel, Photoelectrochemical cells, Nature 414 (6861) (2001) 338. [7] T. Zeng, G. Chen, Interplay between thermoelectric and thermionic effects in heterostructures, J. Appl. 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