MEMS devices require a clear design-for-manufacturability strategy that establishes concurrent-design principles through a common CAD framework. Detailed process and material-property characterization early in the design phase can save enough time to lead to low-cost volume manufacturing of MEMS. Toward this end, a comprehensive, integrated suite of MEMS design tools that incorporates a top-down design methodology with reusable parametric libraries of MEMS components, standard MEMS package libraries and relevant system components can reduce time-to-market.
Design-for-manufacturability is an approach to product design that systematically includes considerations for manufacturability in the design process. It fundamentally includes organizational changes, systematic design principles and a common CAD methodology and framework for evaluating device designs.
For example, during the conceptual-design phase, it is critical to have many design concepts simultaneously to increase the likelihood of a successful design that meets target specifications. The key to multiple viable concepts is the ability to rapidly create, analyze and evaluate designs. For MEMS, this means making libraries of parameterized design elements available for engineers to use (and reuse) as building blocks.
Parameterized libraries of elements were developed as the backbone of integrated MEMS design tools. Today, libraries exist for multiple physical domains such as electromechanical, RF, microfluidics, magnetics and optics. Each element of the library is essentially an analytic or semianalytic macromodel that captures the behavior of the element accurately over the possible actuations of the element. Therefore, each component model has at least six mechanical degrees of freedom and will include all geometric and material-property parameters of each element, allowing the designer to then set variables accordingly.
Concurrent-engineering practices are more efficient using design environments that are compatible between the MEMS and IC design worlds. Standard IC design tools should be the environment of choice for MEMS design. In this environment, the schematic model of the MEMS device assembled from parameterized library components immediately enables rapid and accurate characterization of a specific conceptual design. Also, models of multiple conceptual-design topologies are easily and rapidly modeled and ready for evaluation in ways that dramatically save time.
The first advantage of this simulation approach is the ability to set fabrication process constraints; the second is the ability to include the system-level ASIC (signal conditioning) during the evaluation of conceptual designs. Third, the approach offers the ability to perform virtual manufacturing through parametric sensitivity, stress and statistical Monte Carlo analyses to determine the influence of the specific design variables. This provides immediate feedback on the viability of certain concepts over others, and can literally save years in design time.
In the next phase of design, detailed engineering analysis is performed. The key steps involve applying design-of-experiments techniques to define the simulation space and then use a variety of analysis techniques to do optimization studies, and tolerant and robust design.
The availability of various physics-based solvers to simulate electrostatics, mechanics, coupled electromechanics, magnetics, eletrothermal effects and fluidic-structure coupling effects is also a vital part of a CAD framework. Although it may be possible to perform such simulations in multiple point tools, it is invariably faster and more efficient to have them within one integrated tool.
In parallel with design group activities, the process group begins to create a manufacturing specification, which essentially contains three major activities: develop process requirements, identify and evaluate existing processes, and determine packaging and test requirements.
Since MEMS devices are essentially mechanical, accurately characterizing material properties is also crucial for design optimization. In addition, design-for-manufacturability requires detailed process characterization. In some cases, this requires process short-loop experiments using standard process-characterization structures on a test die.
Having such test dice to identify and characterize critical process characterizations (e.g. sacrificial-layer thickness, line widths, verification of design rules, etc.) substantially improves all MEMS design activity from initial conceptual development to virtual manufacturing. Iteration between the process and design groups takes place through continuous enhancement of the reusable libraries of design kits and models.
Lastly, in MEMS design, the availability of process design kits, which contain materials and process data for use in simulation models throughout the design, is essential. It has been demonstrated that initial investment in creating such design kits through material-property and process characterization pays off in fewer wafer starts and higher yield.
To ensure manufacturability of the product, packaging and testing issues of the MEMS device also need to be solved in a systematic way, congruent with other engineering tasks. In MEMS, packaging is costly, typically accounting for a substantial portion of the overall product development cost, primarily due to device redesign or custom packages Both of these issues can be controlled by simultaneous initiation of the package design. Package analysis capabilities are now available as integrated libraries within MEMS design tools.
Mark daSilva (email@example.com) is the director of MEMS technology for Coventor Inc. (Cary, N.C.).
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