MEMS' Fantastic Voyages

Journey with us now to the ultra-miniature world of MEMS, where microscopically small accelerometers, valves, switches, and gears perform much of the same work as their bigger brothers but in impossibly small spaces.

Born in R&D labs more than a generation ago, MEMS devices took their nurturing from the semiconductor-manufacturing industry and have grown steadily in status if not stature. They rule the airbag-sensor world where, as extremely small accelerometers, they detect the slightest change in acceleration, and the ink-jet printer domain where, as miniature flow valves, they direct the dispersion of ink to produce printed pages.

They also act as pressure sensors inside tire-valve stems, inertial sensors, resonators and mechanical filters, micro-capacitors, micro-inductors, micro-probes, DNA sequencers, and chemical- and biological-agent sensors.

And that's just the beginning. Researchers and engineers alike say that MEMS are like early teens, continually asserting themselves and their worth and poised for explosive growth. Says market research firm In-Stat/MDR, they'll grow in consumer electronics to nearly 190 million devices in 2006 from about 5 million in 2001. In the automotive world, the firm predicts, there will be twice as many MEMS applications in 2007 as there were in 2002. The firm also predicts MEMS use in gyros to grow from $279 million in 2002 to $396 million in 2007.

With numbers like that, it's only natural that the tools for designing MEMS would likewise grow in number and sophistication. In the last ten years, software developers have been incorporating algorithms in their codes to handle the special problems inherent in MEMS design. On the CAD side, SolidWorks now has customers using its software in the design of commercial MEMS-based products as does ANSYS on the analysis/simulation front. MSC.Software and ALGOR also are working with customers in analysis of products.

While MEMS share many of the same design problems as products in the macro world, they pose a few different challenges that software developers have had to take into account in their codes. For example, says MSC's Brian Cheung, surface-to-volume ratios are different in the micro world from what they are in the macro world.

"In the macro world, thickness could be comparable to length and width, but micros are very thin and can be out of scale to the others," he says. Additionally, he says, because mechanical MEMS devices live in an electrical world, MEMS designers have to simulate electronics as well as mechanics.

Paul Lethbridge of ANSYS says fluid damping effects are different too. "They can be a thousand times greater," he says, "so MEMS designers have to think like aerospace engineers, figuring out how the device might move through a fluid."

Electrostatic forces are also greater on MEMS devices than macro devices. And, say Motorola engineers Dave Monk and Andrew McNeil, engineers have to be especially concerned with variations in manufacturing processes.

But, what makes MEMS especially challenging in analysis, says Lethbridge, is dealing with all of these issues and more simultaneously. "In the macro world, engineers have to consider maybe one or two physics," he says. "With MEMS, you have to look at several, including electrostatic, structure, and fluid, and they're all interrelated—a multi-physics approach."

In practical terms, that means engineers might have to use a different finite-element mesh for each physics problem. "In the macro world, you can get good analysis results with a coarse mesh," Lethbridge says. "But with a MEMS device, you may have to solve for a fluid domain and that requires computational fluid dynamics, a much more complex process."

One other difference in the use of software for design of MEMS devices is the lack of inter-operability among different software packages, particularly 2D and 3D packages. MEMS design work is generally 2D. The missing dimension is thickness, but you need to know thickness to construct finite-element models. Motorola engineers extrude the 2D models and add the required thickness, then use CIF (Caltech Intermediate Form) and GDSII (Graphic Data System file format) to translate the models to an analysis software. "Getting the different software packages to talk to each other is a bigger problem than it is in the macro world," says Motorola's McNeil.

Motorola has used ANSYS multi-physics software in the development of low-g Z- and X-access accelerometers. Among other goals, they wanted to ensure the accuracy of the devices in the face of the high sensitivity of low-g parts. For example, they used the software to model performance of various metals before choosing the best one for minimizing the effect of metal-stress hysteresis on device performance. They also have been using ANSYS software to analyze how their MEMS devices would work in applications other than those they were designed for. In one case, they analyzed a medium-g (about 40g) device originally designed for airbag deployment to see if they could modify it to produce a low-g (1.5g to 8g) or high-g (100 to 500g) device.

Engineers at SiWave Inc. have used ALGOR to calculate stress and displacement for applied shock loads on their MEMS optical switches. They have also used that software for electrostatic analysis. Meanwhile, engineers at the University of Alberta have used ALGOR to demonstrate the usefulness of FEA in design of MEMS pumps.

On the CAD side, the emergence of 3D design capability is facilitating MEMS design. Because of its origin in the semiconductor-manufacturing world, MEMS design has traditionally been a 2D process. The typical sequence is to begin with a model of the component created out of multiple semiconductor layers, followed by photo masks and 2D layouts for each layer, which match each specific cross-section configuration to drive manufacturing. Often, there can be photo masks for several cross sections of a solid model at the micron and sub-micron level for a device that's eventually going to wind up in a larger assembly. SolidWorks' Ilya Mirman says MEMS-specific CAD functionality allows engineers to move back and forth between different dimensional scales for 3D visualization of the complete assembly.

Axsun Technologies' Chief Technology Officer Dale Flanders uses SolidWorks in design and manufacture of the company's miniaturized optical micro-instrumentation devices. He says he and his team use the assembly and interference-checking capabilities of the software and the associative nature of 3D solid models to support mechanical, thermal, and electro-magnetic analyses as well as rapid prototyping activities. In fact, he says the associativity of the software is key for fast design.

The benefits of 3D design are extending to manufacturing too, helping MEMS designers break the stranglehold semiconductor-manufacturing process have held on them. For a long time, says Microfabrica Inc. Applications Engineering Director Nelsimar Vandelli, micro devices could only be created by experts using specialized tools and exotic silicon-based micro-machining techniques. Now, he says, Microfabrica Inc. is using SolidWorks to create arbitrary geometries. That opens up lots of new possibilities for engineers, he says. As an example, he points to helical springs. They're efficient for controlling force and displacement, but tough to produce based on four or five layers of silicon. "We're eliminating that restriction," he says.

National Editor Paul E. Teague can be reached at[email protected].

Research on MEMS
Sandia National Laboratories has an extensive research program on micro electromechanical systems (MEMS. The research has resulted in several breakthroughs. For information on one of the Lab's most recent MEMS projects, click on the following: http://www.sandia.gov/ldrd/stacelmi.htm

The Massachusetts Institute of Technology also has several programs on MEMS. For information, click on http://www-mit.edu/mtlhome/mems/opticalmems.shmtl

Occupying a package no bigger than a grain of sand, new smart-dust circuits combine sensing, computing, and communications functions.
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