Simulation of convection-driven wet-chemical etching

Abstract

In a wet-chemical etching process, the resulting etched shape is smaller than the originally designed shape at the mask. This is caused by the fact that, as soon as material next to the mask is dissolved, material under the mask will be dissolved too. This is the so-called undercut effect. During an etching process, eddies arise inside the etch-hole cavity, which decrease the etch-speed. These eddies may lead to an increase of the undercut. In order to improve the production process, it is necessary to have insight into these phenomena. Therefore, in this thesis the simulation of convection-driven wet-chemical etching is studied. It is known that the boundary of an etch hole moves slowly during a wet-chemical etching process. Therefore, the velocity and concentration fields at each time step can be computed as if the boundary were stationary. Since a typical etch hole is small, the corresponding Reynolds number based on the associated length-scale is sufficiently small to allow the use of the Stokes equations in the description of the flow. If we assume that the fluid has a uniform density in the etch hole, the flow problem can be solved independently of the concentration problem. In a convection-driven etching process the fluid flow and the eddy structure inside an etch-hole geometry play an important role. Therefore a large part of the thesis is devoted to the flow problem.