Low dielectric constant material which substitutes for conventional SiO₂ thin film as intermetal dielectric (IMD) is imperative for the reduction of interconnect resistance/capacitance(RC), cross talk and power consumption. SiO₂ aerogel film with unique properties such as ultra-low dielectric constant, low density and high thermal stability is one of the most possible candidates as IMD. SiO₂ aerogel films on a silicon wafer have been synthesized via a process of sol-gel, spin coating and supercritical drying. SiO₂ sol is prepared using a two-step procedure with acid/base as catalyst to investigate the effect of different amount of isopropanol(IPA) and NH₄OH on sol viscosity which is important for spin coating and the microstructure of SiO₂ aerogel film. The optimized viscosity of SiO₂ sol is in the range of 9～15 mPa·s. It is found that the time of viscosity in 9～15 mPa·s becomes longer as IPA increases or NH₄OH decreases.