Effect of surfactants on the porogen size in the low-k methylsilsesquioxane/polystyrene hybrid films

An anionic surfactant, sodium dodecylbenzenesulfonate (NaDBS) and a cationic surfactant, domiphen bromide (DB) were used to modify the surface potential of polystyrene (PS) porogen in a post-integration porogen removal scheme. The effect of surfactant modification on the porogen and pore size in the low-k methylsilsesquioxane (MSQ)/PS hybrid films at 10 wt% PS loading, under a slow curing rate, was studied by grazing incidence small-angle X-ray scattering, viscosity measurement, and Fourier-transform infrared analysis. For PS porogen without modification, the porogen aggregated and its size increased from 10.0 to 16.5 nm at different rates, up to 200 °C, depending on the porogen diffusivity and steric hindrance by the successively cross-linked MSQ matrix at T ⩾ Tg. In contrast, the NaDBS- and DB-modified porogens with higher surface potential impede their aggregation within the cross-linking MSQ matrix, resulting in a smaller porogen size, by electrostatic repulsion and increased viscosity due to electroviscous effect. Also, the columbic attraction between Si−OH groups of MSQ matrix and the positively charged, DB-modified PS, restrains the PS porogen, thus reduces its aggregation during cure step, leading to a small porogen size and tight distribution (8.7 ± 2 nm) at 200 °C and later a similar pore size after porogen removal at 400 °C.

Figure optionsDownload as PowerPoint slideHighlights
► Developed a simple method controlling the pore size/distribution under slow cure.
► Aggregation is impeded by surfactants’ electrostatic and electroviscous effects.
► DB-modified PS is effective in controlling the porogen and pore size/distribution.
► Columbic attraction between DB-modified PS and Si–OH restrains PS aggregation.
► Porogen aggregation behavior was elucidated by in situ GISAXS and viscosity test.