Kyeong-Jae Lee

Fully Integrated Graphene and Carbon Nanotube Interconnects for Gigahertz High-Speed CMOS Electronics

Carbon-based nanomaterials such as metallic single-walled carbon nanotubes, multiwalled carbon nanotubes (MWCNTs), and graphene have been considered as some of the most promising candidates for future interconnect technology because of their high current-carrying capacity and conductivity in the nanoscale, and immunity to electromigration, which has been a great challenge for scaling down the traditional copper interconnects. Therefore, studies on the performance and optimization of carbon-based interconnects working in a realistic operational environment are needed in order to advance the technology beyond the exploratory discovery phase. In this paper, we present the first demonstration of graphene interconnects monolithically integrated with industry-standard complementary metal-oxide-semiconductor technology, as well as the first experimental results that compare the performance of high-speed on-chip graphene and MWCNT interconnects. The graphene interconnects operate up to 1.3-GHz frequency, which is a speed that is commensurate with the fastest high-speed processor chips today. A low-swing signaling technique has been applied to improve the speed of carbon interconnects up to 30%.

Breakdown Current Density of CVD-Grown Multilayer Graphene Interconnects

Graphene wires have been fabricated from large-area multilayer graphene sheets grown by chemical vapor deposition. As the methane concentration increases, a larger percentage of thicker graphene layers are grown. The multilayer graphene sheets have an average thickness of 10-20 nm with sheet resistances between 500 and 1000 Ω/sq. The sheet resistance shows a strong correlation with the average surface roughness. This letter reports measured breakdown current densities up to 4×107 A/cm2, where resistive heating is proposed as the main breakdown mechanism. Increasing the uniformity of the graphene layers is important in achieving a higher breakdown current density.

A Low Power Carbon Nanotube Chemical Sensor System

This paper presents a hybrid CNT/CMOS chemical sensor system that comprises of a carbon nanotube sensor array and a CMOS interface chip. The full system, including the sensor, consumes 32 muW at 1.83 kS/s readout rate, accomplished through an extensive use of CAD tools and a model-based architecture optimization. A redundant use of CNT sensors in the frontend increases the reliability of the system.

A 32-

This paper presents an energy-efficient chemical sensor system that uses carbon nanotubes (CNT) as the sensing medium. The room-temperature operation of CNT sensors eliminates the need for micro hot-plate arrays, which enables the low energy operation of the system. An array of redundant CNT sensors overcomes the reliability issues incurred by the CNT process variation. The sensor interface chip is designed to accommodate a 16-bit dynamic range by adaptively controlling an 8-bit DAC and a 10-bit ADC. A discrete optimization methodology determines the dynamic range of the DAC and the ADC to minimize the energy consumption of the system. A simple calibration technique using off-chip reference resistors reduces the DAC non-linearity. The sensor interface chip is designed in a 0.18-mum CMOS process and consumes, at maximum, 32 muW at 1.83 kS/s conversion rate. The designed interface achieves 1.34% measurement accuracy across the 10 kOmega-9 MOmega range. The functionality of the full system, including CNT sensors, has been successfully demonstrated.

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In this paper, we characterize the performance of monolithically integrated graphene interconnects on a prototype 0.35-μm CMOS chip. The test chip implements an array of transmitter/receivers to analyze the end-to-end data communication on graphene wires. Large-area graphene sheets are first grown by chemical vapor deposition, which are then subsequently processed into narrow wires up to 1 mm in length. A low-swing signaling technique is applied, which results in a transmitter energy of 0.3-0.7 pJ/b·mm-1 and a total energy of 2.4-5.2 pJ/b·mm-1. Bit error rates below 2 × 10-10 are measured using a 231 - 1 pseudorandom binary sequence. Minimum voltage swings of 100 mV at 1.5-V supply and 500 mV at 3.3-V supply have also been demonstrated. At present, the graphene wire is largely limited by its growth quality and high sheet resistance.

W 1.83-kS/s Carbon Nanotube Chemical Sensor System

We have demonstrated a subthreshold FPGA system using monolithically integrated graphene wires. The graphene wires replace double-length lines in the interconnect fabric of a custom FPGA implemented in 0.18-μm CMOS. The four-layer graphene wires have lower capacitance than the CMOS aluminum wires, resulting in up to 2.11× faster speeds and 1.54× lower interconnect energy when driven by a low-swing voltage of 0.4 V. This paper presents the first graphene-based system application and experimentally demonstrates the potential of using low-capacitance graphene wires for ultralow power electronics.

Low-Swing Signaling on Monolithically Integrated Global Graphene Interconnects

This paper presents an energy efficient chemical sensor system that uses carbon nanotubes (CNT) as the sensor. The room-temperature operation of CNT sensors eliminates the need for micro hot-plate arrays, which enables the low energy operation of the system. The sensor interface chip is designed in a 0.18 mum CMOS process and consumes, at maximum, 32 muW at 1.83 kS/s conversion rate. The designed interface achieves 1.34% measurement accuracy over 10 kOmega -9 MOmega dynamic range. The functionality of the full system, including CNT sensors, has been successfully demonstrated.

Demonstration of a Subthreshold FPGA Using Monolithically Integrated Graphene Interconnects

SWNTs possess unique properties that make them excellent candidates for sensing technology. Because the properties of a SWNT depend sensitively on its structure and because a SWNT is composed entirely of surface atoms, a slight variation of its environment tends to have a noticeable effect on its properties. Many types of sensors have been demonstrated using nanotubes, such as chemical [1–3], biological [4], flow [5], strain [6], pressure [7, 8], thermal [9], and mass [10] sensors. In this chapter, we will focus on the chemical sensing using SWNT FET devices. Chemical sensors based on individual SWNTs were first demonstrated in year 2000 [11, 12]. The devices were constructed in the field effect transistor scheme. In Ref. [11], a constant bias was applied between the source and the drain electrodes and the current of the SWNT was monitored while gas molecules were introduced into the chamber. It was found that the electrical conductance of a semiconducting SWNT dramatically increases and decreases upon exposure to gaseous molecules of NO2 and NH3, respectively, as shown in Fig. 8.1. These SWNT sensors have many advantages, including fast responses, small sizes (therefore high packing density), high sensitivities (sub-ppm levels) and room-temperature operation, etc. As a result, there has been tremendous interest in using SWNT FETs as chemical and biological sensors. Two different sensing mechanisms have been proposed in literature. The first one suggests the charge transfer between the SWNTs and the analyte molecules that adsorb on the SWNT surface [13], which then gives rise to the carrier density, and thus channel conductance change. The second mechanism resorts to the modification of the Schottky barriers (SB) at the metal-SWNT contacts due to the adsorption of molecules on both the metal and the SWNT. Depending on the specific analyte and the contact material, reports vary on which effect dominates the