Nanoporous Organosilicate Films Prepared in Acidic Conditions Using Tetraalkylammonium Bromide Porogens

Organosilicate films with narrow pore size distribution tunable in the range of 1-3.4 run were prepared by spin-coating of silicon wafers with sols prepared in acidic conditions using tetraethyl orthosilicate, methyltrimethoxysilane, and tetraalkylammonium bromide (TAABr) porogen. The pore size was defined by the alkyl chain length of the quaternary ammonium molecule and the porogen concentration. The pore network in the films and the hydrophobicity of the pore surfaces were characterized using ellipsometric porosimetry with toluene and water adsorbates. In the absence of TAABr, the pore volume was 9 vol% and the pore size 1 nm. By using TAABr porogens, monomodal pore size distributions were obtained in the range of 1.2-3.5 nm. The pore volume was in the range from 18 vol % at 1.2 nm pore diameter up to 54 vol % at 3.5 nm diameter. The hydrophobicity of the pores was dependent on the pore diameter, with smaller pores being the least hydrophobic. The increase of hydrophobicity with pore size was explained by an increased distance between silanol groups on the curved pore surfaces. The mechanical properties and dielectric constant of these films were comparable to reference materials prepared using more sophisticated porogens reported in the literature.