Experimental and theoretical study of structure-dielectric property relationships for polysilsesquioxanes

Structures of polysilsesquioxanes {(R-SiO1.5)n with R=H, CH3, C6H5} in spin-on thin films are investigated in relation to their dielectric properties. IR spectroscopy in combination with results from quantum chemistry calculations shows that the initial hydrido-silsesquioxane films (cured to 250°C) exhibit more symmetric ring structures than those for methyl-silsesquioxanes. Moreover, IR spectra indicate that increasing the cure temperature above a critical value, which varies with the Si-R moiety, causes extensive three-dimensional cross-linking in silsesquioxanes via breakage of the Si-R bond and formation of networks of O-Si-O structures with a lower ring symmetry than the initial materials. Dielectric properties do not appear to vary with the structural symmetry about the O-Si-O moiety nor with the R substituents, but rather depend on the extent of three-dimensional cross-linking as seen by the loss of Si-R absorbance. Highly cross-linked silsesquioxanes show a higher dielectric value and no variation with temperature from -100°C to 150°C range, whereas a lower value and a negative temperature dependence are seen for dielectric constants of silsesquioxane samples with little loss of Si-R absorbance.