Assembling Carbon Nanotube Bundles Using Transfer Process for Fine-Pitch Electrical Interconnect Applications

As integrated circuit (IC) performance increases, many technical challenges appear in the areas of current-carrying capacity, thermal management, I/O density, and thermal-mechanical reliability. To address these problems, the use of aligned carbon nanotubes (CNTs) has been proposed in IC packaging as electrical interconnects and thermal interface materials. For electronic device and packaging applications, chemical vapor deposition (CVD) methods are particularly attractive due to characteristic CNT growth features such as selective spatial growth, large area deposition capability and aligned CNT growth. However, the CVD technique suffers from several drawbacks. One of the main challenges for applying CNTs to the circuitry is the high growth temperature (>600degC). Such temperatures are incompatible with microelectronic processes. To fabricate microelectronics devices that incorporate CNT blocks, the CNTs should be selectively positioned and interconnected to other materials such as metal electrodes or bonding pads. However, the adhesion between CNTs and the substrates is usually very poor, 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 "CNT transfer technology". 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. The open-ended multi-walled CNT arrays could carry higher current density than close-ended CNTs, since the internal walls can participate in the electrical transport. We for the first time developed an in-situ process to grow well-aligned CNT bundles by water-assistant selective etching. The process is very efficient, with CNT growth rate of 80 mum/min. 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 mum, 4, and 80 mum, respectively, were assembled on the copper substrates. The measured resistivity of the long CNTs is ~2.3times 10^-4 Omega-cm. The CNT-solder alloy interfaces were analyzed by the SEM. The results indicated that molten SnPb solder form strong mechanical bonding with CNTs. Overall, the advantages of CNT transfer technology are : low process temperature, improved adhesion and the feasibility of transferring CNT bundles to different substrates for fine-pitch interconnect applications.