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

Structures of polysilsesquioxanes {(R- SiO1.5)(n) with R=B, CH3, C(6)H(5)1} 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 degrees 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 crosslinked silsesquioxanes show a higher dielectric value and no variation with temperature from -100 degrees C to 150 DC 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.