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Signal Integrity Comparison between Copper and CNT-based TSVs
Keywords: Signal integrity, Carbon nanotube, TSV
TSV technology has presented itself as a viable solution to rapidly emerging 3D integration challenges [1]. “More than Moore” approach concurrent with demands of higher clock frequency and gigahertz operation from technology market, such as cellphones, computers and tablets, mandates novel IC design architectures with TSVs [2]. However, identification of new materials for TSV implementation and fabrication is necessitated in addition to exhaustive research on design and test of these TSV structures at such high frequencies. Therefore, this research focuses on a comparative analysis of signal integrity performance of conventional Cu-TSV against a Carbon Nanotube (CNT)-based TSV. We have developed an integrated model of CNT-based TSV by taking into consideration quantum effects of CNTs along with kinetic inductance of CNT-based TSV, which is a crucial factor at high frequencies. Resonant line technique [3], using two-polynomial equation, was used to evaluate losses in reactive components at high frequencies. We have compared the electrical performance and signal integrity of conventional Cu TSVs with CNT-based TSVs. We have considered different via dimensions and distance of separation between vias in our case studies and we also varied the number of conducting CNTs by varying their density inside vias. We performed S-parameter simulation to validate the electrical performance at high frequency range up to 10 GHz. The height of the TSV is 10 µm, the diameter is 3 µm, diameter of CNT is 1 nm and the pitch is 15 µm. For this geometry, the conventional Cu TSV has an insertion loss of -14.2 dB and an output transmission of -5.22 dB, while the CNT-based TSV with a density of 18,761 bundles of SW-CNTs has an insertion loss of -16.96 dB and an output transmission of -3.69 dB. We then varied TSV dimensions with height of TSV being changed to 15 µm, the diameter to 6 µm. The diameter of CNT used was also changed to 2 nm. With a density of 7,346 bundles of SW-CNTs, this CNT-TSV configuration has an insertion loss of –24.98 dB and an output transmission of –1.53 dB, while the conventional Cu TSV has an insertion loss of -11.7 dB and an output transmission of -2.87 dB. This is indicative of CNT-based TSVs having very low insertion loss and high transmission rates. We performed eye diagram analysis of the order of 15 Gbps for large volume transmission to exploit high bandwidth. The eye diagram analysis, for first case presented above, shows CNT-based TSVs have a high density eye with an eye height of 0.9 and eye width of 60 psec with very low distortions and high rate of transmission. However, the conventional Cu TSV has an eye height of 0.48 with an eye width of 42 psec, which indicates that CNT-based TSVs show better performance when compared with conventional Cu TSVs. Based on results presented above it can be seen that high signal throughput in CNT-based TSV is obtained as compared to Cu-TSV. Optimization of modeling parameters, such as geometry of via, CNT density and data rates, for frequency of up to 10 GHz was performed.
Bruce Kim, Associate Professor
City University of New York
New York, NY

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