CARBON NANOTUBES (A SOLUTION FOR IC INTERCONNECT) By G. Abhilash 10H61D5720

Introduction:  As the process technology scales into the nanoscale regime, the impact of onchip communication on performance and reliability continues to increase.  As the interconnect performance depends on both wire and driver transistor characteristics, alternative interconnect and device technologies must be investigated for onchip communication in future integrated circuits.  Increasing resistivity, rising demands on current density and problems due to electromigration of copper interconnects at nanoscale regime, are driving the need for CNT’s as interconnects.

IC Interconnects  Interconnects in Integrated Circuit distribute Clock and other signals and provide power/ground to the various integrated circuits.  The interconnect in an integrated circuit becoming the dominant factor in determining system performance and power dissipation. Interconnects are three types:  Local Interconnects Connecting gates and transistors with in a functional block.  Intermediate Interconnects are provides clock and signal distribution with in a functional block.  Global interconnects provides clock and signal distribution between the functional blocks and deliver power/ground to all functions.

Disadvantages of Using Cu Interconnects:  The traditional copper interconnects will suffer from significant increase in resistivity and from electromigration problems due to lower current densities supported by the copper conductor.  The increase in resistivity leads to increase in propagation delay of the signal.  The standard copper (Cu) interconnect will become a major hurdle for onchip communication due to high resistivity and electromigration.  Researches have proved that in copper, burn out occurred at current densities 80 MA/cm2.

CNT’s as Interconnects  Carbon nanotubes (CNTs) have been proposed as possible replacements for copper interconnect due to their large conductivity and current carrying capabilities.  Carbon Nanotubes are allotropes of carbon with a cylindrical nanostructure.  In 1991 Carbon nanotubes (CNTs) were discovered by Sumio Lijima.  They are mainly classified into two types. SWNTMWNTGraphene

Properties of CNTs  Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. Tensile strength of 63 gigapascals (GPa).  Nanotubes are very good thermal conductors, Low resistivity (~1 μΩ-cm), High current carrying capacity.  The temperature stability of carbon nanotubes is estimated to be up to 2800 °C in vacuum and about 750 °C in air.  Due to nanoscale dimensions, electrons propagate only along the tube's axis. Hence, carbon nanotubes are One-Dimensional.  CNT’s have large current density(10^10A/cm2). which is 1000 times more than copper.

Modeling of Carbon Nanotube Interconnects Consider a system of parallel carbon nanotubes as shown. Radius of Nanotube = r Separation between nanotubes = d Length of Nanotube = l R = Resistance L = Kinetic Inductance Cc= Coupling Capacitance Cq = Quantum Capacitance RLC Model Resistance: By Ji-Yong Park have measured the dc resistance of CNT’s of diameter 1.8 nm. In the incoherent limit for four channels of conduction, the differential resistance of a Nanotube of length (l) is given by for l >for l < Where = Mean free path(mfp) h = Plank’s constant e = Electronic charge

Inductance: For one dimensional CNT conductor the kinetic inductance dominates magnetic inductance. Kinetic inductance(L k ) is V f = Fermi velocity of CNT(~8x10^5 m/s) Capacitance: For CNT interconnects laid down in parallel, two distinct capacitances are imported, they are coupling capacitance, quantum capacitance.

Other Applications of CNTs  The joining of two carbon Nanotubes with different electrical properties to form a diode has been proposed.  CNT based Ultra-Capacitors have high energy density, Capacitances up to 5000 Farads can be achieved.  CNT based Transistors work at room temperature and are capable of fast digital switching using a Single Electron.  CNT sponge can filter any toxic sludge from water. As CNT sponge is Hydrophobic, it absorbs everything but water.

Conclusion Inspite of the high current density, CNT interconnects suffer from very high intrinsic resistance and kinetic inductance and severely limits high frequency of operation. Performance can be improved by routing the same signal through parallel CNTs. It has been shown that it requires a substantial increase in the number of metal layers to meet the performance of copper in the same die area. Thus CNTs provide reliable interconnect solutions but are limited in terms of performance in the high performance digital VLSI.

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