Thermal conductivity of periodically microporous silicon membranes

Thermal conductivity reduction in thin films due to phonon boundary scattering is an important phenomenon that helped revive the development of efficient thermoelectric materials. Nanoscale and microscale porous structures in thin films can amplify such phonon size effect and further reduce their thermal conductivity. The understanding of the effect of microporous structure on phonon transport requires a careful comparison of the theoretical modeling and experimental characterization. The objective of this study is to fabricate and characterize a porous structure in a thin film that clearly exhibits phonon size effect and is relatively simple to model. Specifically, experimentally characterized in-plane thermal conductivity of freestanding, periodically micro-porous single-crystal silicon membranes is presented. These porous silicon membranes are created from a silicon-on-insulator wafer using microfabrication technique including KOH bulk etching for membrane releasing and deep-reactive ion etching for hole drilling. The porous membranes contain 2-/spl mu/m or 10-/spl mu/m diameter pores arranged in aligned or staggered rows, and they are compared with to a solid, non-porous membrane. The temperature-dependent in-plane thermal conductivity of porous silicon membranes is measured from 50 K to 300 K using microfabricated heaters and temperature sensors on the membrane. The experimental in-plane thermal conductivity of the solid and porous silicon membranes clearly shows phonon size effect, even at room temperature. Membranes with small pores exhibit clearly lower thermal conductivity values than membranes with larger pores, despite similar porosity values. For small-pore membranes, staggered hole pattern shows much lower thermal conductivity than aligned one, indicating that backward phonon reflection becomes important as size scale decreases. As temperature decreases, the thermal conductivity of the porous membranes steadily decrease further below the effective thermal conductivity due to porosity alone.