Page 270 - The Mechatronics Handbook

We also survey available tools which a MEMS developer can use to achieve good simulation results. Many
of these tools build MEMS development systems on platforms already in existence for other technologies,
thus leveraging the extensive resources which have gone into previous development and avoiding “rein-
venting the wheel.”
For our discussion of modeling and simulation, the salient characteristics of MEMS are:

1. inclusion and interaction of multiple domains and technologies,
2. both two- and three-dimensional behaviors,
3. mixed digital (discrete) and analog (continuous) input, output, and signals, and
4. micro- (or nano-) scale feature sizes.

Techniques for the manufacture of reliable (two-dimensional) systems with micro- or nano-scale feature
sizes (Characteristic 4) are very mature in the field of microelectronics, and it is logical to attempt to extend
these techniques to MEMS, while incorporating necessary changes to deal with Characteristics 1–3. Here
we survey some of the major principles which have made microelectronics such a rapidly evolving field,
and we look at microelectronics tools which can be used or adapted to allow us to apply these principles
to MEMS. We also discuss why applying such strategies to MEMS may not always be possible.

13.2 The Digital Circuit Development Process: Modeling
and Simulating Systems with Micro- (or Nano-) Scale
Feature Sizes

A typical VLSI digital circuit or system process flow is shown in Fig. 13.1, where the dotted lines show
the most optimistic point to which the developer must return if errors are discovered. Option A, for a
“mature” technology, is supported by efficient and accurate simulators, so that even the first actual
implementation (“first silicon”) may have acceptable performance. As a process matures, the goal is to
have better and better simulations, with a correspondingly smaller chance of discovering major perfor-
mance flaws after implementation. However, development of models and simulators to support this goal
is in itself a major task. Option B (immature technology), at its extreme, would represent an experimental
technology for which not enough data are available to support even moderately robust simulations. In
modern software and hardware development systems, the emphasis is on tools which provide increasingly
good support for the initial stages of this process. This increases the probability that conceptual or design
errors will be identified and modifications made as early in the process as possible and thus decreases
both development time and overall development cost.
At the microlevel, the development cycle represented by Option A is routinely achieved today for many
digital circuits. In fact, the entire process can in some cases be highly automated, so that we have “silicon
compilers” or “computers designing computers.” Thus, not only design analysis, but even design synthesis
is possible. This would be the case for well-established silicon-based CMOS technologies, for example.
There are many characteristics of digital systems which make this possible. These include:
• Existence of a small set of basic digital circuit elements. All Boolean functions can be realized by
combinations of the logic functions AND, OR, NOT. In fact, all Boolean functions can be realized
by combinations of just one gate, a NAND (NOT-AND) gate. So if a “model library” of basic gates
(and a few other useful parts, such as I/O pins, multiplexors, and flip-flops) is developed, systems
can be implemented just by combining suitable library elements.
• A small set of standardized and well-understood technologies, with well-characterized fabrication
processes that are widely available. For example, in the United States, the MOSIS service [3]
provides access to a range of such technologies. Similar services elsewhere include CMP in France
[4], Europractice in Europe [5], VDEC in Japan [6], and CMC in Canada [7].
• A well-developed educational infrastructure and prototyping facilities. These are provided by all of
the services listed above. These types of organization and educational support had their origins in
the work of Mead and Conway [8] and continue to produce increasingly sophisticated VLSI engineers.

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