A composite insulating film including three layers is formed on a substrate having a gap. The first layer partially fills the gap and contains a dielectric material having a low dielectric constant, such as halogen-doped silicate glass. The second layer is formed over the first layer, and contains an undoped dielectric material such as silicon oxide, nitride, or oxynitride. The second layer is more stable and integrable, and less susceptible to moisture absorption and outgassing, than the first layer. The second layer is substantially smaller in thickness than the first layer, and at least substantially fills the gap. The third layer is formed over the second layer, and contains a dielectric material having a low dielectric constant, such as halogen-doped silicate glass. In a specific embodiment, the first layer is formed by plasma-enhanced chemical vapor deposition in which reactive species are generated from a process gas mixture by plasma for sputtering the first layer.