Silylation of porous methylsilsesquioxane films in supercritical carbon dioxide

Silylation reactions using a supercritical carbon CO₂ solvent were performed on porous methylsilsesquioxane (p-MSQ) thin films. The addition of alkylsilyl moieties to the films repaired damage due to oxygen plasma processing. The films (JSR LKD 5109) were characterized using Fourier transform infrared (FTIR) spectroscopy, ellipsometry, contact angle, and electrical measurements. The silylation chemistries included <1.5 wt% hexamethyldisilazane (HMDS), tetramethyldisilazane (TMDS), and trimethylchlorosilane (TMCS) mixed with supercritical CO₂ (scCO ₂). Blanket films with a dielectric constant of 2.4 before oxygen ashing and 3.5 after ashing were processed at 150-300 atm and 50-60°C for a 2 min soak time (after a 15 min ramp to steady-state). The disilazanes and chlorosilane reacted with both lone and hydrogen-bonded silanol (SiO-H) groups on the surface of the p-MSQ. The hydrophobicity of the p-MSQ surface was recovered after treatment as shown by contact angles >80°. The dielectric constant of ashed p-MSQ was reduced to 2.5, 3.3, and 2.6 ± 0.1 by HMDS, TMDS, and TMCS addition, respectively. The mechanism proposed involves the direct reaction of the Si-bearing precursor molecules with the p-MSQ surface. Supercritical CO₂ is a good solvent for silylation reactions and is an effective approach to restore the degradation of porous MSQ films due to plasma ashing.