Modelling of gas-surface interactions using atomistic approaches

Abstract

The oxidation of Si is one the basic steps in the manufacture of microchips in electronic devices. With integrated circuits increasingly getting smaller, the controlled deposition of the thin insulating SiO2 layers becomes critical. During rf reactive magnetron sputter deposition of silicon suboxides, various relevant ionic and molecular Si- and O-containing species relevant are present in the reaction chamber. These species impinge on the deposition surface, i.e., the Si substrate, oxidizing it and then forms a thin insulating layer of SiOx material. Precise control of vapor deposition of Si and O containing species requires understanding of the deposition process at the atomic or molecular. Here ab-initio methods, mainly density functional theory based techniques, are used to theoretically investigate the ability of relevant molecules in gas phase to physically or chemically adsorb on the clean Si surface. The most stable (clean) Si surface model with p(2x2) reconstruction was utilized in this work. Relevant species for SiOx deposition are O2, SiO, SiO2, as well as the Si and O atoms. O and O2, the well-adsorbed species on the Si surface have adsorption energies up to -6.00 eV. The SiO molecule, which is abundant at the deposition chamber, is adsorbed on Si surface with adsorption energies up to -2.50 eV. While SiO2 molecule on Si surface has adsorption energies up to -4.90 eV. Various adsorption sites of both SiO and SiO2 on the Si surface were identified, showing negligible adsorption barrier, an indication that these molecules are readily adsorbed on the Si surface. Observed red-shifts in the vibrational frequencies of both the adsorbed SiO and SiO2 molecules, indicate weakening of the Si-O bonds. Simultaneous coadsorption of O2 with a SiO molecule on the Si surface indicates an energy gain of -2.90 eV, higher than the energies gained when O2 and SiO are individually adsorbed, in case where O2 and SiO share the same Si surface atom to bond with. A SiO-precovered Si surface further promotes the adsorption of an incoming O2 molecule. Argon atoms are inadvertently introduced in the interface of Si/SiO2 during the growth process. Results of the static DFT calculations indicate that an argon atom does not bind to any of the atoms of the Si/SiO2 interface, and settles in the interface region with least atomic coordination. These calculations are done for a temperature of 0K so investigation of the dynamics at realistic temperatures is recommended. The electron impact ionization and the photoionization cross-sections of SiO calculated here have values typical of small diatomic molecules. Neither of the calculated cross-sections is unusually large and the ionization processes mentioned here are not the causes of the reported large flux of SiO+ ions impinging the deposition surface. Finally in a related topic, calculations of the initial steps of the free-radical polymerization process of PGMA, a polymer coating material that can potentially prevent device degradation, indicate that this process happens exothermically and spontaneously with the presence of a TBPO radical. Introduction of more radicals into the system causes the polymerization process to terminate. Both the GMA monomer and TBPO radical are well adsorbed on the Si surface.