Thermal oxidation of silicon is one of the most important steps in the fabrication of highly integrated electronic circuits, being mainly used for efficient isolation of adjacent devices. If a surface of a silicon body has contact with an oxidizing atmosphere, the chemical reaction of oxidants (oxygen or water vapor) with silicon (Si) forms silicon dioxide (SiO2). If a silicon dioxide domain already exists, the oxidants diffuse through the SiO2 domain to the Si-SiO2 interface. The parts of silicon which should not be oxidized are masked by a layer of silicon nitride.
During the oxidation process the chemical reaction consumes Si and the newly formed SiO2 has more than twice the volume of the original Si. This significant volume increase is the main source of mechanical stress in the materials, if the additional volume is prevented from expanding as desired. So thermal oxidation is a complex process in which the three subprocesses oxidant diffusion, chemical reaction, and volume increase occur simultaneously. From the mathematical point of view, the problem is described by a coupled system of partial differential equations, one for the diffusion of the oxidant through the oxide, the second for the conversion of Si into SiO2 at the interface, and a third for the mechanical problem. For the mechanics, an elastic or viscoelastic can be applied. In order to solve the numerical formulation of the oxidation process, the finite element method is applied.
Since the oxidant diffusion and the chemical reaction is strongly stress dependent, the oxidation process itself is influenced by stress. So during the last year a stress-dependent oxidation model which agrees with the real physical behavior has been designed and implemented. Also, more physical simulation results with a sharp interface between Si and SiO2 were realized.