Synthesis of Vertically Aligned Multi-Walled Carbon Nanotubes on Copper Substrates for Applications as Thermal Interface
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Synthesis of Vertically Aligned Multi-Walled Carbon Nanotubes on Copper Substrates for Applications as Thermal Interface Materials Wei Lin and C. P. Wong School of Material Science & Engineering Georgia Institute of Technology 771 Ferst Drive NW. Atlanta, GA 30332 ABSTRACT Vertically aligned carbon nanotubes (VACNTs) grown on bulk copper substrate are of great importance for CNT real-life applications as thermal interface materials in microelectronic packaging. However, their reproducible synthesis has been a great challenge so far. In this study, by introducing a well-controlled conformal Al2O3 support layer on the bulk copper substrate by atomic layer deposition (ALD) prior to the deposition of the iron catalyst layer, we reproducibly synthesize VACNTs of good alignment and high quality on the copper substrate, using a conventional thermal chemical vapor deposition process. The alignment and the quality are characterized by scanning electronic microscopy and Raman spectroscopy, respectively. The roles of the conformal Al2O3 support layer are discussed. A kinetics-controlled growth mechanism is shown. This progress provides a viable VACNT commercial application for thermal management, on the basis of which, we show a recent progress on a state-of-art Si/VACNT/Cu assembling process, named “chemical anchoring”. The high quality of the VACNTs on the copper growth substrate and the covalent bonding formed between the VACNTs and the silicon mating substrate greatly reduces the thermal resistance of the VACNTmediated thermal interface. INTRODUCTION Carbon nanotubes (CNTs) have attracted extensive attention due to their outstanding electrical, thermal and mechanical properties, and their wide range of potential applications.[1-4] One promising application of CNTs in microelectronic packaging is to use vertically aligned carbon nanotubes (VACNTs) as thermal interface materials (TIMs) to enhance heat dissipation.[5-14] Vertically aligned CNTs (VACNTs) utilize the superior longitudinal thermal conductivity of individual nanotubes and exhibit the overall thermal conductivities of ~ 80 W m-1 K-1 or higher.[5, 8-10, 12] For a typical VACNT TIM assembly, a VACNT layer is synthesized on a growth substrate and then brought into contact with a mating substrate by, for example, compression force or solder anchoring.[5, 6, 8, 13] Although VACNT synthesis on silicon as the growth substrate has been widely investigated, direct synthesis of a VACNT layer on the backside of a silicon device is not compatible with a current front-end semiconductor fabrication process due to the high temperature (typically > 600 oC) required for VACNT syntheses via chemical vapor deposition (CVD). Thus, a silicon substrate is not the growth substrate of choice for real-life VACNT TIM applications; instead, VACNT synthesis on a bulk copper substrate, i.e. a copper lid, is preferred. A few papers have reported VACNT syntheses on various metal substrates, however, the direct VACNT synthesis on a bulk copper substrate is still a great challenge.[1
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