(275e) Thermal Interface Properties of Carbon Nanotubes on Diamond

Recently, significant attention has focused on using highly thermal conductive carbon nanotubes (CNTs) for thermal contact conductance enhancement. Among the several reported works, Xu and Fisher have reported the lowest resistance values of 19.8mm2K/W and 5.2mm2K/W under moderate pressures for copper-silicon interfaces with dry CNT arrays and PCM-CNT arrays, respectively. Ngo et al. used copper as a gap filler to enhance the stability and thermal conductance of carbon nanofiber (CNF) arrays. They reported a resistance of 25mm2K/W under a pressure of 60psi for Cu-Si interfaces. However, the CNT/CNF arrays are electrically conductive and therefore may not be suitable for all electronics applications. Meanwhile high-quality PECVD diamond is a good electrical insulator and with high resistance to abrasion. Further, with in-plane and transverse thermal conductivities of 500W/(mK) and more than 1000W/(mK) respectively, diamond films can also be excellent heat spreaders. This heat spreading ability could be useful for thermal interface enhancement in combination with CNTs in that the diamond layer could act to spread the heat to local regions where CNTs bridge interface gaps most effectively. In the present work, a combination of polycrystalline diamond films and CNT arrays have been fabricated for possible thermal contact conductance enhancement. Diamond thin films were deposited on a bare silicon wafer surface and then coated with a well anchored CNT array. Both layers were directly synthesized by plasma enhanced chemical vapor deposition (PECVD) in the same reactor, and thermal contact resistances have been experimentally measured. The effects of the CNT array thickness and topologic characteristics of the diamond film are discussed. Significant substrate deformation was found to exist in the diamond film, and the resulting contact area at Cu-diamond interface was estimated to be only less than 0.8% of the nominal interface area. Measured bulk contact resistance at different interface pressures for diamond-only samples ranged from 527 to 974 mm2K/W, which are approximately 400 mm2K/W higher than the results for a Cu-Cu interface. The CNT-diamond composite fabricated in this study reduced the contact resistances of copper-diamond interface, but the 8ìm tall CNT arrays on diamond films were not capable of bridging all the large gaps resulted from the substrate deformation; therefore the measured bulk Cu-CNT/diamond interface resistances were still very high and ranged from 316 to 764 mm2K/W. In spite of the high resistances obtained with the present specimens, this study has revealed the important effects of substrate deformation in applications of diamond thin films. Our group continues to work on thermal conductance enhancement with CNT-diamond composites including flat sample fabrication, variation of properties for CNT arrays and diamond films.