Graphene Heat Spreaders for Electronics Thermal Management Applications
Abstract: Graphene shows great potential for applications in electronics due to its outstanding physical properties such as extremely high electron mobility, high thermal conductivity, high Young’s modulus and very high surface-to-volume ratio. Among these attractive properties, the high intrinsic thermal conductivity is a critical advantage for the application of graphene in electronics to alleviate heat dissipation problems. The work described in this thesis attempts to apply graphene as heat spreader for thermal management in electronic packaging. To apply graphene as a potential alternative to metals for heat spreading applications, high-quality material and large-area synthesis is required. In the current thesis work, thermal chemical vapor deposition (TCVD), liquid phase exfoliation (LPE) from graphite, and reduction of graphene ox- ide (GO) are used to synthesize graphene, and transfer methods were also demonstrated. In the TCVD approach, high quality graphene was fabricated over a large- area, controlling the graphene layer thickness. The thermal performance of graphene heat spreaders was evaluated by the temperature drop of the hotspots after the graphene transfer. To further enable the development of graphene heat spreaders, phonon scattering on the graphene-substrate interface, phonon-grain boundary scattering, thermal resistance boundary (TBR), and the effect of the number of graphene layers are discussed. In the LPE approach, following LPE films were made by two different methods, vacuum filtration and drop coating. Three different methods were combined to evaluate and predict the thermal performance of such graphene- based films. Resistance thermometers were used to monitor the hotspot temperature decrease versus the Joule heat flow as a result of using graphene- based heat spreaders. The 3ω method was used to experimentally deter- mine the in-plane and through-plane thermal conductivities of such films. A finite element (FE) model of the hotspot test structure was setup using the in-plane and through-plane thermal conductivities obtained from the 3ω measurements. Simulations were performed to predict the hotspot temperature decrease with excellent agreement obtained between all methods. The results indicate that the alignment and purity of the graphene-based films, as well as their thermal boundary resistance with respect to the chip, are key parameters when determining the thermal performance of graphene-based heat spreaders. In the reduction of GO approach, a graphene-based film heat spreader was fabricated from the reduced graphene oxide (RGO). However, these free- standing materials were poorly adhered to the substrate because only weak van der Waals interactions provide any adhesion. The enhanced heat transfer by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules is demonstrated. Different techniques such as resistance thermometers, IR test, photothermal reflectance and molecular dynamics simulations were employed to reveal that the functionalization mediates heat transport in graphene nanoflakes. These studies suggest a significant package level solution for the thermal management of hotspots in high-power electronics at the micro- and nanometer scale.
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