Toward Carbon Nanotube Based Thermal Interface Materials
Authors: Siegal Michael P. , Rochford Caitlin , Yelton W. Graham , Beechem Thomas E. , Howell Stephen W. , Medlin Douglas L. 
Published: February 2020

Vertically aligned, untangled planarized arrays of multiwall carbon nanotubes (CNTs) with Ohmic back contacts are demonstrated in anodized aluminum oxide (AAO) nanopore templates on arbitrary substrates for use as thermal interface materials (TIMs). Such CNT-based TIMs are a promising alternative to conventional metal-filled epoxy TIMS, (e.g. silver paste), due to the large thermal conductivity of CNTs and the ability to eliminate epoxy in the thermal pathways, a major bottleneck for phonon transport. Without an efficient TIM between heat-producing devices and their packaging material (heat sink), the advantages of advanced power electronics are obscured due to the degradation of both performance and lifetime that occur with increasing temperatures.
Briefly, we prepare templates by sputter depositing low residual stress Nd-doped Al films onto W-coated substrates, followed by electrochemical anodization to form aluminum oxide (AAO) nanopore arrays. The W underlayer helps eliminate the aluminum oxide barrier that typically occurs at the nanopore bottoms by instead forming a thin WO3 layer that can be selectively etched away. This leaves an electrically conductive W film at the bottom of every nanopore, provides a back electrical contact for both the electrodeposition of Co catalyst nanowires and later transport measurements. These structures are then use to grow CNTs arrays via thermal chemical vapor deposition (CVD).
Critical issues include (1) optimizing the crystalline quality of individual CNTs to ensure high thermal conductivity, (2) achieving high densities of CNTS to maximize the thermal pathway, and (3) planarizing the CNT array tips to enable direct contact to a heat-producing device surface with as many of the CNTs as possible. CNT crystalline quality is primarily controlled via the CVD process choices such as carbon feedstock gas, catalyst material, and growth conditions (temperature and time). The array density is primarily impacted by the initial fraction of nanopores filled with Co, and a competition during CVD between (a) CNT growth from the Co nanowires in the nanopores and (b) decomposition of carbonaceous material on the AAO template surface that chokes the pores, solved by controlling the aspect ratio of the open pore above the Co catalyst. Finally, we demonstrate the use of ultrasonication to cut all the CNTs to a uniform height as well as ohmic contacts between CNT tips and the W backplane, implying that the CNT-Co-W structure is unbroken, critical for thermal transport. These properties are studied using scanning and transmission electron microscopies, Raman spectroscopy, and thermal/electrical transport measurements.
This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.