Highly Anisotropic Thermal Conductivity in Spin-Cast Polystyrene Nano-Films

The development of advanced electronics packaging materials has been limited because the underlying physics regarding nanoscale transport phenomena in polymers remains poorly understood. The research community is in need of a measurement that can be readily applied to a variety of samples in order to strengthen the fundamental understanding of thermal transport in soft materials, to develop packaging materials for improved thermal management. Here, we present the application of the 3ω method, a common nanoscale thermal characterization technique, to the characterization of anisotropic thermal conductivity in a 136 nm thin spin coated polystyrene (PS) film. We demonstrate that this technique can be sensitive to anisotropy ratio for heater widths more than an order of magnitude wider than the film thickness. Narrow heaters are sensitive to both in-plane and through-plane conductivity while wider heaters are primarily sensitive to through-plane conductivity; thus, using three heaters with widths of 1.7μm, 5μm, and 10μm, we are able to measure the polymer film conductivity along both the vertical and radial directions of the film. We report the vertical thermal conductivity, i.e. through the plane of the film, to be 0.157 ± 0.004 Wm^-1 K^-1, and the radial component to be 13.7 ± 2.7 Wm^-1 K^-1, corresponding to an anisotropy ratio of 87 ± 17. This article represents a step towards large area, high thermal conductivity soft materials for electronics packaging and other applications, which facilitates improved device performance and lifetime.