Structural control of oligomeric methyl silsesquioxane precursors and their thin-film properties

Oligomeric methyl silsesquioxane (O-MSSQ) precursors were prepared from methyl trimethoxysilane (MTMS) in a mixed solvent of methyl isobutyl ketone and tetrahydrofuran by variations in the pH and molar ratio of water to MTMS (R-1). The molecular structures of O-MSSQ were controlled by the reaction conditions. At a fixed pH value, the percentage of the end group, Si-OCH3, decreased with increasing R-1, but that of Si-OH increased. With the pH increasing, the ratio of Si-OCH3 groups to Si-OH groups was enhanced, but ratio of the molecular weights was reduced. The molecular weight distribution was progressively broader as the pH value decreased. These results were explained by the effects of R-1 and pH on the hydrolysis and condensation reactions. The prepared O-MSSQ precursors consisted of mixed cage and network structures. The ratio of cage structures to network structures increased at low pH and high R-1 values. Highly uniform thin films were spin-coated from the O-MSSQ precursors, and this was followed by multistep curing. The content of cage structures in O-MSSQ films decreased with increasing curing temperatures, whereas the network content in O-MSSQ films showed the opposite trend. Such a structural transformation resulted in significant variations in the physical properties. Both the refractive index and dielectric constant decreased with higher cage/network ratios because of changes in the molar volume. The prepared O-MSSQ has potential applications as a low dielectric constant material. (C) 2002 Wiley Periodicals, Inc.