Sacrificial etching is one of the most important process steps in Micro-Electro-Mechanical Systems (MEMS) technology, since it enables the generation of free-standing structures. These structures are often the main part of micro-mechanical devices, intended to sense or induce a mechanical movement. The etching process transforms an initial multisegmented geometry and depends on material properties and several process conditions. One of the crucial issues for etching is the etching selectivity on different materials. The major task for the simulation is to answer how sacrificial layer surfaces regress in time under the influence of process parameters and to which magnitude surrounding material segments are affected by the etching process. In order to analyze these effects a full, three-dimensional etching simulation tool was developed.
During simulation the topography of the initial multisegment geometry is changed. This change is handled by a level-set algorithm, which describes the movement of a boundary during time with a certain speed. In our case, the moving boundary is the material surface which is exposed to the etching agent. The etch rates determine the speed function of this level-set equation.
Because of additional factors, like the transport of the etching medium on the surface, including diffusion and evacuation of the etch products, lower etch rates than the blanket ones are observed. The etching medium is transported through a relatively narrow channel, which dilutes the concentration of the etching medium at the interface where the reaction takes place. This behavior is modeled by diffusive transport of the etching agent from the etchant source to the etcher/material interface. Special care must be taken in the discretization of the boundary conditions of the diffusion equation, since the level-set function is located mainly between the grid points.
After the etching simulation is finished, the resulting boundary representation has to be converted to a volume mesh to make it applicable for further investigations.