Tetraethylorthosilicate reaction rates on SiO2 at 1000 K: Zero-order dependence on hydroxyl coverage and implications for reactions with three-membered siloxane rings

We have determined key kinetic parameters for the reaction of tetraethylorthosilicate (TEOS) on SiO2. This was accomplished under conditions (20-500 mTorr at 1000 K) that pertain directly to TEOS-based chemical vapor deposition processes. TEOS reactions were carried out using deuterated silanols (SiOD) on the initial SiO2 surface. This allowed Fourier transform infrared spectroscopy measurements to distinguish the consumption of SiOD by TEOS from the concurrent formation of SIGH which results from TEOS decomposition at 1000 K. While SiOD consumption did exhibit a first-order dependence on SiOD coverage, TEOS decomposition exhibited a zero-order dependence on the total coverage of hydroxyl groups. This suggests that reactions with hydroxyl groups alone cannot account for all of the TEOS decomposition reactions at 1000 K. Since the low coverage of two-membered siloxane [(Si-O)(2)] rings was consumed during the initial TEOS exposure, siloxane (Si-O-Si) bridges in three-membered siloxane [(Si-O)(3)] rings may be the additional species responsible for the constant rate of TEOS decomposition. However, it is not conclusive that this type of site-specific mechanism controls the chemistry. The data may also be explained with a site-independent mechanism in which intramolecular decomposition of TEOS on the surface provides a common rate-determining step for subsequent consumption of hydroxyls and siloxane bridges on SiO2. Regardless of the specific mechanism, our results predict that deposition rates will be insensitive to the relative coverages of siloxane bridges and hydroxyls on SiO2. Therefore, a precise knowledge of the coverages of these species on SiO2 is not essential for modeling thermal TEOS decomposition rates. (C) 1996 American Vacuum Society.