Interconnects plays an important role in integrated circuits. Copper is used as an interconnect material, but beyond 22[Formula: see text]nm technology node it faces many problems due to grain boundary scattering, and therefore carbon nanotubes are the most promising future interconnect materials. Various techniques and approaches such as driver sizing, repeater sizing, repeater insertion, wire sizing, wire spacing, shielding, boos table repeater were used by various researchers. Many of these techniques can be utilized for future CNT based VLSI interconnects as well. This paper presents a detailed discussion on the techniques and approaches of past, present and future relevant for interconnects of VLSI circuits.

Partha Sharathi Mallick

Department of Instrumentation

School of Electrical Engineering

Vellore Campus

Fulltext

Vellore Institute of Technology (VIT) is a private university located in&nbsp;Tamil Nadu, India. Founded in 1984, as Vellore Engineering College, the institution offers 20 undergraduate, 34 postgraduate, four integrated and four research programs. It has campuses in Vellore, Amravati, Bhopal and Chennai.

VIT is one of the top ranked private universities in India according to NIRF, THE and QS Rankings.&nbsp;Govt. of India has recognized&nbsp;VIT, Vellore as an&nbsp;Institution of Eminence. This has allowed VIT to take independent quality initiatives and move up in world ranking.

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VIT University

Optimization Techniques for CNT Based VLSI Interconnects — A Review

Journal of Circuits, Systems and Computers

This paper proposes novel triangular cross-sectioned geometry of carbon nanotube (CNT) bundles for crosstalk and delay reduction in CNT bundle interconnects for VLSI circuits. First, we formulate the equivalent single conductor (ESC) transmission line models of the interconnects. Through SPICE analysis of the ESC circuits, we find the propagation delays of the proposed CNT bundles. Next, we model the capacitively coupled interconnects for crosstalk analysis. It is found that the coupling capacitance of triangular CNT bundle is 29% lesser than the traditionally used square CNT bundles. Further, the crosstalk-induced delay of triangular interconnects is found to be 30% lesser when compared to square bundle interconnects. The reduction in delay is found to increase as the number of CNTs in the bundle increases. So, we suggest that triangular CNT bundles are the most suitable candidates as global interconnects.

Geometry-Based Crosstalk Reduction in CNT Interconnects

Sub-threshold voltage operated circuits are the future for ultra-low-power applications. These circuits are inherently slow due to the very small sub-threshold currents. Here, the authors propose two approaches for improving the speed of SWCNT bundle interconnects driven by CNTFET-based circuits under sub-threshold conditions. First, the authors modulate the channel length of the CNTFETs that are used in the driver circuits to increase sub-threshold output current. The output current is maximum when the channel length is optimised to 15 nm. Second, the authors design driver circuits made of CNTFET-based inverters and transmission gates for SWCNT bundle interconnects at sub-threshold voltages. The authors consider five different configurations of the driver and load circuits. SPICE simulations show that transmission gates play a vital role in driver circuits by reducing the propagation delay and increasing the switching speed at high frequencies. Finally, the authors perform temperature-dependent analysis of the best cases from the proposed circuits and show that the propagation delay and power dissipated by them increases drastically at increased temperatures up to 500 K.

IET Circuits, Devices & Systems

High-speed sub-threshold operation of carbon nanotube interconnects

Current mode signaling (CMS) is found to be advantageous than voltage mode signaling (VMS) in VLSI interconnects. So, we propose the use of triangular carbon nanotube bundles as VLSI interconnects for CMS in VLSI circuits. T-CNT bundles have inherent advantages of lesser propagation delay and crosstalk delay than traditionally proposed square CNT bundle interconnects. We analyze the propagation delays of both the types of bundles in voltage and current mode signalling schemes and compare the improvement of propagation delay in both the cases. We find that T-CNT bundles perform better than square bundles in both the signalling schemes. © 2019 IEEE.

2019 Innovations in Power and Advanced Computing Technologies (i-PACT)

Analysis of triangular CNT bundle interconnects for current mode signalling

This paper proposes a body-biased bootstrapped CMOS driver for subthreshold applications. The proposed driver has been implemented with the same number of transistors as conventional bootstrapped CMOS driver. The performance of the subthreshold bootstrapped CMOS driver has been compared with the conventional bootstrapped CMOS driver. Our results show that the proposed body-biased subthreshold bootstrapped CMOS driver has 37% reduction in delay and 39% reduction in power dissipation compared to conventional bootstrapped CMOS driver. The proposed driver is more suitable to drive large loads compared to the conventional driver and operates better at subthreshold region.

Body-Biased Subthreshold Bootstrapped CMOS Driver

In this letter, we present a new method to reduce crosstalk of carbon nanotube (CNT)-based VLSI interconnects. For this, proper integration of semiconducting CNTs (s-CNTs) and a new contact geometry, where metallic CNTs are in the core and s-CNTs are in the periphery of the CNT bundle, is proposed. The coupling capacitance between adjacent interconnects is modeled and compared with and without s-CNTs in the CNT bundle periphery. SPICE analysis and EM simulations are carried out which show that the coupling capacitance can be reduced by 82.5% and the resulting delay by 8.41%. We suggest that the crosstalk effect between neighboring wires can be reduced by using s-CNTs in the design.

IEEE Transactions on Nanotechnology

Role of Semiconducting Carbon Nanotubes in Crosstalk Reduction of CNT Interconnects

This article presents a realistic inter-carbon nanotube (CNT) electrostatic coupling capacitance and tunnelling conductance model for a mixed CNT bundle. The change of potential across such a bundle necessitates the need to consider the inter-CNT capacitance in the equivalent circuit of CNT interconnects for very large scale integration circuits. The equivalent transmission line circuit model of a unit bundle containing one single-walled CNT (SWCNT) and one multi-walled CNT (MWCNT) has been shown. This new model is then used to calculate the delay induced by the inter-CNT capacitance and tunnelling conductance, which predicts the relative positioning of MW/SWCNTs in mixed CNT bundle.

International Journal of Electronics

Inter-CNT capacitance in mixed CNT bundle interconnects for VLSI circuits

Carbon nanotube based interconnect technology is becoming popular for its various merits over the current copper technology. The ability to conduct high current at high temperatures through the 1D structure is making the CNT interconnect technology an attractive one. Single wall and multi wall CNT interconnects have received greater interest initially when the mixed CNT bundles (MCBs) did not get much attention as VLSI interconnects. But, it was only in 2007 when the conductance study of mixed CNT bundles was done and since then many works have been reported. At this stage, it is essential to review the reports of MCB based interconnect technology. We present in this paper, the first review on MCB VLSI interconnects.

Nano Communication Networks

Towards realisation of mixed carbon nanotube bundles as VLSI interconnects: A review

Presented for the first time is an accurate modelling hierarchy for mixed CNT bundle interconnects. Single-walled CNTs and multi-walled CNTs have been modelled as equivalent single conductor transmission lines and then combined to form a mixed CNT bundle interconnect, which is basically a multiple equivalent single conductor model. Two multiple equivalent single conductor interconnects have been taken to form the multiconductor transmission line model. The delays from transient analysis for both models for a unit bundle have been compared with the corresponding ones for SWCNT bundles and MWCNTs that exist in the literature. It is found that mixed CNT bundle interconnects are superior to both SWCNT bundle and MWCNT interconnects in terms of delay.

Journal	Journal of Circuits, Systems and Computers
Publisher	World Scientific Pub Co Pte Lt
ISSN	0218-1266
Open Access	No