Condensation of silanol groups in porous methylsilsesquioxane films using supercritical CO₂ and alcohol cosolvents

Fourier transform infrared (FTIR) spectroscopy, goniometry, and electrical measurements were used to investigate the effect of adding alcohol and carboxylic acid cosolvents to supercritical carbon dioxide (scCO₂) to condense silanol groups in blanket porous methylsilsesquioxane (p-MSQ) films (JSR LKD 5109). The aliphatic C1-C6 alcohols removed approximately 50% more hydrogen-bonded silanol (SiO-H) groups than pure scCO₂, leaving isolated silanol groups on the surfaces of the pores. Acetic acid removed H-bonded silanols but left fewer isolated moieties. On a molar basis, n-propanol, isopropanol, and n-butanol removed the largest percentage of silanols per molecule of cosolvent. These cosolvents were also among the lowest vapor pressure cosolvents studied, making them the most environmentally acceptable. As-received ashed ultralow-k MSQ had a contact angle of less than 10° and a dielectric constant of 3.5±0.1. After processing in a mixture containing 7% n-propanol and scCO₂, the contact angle was 15° and the dielectric constant decreased to 3.2±0.1. The surface was hydrophilic after processing in mixtures of cosolvents and scCO₂ because of the isolated silanol groups on the surface. A comparison of the trends across the alcohol series indicates that cosolvent addition to scCO ₂ increased the solubility of water in the supercritical fluid mixture compared to pure scCO₂. Within the same class of molecules, the solubility of the cosolvent in the supercritical fluid is a more important selection criterion than the solubility of water in the cosolvent.