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CAD tools shrink small world of MEMS design

Posted: 16 Mar 2005 ?? ?Print Version ?Bookmark and Share

Keywords:mems? microdesign technology? micromanufacturing? micromechanical design? cad?

The introduction of MEMS-specific capabilities in mainstream design tools is paving the way to greater use of microtechnology. These tools will help companies facing miniaturization challenges break new ground; innovate products, devices and micromanufacturing technologies; and broaden the use of MEMS for a variety of applications.

Much of the groundbreaking work in this emerging field took place in the semiconductor industry, where the drive to produce silicon ICs steered R&D into the creation of manufacturing processes that yield features and components one can barely see with the naked eye. Although ICs were only the initial focus for the development of microdesign methodologies, manufacturers are leveraging these technologies to develop and produce MEMS and miniaturized mechanical devices to satisfy market demand for miniaturization.

Until the last few years, MEMS and micromechanical design and production were more often research-oriented activities that took place in university labs than commercially viable manufacturing enterprises. The design and manufacturing tools used for MEMS and micromechanical design were highly specialized, and few engineers knew how to accomplish microdesign without developing manufacturing techniques and enabling technologies as part of the design process. Much of this work was done using 2D layouts to represent configurations of the separate layers of silicon that are deposited sequentially to create a conventional MEMS component.

However, the availability of MEMS-specific design functionality in mainstream CAD tools is driving the cost-effective development of MEMS and miniaturized mechanical devices for commercial products.

Potential applications for MEMS are wide ranging. Any miniaturized electrical system that requires a mechanical component is a candidate, including inertial sensors, switches and relays, resonators and mechanical filters, microcapacitors, inductors and probes, as well as inclinometers, valves, DNA sequencers and chemical- and biological-agent sensors.

Whatever the application, the driving force behind micromechanical system development is size and weight. Unlike ICs, which focus on passing electrical current through extremely small circuits, MEMS components all have some mechanical element and most have at least one movable part. While depositing several thin layers of silicon and etching material away to create layer configurations works well for manufacturing ICs and some MEMS devices, new manufacturing processes are emerging that give designers more options for optimizing the mechanical aspects of MEMS.

For years, the primary material available to MEMS designers was silicon and the only manufacturing processes available emanated from the semiconductor industry. These processes generally required highly specialized tools, limited designs to the use of four or five layers of silicon of specific thicknesses, and were time-consuming and costly for creating MEMS components. But as more and more manufacturers turn to MEMS and microdesign to meet demands for miniaturization, new automated processes have been developed that focus on the mechanical aspects of MEMS design.

One company, Microfabrica Inc., has developed a proprietary micromanufacturing technology called EFAB that leverages 3D CAD data. EFAB technology is similar to silicon depositionthe process is based on selective electrolytic deposition of metal onto a substratebut is more robust, automated and flexible. EFAB allows engineers to design arbitrary, complex 3D geometries based on electroplatable materials such as nickel, silver, copper, gold and platinum, instead of silicon, in tens to hundreds of layers that can range in thickness from 2-10?m.

The ability to create arbitrary geometries rather than those utilized by the IC industry opens a whole new world of possibilities to MEMS designers. Take the mechanical helical spring, for example. This is one of the most efficient designs and most useful devices for controlling force and displacement ever developed. It is difficult, however, to produce an effective helical spring based on four or five layers of silicon. Thus, you cannot take advantage of one of the most proven mechanical designs at the miniature level using traditional silicon-based micromanufacturing. Microfabrica, however, is working to eliminate these types of limitations for mechanical engineers doing microdesign.

One development that helps facilitate the widespread use of MEMS and microdesign techniques is the inclusion of MEMS-specific functionality in a mainstream 3D CAD application. The ability to use a familiar tool and design environment for designs ranging from MEMS and microdevices to larger assemblies and components eliminates the time, effort and cost involved with learning specialized tools.

The typical sequence for designing a MEMS device is to begin with a model of the component created out of multiple semiconductor layers. Designers then produce photomasks, 2D layouts for each layer that match each specific cross-section configuration, to drive manufacturing.

A significant challenge in MEMS design arises in working with photomasks for several cross-sections of a solid model at the micron and submicron level for a device that will be packaged in a much larger assembly. Without MEMS-specific CAD functionality, designers must move back and forth in their CAD systems between different dimensional scales, from microns to millimeters to meters. They are unable to truly visualize the complete assembly and cannot take advantage of basic solid modeling features such as parametric associative design.

Fortunately, 3D solid modeling systems with the ability to operate on this scale simplify the complexity of the process by providing capabilities that specifically address MEMS design functions. For example, some mainstream 3D CAD software packages enable a broad geometric range, which allows designers to work on the same assembly at the micron level all the way up to many meters. Engineers can simply zoom into the MEMS detail and zoom out to the larger assembly, providing full 3D visualization of both the MEMS component and its packaging. The software also automatically cross-sections the MEMS component and creates fully associative photomasks for each layer, eliminating the time and effort involved in manually creating each 2D layout.

As the design is modified and refined, changes propagate to all associated design documents, including components, assemblies, detail and photomask drawings. Submicron feature definition, collision/interference detection of components and the creation of feature patterns and patterns of patterns are capabilities that are also useful for MEMS design.

- John McEleney

Chief Executive Officer

SolidWorks Corp.

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