Vertically aligned carbon nanotubes on copper substrates for applications as thermal interface materials: From synthesis to assembly

One promising application of CNTs in microelectronics is to use vertically aligned CNT (VACNT) arrays as novel thermal interface materials (TIMs). No doubt that the vertical alignment makes the best of the extremely high longitudinal thermal conductivity of individual CNTs; however, it is the CNT/substrate interface that exerts the main restriction on phonon transport through a TIM layer. There are two VACNT TIM/substrate interfaces in a typical TIM assembly: the VACNT/growth substrate interface and the VACNT/mating substrate interface. In terms of the growth substrate, given the high temperature required (>700degC) for VACNT syntheses, e.g. by chemical vapor deposition (CVD), direct synthesis of a VACNT TIM layer on the backside of a silicon chip is not compatible with current electronic packaging systems. Instead, VACNT synthesis on the copper lid surface is preferred. Researchers have reported successful VACNT syntheses on some metal surfaces, however, with very limited success on copper substrates, especially based on common thermal CVD processes. In this study, we deliver a remarkable progress on fast synthesis of high-quality VACNTs on copper substrates, based on a common thermal CVD process, by introducing a well-controlled conformal alumina layer as the support layer on the bulk copper substrate. It is the fundamental understanding of the role by the support layer that leads to the successful synthesis of VACNT on the bulk copper in our study. Raman spectroscopy and scanning electronic microscopy showed well aligned CNTs of high quality. The key role of the support layer was discussed. As for reducing the contact thermal resistance at the VACNT/growth substrate interface, i.e. chip backside, chemical anchoring was proposed and developed. A right chemical compound was chosen to covalently bond the VACNTs to the Si substrate surface and, more importantly, to serve as ldquomolecular phonon couplersrdquo at the contact interface to enhance interfacial therma- l transport. Experimental results indicated that such an interface modification improved the effective thermal diffusivity of the carbon nanotube-mediated thermal interface by an order of magnitude and conductivity by two orders of magnitude. The interfacial adhesion was dramatically enhanced as well, which is significant for reliability improvement of the TIMs. This remarkable breakthrough undoubtedly provides a viable commercial VACNT application for thermal management in microelectronic and photonic packaging, and opens up a new field in the design of CNT/substrate interfaces.