A cell phone that fits inside a pinky ring. A television screen you wear like a wristwatch. Incredibly small robots that travel through the bloodstream to the site of a malignant tumor, which they quickly attack and dispatch. While these types of products sound like the stuff of science fiction, advances in micro-mechanical design and manufacturing technologies are making their development less of a futuristic dream and more of a realistic possibility.
In fact, the miniaturization of mechanical systems with features and components at the micron and sub-micron level is already upon us and will eventually lead to these very types of products. Much of the ground-breaking work in this emerging field took place in the semiconductor industry, where the need to produce integrated electrical circuits based on silicon drove research and development into the creation of manufacturing processes that can produce features and components that one can barely see with the naked eye. Although integrated circuits were the initial focus for the development of micro-design methodologies, manufacturers are leveraging these technologies to develop and produce micro-electromechanical systems (MEMS) and miniaturized mechanical devices to satisfy market demand for miniaturization.
Until the last few years, MEMS and micro-mechanical 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 micro-mechanical design were highly specialized, and very few engineers knew how to accomplish micro-design 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, such as the SolidWorks® mechanical design system from SolidWorks Corp (Concord, MA) is driving the cost-effective development of MEMS and miniaturized mechanical devices for commercial products.
MEMS and Micro-Mechanical Applications
Even though the MEMS market is relatively young, the use of MEMS components is already well established in some industry applications. The most widely used application for MEMS components involves car airbag sensors used in the automotive industry. These sensors utilize extremely small accelerometers to detect the abrupt change in acceleration that will activate the airbags. Inside the MEMS circuit, a small cantilever of silicon hangs in free space and vibrates with different amplitude depending on the rate of acceleration. Using this simple mechanical element, the sensor measures the change in capacitance between the cantilever and the ground plane and deploys the airbag during sudden deceleration.
Other current MEMS applications include the flow valves used in inkjet printers and the switches used in fiber optic and wireless communications networks. Potential applications for MEMS are almost limitless. Any miniaturized electrical system that requires a mechanical component is a candidate for MEMS, including inertial sensors, switches and relays, resonators and mechanical filters, micro-capacitors, micro-inductors, micro-probes, inclinometers, valves, DNA sequencers, and chemical and biological agent sensors.
Whatever the function, the driving force behind micro-mechanical system development is size and weight. Whenever the application provides little real estate for placing or mounting components or has low weight as a key requirement, micro-devices become the preferred approach.
MEMS and Micro Manufacturing
Unlike integrated circuits, 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 integrated circuits 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 micro-design to meet demands for miniaturization, new automated processes have been developed that focus on the mechanical aspects of MEMS design.
One such company is MEMGen Corp (Burbank, CA), which has developed a proprietary micro-manufacturing technology called EFAB™ that leverages 3D CAD data. "For many years, micro-devices could only be created by experts using specialized tools and exotic silicon-based micro-machining techniques," explains Nelsimar Vandelli, Director of Applications Engineering at MEMGen.
"These approaches were an outgrowth of the integrated circuit needs of the semiconductor industry and were not specifically developed for the micro-design needs of mechanical engineers. In many ways, the process was driving the design, limiting what mechanical engineers could do and basically telling designers that they could design anything they wanted as long as it was in silicon and used only four layers of one or two thicknesses," Vandelli says. "We are developing technologies that leverage existing mechanical design tools, such as the SolidWorks 3D solid modeling system, and focus on the mechanical elements of MEMS design."
MEMGen's EFAB technology is similar to silicon deposition -- the process is based on selective electrolytic deposition of metal onto a substrate -- but 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 microns.
Vandelli says the ability to create arbitrary geometries rather than those utilized by the semiconductor industry opens a whole new world of possibilities to MEMS designers. "Take the mechanical helical spring, for example," he notes. "It's one of the most efficient designs and most useful devices for controlling force and displacement ever developed. It's very difficult to produce an effective helical spring based on four or five layers of silicon. In other words, you cannot take advantage of one of the most proven mechanical designs at the miniature level using traditional silicon-based micro manufacturing. We are working to eliminate these types of limitations for mechanical engineers doing micro design."
Mainstream CAD Replacing Specialty Applications
One development that is helping to facilitate the widespread use of MEMS and micro-design 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 micro devices 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 photo masks, 2D layouts for each layer, which match each specific cross-section configuration to drive manufacturing. A significant challenge in MEMS design arises in working with photo masks for several cross-sections of a solid model at the micron and sub-micron 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, are unable to truly visualize the complete assembly, and cannot take advantage of basic solid modeling features such as parametric associative design.
The SolidWorks solid modeling system simplifies the complexity of the process by providing capabilities that specifically address MEMS design functions. For example, SolidWorks software enables 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 photo masks 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 photo-mask drawings. Sub-micron 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.
Another important benefit in using mainstream CAD for MEMS design is the ability to conduct structural, thermal, electromagnetic and fluid flow analyses directly on the solid model. Finite element analysis (FEA) is extremely important for predicting the behavior and performance of a MEMS component or micro device before producing costly prototypes.
"The MEMS-specific functionality in SolidWorks software is tapping the mainstream potential for MEMS," MEMGen's Vandelli says. "Working with MEMS involves creating features that are smaller than some of the tolerances used for many macro-manufacturing operations. SolidWorks automates many of the steps involved with MEMS, making much of the complexity and tedium completely transparent to the user."
Single Associative Platform Sparks Productivity
Axsun Technologies (Billerica, MA) is a company that has standardized on mainstream mechanical CAD for MEMS design since its founding in 1998. Axsun specializes in the design and manufacture of miniaturized optical micro-instrumentation for use in a variety of industries. As someone who had worked in the trenches of MEMS design using 2D tools, Chief Technology Officer Dale Flanders said the company had to use SolidWorks CAD software from the very start for several reasons.
"One of the reasons we needed SolidWorks software from the very beginning was we needed to be able to show visually compelling images of very complex micro-electro-optical instrumentation and telecommunications devices to both investors and customers before building them," Flanders explains.
"We also wanted to leverage the software's assembly and interference-checking capabilities as well as the associative nature of 3D solid models to support other engineering functions, such as mechanical, thermal, and sophisticated electromagnetic analyses and rapid prototyping activities. SolidWorks models integrate very nicely with finite element analysis and work well with our optical design tools, which we use for diffractive optics and beam propagation," he adds.
Flanders notes that the associative nature of SolidWorks data is a key element in fast, cost-effective MEMS design. "SolidWorks software was the beginning of our inexorable trend toward integrated tools that really work. We do not expend a lot of overhead in translating files from one package to another and having software with a range of capabilities helps us to solve system-level problems seamlessly."
Michael Conti, a CAD engineer at Axsun, says using a mainstream CAD tool when working at such small dimensions is a big plus. "SolidWorks helps us avoid any delays associated with data conversion or file translations. It's particularly useful when working with our analyst. She can run analysis on a MEMS component and show me where we need to add a flexure or increase stiffness directly on the solid model. Analysis also shows if we have any part interference, and I can use SolidWorks to locate and address it. It's really efficient to stay in one software package."
Richard Dee, principal of LightWorks Machine Design, Inc (Acton, MA), a contract designer who works with many companies utilizing MEMS, including Axsun, says using a mainstream CAD tool like SolidWorks makes collaborating on product designs involving MEMS more efficient.
"You only need one piece of software to work with and collaborate across many customers and different types of components. I'm not involved in MEMS design per se, but I do design a lot of MEMS packaging and machines that employ MEMS components. In SolidWorks, it's easy for me to put in a placeholder for the MEMS device that my client can pop in later without having to adjust the scale or dimensions. There's no need for model re-creation, translations, or rescaling -- no clumsiness at all. It's great to work in one piece of software from the sub-micron level up to as far as you need to go," Dee says.
The introduction of MEMS-specific capabilities in mainstream design tools such as the SolidWorks modeling system is helping to pave the way to greater use of micro-technology. These tools will help companies facing miniaturization challenges to break new ground; to innovate products, devices, and micro-manufacturing technologies; and to broaden the use of MEMS systems for a variety of applications. It's often said that technology is making the world smaller, and in the world of mechanical design, mainstream tools for leveraging MEMS and micro-design technologies have the potential to make something really small turn out to be quite big.
