Fine-pitch carbon nanotube bundles assembly using CNT transfer for electrical interconnects

Carbon nanotubes (CNTs) have been proposed as electrical interconnects in microelectronic devices to address such problems as stress, electromigration, and heat removal. 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 circuitry is the high growth temperature (≫600°C). 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 propose 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 interconnect 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 μm/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 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. Due to the capillary force effects, the Sn/Pb show improved wetting properties on open-ended CNT films. It is desirable for CNT interconnect with metal electrode by solder reflow process. The CNT-solder 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 embodied in the low process temperature, adhesion improvement and the feasibility of transferring CNT bundles to different substrates for fine-pitch interconnect applications.