Supercritical CO2 Extraction of Porogen Phase: An Alternative Route to Nanoporous Dielectrics

We present a supercritical CO 2 (SCCO 2) process for the preparation of nanoporous organosilicate thin films for ultra low dielectric constant materials. The porous structure was generated by SCCO 2 extraction of a sacrificial poly(propylene glycol) (PPG) from a nanohybrid film, where the nanoscopic domains of PPG porogen are entrapped within the crosslinked poly(methylsilsesquioxane) (PMSSQ) matrix. As a comparison, porous structures generated by both the usual thermal decomposition (at ca. 450°C) and by a SCCO 2 process for 25 wt% and 55 wt% porogen loadings were evaluated. It is found that the SCCO 2 process is effective in removing the porogen phase at relatively low temperatures (< 200°C) through diffusion of the supercritical fluid into the phase-separated nanohybrids and selective extraction of the porogen phase. Pore morphologies generated from the two methods are compared from representative three-dimensional (3D) images built from small angle x-ray scattering (SAXS) data.