Abstract: A method for fabricating a thermally stable ultralow dielectric constant film comprising Si, C, O and H atoms in a parallel plate chemical vapor deposition process utilizing plasma enhanced chemical vapor deposition (“PECVD”) process is disclosed. To enable the fabrication of thermally stable ultralow dielectric constant film, specific precursor materials are used, such as, cyclic siloxanes and organic molecules containing ring structures, for instance, tetramethylcycloterasiloxane and cyclopentene oxide. To stabilize plasma in the PECVD reactor and thereby improve uniformity of the deposited film, CO2 is added to TMCTS as a carrier gas, or CO2 or a mixture of CO2 and O2 are added to the PECVD reactor.