Assembly of Fine-Pitch Carbon Nanotube Bundles for Electrical Interconnect Applications
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Assembly of Fine-Pitch Carbon Nanotube Bundles for Electrical Interconnect Applications Lingbo Zhu1,2, Dennis W. Hess1, and ChingPing Wong2 1 School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, 30332 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332
ABSTRACT Carbon nanotubes (CNTs) have been proposed as electrical interconnects, due to their excellent properties. However, the current nanotube growth techniques suffer from several drawbacks. One of the main challenges for applying CNTs to the circuitry is the high growth temperature (>600∞C). Such temperatures are incompatible with microelectronic processes. The other issue is the poor adhesion between CNTs and the substrates, which will result in long term reliability issues and high contact resistance. To overcome these disadvantages, we have successfully demonstrated a methodology that we term ìCNT transfer technologyî. The distinctive CNT-transfer-technology features are separation of CNT growth and CNT device assembly at solder reflow temperature. In this paper, we combined our expertise in growth of well-aligned open-ended CNT bundles with the CNT transfer process to assemble CNT bundles for fine-pitch interconnects applications. To demonstrate the feasibility of transfer process to assemble the fine-pitch CNT bundles, the CNT bundles with diameter, aspect-ratio and pitch of 25 µm, 4, and 80 µm, respectively, were assembled on the copper substrates. The measured resistivity of the long CNTs is ~2.3◊10-4 Ω-cm. The CNT-solder alloy interfaces were observed by the SEM. The results indicated that molten SnPb solder form strong mechanical bonding with open-ended CNTs, suggesting the superior CNT/solder interfacial properties by solder reflow process. INTRODUCTION For applications of the nanotubes in microelectronics, the most interesting features are the ballistic transport of electrons and the extremely high thermal conductivity along the tube axis [1]. Metallic CNTs show ballistic conductivity at room temperature [2]. Electrons transport ballistically (without scattering) in metallic single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs) over reasonable lengths (~1µm), thereby enabling CNTs to carry very high currents (> 109 A/cm2) without electromigration failure [3]. Phonons also propagate easily along the nanotubes [3]. The measured thermal conductivity of an individual MWNT at room temperature is >3000 W/m∑K [4], which exceeds the conductivity of diamond (2000 W/m∑K). Based on these advantageous properties of CNTs, researchers have reported the integration of CNTs into electrical interconnect applications [5-8]. The high resistance of an individual CNT [9, 10] indicates that an array of thousands of parallel CNTs will be necessary for interconnects applications. Especially, recent studies have demonstrated that the internal walls of MWNTs can participate in electrical transport, thereby enabling large current-carrying capacity [11]
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