Micro-electromechanical
systems (MEMS) are tiny systems (from a few micrometers to
millimetres in size) that combine mechanical and electrical components.
In the
most general form, MEMS consist of mechanical microstructures, like
sensors, actuators, gears, motors etc., and microelectronics, all
integrated onto the same silicon chip. Microsensors detect changes in
the systems environment by measuring mechanical, thermal, magnetical,
chemical or electromagnetical information. Microelectronic components
process this information and signal the microactuators to react and
create some form of change to the environment. The major steps in the
MEMS fabrication technology include film growth, doping, lithography,
etching, dicing, and packaging. Devices are usually fabricated on Si
substrates, thin films are grown on these substrates and are used to
build active and
passive components and interconnections between circuits. These films
are: (1) epitaxial Si, (2) SiO2, (3) silicon nitride Si3N4, (4)
polycristalline Si, and (5) metal films. To modify electrical or
mechanical properties, films are doped with impurities by thermal
diffusion or ion implantation.
A specific issue for
MEMS applications are sacrificial layers used to form the mechanical
components. After formation of the mechanical components the
sacrificial layer is selectively etched away, leaving the desired
pattern in the film. The chemical etching of the sacrificial layer is a
crucial step in the fabrication of MEMS. For example, SiO2 or
phosphosilicate-glass (PSG) can be easily etched in a hydroflorid acid
(HF)
based solution. HF based etchants etch the material isotropic. In some
cases anisotropic etching is required, which means that etchants like
potassium hydroxide (KOH) etch faster in a preferred direction.
The focus of the work is the development of etching models for
sacrificial etching and the simulation of the moving etch front, which
is calculated by means of a diffusion equation (for the etchant
transport) with appropriate initial and boundary conditions in
combination with the level set algorithm. The solution of the diffusion
equation delivers the etching speed neccesary for the level set method,
which describes the moving boundary. The advantage of combining both
methods is that a single grid can be used. The level set equation will
be solved using the narrow band approach, which allows a fast
calculation.
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