For future miniaturization of electronic systems using 3D chip stacking, new fine-pitch materials for through-silicon-via (TSV) applications are likely required. In this paper, we propose a novel carbon nanotube (CNT)/copper nanocomposite material consisting of high aspect ratio, vertically aligned CNT bundles coated with copper. These bundles, consisting of hundreds of tiny CNTs, were uniformly coated by copper through electroplating, and aspect ratios as high as 300:1 were obtained. The resistivity of this nanomaterial was found to be as low as ∼10(-8) Ω m, which is of the same order of magnitude as the resistivity of copper, and its temperature coefficient was found to be only half of that of pure copper. The main advantage of the composite TSV nanomaterial is that its coefficient of thermal expansion (CTE) is similar to that of silicon, a key reliability factor. A finite element model was set up to demonstrate the reliability of this composite material and thermal cycle simulations predicted very promising results. In conclusion, this composite nanomaterial appears to be a very promising material for future 3D TSV applications offering both a low resistivity and a low CTE similar to that of silicon.
This paper presents the design and fabrication of a broadband microstrip attenuator, operating at 1-20 GHz, based on few layer graphene flakes. The RF performance of the attenuator has been analyzed in depth. In particular, the use of graphene as a variable resistor is discussed and experimentally characterized at microwave frequencies. The structure of the graphene-based attenuator integrates a micrometric layer of graphene flakes deposited on an air gap in a microstrip line. As highlighted in the experiments, the graphene film can range from being a discrete conductor to a highly resistive material, depending on the externally applied voltage. As experimental evidence, it is verified that the application of a proper voltage through two bias tees changes the surface resistivity of graphene, and induces a significant change of insertion loss of the microstrip attenuator.
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