Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 59

they exhibit very high energy densities; they have enormous recoverable
strain levels (strains of over 8% have been demonstrated); since they are ther-
mally activated, the power consumption can be high and the mechanical
bandwidth low; the processing is somewhat complicated; and the material
can fatigue if repeatedly cycled at high strain levels.
MEMS Design Tools
MEMS design is frequently more demanding than the design of integrated
circuits. In the integrated circuits domain, a process technology and the asso-
ciated design rules usually already exist. The designers merely need to build
these into their computer-aided design tool, which usually only considers the
electrical effects, and come up with a design. In MEMS technology, the situa-
tion is much more complicated. As we have already mentioned, a customized
process sequence must frequently be developed for each device type as part of
the product development effort, and design rules are not known until a pro-
cess sequence is finalized. Furthermore, the material properties are usually
not fully known and are highly dependent on the process sequence and condi-
tions, which also are not known beforehand. Also, many MEMS devices have
several physical phenomena (electrical, mechanical, thermal, chemical, etc.)
occurring simultaneously that leads to many strongly coupled fields, thereby
making the design process more challenging. Importantly, the IC designer
usually does not need to know much about the fabrication, whereas in MEMS
design, the designer must be an expert in MEMS fabrication [22, 43].
Fortunately, there are some design tool capabilities now available to the
MEMS community that are suitable for process, physical, device, and systems
modeling [44]. The process-modeling tools are essentially the same as those
used by the integrated circuit industry and enable the designer to create pro-
cess models and mask artwork. Numerical techniques are available to simulate
the processing steps. Although these tools are quite good for predicting electri-
cal behavior, they are not very good for predicting mechanical material prop-
erties. One important feature of MEMS process design tools is the ability to
create representative 3-D renderings of the devices. The physical-level design
tools are used to model the behavior of components in the real 3-D continuum
using partial differential equations. These tools can be analytical or numeric;
the numeric techniques include finite-element, boundary-element, and finite-
difference methods. These tools are based mostly on finite-element tools that
are modified versions of FEM tools used in macroscale design. The device level
models are macro-models or reduced-order models that capture the physical
behavior of a component (over a limited range) and are compatible with the
system level models. Care must be exercised to ensure that the dynamic range
of the model is not overextended. System level models are high-level block