Ab initio Molecular Orbital Study of Water Absorption and Hydrolysis of Chemical Vapor Deposited SiOF Films I

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Abstract
We investigated the mechanism of water absorption and hydrolysis of F-doped SiO2 (SiOF) films using the ab initio molecular orbital (MO) method with small SiF n (OH)4- n (n=0–3) tetrahedral model clusters. We focused on the dependence of the reactivity of these clusters with both OH- and H2O on the number of F atoms bonded with a Si atom. The reactivity of defect sites such as non bridging oxygens (NBOs) and free volumes around Si–F bonding were also investigated. The calculations revealed the following. SiOF films are more reactive with OH- than with H2O. Si atoms linked with multiple F atoms become more reactive with H2O by reducing the activation barrier of the –OH coordination to Si, and with OH- by increasing the stabilization energy of the pentacoordinated-Si formation. Multiple H2O attacks at sparse, i.e., sparsely packed, network sites increase the adsorption energy and reduce the activation barrier. NBO defects also promote successive H2O adsorption.