Sandia LabNews

Nanoscience and computer modeling come to the aid of MEMS-scale engineering

Sandia’s determination to do big things with tiny technology is evident across the Labs. It has spawned an unprecedented cross-disciplinary surge of interest, attracting new talent in a number of disciplines, from mechanical engineering to microbiology.

At the micro scale, Sandia has pioneered a number of microelectromechanical systems (MEMS) innovations. At the nano scale, as researchers expand their understanding of the science and learn to take advantage of nano’s unique properties, the opportunities for innovation are limitless: increased energy efficiency, improved healthcare, and enhanced national security are only the beginning.

The technical community is fairly well agreed that, with time, micro- and nano-scale devices will revolutionize engineering, and that the manufacture of micro-scale devices will be transformed by nano-scale assembly. For now, though, progress in developing complex MEMS devices has been slow, slower than early champions may have hoped or anticipated. To date, the most successful MEMS innovations have been relatively simple devices

like mass-produced accelerometers for airbag sensors, inkjet printer heads, and digital mirrors for video projection. In the case of more complex systems with multiple components, unexpected and little-understood problems of reliability begin to appear.

Using modeling and simulation tools originally developed for Sandia’s weapons-related work, Labs researches have begun to speed up MEMS development by learning more about the unusual nature of micro-engineered mechanisms and devising ways of turning their peculiarities into assets. Building on this promising approach, continued advances in modeling and simulation are revealing ever more about the world at the nano scale. The latest modeling and simulation results — where nano-scale physical effects can be formulated to describe more general aspects of MEMS systems — are generating a high level of enthusiasm among researches.

A different world

One of the great discoveries of engineering at the micro and nano scales is that the world down there is quite, quite different from the world we can see and touch. Sandia researchers are discovering that if we don’t learn to operate in that realm — and essentially that means that engineers and physicists will need to cooperate more closely than ever — then the full potential of MEMS technology will most likely remain beyond our grasp.

In other words, one cannot scale down confidently from macro-dimensional assumptions when the final product is measured in micrometers rather than fractions of an inch. When they attempt to apply traditional methods at such small scales, engineers may find themselves face-to-face with the unexpected — a host of situations in which the experiences of designing in the macro world can no longer provide all the answers.

Because many of the old assumptions don’t work and the new ones are much more complex, micro-scale engineering is likely to reap great dividends from the growing interest in modeling and simulation, while relying less on conventional engineering problem solving.

“We are moving from the early, relatively unenlightened days of ‘making macro solutions smaller’ to doing things a new way, through micro-scale-enabled solutions,” wrote Art Ratzel, director of Engineering Sciences Center 1500, in the March 2007 issue of Mechanical Engineering,

flagship magazine of the American Society of Mechanical Engineers. “Engineering at the micro scale introduces an appreciation of the complex physics at the feature scales of the devices. It demands the appreciation of a ground-up approach to design and problem solving.

“While MEMS has not yet lived up to the optimism of the 1990s, enhanced understanding of scale-dependent physics is helping us to make progress toward the buoyant expectations voiced during those times.”

‘A lot of new things to do’

As a result, emphasis on computerized design support is increasing dramatically, and modern mechanical engineers are becoming software experts, some arguing that, since MEMS production is an automated two-month process, model-based design verification should be completed before fabrication begins.

“It’s exciting that you can review the design and check out the 3-D solid model before it’s actually made,” says Channy Wong, manager of Applied Mechanics Development Dept. 1526. “You predict the performance, evaluate the responses from different design variations and analyze the results. Applying modeling and simulation allow us to conduct concurrent engineering and optimize the design, reducing cost significantly.

“There are a lot of new things to do,” he adds. “I think that’s the most exciting thing about working in the micro and nano world. Science and engineering can be very different at that length scale.”

Lessons learned: the 1995 micro engine experience

Interest in modeling and simulation blossomed as the result of lessons learned with the Sandia micro engine, an early MEMS product initially developed in 1995. The basic design was simple, and the first micro engines were built under stringent clean room conditions, but proved to be only somewhat reliable. After 477,000 cycles, an electron microscope image clearly showed the reason — accumulation of debris detached from rubbing surfaces. The Sandia development team, conceding they had insufficient understanding of the wear mechanisms, accelerated the Labs’ research into micro-scale science — and engineering. The “build-and-test” approach, which was relatively expensive, was supplemented with discovery experiments, model development, and computational simulation. This shed new light onto the mechanisms that were causing imprecise precision control and lateral instability (“clamping”), as well as wear.

Since the micro engine experiments, researchers in Sandia’s micro-scale modeling and simulation community in 1000, 2000, and 8000 have been occupied with forensics (asking what went wrong?), exploration (can modeling and simulation help ensure that a component will perform up to expectations?), and improved performance (can modeling and simulation suggest better operation and perhaps improved design?). Their next step is to bring about micro-scale-enabled engineering solutions — ultimately a transformation in component design — through shared innovation by individuals specializing in components, engineering, and microelectronics.

Down the rabbit hole

Some engineers compare moving from macro to micro to nano to Alice’s trip down the rabbit hole. Physical models must change, sometimes drastically, as length scales shrink. For example, at the world down there gravity is more easily overcome by adhesion; friction models break down; solids melt at lower temperatures; a tiny moving “front” of changing temperature must be described as a quantum effect — as a spreading cloud of ballistic phonons.

Such effects are especially important in polycrystalline silicon (including structures made with Sandia’s SUMMiT® V process), which have several levels and have geometry features of 1 to 10 micro-meters and grains of 10s to 100s of micrometers.

The fledgling MEMS industry has a limited knowledge of materials physics at micrometer size, and currently commercialized devices are designed for specialized purposes. Partly for this reason, they do not have a broad user base, and therefore have not generated industry standards or the design and process software that would be built based on those industry standards. However, this may change as Sandia’s materials science, engineering, and computer sciences organizations develop engineering systems that, while designed for their own use, could migrate into the private sector and revitalize the pace of invention.

While Sandia is seeing a major effort to harness nanoscience for the improvement of esoteric applications such as micromachines, big benefits for everyday products also are visualized by Jim Redmond, manager of Strategic Initiatives Dept. 1525. Jim calls this “the connection of nano-scale phenomena to the engineering of macro-scale solutions for the world we live in, like lighter, stronger, more robust materials.”

“For example, carbon black is a ‘nano material’ that has been used for years to enhance the performance of tires, a product we’re all familiar with,” he says. “This benefit was determined by trial and error. With modern computing and production tools, we ought to be able to engineer similar improvements for many products.”

“As these new perspectives evolve into reality, a new breed of engineer is also coming into existence,” says Art in Mechanical Engineering. “In fact, the line among the computer scientist, the materials

scientist, and the engineer is becoming blurred and indistinct. Mechanical engineering cannot help but benefit from this exciting new horizon. MEMS is here to stay, and it will transform the future.”

Some of the challenges of scaling down

The physics of the “normal” world, the macro world we live in every day, begins to break down in odd and unanticipated ways at the micro and nano scale. For example, many surface interaction models depend on a statistical description of the asperity heights — that is, points of roughness — for high-contact forces, since the real contact area increases with load as asperities are flattened and more come into contact. But with light contact, only the outlier asperities are engaged. Thus, at the macro scale, where a surface will have many contacting asperities, the real contact area varies directly with load — the heavier the package, the harder it is to slide along a counter top. But at the micro scale, unfamiliar effects can increase static friction (stiction, the force required to get an object moving), as a result of the interaction between single asperities.

Understanding the effect of asperities is vital, because asperity points are an artifact of MEMS fabrication. MEMS structures are built from depositing and etching away polysilicon at selected areas using a multilayer, multistage photolithographic process. Each deposition and etching process exposes a new surface which, when examined with an electron microscope, can be seen to include a large number of unwanted rough points, or asperities, which project at various heights above the “real” part of that surface. In the macro world, the effect of these asperities gives rise to an averaging-out notion of “slick” or “rough” surfaces and thence to the idea of sliding friction, the force that resists relative motion between two bodies in contact. On the micro scale, friction behavior depends on discrete contacts because each asperity is relatively large and a small number of them will interact and produce effects of their own — for example breaking off and generating debris. The physics of friction at the nano scale is not completely understood. This is still an active area of research.

Computational simulation of a Sandia-designed micro-scale thermal actuator provides another example of how size affects how things work. In this device, electrical current passes through four mechanical legs, causing them to expand and displace a shuttle with a reciprocating motion. Remarkably, conventional methods predict the beam temperature at 750 kelvin, whereas Sandia’s “non-continuum” calculation, using nano-scale data, puts it at a more accurate 900 K. With some materials, this could make the difference between melting and not melting.