An apparatus for thermal treatment of dielectric films on substrates includes: a microwave applicator cavity and microwave power source; a workpiece to be heated in the cavity, having a porous coating on a selected substrate; and, an apparatus for introducing a controlled amount of a polar species into the porous coating immediately before heating by the microwave power. The interaction of the polar species with the microwaves enhances the efficiency of the process, to shorten process time and reduce thermal budget. A related method includes: depositing a porous film on a substrate; soft baking the film to a selected state of dryness; introducing a controlled amount of a polar species into the soft baked film; and, applying microwave energy to heat the film via interaction with the polar species.