Several kinds of homogeneous organic-inorganic hybrid polymer thin films were designed with improved mechanical properties and low dielectric constants (<3.0). Novel soluble siloxane-silsesquioxane hybrid polymers were synthesized with cyclic and/or cage silane monomers, which had triorganosiloxy (R3Si1/2), diorganosiloxane (R2SiO2/2), and organosilsesquioxane (RSiO3/2) moieties with ethylene bridges at the molecular level, by the hydrolysis and condensation of 2,4,6,8-tetramethyl-2,4,6,8-tetra(trimethoxysilylethyl)cyclotetrasiloxane (a cyclic monomer). The electrical properties of these films, including the dielectric constant (similar to2.51), leakage current (6.4 X 10(-11) A/cm(2) at 0.5 MV/cm), and breakdown voltage (similar to5.4 MV/cm) were fairly good. Moreover, the mechanical properties of the hybrid films, including the hardness (similar to7 GPa), modulus (similar to1.2 GPa), and crack-free thickness (<2 mum), were excellent in comparison with those of previous spin-on-glass materials with low dielectric constants. The excellent mechanical properties were proposed to be due to the high contents of Si-OH groups (>30%) and the existence of ethylene bridge and siloxane moieties in the hybrid polymer precursors. In addition, the mechanical properties of the hybrid films were affected by the contents of the cagelike structures. The more cagelike structures a hybrid film contained, the worse its mechanical properties were. (C) 2003 Wiley Periodicals, Inc.