Dielectric composite films characterized by a dielectric constant (k) of less than about 7 and having a density of at least about 2.5 g/cm3 are deposited on partially fabricated semiconductor devices to serve as etch stop layers. The dielectric composite film in one embodiment includes Al, Si, and O and has a thickness of between about 10-100 Å. The dielectric composite film can reside between two layers of inter-layer dielectric, and may be in contact with metal layers. An apparatus for depositing such dielectric composite films includes a process chamber, a conduit for delivering an aluminum containing precursor to the process chamber, a second conduit for delivering a silicon-containing precursor to the process chamber and a controller having program instructions for depositing the dielectric composite film from these precursors, e.g., by reacting the precursors adsorbed to the substrate with an oxygen-containing species.