Thermal Conductivity of Confined Ultrathin Polymers
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1172-T02-03
Thermal Conductivity of Confined Ultrathin Polymers M. G. Ghossoub1, J. H. Lee1, O. T. Baris1, D. G. Cahill2, 3 and S. Sinha1 1 Department of Mechanical Science and Engineering, 2 Department of Materials Science and Engineering and 3Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana IL 61801, U.S.A.
ABSTRACT We report ultrafast measurements of thermal transport in plasma polymerized CHF3 films deposited on standard Si substrates with Al sputtered on top. We characterize the thin films by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and spectroscopic ellipsometry and measure polymer thicknesses ranging from 33 nm down to 6 nm. Time-domain thermoreflectance (TDTR) provides quantitative data on the polymer thermal response to periodic heating from a pulsed laser source. A pump beam heats the Al layer, which acts as an opto-thermal transducer to the stack (Al-Polymer-Si) and a delayed probe beam measures the change in Al surface reflectance. We extract the polymer thermal conductivity by comparing TDTR data to a thermal diffusion model and find it to increase with decreasing polymer thicknesses below 30 nm.
INTRODUCTION Polymers are industrially interesting materials that may contribute inexpensive solutions to nanoscale heat transfer and its numerous applications [1]. In particular, polymers exhibit a shift in their thermal properties when confined to nanometer scales. For instance, ultrathin polymers, with thicknesses comparable to the radius of gyration, show a glass transition temperature that is both different from the bulk and largely dependent on surface interactions [2]. Furthermore, incoherent neutron scattering studies [3] on confined ultrathin films reveal a suppression of anharmonicity near the glass transition but its impact on thermal transport is not well understood at present. While bulk thermal properties have been extensively studied for a variety of polymers, published thermal conductivity measurements for thin polymer films typically cover thicknesses greater than 100 nm. Kurabayashi et al. showed that the thermal conductivity of polyimide films is anisotropic and independent of thickness for polymers exceeding 500 nm [4]. A more recent study on polyaniline [5] measured a five-fold decrease in thermal conductivity when the film thickness is decreased from 5 μm to 110 nm. In this work, we use time-domain thermoreflectance (TDTR) to measure the thermal conductivity of plasma polymerized ultrathin CHF3 polymer films that are 35 nm and thinner. The polymer films are deposited on a standard Si substrate and a thin Al layer is sputtered on top. We use X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), atomic force microscopy (AFM), and spectroscopic ellipsometry to characterize these films and report thicknesses in the range 6 to 33 nm. An ultrafast femtosecond pulsed laser pump beam heats the Al surface and a delayed probe beam measures the decay in Al surface temperature with a
picosecond time resolution. A comparison of
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