Mixed carbon nanotube bundles for interconnect applications

The increasing resistivity of copper with scaling and demands for higher current density are the driving forces behind the ongoing investigation for new wiring solutions for deep nanometer scale VLSI technologies. Metallic carbon nanotubes (CNTs) are promising candidates that can potentially address the challenges faced by copper, and thereby extend the lifetime of electrical interconnects. This article examines the state of the art in CNT applications with focus on CNT interconnect research. It is observed that individually, single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) exhibit characteristics that can be better exploited when a combination of the two is used – in the form of a CNT bundle that plays a vital role in interconnect applications. The focus here is that the usage of a combination of SWCNT (at the centre area of the bundle) and MWCNT (on the outside) provides great performance boost with lower interaction and crosstalk between neighbouring CNT bundles. Simulation results show that the resistance, capacitance, and inductance of a CNT depend on the probability of metallic CNTs present in the bundle and the length of the nanotube. Because Cu is metallic, it indicates that using a higher number of metallic nanotubes in the bundle would aid the CNT bundle performance. In addition, using MWCNT on the outer periphery of the bundle and SWCNT in the centre of the bundle would be the ideal way to maximise the performance of the bundle. Based on the observations we provide an analysis of why a mixed CNT bundle would be highly suitable as interconnections.     

Beam to String Transition of Vibrating Carbon Nanotubes Under Axial Tension

State‐of‐the‐art nanoelectromechanical systems have been demonstrated in recent years using carbon nanotube (CNT) based devices, where the vibration of CNTs is tuned by tension induced through external electrical fields. However, the vibration properties of CNTs under axial tension have not been quantitatively determined in experiments. Here, a novel in situ method for precise and simultaneous measurement of the resonance frequency, the axial tension applied to individual CNTs and the tube geometry is demonstrated. A gradual beam‐to‐string transition from multi‐walled CNTs to single‐walled CNTs is observed with the crossover from bending rigidity dominant regime to extensional rigidity dominant regime occur much larger than that expected by previous theoretical work. Both the tube resonance frequency under tension and transition of vibration behavior from beam to string are surprisingly well fitted by the continuum beam theory. In the limit of a string, the vibration of a CNT is independent of its own stiffness, and a force sensitivity as large as 0.25 MHz (pN)^?1 is demonstrated using a 2.2 nm diameter single‐walled CNT. These results will allow for the designs of CNT resonators with tailored properties.     

High-Frequency Analysis of Carbon Nanotube Interconnects and Implications for On-Chip Inductor Design

This paper presents a rigorous investigation of high-frequency effects in carbon nanotube (CNT) interconnects and their implications for the design and performance analysis of high-quality on-chip inductors. A frequency-dependent impedance extraction method is developed for both single-walled CNT (SWCNT) and multiwalled CNT (MWCNT) bundle interconnects. The method is subsequently verified by comparing the results with those derived directly from the Maxwell's equations. Our analysis reveals for the first time that skin effect in CNT (particularly MWCNT) bundles is significantly reduced compared to that in conventional metal conductors, which makes them very attractive and promising material for high-frequency applications, including high-quality (Q) factor on-chip inductor design in high-performance RF/mixed-signal circuits. It is shown that such unique high-frequency properties of CNTs essentially arise due to their large momentum relaxation time (leading to their large kinetic inductance), which causes the skin depths to saturate with frequency and thereby limits resistance increase at high frequencies in a bundle structure. It is subsequently shown that CNT-based planar spiral inductors can achieve more than three times higher Q factor than their Cu-based counterparts without using any magnetic materials or Q factor enhancement techniques.     

Carbon nanotubes grown from nickel catalyst pretreated with H2/N2 plasma

To find the possibility of using a low-temperature process in growing carbon nanotubes (CNTs), nickel catalyst converted from film into particles by microwave H₂/N₂ plasma and the following CNT growth are all kept at a low temperature of 250 °C. The flat panel display industry requests low-temperature rather than the traditional high-temperature process for CNT growth. It was found that H₂/N₂ proportion is very sensitive to nickel morphology and the subsequent CNT growth. Better nickel and CNTs morphology are obtained for the proportion H₂/N₂=3/1 than those for the generally used pure hydrogen environment. The process pressure selection during pretreatment can determine whether CNTs are grown or not. The diameter of growing CNTs is proportional to nickel particle size. Field emission results support field amplification coefficient claim. The long tube length and high tube density of growing CNTs demonstrate low threshold electric field. This work shows the potential to use H₂/N₂ instead of pure hydrogen plasma in growing qualified CNTs applied in display industry.     

Atomistic Investigation of Load Transfer Between DWNT Bundles “Crosslinked” by PMMA Oligomers

The production of carbon nanotube (CNT) yarns possessing high strength and toughness remains a major challenge due to the intrinsically weak interactions between “bare” CNTs. To this end, nanomechanical shear experiments between functionalized bundles of CNTs are combined with multiscale simulations to reveal the mechanistic and quantitative role of nanotube surface functionalization on CNT‐CNT interactions. Notably, the in situ chemical vapor deposition (CVD) functionalization of CNT bundles by poly(methyl methacrylate) (PMMA)‐like oligomers is found to enhance the shear strength of bundle junctions by about an order of magnitude compared with “bare” van der Waals interactions between pristine CNTs. Through multiscale simulations, the enhancement of the shear strength can be attributed to an interlocking mechanism of polymer chains in the bundles, dominated by van der Waals interactions, and stretching and alignment of chains during shearing. Unlike covalent bonds, such synergistic weak interactions can re‐form upon failure, resulting in strong, yet robust fibers. This work establishes the significance of engineered weak interactions with appropriate structural distribution to design CNT yarns with high strength and toughness, similar to the design paradigm found in many biological materials.     

Performance Comparisons Between Carbon Nanotubes, Optical, and Cu for Future High-Performance On-Chip Interconnect Applications

Optical interconnects and carbon nanotubes (CNTs) present promising options for replacing the existing Cu-based global/semiglobal (optics and CNT) and local (CNT) wires. We quantify the performance of these novel interconnects and compare it with Cu/low-kappa wires for future high-performance integrated circuits. We find that for a local wire, a CNT bundle exhibits a smaller latency than Cu for a given geometry. In addition, by leveraging the superior electromigration properties of CNT and optimizing its geometry, the latency advantage can be further amplified. For semiglobal and global wires, we compare both optical and CNT options with Cu in terms of latency, energy efficiency/power dissipation, and bandwidth density. The above trends are studied with technology node. In addition, for a future technology node, we compare the relationship between bandwidth density, power density, and latency, thus alluding to the latency and power penalty to achieve a given bandwidth density. Optical wires have the lowest latency and the highest possible bandwidth density using wavelength division multiplexing, whereas a CNT bundle has a lower latency than Cu. The power density comparison is highly switching activity (SA) dependent, with high SA favoring optics. At low SA, optics is only power efficient compared to CNT for a bandwidth density beyond a critical value. Finally, we also quantify the impact of improvement in optical and CNT technology on the above comparisons. A small monolithically integrated detector and modulator capacitance for optical interconnects (~10 fF) yields a superior power density and latency even at relatively lower SA (~20%) but at high bandwidth density. At lower bandwidth density and SA lower than 20%, an improvement in mean free path and packing density of CNT can render it most energy efficient.     

Cover Picture: Fabrication and Electrical and Mechanical Properties of Carbon Nanotube Interconnections (Adv. Funct. Mater. 11/2005)

The fabrication of carbon nanotube (CNT) structures, including simple tube–tube connections, crossed junctions, T‐junctions, zigzag structures, and even nanotube networks, has been achieved by cutting and soldering CNTs using electron‐beam‐induced deposition of amorphous carbon (a‐C), as detailed in the work of Peng and co‐workers on p. 1825. These CNT structures have been constructed with a high degree of control, and it is found that the electric conductance and mechanical strength of the junctions can be improved by the deposition of a‐C and by increasing the contact area of the junctions. Individual carbon nanotubes (CNTs) have been cut, manipulated, and soldered via electron‐beam‐induced deposition of amorphous carbon (a‐C) and using a scanning tunneling microscope inside a transmission electron microscope. All CNT structures, including simple tube–tube connections, crossed junctions, T‐junctions, zigzag structures, and even nanotube networks, have been successfully constructed with a high degree of control, and their electrical and mechanical properties have been measured in situ inside the transmission electron microscope. It is found that multiple CNTs may be readily soldered together with moderate junction resistance and excellent mechanical resilience and strength, and the junction resistance may be further reduced by current‐induced graphitization of the deposited a‐C on the junction.     

Compact Performance Models and Comparisons for Gigascale On-Chip Global Interconnect Technologies

The on-chip global interconnect with conventional Cu/low-k and delay-optimized repeater scheme faces great challenges in the nanometer regime owing to its severe performance degradation. This paper describes the analytical models and performance comparisons of novel interconnect technologies and circuit architectures to cope with the interconnect performance bottlenecks. Carbon nanotubes (CNTs) and optics-based interconnects exhibit promising physical properties for replacing the current Cu/low-k-based global interconnects. We quantify the performance of these novel interconnects and compare them with Cu/low-k wires for future high-performance integrated circuits. The foregoing trends are studied with technology node and bandwidth density in terms of latency and power dissipation. Optical wires have the lowest latency and power consumption, whereas a CNT bundle has a lower latency than Cu. The new circuit scheme, i.e., “capacitively driven low-swing interconnect (CDLSI),” has the potential to effect a significant energy saving and latency reduction. We present an accurate analytical optimization model for the CDLSI wire scheme. In addition, we quantify and compare the delay and energy expenditure for not only the different interconnect circuit schemes but also the various future technologies, such as Cu, CNT, and optics. We find that the CDLSI circuit scheme outperforms the conventional interconnects in latency and energy per bit for a lower bandwidth requirement, whereas these advantages degrade for higher bandwidth requirements. Finally, we explore the impact of the CNT bundle and the CDLSI on a via blockage factor. The CNT shows a significant reduction in via blockage, whereas the CDLSI does not help to alleviate it, although the CDLSI results in a reduced number of repeaters due to the differential signaling scheme.      

Proposal and analysis of relative stability in mixed CNT bundle for sub-threshold interconnects

We introduce a Nyquist stability analysis of coupled mixed CNT bundle (MCB) for sub-threshold interconnects. In this analysis, the dependence of relative stability of sub-threshold MCBs with specific and probabilistic distribution of CNTs, on the geometry and probability of metallic CNTs, has been obtained. Using the proposed ABCD model and Nyquist stability criterion for sub-threshold MCBs, we show that, by increasing the diameter of each individual CNT and the length of MCB, the sub-threshold MCB interconnect system becomes more stable, while a densely packed MCB reduces the relative stability. Moreover, the crosstalk impact results in the greater stability of sub-threshold MCB system in comparison to a single interconnect. The crosstalk delay of MCB and composite Cu-MWCNT interconnects is also compared at various lengths. This is, so far, the first instance that such an analysis has been presented for coupled sub-threshold MCB interconnects.     

High‐Performance Epoxy Nanocomposites Reinforced with Three‐Dimensional Carbon Nanotube Sponge for Electromagnetic Interference Shielding

Light‐weight and high‐performance electromagnetic interference (EMI)‐shielding epoxy nanocomposites are prepared by an infiltration method using a 3D carbon nanotube (CNT) sponge as the 3D reinforcement and conducting framework. The preformed, highly porous, and electrically conducting framework acts as a highway for electron transport and can resist a high external loading to protect the epoxy nanocomposite. Consequently, a remarkable conductivity of 148 S m^?1 and an outstanding EMI shielding effectiveness of around 33 dB in the X‐band are achieved for the epoxy nanocomposite with 0.66 wt% of CNT sponge, which is higher than that achieved for epoxy nanocomposites with 20 wt% of conventional CNTs. More importantly, the CNT sponge provides a dual advantage over conventional CNTs in its prominent reinforcement and toughening of the epoxy composite. Only 0.66 wt% of CNT sponge significantly increases the flexural and tensile strengths by 102% and 64%, respectively, as compared to those of neat epoxy. Moreover, the nanocomposite shows a 250% increase in tensile toughness and a 97% increase in elongation at break. These results indicate that CNT sponge is an ideal functional component for mechanically strong and high‐performance EMI‐shielding nanocomposites.     

Quantum modeling and proposed designs of CNT-embedded nanoscale MOSFETs

We propose a novel MOSFET design that embodies single-wall zigzag semiconducting carbon nanotubes (CNTs) in the channel. Investigations show that CNTs have high low-field mobilities, which can be as great as 1 /spl times/ 10/sup 5/ cm/sup 2//V/spl middot/s. Thus, we expect that MOSFET performance can be improved by embedding CNTs in the channel. To investigate the performance of a newly proposed CNT-MOSFET device, we develop a methodology that connects CNT modeling to MOSFET simulations. Our calculations indicate that by forming high mobility regions in the channel, MOSFET performance can be boosted. However, barriers formed between the CNT and silicon due to the variations of the bandgaps and electron affinities can degrade MOSFET performance improvements. Our calculations were obtained by building on our existing CNT Monte Carlo simulator , and quantum-based device solver ,.     

Electrically Addressable Hybrid Architectures of Zinc Oxide Nanowires Grown on Aligned Carbon Nanotubes

The fabrication and characterization of hybrid architectures of ZnO nanowires (ZNWs) grown on organized carbon nanotubes (CNTs), by a two‐step chemical vapor deposition (CVD) process involving CNT growth from a hydrocarbon source followed by ZNW growth using a Zn metal source, is reported. The ZNWs grow uniformly and radially from individual CNTs and CNT bundles, and the aligned morphology of the CNTs is not disturbed by the ZNW growth process. The nucleation and growth of ZnO crystals on CNTs are analyzed in relation to the classical vapor–solid mechanism. Importantly, the CNTs make uniform and distributed electrical contact to the ZNWs, with up to a 1000‐fold yield advantage over conventional ZNW growth on a flat substrate. Hybrid ZNW/CNT sheets are fabricated by scalable CVD, rolling, and printing methods; and their electrical properties, which are governed by transport through the anisotropic CNT network, are characterized. Functional interaction between the ZNWs and CNTs is demonstrated by photoconductive behavior and photocurrent generation of the hybrid material under UV illumination. There is significant future opportunity to extend these processing methods to fabricate other functional oxides on CNTs, and to build devices that harness the attractive properties of ZNWs and CNTs with high volumetric efficiency over large areas.     

基于单壁碳纳米管束的三维互连线设计与分析

Metallic carbon nanotubes(CNTs) have been proposed as a promising alternative to Cu interconnects in future integrated circuits(ICs) for their remarkable conductive, mechanical and thermal properties. Compact equivalent circuit models for single-walled carbon nanotube(SWCNT) bundles are described, and the performance of SWCNT bundle interconnects is evaluated and compared with traditional Cu interconnects at different interconnect levels for through-silicon-via-based three dimensional(3D) ICs. It is shown that at a local level, CNT interconnects exhibit lower signal delay and smaller optimal wire size. At intermediate and global levels, the delay improvement becomes more significant with technology scaling and increasing wire lengths. For 1 mm intermediate and 10 mm global level interconnects, the delay of SWCNT bundles is only 49.49% and 52.82% that of the Cu wires, respectively.     

Covalently Bonded Graphene Sheets on Carbon Nanotubes: Direct Growth and Outstanding Properties

Integrating 1D carbon nanotubes (CNTs) and 2D graphene with covalent bonds can inherit the outstanding properties of both components and obtain additional advantages. Here, this work reports the preparation of covalently bonded graphene/CNT (G/CNT) structure by a normal chemical vapor deposition method. Specifically, the pre-synthesized defects on the sidewall of CNTs act as nucleation sites for the growth of graphene sheets to form a branch-leaf structure. Graphene leaves restrict the sliding and re-stacking of CNTs, endowing G/CNT hybrid demonstrates excellent anti-agglomeration properties that are not present in either graphene or CNTs. In addition, the covalently bonded structure and high graphitization degree of graphene sheets and CNTs significantly enhance the comprehensive properties of the G/CNT hybrid material, such as large specific surface area, excellent thermal stability, and high electrical conductivity. Consequently, the microwave absorption properties of G/CNT are significantly enhanced compared with CNTs. This work provides a feasible pathway to synthesize high-performance covalently bonded G/CNT hybrids.     

Carbon nanotube electronics

We evaluate the potential of carbon nanotubes (CNTs) as the basis for a new nanoelectronic technology. After briefly reviewing the electronic structure and transport properties of CNTs, we discuss the fabrication of CNT field-effect transistors (CNTFETs) formed from individual single-walled nanotubes (SWCNTs), SWCNT bundles, or multiwalled (MW) CNTs. The performance characteristics of the CNTFETs are discussed and compared to those of corresponding silicon devices. We show that CNTFETs are very competitive with state-of-the-art conventional devices. We also discuss the switching mechanism of CNTFETs and show that it involves the modulation by the gate field of Schottky barriers at the metal-CNT junctions. This switching mechanism can account for the observed subthreshold and vertical scaling behavior of CNTFETs, as well as their sensitivity to atmospheric oxygen. The potential for integration of CNT devices is demonstrated by fabricating a logic gate along a single nanotube molecule. Finally, we discuss our efforts to grow CNTs locally and selectively, and a method is presented for growing oriented SWCNTs without the involvement of a metal catalyst.     

Synthesis of carbon nanotubes by catalytic chemical vapour deposition: A review on carbon sources,catalysts and substrates

Carbon nanotubes (CNTs) have been extensively studied during the past two decades and Catalytic Chemical Vapour Deposition (CCVD) technique has been untiredly employed by researchers to produce CNTs of various crystallographic configurations. In this paper the material aspects carbon sources, catalysts and substrates with regard to CCVD synthesis of carbon nanotubes are reviewed in light of latest developments and understandings in the field. The role of these materials in synthesis of CNTs is explained keeping the upto date literature in view. Latest research reports and their findings are presented with regard to effects of growth control aspects such as temperature, vapour pressure and catalyst concentration on CNT formation. Besides recent understandings with regard to preferential growth of CNTs are also discussed. From this literature review it is found that carbon diffusibility and carbon solubility of any catalyst are two important factors in determining CNT nucleation and growth. Moreover, addition of catalyst species to any transition metal catalyst can improve the catalyst performance and addition of water, air, alcohol etc. during CCVD process can increase the activity and lifetime of the catalyst besides enhances the production of CNTs.     

Covalently Bonded Graphene–Carbon Nanotube Hybrid for High‐Performance Thermal Interfaces

The remarkable thermal properties of graphene and carbon nanotubes (CNTs) have been the subject of intensive investigations for the thermal management of integrated circuits. However, the small contact area of CNTs and the large anisotropic heat conduction of graphene have hindered their applications as effective thermal interface materials (TIMs). Here, a covalently bonded graphene–CNT (G‐CNT) hybrid is presented that multiplies the axial heat transfer capability of individual CNTs through their parallel arrangement, while at the same time it provides a large contact area for efficient heat extraction. Through computer simulations, it is demonstrated that the G‐CNT outperforms few‐layer graphene by more than 2 orders of magnitude for the c‐axis heat transfer, while its thermal resistance is 3 orders of magnitude lower than the state‐of‐the‐art TIMs. We show that heat can be removed from the G‐CNT by immersing it in a liquid. The heat transfer characteristics of G‐CNT suggest that it has the potential to revolutionize the design of high‐performance TIMs.     

Luminescence of Functionalized Carbon Nanotubes as a Tool to Monitor Bundle Formation and Dissociation in Water: The Effect of Plasmid‐DNA Complexation

Functionalized carbon nanotubes (f‐CNTs) are explored as novel nanomaterials for biomedical applications. UV‐vis luminescence of aqueous dispersions of CNT–NH₃⁺ and CNT–NH–Ac (NH–Ac: acetamido) is observed using standard laboratory spectrophotometric instrumentation, and the measured fluorescence intensity is correlated with the aggregation state of the f‐CNTs: a high intensity indicates improved f‐CNT individualization and dispersion, while a decrease in fluorescence intensity indicates a higher degree of nanotube aggregation and bundling as a result of varying the sodium dodecyl sulfate (SDS) concentrations and pH in the aqueous phase. Moreover, utilization of this relationship between fluorescence intensity and the state of f‐CNT aggregation is carried out to elucidate the interactions between f‐CNTs and gene‐encoding plasmid DNA (pDNA). pDNA is shown to interact with CNT–NH₃⁺ primarily through electrostatic interactions that lead concomitantly to a higher degree of f‐CNT bundling. The CNT–NH₃⁺/pDNA interactions are successfully competed by SDS/f‐CNT surface interactions, resulting in the displacement of pDNA. These studies provide exemplification of the use of fluorescence spectrophotometry to accurately describe the aggregation state of water‐soluble f‐CNTs. Characterization of the complexes between pDNA and f‐CNTs elucidates the opportunities and limitations of such supramolecular systems as potential vectors for gene transfer.     

Bifunctional Carbon Nanotubes by Sidewall Protection

A vertically aligned array of multiwalled carbon nanotubes (MWCNTs) impregnated with polystyrene is utilized to independently functionalize each end of the MWCNTs. The presence of the polystyrene matrix prevents sidewall oxidation of the CNTs, resulting in carboxylate derivatization at the CNT tips during processing via plasma oxidation. The membrane is subsequently dissolved in toluene, resulting in a suspension of CNTs with carboxylate‐derivatized tips. The CNT tips are further functionalized using carbodiimide‐mediated linking of carboxylate at the CNT tips with an amine of 2‐aminoethanethiol. This treatment results in thiol functionality and Fourier‐transform infrared (FT‐IR) studies confirm amide‐bond formation. Gold nanoparticles that are readily observed using transmisison electron microscopy (TEM) are then covalently linked to the thiol functional groups. Estimates of the average nanoparticle density are observed to decrease from ～ 526 particles μm^–1 near the CNT tips to negligible values (< 7 particles μm^–1) at locations beyond 700 nm from the CNT tips. This is consistent with a membrane geometry where CNTs tips are above the polystyrene surface owing to differing oxidation rates. Bifunctional CNTs (with different chemical functionality at either end of each CNT) is achieved by thiol functionalization on only one side of the oxidized CNT membrane floating on top of a 2‐aminoethanethiol functionalization reaction solution. After dissolution of the polystyrene matrix, TEM analysis shows gold‐nanoparticle decoration at the thiol‐functionalized end of the CNT.     

Towards Efficient Dispersion of Carbon Nanotubes in Thermotropic Liquid Crystals

Motivated by numerous recent reports indicating attractive properties of composite materials of carbon nanotubes (CNTs) and liquid crystals (LCs) and a lack of research aimed at optimizing such composites, the process of dispersing CNTs in thermotropic LCs is systematically studied. LC hosts can perform comparably or even better than the best known organic solvents for CNTs such as N‐methyl pyrrolidone (NMP), provided that the dispersion process and choice of LC material are optimized. The chemical structure of the molecules in the LC is very important; variations in core as well as in terminal alkyl chain influence the result. Several observations moreover indicate that the anisotropic nematic phase, aligning the nanotubes in the matrix, per se stabilizes the dispersion compared to a host that is isotropic and thus yields random tube orientation. The chemical and physical phenomena governing the preparation of the dispersion and its stability are identified, taking into account enthalpic, entropic, as well as kinetic factors. This allows a guideline on how to best design and prepare CNT–LC composites to be sketched, following which tailored development of new LCs may take the advanced functional material that CNT–LC composites comprise to the stage of commercial application.