Two structures of low dielectric constant (low-k) SiOC films were elucidated in this work. Low-k thin film by remote plasma mode was mainly composed of inorganic Si-O-Si backbone bonds and some oxygen atoms are partially substituted by CH3, which lowers k value. The host matrix of low-k- thin films deposited by direct plasma mode, however, was mainly composed of organic C-C bonds and "M" and "D" moieties of organosilicate building blocks, and thus the low dipole and ionic polarizabilities were the important factors on lowering k value. (c) 2006 Elsevier B.V. All rights reserved.