Page 282 - The Mechatronics Handbook

NODAS simulator, basic elements include beams and gaps, as well as plate masses, anchors, and
electrostatic comb drives (vertical and horizontal). For the MUMPS process there is the Consol-
idated Micromechanical Element Library (CaMEL), which contains both a nonparameterized cell
database and a library of parameterized elements (which can be accessed through a component
“generator,” but not directly by the user). CaMEL supports the creation of a limited set of com-
ponents, including motors and resonators, in a fixed surface-micromachined technology. But the
bottom line for MEMS is that no set of basic building blocks has yet been identified which can
support all the designs, in many different energy domains and in a variety of technologies, which
researchers are interested in building. Moreover, there is no consensus as to how to effectively
limit design options so that such a fundamental set could be identified. In addition, the continuous
nature of most MEMS behavior presents the same kinds of difficulties that are faced with analog
elements. Development of higher level component libraries, however, is a fairly active field, with,
for example, ANSYS, CFD, MEMCAD, Carnegie Mellon, and MemsPro all providing libraries of
previously designed and tested components for systems developers to use. Most of these compo-
nents are in the electromechanical domain. As mentioned above, a few VHDL-AMS models are
also available, but these will not be of practical value until more robust and complete VHDL-AMS
simulators are developed and more experimental results can be obtained to validate these models.
• Is there a small set of well-understood technologies? Again the answer must be no. Almost all
digital and analog circuits are essentially two-dimensional, but, in the case of MEMS, many designs
can be developed either in the “2.5-dimensional” technology known as micromachining or in the
true three-dimensional technology known as bulk micromachining. Thus, before doing any mod-
eling or simulation, the MEMS developer must first choose not only among very different fabri-
cation techniques but also among actual processes. Both the Carnegie Mellon and Cronos tools,
for example, are based on processes that are being developed in parallel with the tools. MOSIS
does provide central access to technology in which all but the final steps of surface micromachining
can be done, but no other centrally maintained processing is available to the community of MEMS
researchers in general. For surface micromachining, the fact that the final processing steps are
performed in individual research labs is problematic for producing repeatable experimental
results. For bulk micromachining examples, fabrication in small research labs rather than in a
production environment is more the norm than the exception, so standardization for bulk pro-
cesses is difficult to achieve. In addition, because much MEMS work is relatively low-volume,
most processes are not well enough characterized for low-level modeling to be very effective. In
such circumstances it is very difficult to have reliable process characterizations on which to build
robust models.
• Is there a well-developed educational infrastructure and prototyping facilities? Again we must
answer no. Introductory MEMS courses, especially, are much more likely to emphasize fabrication
techniques than modeling and simulation. In [66] a set of teaching modules for a MEMS course
emphasizing integrated design and simulation is described. However, this course requires the use
of devices previously fabricated for validating design and simulation results, rather than expecting
students to complete the entire design-simulate-test-fabricate sequence in one quarter or semester.
In addition, well-established institutional practices make it difficult to provide the necessary support
for multidisciplinary education which MEMS requires.
• Are encapsulation and abstraction widely employed? In the 1980s many researchers believed that
multiple levels of abstraction were not useful for MEMS devices. Currently, however, the concept
of intermediate-level “macromodels” has gained much support [57,70], and increasing emphasis
is being placed on developing macromodels for MEMS components that will be a part of larger
systems. In addition, there are several systems in development that are based on sets of more primitive
components. But this method of development is not the norm, in large part because of the rich set
of possibilities inherent in MEMS in general. In Fig. 13.2(b) we have given a partial classification of
MEMS corresponding to the classification for digital devices in Fig. 13.2(a). At this point it is not

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