Thermal conductivity and nanoindentation hardness of as-prepared and oxidized porous silicon layers

Porous silicon (PS) offers promising possibilities to be applied as thermal insulating material in thermal effect microsystems for its thermal conductivity (TC) is up to two orders smaller than that of bulk silicon. In order to find a compromise between efficient thermal isolation and good mechanical stability of PS, thermal oxidation of PS is commonly used to tune the mechanical and thermal properties of PS. Both TC and the hardness of as-prepared and oxidized PS have been thoroughly investigated. TC and the hardness of as-prepared and oxidized PS were measured using micro-Raman scattering and nanoindentation, respectively. Experimental results revealed that TC and the hardness of as-prepared PS, exhibiting a strong dependence on the preparing conditions, decrease with increasing porosities. After oxidization at different temperatures, TC of oxidized PS decreases with increasing oxidation temperatures, whereas the hardness increases a lot. PS with a moderate porosity of 73.4% oxidized at 600 °C has a compromise between low TC [2.100 W/(m K)] and high hardness (∼1.160 GPa). So this process finalizes this kind of oxidized PS to be used as a suitable thermal insulation substrate in thermal effect microsystems.