Department of Mechanical Engineering
Kenneth E. Goodson
The basic physics of phonon conduction in dielectrics and semiconductors has been the focus of research for more than a century. However, recent improvements in nanofabrication technologies have enabled new classes of experiments involving silicon films of near perfect crystalline quality. The MEMS and semiconductor communities need the thermal conductivities of these materials for a variety of applications, and these data are can provide breakthrough information about phonon conduction physics.
We developed measurements of in-plane silicon thin film thermal conductivities in the late 1990's using Joule heating and thermometry and the best contemporary SOI substrates, which augment lateral heat spreading. The thermal conductivity data over a broad range of temperatures and film thicknesses were consistent with solutions to the phonon Boltzmann transport equation accounting for boundary scattering. Room temperature data for very thin silicon films (below 100nm) showed a size effect of nearly 50% at room temperature and this result was compared with predictions to help separate the relative contributions of phonons to the overall thermal conductivity in bulk silicon. A suspended MEMS structure was developed for measuring the thermal conductivities of doped and undoped polycrystalline silicon films, in which the coupled scattering effects of grain boundaries, interfaces, and dopant impurities were investigated as a function of film thickness and temperature. More recent data have determined the impact of localized phonon nonequilibrium on lateral conduction in SOI films, which elevates the thermal resistance compared to solutions based on Fourier's law.