Application of scanning thermal microscopy for thermal conductivity measurements on meso-porous silicon thin films

A scanning thermal microscope (SThM) in the dc regime was used to study the thermal conductivity of homogeneous in-depth meso-porous silicon in the form of thin films on a monocrystalline silicon substrate. Measurements for different film porosities (30-80%) and thicknesses (100 nm-8 mu m) were performed in order to estimate the influence of both layer porosity and thickness on the thermal conductivity values of porous silicon (PS). An analytical model predicting the SThM measurement in the case of ultra-thin monolayered samples was used to calibrate the technique, to analyse experimental data and to determine the thermal conductivity of meso-porous layers. Effective thermal conductivity of meso-PS films was found to decrease when the porosity increases. The effective thermal conductivities measured for thick porous layers (several mu m) are in good accordance with those measured by micro-Raman-spectroscopy on bulk meso-PS samples. For submicrometric thicknesses (< 1 mu m), the effective thermal conductivity of layers decreases significantly with decreasing layer thickness due to the increased sensitivity of measurements to the thermal resistance of the film/substrate interface. An intrinsic thermal conductivity of PS was calculated independently of the film thickness and the values of interfacial thermal resistances were thus estimated. From the apparatus point of view, the results obtained show that the depth being sensed is of the order of a few micrometres for insulating materials and depends on the thermal conductivity of the films.