Reinforcement mechanism for mechanically enhanced xerogel films

Abstract

The weak mechanical properties of xerogel films have limited their implementation as low dielectric constant materials in the semiconductor industry. Recently, we reported that triethoxyfluorosilane-based xerogel films demonstrated enhanced mechanical properties. Nanoindentation results indicate that these materials have over twice the elastic moduli of comparably porous xerogel films. The transmission electron microscopy (TEM) images indicate that the gel networks comprise structures with thick necks between clusters. Triethoxyfluorosilane (TEFS) forms gels rapidly making it well suited for commercial processes; however, the gel time appears to be inversely related to mechanical properties. On the basis of TEM and FTIR experiments, we propose a mechanism in which particle growth ceases when adjacent particles impinge on one another; additional condensation reactions occur at regions of high curvature between particles resulting in thickened neck morphologies. As interconnectivity is critical to the robustness of these films, thickened necks mechanically stabilize the solid network and impart an enhanced elastic modulus. This mechanism of strengthening (particle growth until impingement followed by reinforcement of weaker sites) may be useful in future material design.