Page 876 - Mechanical Engineers' Handbook (Volume 2)

3 Design and Simulations 867

Silicon substrate

(a)

(b)
Figure 4 (a) Bulk micromachining anisotropic etch. (b) Examples of bulk micromachining devices.

the device. Finite-element techniques are used to solve these modeling equations. There are
a variety of computer aided design (CAD) tools to aid the designer in the simulation and
modeling of the device. In a very fundamental way, these tools are more complicated than
the software for design of either solely ICs or solely mechanical devices. This is due to the
close coupling of both electrical and mechanical effects within many MEMS. Consider a
microcantilever that is pulled down by electrostatic forces. Its simulation has to take into
account both the flow of electrical charge and mechanical elasticity in an iterative and self-
consistent fashion.
Thermal, optical, magnetic, fluidic, and other mechanisms are also active in some
MEMS and have to be handled self-consistently in the simulation phase.
Two basic approaches have been taken in the past decade to the need for specialized
software for the design and simulation of MEMS. In the first approach, CAD design, tools
and available software from electronic design were modified to accommodate the requirement
for MEMS design. In the second approach finite-element modeling was applied to MEMS.
Software from the Tanner Tools very large scale integrated (VLSI) design suite were used
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for MEMS, for example MEMS-PRO, which was recently acquired by MEMSCAP, as was
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the popular mechanical engineering software from ANSYS. Recently, new suites of software
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specifically developed for MEMS were marketed. Most of them include electronic, mechan-
ical, and thermal simulation, and some have other physical mechanisms as well as processing
simulation tools. Such software is available from CFD Research Corporation, Coventor (for-
merly called CRONOS Technologies), IntelliSense Corporation, and Integrated Systems En-
gineering. These tools vary widely in the mechanisms and material parameters that they
include, the details of design and simulation of devices, and the fabrication facilities with
which they interface. The choice of suitable software to use for MEMS design is still chal-
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lenging. MEMSCAP is based on CADENCE (which is the most popular IC design tool ).
It consists of a set of tools which enable the design flow either bottom up or top down. It