Physical properties of low-k films based on the co-condensation of methyltrimethoxysilane with a bridged silsesquioxane

Though spin-on organosilicates are considered as the promising candidates of low dielectric constant materials, it is necessary for a successful integration to improve mechanical strength such as modulus and fracture toughness. In this study, five sets of MTMS–BTMSE copolymers were synthesized and characterized while the monomer content of BTMSE (bis(trimethoxysilyl)ethane) was varied from 9.1 to 47.3 mol% vs. MTMS (methyltrimethoxysilane). In parallel with the measurement of dielectric constant, four different tests were carried out to evaluate mechanical properties of the MTMS–BTMSE copolymers. Modulus was measured by the nanoindentation technique and the modified edge lift-off test (MELT) was employed to extract adhesive fracture toughness quantitatively. In addition, residual stress was calculated by sensing the change in radius of curvature of the substrate. The chemical structure of the copolymers was also analyzed with FTIR and NMR. Network formation was enhanced as the amount of BTMSE increased, which led to improvement of modulus and the increase in refractive index and dielectric constant. However, an increasing rate of fracture toughness by the MELT was not proportional to the increase in the amount of BTMSE, which implied that it was necessary to optimize the composition of the copolymers since adhesion strength was conjectured the most critical factor for a successful integration.