Assembling Carbon Nanotube Films as Thermal Interface Materials

As the feature size of integrated circuits (ICs) decreases and the IC clock frequency and transistor density per chip increase, the power density increases exponentially. As such, heat generation has become a critical issue in advanced high performance ICs in recent years. The current thermal solutions will soon be inadequate for heat dissipation. New thermal management strategies and high thermal conductivity materials are urgently needed for electronic thermal management within the chip and surrounding packaging. Carbon nanotubes (CNTs), which are reported to be the most thermally conductive material (>3000 W/K.m), are a promising candidate for thermal management in microelectronics. However, several existing technical barriers have restrained their application of CNTs in microelectronic devices. One of the main challenges is that the high temperature required to grow high quality CNTs (>600degC) which is too high to be compatible with back-end microelectronic fabrication processes. Another major obstacle is the poor adhesion between CNTs and substrates, which results in high interface thermal resistance and poor long term reliability. To address these challenges, we proposes to use a novel CNT transfer process, which features separated steps of an in-situ open-ended CNT synthesis and a low-temperature CNT assembly to engineer the well-aligned open-ended CNT architectures for microelectronic thermal management. In-situ growth of high density open-ended CNTs with vertical alignment was first developed in our laboratory. To successfully achieve CNT assembly with good thermal performance, the following issues should be addressed: (1) Appropriate metals/solders should be selected to form good thermal and electrical coupling with CNTs. (2) The wetting of solder metal alloys in CNT channels and on the CNT outer walls by capillary force should be further clarified. (3) An appropriate solder reflow process should be developed to guarantee good wetting on CNTs. Pre- liminary results show that the transferred open-ended CNT structures can have very strong adhesion to the substrate, which promise to improve the CNT-metal interface properties. Initial thermal measurements of the CNT assembly with CNT height of ~180 mum show that the thermal conductivity and total thermal resistance of the assembly were 81 W/m.K and 0.43 cm² K/W, respectively. The work on the improvement of thermal performance of the CNT assembly is ongoing, including the interfacial solder layer thickness optimization and material selection, and CNT film quality.