Spectroscopic ellipsometry investigation of the swelling of poly(dimethylsiloxane) thin films with high pressure carbon dioxide

The thickness and refractive index of poly(dimethylsiloxane) (PDMS) thin fi lms an silicon, exposed to high-pressure carbon dioxide, have been measured in-situ by spectroscopic ellipsometry. The swelling of the film is determi ned from the thickness and the sorption of CO2 from the refractive index. T he sorption and swelling Values for the thin films exceed those of the bulk films. These increases may be attributed to excess CO2 at the polymer thin film-CO2 and polymer-silica interfaces and the influence of the silica sur face and CO2 on the orientation of the polymer. Pressure-induced birefringe nce of the fused-silica windows was addressed in the model calculations and corrections were found to be successful for a thermal oxide reference wafe r and the PDMS films. Large surface excesses of CO2 on the reference wafer were observed in regions where CO2 is highly compressible due to critical a dsorption.