Abstract: An apparatus and method for forming thin film aerogels on semiconductor substrates is disclosed. It has been found that in order to produce defect.about.free nanoporous dielectrics with a controllable high porosity, it is preferable to substantially limit evaporation and condensation of pore fluid in the wet gel thin film, e.g. during gelation, during aging, and at other points prior to obtaining a dried gel. The present invention simplifies the atmospheric control needed to prevent evaporation and condensation by restricting the atmosphere in contact with the wet gel thin film to an extremely small volume. In one embodiment, a substrate 26 is held between a substrate holder 36 and a parallel plate 22, such that a substantially sealed chamber 32 exists between substrate surface 28 and chamber surface 30. Preferably, the average clearance between surfaces 28 and 30 is less than 5 mm, or more preferably, less than 1 mm.