Simultaneous measurements of thermal conductivity and electrical conductivity of micro-machined Silicon films

The in-plane effective thermal conductivity of free-standing Si thin films with periodic micropores was measured at -100 to 0°C. The Si thin films with micropores were prepared from silicon-on-insulator (SOI) wafers by standard microfabrication processes. The dimensions of the free-standing Si thin films were 200μmx150xmx2 μm, with staggered 4 μm pores having an average pitch of 4 mm. The Si thin film serves both as a heater and thermometer. The average temperature rise of the thin film is a function of its in-plane thermal conductivity. The effective thermal conductivity was calculated using a simple one-dimensional heat conduction model. The measured thermal conductivity was much lower than that expected based on classical model evaluations. A significant phonon size effect was observed even in the microsized structures, and the mean free path for phonons is very long even at the room temperature.