Azad Naeemi

Performance comparison between carbon nanotube and copper interconnects for gigascale integration (GSI)

Physical models are used to determine the ultimate potential performance of carbon nanotube interconnects and compare them with minimum-size copper wires implemented at various technology generations. Results offer important guidance regarding the nature of carbon nanotube technology development needed for improving interconnect performance. Since wave propagation is slow in a single nanotube, nanotube bundles with larger wave speeds must be used. At the 45-nm node (year 2010), the performance enhancement that can be achieved by using nanotube bundles is negligible, and at the 22-nm node (year 2016) it can be as large as 80%.

Compact physical models for multiwall carbon-nanotube interconnects

Compact physical models are derived for conductivity of multiwall carbon-nanotube (MWCN) interconnects. It is proven that for MWCNs shorter than the critical length (typically around 7 /spl mu/m), the conductivity decreases as diameter increases, whereas for MWCNs longer than the critical length, increasing the diameter results in higher conductivities. For long lengths (hundreds of micrometers), MWCNs can potentially have conductivities several times larger than that of copper or even single-wall carbon nanotube (SWCN) bundles. For short lengths (<10 /spl mu/m), however, SWCN bundles offer more than two times higher conductivities compared to MWCNs.

Design and Performance Modeling for Single-Walled Carbon Nanotubes as Local, Semiglobal, and Global Interconnects in Gigascale Integrated Systems

Based on physical models, distributed circuit models are presented for single-walled carbon nanotubes (SWCNs) and SWCN bundles that are valid for all voltages and lengths. These models can be used for circuit simulations and compact modeling. It is demonstrated that by customizing SWCN interconnects at the local, semiglobal, and global levels, several major challenges facing gigascale integrated systems can potentially be addressed. For local interconnects, monolayer or multilayer SWCN interconnects can offer up to 50% reduction in capacitance and power dissipation with up to 20% improvement in latency if they are short enough (<20 mum). For semiglobal interconnects, either latency or power dissipation can be substantially improved if bundles of SWCNs are used. The improvements increase as the cross-sectional dimensions scale down. For global interconnects, bandwidth density can be improved by 40% if there is at least one metallic SWCN per 3-nm2 cross-sectional area

Conductance Modeling for Graphene Nanoribbon (GNR) Interconnects

Graphene nanoribbons (GNRs), which are single graphene sheets, share many of the fascinating electronic, mechanical, and thermal properties of carbon nanotubes. Compact physical models for conductance of GNRs as functions of chirality, width, Fermi level, and the type of electron scatterings at the edges are presented. For widths below 8 nm, the models demonstrate that single-layer GNRs can potentially outperform copper wires with unity aspect ratio

Compact Physics-Based Circuit Models for Graphene Nanoribbon Interconnects

Physics-based equivalent circuit models are presented for armchair and zigzag graphene nanoribbons (GNRs), and their conductances have been benchmarked against those of carbon nanotubes and copper wires. Atomically thick GNRs with smooth edges can potentially have smaller resistances compared with copper wires with unity aspect ratios for widths below 8 nm and stacks of noninteracting GNRs can have substantially smaller resistivities compared to Cu wires. It is shown that rough edges can increase the resistance of narrow GNRs by an order of magnitude. This fact highlights the need for patterning methods that can produce relatively smooth edges to fabricate low resistance GNR interconnects.

Performance Modeling for Single- and Multiwall Carbon Nanotubes as Signal and Power Interconnects in Gigascale Systems

Using physics-based circuit models, the performances of carbon nanotube (CNT) interconnects, both single- and multiwall (SWNT and MWNT), are benchmarked against their copper counterparts at a realistic operating temperature (100degC). The models used capture various electron phonon scattering mechanisms and the dependence of quantum conductance on temperature and diameter. It is demonstrated that any performance comparison between CNT and copper wires needs to be done at realistic temperatures because changes in temperature affect copper and CNT interconnects quite differently. The results of this paper demonstrate that a hybrid system of copper/SWNT/MWNT offers the highest performance enhancement for interconnects.

Physical Modeling of Temperature Coefficient of Resistance for Single- and Multi-Wall Carbon Nanotube Interconnects

Equivalent circuit models are presented for the resistance of single- and multi-wall carbon nanotubes (MWCNs) that capture various electron-phonon scattering mechanisms as well as changes in the number of conduction channels as a function of temperature. For single- and few-wall nanotubes, the temperature coefficient of resistance (TCR) is always positive and increases with length. It reaches 1/(T-200 K) for lengths much larger than the electron mean free path, where T is the temperature in kelvin. For MWCNs with large diameters (>20 nm), TCR varies from -1/T to +0.66/(T-200 K) as the length varies from zero to very large values

Monolayer metallic nanotube interconnects: promising candidates for short local interconnects

Mono- or bi-layer metallic single-wall carbon nanotube interconnects have lateral capacitances more than four times smaller than those of copper interconnects. The resistance and time-of-flight of these monolayer nanotubes would be larger than that of copper interconnects. For short lengths, however, driver resistance is quite dominant, and latency is determined by interconnect capacitance. Monolayer nanotube interconnects are therefore promising candidates for local interconnects. The average capacitance per unit length of these nanotube interconnects can be 50% smaller than that of copper interconnects and that leads to significant saving in power dissipation.

3D heterogeneous integrated systems: Liquid cooling, power delivery, and implementation

This paper describes a novel 3D integration technology that enables the integration of electrical, optical, and microfluidic interconnects in a 3D die stack. The electrical interconnects are used to provide power delivery and signaling, the optical interconnects are used to enable optical signal routing to all levels of the 3D stack, and the microfluidic interconnects are used to cool each level in the 3D stack and thus enable stacking of high-performance (high-power) dice. These interconnects are integrated in a 3D stack both as through-silicon vias (TSVs) and as input/output (I/O) interconnects. Design trade-offs (TSV density, power supply noise, thermal resistance, and pump size), fabrication, and assembly are reported.

Impact of electron-phonon scattering on the performance of carbon nanotube interconnects for GSI

While electron mean-free path in carbon nanotubes can be as large as several micrometers for small bias voltages, for large biases electrons get backscattered by optical and zone-boundary phonons and nanotube resistance can increase by more than 100 times. This letter reveals this kind of backscattering has a small impact (error <25%) in most interconnect applications of carbon nanotubes in which adequate numbers of nanotubes are connected in parallel. This is mainly due to relatively small electric fields along nanotubes when they are used as interconnects. This is in sharp contrast with transistor applications of carbon nanotubes in which transconductance degrades considerably by electron-phonon scatterings unless their channels are made ultrashort (/spl sim/10 nm).