Thermally Aware Performance Analysis of Mixed Carbon Nanotube Based VLSI Interconnects

Abstract

Nowadays, excessive technology scaling in the deep sub-micron (DSM) regime allows the integration of immense functions on a single chip. As a result of excessive scaling, copper-based on-chip interconnect performance degrades, thus necessitating the search for an alternative suitable material for future nano interconnects. For the next generation of interconnects, the potential material should be capable of manifesting minimized propagation delay, power dissipation, and the effects of functional as well as dynamic crosstalk in the coupled interconnects. The current work aims to find the best material possible for making future generation interconnects which provide better performance by eliminating coupling effects between adjoining interconnects. The most befitting material has turned out to be carbon-based interconnects such as CNT and GNR. In the present work, a realistic CNT bundle such as mixed CNT (m-CNT) bundle made up of both single wall carbon nanotube (SWCNT) and multi wall carbon nanotubes (MWCNT) with different placements is considered for performance analysis under temperature as an obligatory component. A temperature-dependent crosstalk model based on ABCD parameter matrix along with decoupling technology, with application of driver interconnect load (DIL) in coupled m-CNT bundle (MCB) interconnects is taken. A study is carried out at 14nm technology node to examine the effects of irradiation-induced defects on MCB interconnects by considering tunneling and inter-shell capacitances in an equivalent capacitance model over a temperature range from 300K to 500K. In addition, the performance of capacitively coupled interconnects of MCB has also been discussed and compared with copper interconnects in terms of propagation delay, power dissipation, crosstalk induced noise voltage, and frequency spectrum. The propagation delay of the four different structures of MCB, e.g. S1, S2, S3 and S4 (aka ST-1, ST-2, ST-3 and ST-4, increases with defects. Propagation delay is appreciably higher when crosstalk effects are taken into consideration. For the entire temperature range, the crosstalk-induced peaks and time durations of victim output pulses in copper are higher than those of all other structures of MCB. A temperature-dependent frequency spectrum of MCB and copper are also analyzed at 14nm technology node. Under this analysis, the time domain signal obtained at the far end of single interconnect and at the far end of victim line in capacitively coupled interconnects are converted into frequency domain to study the band width and crosstalk induced noise voltage, respectively. The effects of temperature variation on noise amplitudes are explore and it showed that the frequency amplitude decreases with increase in temperature which means that the loss in signal power is more at higher temperatures. The structure-4 of MCB interconnect (also known as S4 or ST-4) experiences more loss in signal power when temperature is raised from 300K to 500K. Also, there is decrease in the bandwidth with rise in temperature which is due to increase in resistance. It is also noted that crosstalk induced noise amplitude is becoming larger with rise in temperature. This implies that filtering capability of noise is going to decrease with higher temperatures. Moreover, dielectric surface roughness scattering induced crosstalk performance of coupled MCB interconnects is also done, where, the most typical and consistent dielectric materials like vi silicon dioxide (SiO2), boron nitride (BN) and silicon carbide (SiC) are taken into account and analyzed w.r.t. mean free path(MFP) created by surface roughness scattering. A complete crosstalk (dynamic & functional) effect in terms of positive peaks, time duration and area of rise glitch is studied. Frequency spectrum and bode stability analysis is also done for all four structures of MCB. Structure-4 (also known as S4 or ST-4) placed on SiC substrate material provides the best results in terms of positive peaks, time duration, and area of rise glitches among all the structures in the MCB. The bode stability of S4 placed on SiC interconnects are more stable and have larger phase margin, gain margin and 3-dB frequency bandwidth in comparison to S4 placed on BN and S4 placed on SiO2 with the same dimensions. Moreover, MCB interconnects placed on smooth substrate surface have larger 3-dB frequency bandwidth and smaller delay times than those of MCB placed on rough substrate surfaces over a temperature range (300K-500K). Also, for the S4, an increase in surface roughness parameter (𝞭SR) causes a reduction in frequency bandwidth and a 50% rise time of the transient response at the far end of an aggressor line. The work presented in this thesis demonstrates that temperature, tunneling effect, inter-shell capacitance, defects, and dielectric surface roughness scattering play an important role in studying the performance and reliability of MCB interconnects in DSM regime.