Anisotropic thermal expansion behavior of thin films of polymethylsilsesquioxane, a spin-on-glass dielectric for high-performance integrated circuits

Abstract

Thin films of poly(methylsilsesquioxane) (PMSSQ) are candidates for use as interdielectric layers in advanced semiconductor devices with multilayer structures. We prepared thin films of PMSSQ with thicknesses in the range 25.0-1151.0 nm by spin-casting its soluble precursor onto Si and GaAs substrates with native oxide layers and then drying and curing the films under a nitrogen atmosphere at temperatures in the range 250-400 degreesC. The out-of-plane thermal expansion coefficient alpha(perpendicular to) of each film was measured over the temperature range 25-200 degreesC using spectroscopic ellipsometry and synchrotron X-ray reflectivity, while the in-plane thermal expansion coefficient alpha(parallel to) of each film was determined over the temperature range 25-400degreesC by residual stress analysis. PMSSQ films cured at higher temperatures exhibited reduced thermal expansion, which is attributed to the denser molecular packing and higher degree of cross-linking that arises at higher temperatures. Surprisingly however, all the PMSSQ films were found to exhibit very strong anisotropic thermal expansion; alpha(perpendicular to) and alpha(parallel to) of the films were in the ranges 140-329 ppm/degreesC and 12-29 ppm/degreesC respectively, depending on the curing temperature. This is the first time that cured PMSSQ thin films have been shown to exhibit anisotropic thermal expansion behavior. This anisotropic thermal expansion of the PMSSQ thin films might be due to the anisotropy of cross-link density in the films, which arises because of a combination of factors: the preferential orientation of methyl groups toward the upper film surface and the preferential network formation in the film plane that occurs during curing of the confined film. In addition, the film electron densities were determined using synchrotron X-ray reflectivity measurements and the film biaxial moduli were obtained using residual stress analysis.