Page 101 - An Introduction to Microelectromechanical Systems Engineering

80 MEM Structures and Systems in Industrial and Automotive Applications

structural elements, then proceeds onto analysis and simulation, and finally onto
outlining of the individual steps in the fabrication process. This is often an iterative
process involving continuous adjustments to the shape, structure, and fabrication
steps. The layout of the lithographic masks is the final step before fabrication and is
completed using specialized CAD tools to define the two-dimensional patterns.
Early design considerations include the identification of the general sensing or
actuation mechanisms based on performance requirements. For instance, the output
force requirement of a mechanical microactuator may favor thermal or piezoelectric
methods and preclude electrostatic actuation. Similarly, the choice of piezoresistive
sensing is significantly different from capacitive or piezoelectric sensing. The inter-
disciplinary nature of the field brings together considerations from a broad range of
specialties, including mechanics, optics, fluid dynamics, materials science, electron-
ics, chemistry, and even biological sciences. On occasion, determining a particular
approach may rely on economic considerations or ease of manufacture rather than
performance. For example, the vast majority of pressure sensors use cost-effective
piezoresistive sense elements instead of the better performing, but more expensive,
resonant-type sense structures.
The design process is not an exact analytical science but rather involves develop-
ing engineering models, many for the purpose of obtaining basic physical insights.
Computer-based simulation tools using finite-element modeling are convenient for
analyzing complex systems. A number of available programs, such as ANSYS ®
(ANSYS, Inc., of Canonsburg, Pennsylvania) and CoventorWare™ (Coventor, Inc.,
of Cary, North Carolina), can simulate mechanical, thermal, and electrostatic
structures (see Figure 4.1). Substantial efforts are currently under way to develop
sophisticated programs that can handle coupled multimode problems, (e.g., simulta-
neously combining fluid dynamics with thermal and mechanical analysis). As
powerful as these tools are perceived to be, their universal predictive utility is ques-
tionable. However, they can provide valuable insight into and visualization of the
device’s operation.
In planning a fabrication process, the choice is to use a standard foundry service
with a completely predefined process flow, to use a service that allows the selection

Front side

Frame
Membrane
Back side
Figure 4.1 A finite element simulation using ANSYS modeling program of a quarter of a bulk
micromachined silicon pressure sensor showing contours of mechanical stress in response to an
applied pressure load.