Thermal Interface Properties of Cu-filled Vertically Aligned Carbon Nanofiber Arrays Quoc Ngo, ²,‡ Brett A. Cruden, ‡ Alan M. Cassell, ‡ Gerard Sims, ‡ M. Meyyappan, ‡ Jun Li,* ,‡ and Cary Y. Yang ² Center for Nanostructures, Santa Clara UniVersity, Santa Clara, California 95050, and Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California 94035 Received September 13, 2004; Revised Manuscript Received October 24, 2004 ABSTRACT Nanoengineered materials have emerged as efficient thermal interface materials in a variety of thermal management applications. For example, integrated circuits (IC) are subject to tight thermal budgets to maintain acceptable reliability standards. This letter presents thermal contact resistance measurement results and analyses for copper gap-filled carbon nanofiber-copper composite arrays. Experimental results demonstrate the efficient interfacial thermal conduction of these structures. Using copper as a gap-fill material for improving lateral heat spreading and mechanical stability is discussed. Thermal characteristics of multiwall carbon nanotubes (MWNTs) have been measured, 1-4 revealing their unique thermal conductivity characteristics along the nanotube axis. For a discrete MWNT, thermal conductivity has been measured surpassing 3000 W m -1 K -1 in the axial direction. 1 Other studies have reported values for discrete MWNTs as small as 15 (W m -1 K -1 ) 2 and 27 (W m -1 K -1 ). 3 The wide variation can be attributed to the inherently disordered nature of some carbon nanostructures grown by the chemical vapor deposition (CVD) process. 5 Concerns about the degradation of thermal conductivity in vertically aligned carbon nanofiber (VACNF) arrays due to poorly graphitized structures are valid when considering these structures in thin film applica- tions where the intrinsic film properties are of great importance. In cases of studying thermal contact resistance, however, the physical nature of the contact between the nanofiber ends and hot contact surface tends to take precedence. It has been shown that as-grown carbon nanotube and nanofiber arrays have the potential to significantly improve thermal contact conductance. 6,7 However, to convert this into a manufacturable solution, care needs to be taken so that the array can withstand the rigorous mechanical stress in packaging process flows. Gap-filling copper between VACNFs provides a suitable mechanical anchor for the nanofibers to the substrate while also serving as a lateral heat spreader. The robust physical characteristics of the CNF-Cu composite also allow us to take advantage of increased contact surface area to the target material. Progress in the scaling of integrated circuits has resulted in an alarming rise in power dissipation in high-density, high- frequency, silicon-based microprocessors. 8 The need for addressing this problem is imperative for maintaining reli- ability standards for next-generation IC packaging technol- ogy. 9,10 The knowledge gained from addressing issues in microprocessor packaging can also be generalized to most devices that exhibit high power dissipation. The space program at NASA is also in need of thermal interface materials to draw heat away from hot spots on critical electronic components. One of the current issues with cooling systems for space vessels is the degradation of such systems over time. Liquid-cooled systems and those with moving parts do not fit this requirement due to their lack of stability and the need for constant servicing. Carbon nanofiber composites are a strong candidate material to provide thermal solutions for space missions. Through the use of DC-powered PECVD, 5 we fabricate vertically aligned, free-standing CNF arrays on silicon wafers of ∼500 μm thickness. Copper electrodeposition, a common process used for gap-filling high aspect ratio trenches, is used for the creation of a CNF-Cu composite array. The data presented here demonstrate the mechanical strength and efficient interfacial heat conduction of CNF-Cu composite arrays suitable for next-generation heat-sink devices. CNF arrays were grown using the procedure and reactor conditions detailed in ref 5. A layer of titanium (300 Å) was used as both an adhesion layer for a thin layer of nickel * Corresponding author. E-mail: jli@mail.arc.nasa.gov; phone (650) 604-6459; fax (650) 604-5244. ² Santa Clara University. ‡ NASA Ames Research Center. NANO LETTERS 2004 Vol. 4, No. 12 2403-2407 10.1021/nl048506t CCC: $27.50 © 2004 American Chemical Society Published on Web 11/13/2004