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 scCO₂ 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.