1. An apparatus for thermal treatment of dielectric films on substrates comprising:a microwave applicator cavity to apply microwaves to a workpiece disposed within the microwave applicator cavity to heat the workpiece, the workpiece comprising a porous coating on a selected substrate;
a microwave power source to deliver power to the applicator cavity;
a jar located within the microwave applicator cavity and,
a dispenser located within the jar and positioned above the surface of the porous coating for introducing a controlled amount of a polar solvent into the porous coating immediately before heating by the microwave power.