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The thermal stress of thin and ultrathin polystyrene (PS) films on Si substrate has been studied and the glass transition temperature (Tg) is determined from the thermal stress data. Tg of PS turned out to be thickness independent for thick films but decreases when the film thickness is comparable to the end-to-end distance of the polymer chains (<100 nm). The thermal stress level and the slope of the stress temperature curve of the film also decrease as the film thickness decreases. The slope reduction indicates that the product of the biaxial modulus E/(1−ν) and the coefficient of thermal expansion (CTE) of the film decreases with film thickness. Assuming that the CTE increases for ultrathin films, the modulus is found to decrease significantly with respect to the bulk value.

This letter reports a study of the porosity effect on material properties of methylsilsesquioxane films, including the dielectric constant, thermal conductivity, and thermal stress behavior. In a porosity range from 0% to 50%, both the dielectric constant and thermal conductivity decreased with increasing porosity and no significant change was observed at the percolation point where pores became interconnected. In comparison, the stress–temperature slope also decreased with porosity, but as the porosity approached the percolation point, the slope showed a large drop of 40%, indicating a significant degradation of the thermomechanical properties due to percolation of pores. Assuming the coefficients of thermal expansion remain at 17 ppm/°C within the porosity range, the change in the stress–temperature slope corresponds to a decrease of the biaxial modulus from 7 to 5 GPa around the percolation point.

The effective thermal conductivity of four polymeric thin films with distinct molecular morphologies has been measured as a function of film thickness down to 70 Å using a 3ω technique. Comparing to SiO2, the intrinsic thermal conductivity of polymers is about 5–10 times smaller while the interfacial thermal resistance is about 2–10 times larger. The interfacial thermal resistances are explained in terms of the mismatches of acoustic and mechanical properties between polymers and crystalline materials. Both elastic and inelastic scatterings at the interface are examined.