(45d) Ab Initio Calculations of the Initial Reaction Mechanisms for Tio2 Atomic Layer Deposition Onto Sio2 Surfaces

Two of the most critical issues currently facing the semiconductor industry are the discovery of high-k gate dielectric replacement materials for SiO₂ and the development of deposition processes that will afford high surface uniformity and controlled growth at the atomic scale. Atomic layer deposition (ALD) is an ideal candidate for meeting these challenges, enabling the deposition of a material through highly uniform and conformal growth, with thickness control at the atomic layer level. Our current work involves the use of a multi-scale modeling strategy to gain theoretical insights into ALD at the atomic scale, as applied to the deposition of TiO₂ thin films. First, information about the atomic processes and energetics involved is gathered using ab initio density functional theory (DFT) studies. Then, based on the results of the DFT calculations, the dominant kinetic processes occurring on the surface can be modeled simultaneously using the Kinetic Monte Carlo (KMC) simulation technique. This technique provides information about the kinetics of the processes occurring on the surface, while preserving the essential atomic structural details of the system. Based on these muti-level simulations, a better understanding of the mechanisms and the critical factors contributing to the ALD film growth process can be obtained. In this work, we present results from the first phase of our simulation strategy, which involves DFT calculations of TiO2 film growth on a hydroxylated SiO₂ substrate. A series of cluster calculations were carried out to study the atomistic mechanism and energies of the initial surface reactions. The effects of surface functionalities and precursors (TiCl₄ and TiI₄) on the thin film deposition process are discussed. We predict activation barriers and the reaction pathways of these precursors (as well as their alternating exposure to H₂O) on isolated hydroxyl sites, neighboring hydroxyl sites, and oxygen bridges. The potential energy surface and Gibbs free energy surfaces are predicted at two typical growth temperatures. In conjunction, the temperature dependence of the film growth is discussed. Our DFT calculations show the complexity of the growth mechanisms during ALD processing. The concentration and arrangement of the different surface reactive groups determine the resulting film growth characteristics. Therefore, the factors which are related to the presence of the different surface reactive groups, such as the preparation of the initial SiO₂ substrate and the growth temperature, are predicted to have a marked influence on film structure and growth rate. As far as we know, this is the first ab initio investigation of this system, despite its relevance to the semiconductor industry. We plan to expand our work in the future by investigating bulkier metal precursors, such as titanium alkoxydes, in order to understand the effect of steric hindrance during the deposition process.