A survey of carbon nanotube interconnects for energy efficient integrated circuits

This article is a review of the state-of-art carbon nanotube interconnects for Silicon application with respect to the recent literature. Amongst all the research on carbon nanotube interconnects, those discussed here cover 1) challenges with current copper interconnects, 2) process & growth of carbon nanotube interconnects compatible with back-end-of-line integration, and 3) modeling and simulation for circuit-level benchmarking and performance prediction. The focus is on the evolution of carbon nanotube interconnects from the process, theoretical modeling, and experimental characterization to on-chip interconnect applications. We provide an overview of the current advancements on carbon nanotube interconnects and also regarding the prospects for designing energy efficient integrated circuits. Each selected category is presented in an accessible manner aiming to serve as a survey and informative cornerstone on carbon nanotube interconnects relevant to students and scientists belonging to a range of fields from physics, processing to circuit design

Elucidating Nucleation and Growth Behavior of Single-Walled Carbon Nanotubes obtained via Catalyzed Synthesis

The catalytic growth of single-walled carbon nanotubes (SWCNTs) is studied using reactive molecular dynamics (RMD) simulations and density functional theory (DFT) calculations. Computational calculations are performed in order to achieve a better understanding of the catalytic reaction mechanism at the initial stages of synthesis, where most of the structural characteristics are defined. Different process variables such as catalyst chemical composition and size, temperature, pressure, and the nature of catalyst support, can be optimized with the purpose of tuning the structure and physical properties of SWCNTs. Controlling the structure of SWCNTs during synthesis and avoiding additional purification and/or separation processes are critical for the direct use of SWCNTs in electronic devices. RMD simulations demonstrate that small catalyst particles favor the growth of lengthy nanotubes over catalyst encapsulation as a result of an increase of the curvature energies of the carbon capsule. Furthermore, simulations performed over deposited catalyst particles demonstrate that the catalyst-support adhesion must be controlled in order to grow nanotubes with high structural quality and avoid catalyst poisoning. Results herein reported suggest that growth conditions must be optimum to minimize the nucleation of topological defects in nanotubes. RMD trajectories prove the vital role played by the catalyst surface in healing defects via adsorption and diffusion. These results significantly impact the field of chirality control since the presence of defects introduce misorientation of hexagons, shifts the overall chiral angle, and therefore, modifies the physical properties of the nanotube. DFT calculations are employed to evaluate the interaction between SWCNTs and the ST-cut quartz substrate. The outstanding performance of CNT-based FET relies on the alignment of the horizontally grown nanotubes on silica substrates, as well as on the selective growth of semiconducting nanotubes. It is demonstrated that finite-length zigzag nanotubes are adsorbed stronger than armchair tubes on the quartz support. This suggests that the nanotube electronic band structure is a key factor on the preferential adsorption of zigzag tubes. DFT calculations suggest that patterns of unsaturated silicon atoms of silica surfaces define the crystallographic directions of preferential alignment. These patterns might be chemically altered in order to favor other directions of alignment

Mechanocapillary Forming of Filamentary Materials.

The hierarchical structure and organization of filaments within natural materials determine their collective chemical and physical functionalities. Synthetic nanoscale filaments such as carbon nanotubes (CNTs) are known for their outstanding properties including high stiffness and strength at low density, and high electrical conductivity and current carrying capacity. Ordered assemblies of densely packed CNTs are therefore expected to enable the synthesis of new materials having outstanding multifunctional performance. However, current methods of CNT synthesis have inadequate control of quality, density and order. In pursuit of these needs, a new technique called capillary forming is used to manipulate vertically aligned (VA-) CNTs, and to enable their integration in applications ranging from microsystems to macroscale functional films. Capillary forming relies on shape-directed capillary rise during solvent condensation; followed by evaporation-induced shrinkage. Three-dimensional geometric transformations result from the heterogeneous strain distribution within the microstructures during the vapor-liquid-solid interface shrinkage. A portfolio of microscale CNT assemblies with highly ordered internal structure and freeform geometries including straight, bent, folded and helical profiles, are fabricated using this technique. The mechanical stiffness and electrical conductivity of capillary formed CNT micropillars are 5 GPa and 104 S/m respectively. These values are at least hundred-fold higher than as-grown CNT properties, and exceed the properties of typical microfabrication polymers. Responsive CNT-hydrogel composites are prototyped by combining isotropic moisture-induced swelling of the hydrogel with the anisotropic stiffness of CNTs to induce reversible self-directed shape changes of up to 30% stroke. Centimeter scale sheets are fabricated by mechanical rolling and capillary assisted joining of CNTs. The mechanical stiffness, strength and electrical conductivity of CNT sheets are comparable to those of continuous CNT microstructures; and can be tuned by engineering the morphology of the CNT joints. Finally, the applicability of mechanocapillary forming to other nanoscale filaments is demonstrated using silicon nanowires synthesized by metal assisted chemical etching. Further work using the methods developed in this dissertation could enable applications such as directional liquid transport, adhesives, and biosensors; toward an end goal of creating multifunctional surfaces having arbitrary structural, interfacial, and optical responsiveness.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91466/1/stawfick_1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/91466/2/stawfick_2.pd

Horizontally-aligned carbon nanotubes arrays and their interactions with liquid crystal molecules: Physical characteristics and display applications

We report on the physical characteristics of horizonthally-grown Single-Walled Carbon Nanotubes (h-al-SWNT) arrays and their potential use as transparent and conducting alignment layer for liquid crystals display devices. Microscopy (SEM and AFM), spectroscopic (Raman) and electrical investigations demonstrate the strong anisotropy of h-al-SWNT arrays. Optical measurements show that h-al-SWNTs are efficient alignment layers for Liquid Crystal (LC) molecules allowing the fabrication of optical wave plates. Interactions between h-al-SWNT arrays and LC molecules are also investigated evidencing the weak azimuthal anchoring energy at the interface, which, in turn, leads to LC devices with a high pretilt angle. The electro-optical reponses of h-al-SWNT/LC cells demonstrate that h-al-SWNT arrays are efficient nanostructured electrodes with potential use for the combined replacement of Indium Tin Oxyde and polymeric alignment layers in conventional displays

Dry-transfer of chemical vapour deposited nanocarbon thin films

This thesis presents the development of chemical vapour deposited (CVD) graphene and multi-walled carbon nanotubes (MWCNTs) as enabling technologies for flexible transparent conductors offering enhanced functionality. The technologies developed could be employed as thin film field emission sources, optical sensors and substrate-free wideband optical polarisers. Detailed studies were performed on CVD Fe and Ni catalysed carbon nanotubes and nanofibres on indium tin oxide, aluminium and alumina diffusion barriers. Activations energies of 0.5 and 1.5 eV were extracted supporting surface diffusion limited catalysis for CNTs and CNFs. For the first time an activation energy of 2.4 eV has been determined for Cu-catalysed growth of CVD graphene. Graphene was shown to deviate significantly from the more traditional rate-limited surface diffusion and suggests carbon-atom-latticeintegration limited catalysis. An aligned dry-transferred MWCNT thin film fabrication technique was developed using MWCNTs of varied lengths to control the optical transparency and conductivity. A process based on the hot-press lamination of bilayer CVD graphene (HPLG) was also developed. Transport studies revealed that these thin films behave, in a macroscopic sense, similar to traditional c-axis conductive graphite and deviate toward tunnel dominated conduction with increasing degrees of network disorder. Various MWCNT-based thin film field emitters were considered. Solution processing was shown to augment the surface work function of the MWCNTs resulting in reduced turn-on electric fields. Integrated zinc oxide nanowires were investigated and were shown to ballast the emission, thereby preventing tip burn out, and offered lower than expected turn-on fields due to the excitation of a hot electron population. To obviate nearest neighbour electrostatic shielding effects an electrochemical catalyst activation procedure was developed to directly deposit highly aligned sparse carbon nanofibres on stainless steel mesh. Highly-aligned free-standing MWCNT membranes were fabricated through a solid-state peeling technique. Membranes were spanned across large distances thereby offering an ideal platform to investigate the unambiguous photoresponse of MWCNTs by removing all extraneous substrate interfaces, charge traps and nanotube-electrode Shottky barriers as well as using pure, chemically untreated material. Oxygen physisorbtion was repeatedly implicated through in-situ lasing and in-situ heated EDX measurements, FT-IR and lowtemperature transport and transfer measurements. A MWCNT membrane absorptive polariser was fabricated. Polarisers showed wideband operation from 400 nm to 1.1 μm and offered operation over greater spectral windows than commercially available polymer and glass-support dichroic films. Ab-initio simulations showed excellent agreement with the measured polarisation attributing the effect to long-axis selective absorption

Microwave Inter-Connections and Switching by means of Carbon Nano-tubes

In this work, carbon nanotube (CNT) based interconnections and switches will be reviewed, discussing the possibility to use nanotubes as potential building blocks for signal routing in microwave networks. In particular, theoretical design of coplanar waveguide (CPW), micro‐strip single‐pole‐single‐throw (SPST) and single‐pole‐double‐throw (SPDT) devices has been performed to predict the electrical performances of CNT‐based RF switching configurations. Actually, by using the semiconductor‐conductor transition obtained by properly biasing the CNTs, an isolation better than 30 dB can be obtained between the ON and OFF states of the switch for very wide bandwidth applications. This happens owing to the shape deformation and consequent change in the band‐gap due to the external pressure caused by the electric field. State‐of‐art for other switching techniques based on CNTs and their use for RF nano‐interconnections is also discussed, together with current issues in measurement techniques