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“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 the Labs’ Applied Mechanics Development department. “You predict the performance, evaluate the response from different design variations, and analyze the results. Applying modeling and simulation allows us to conduct concurrent engineering and optimize the design thus reducing costs significantly. There are lots of new things to do. I think that’s the most exciting thing about working in the micro and nano worlds. Science and engineering can be very different at those lengths.”
Interest in computational 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 they 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 team conceded they had insufficient understanding of the wear mechanisms and accelerated the labs’ research into micro- and nano-scale science and engineering.
The built-and-test approach, which was relatively expensive, was supplemented with discovery experiments, model development, and computational simulation. This shed new light on the mechanisms that were causing imprecise precision control and lateral instability, as well as wear.
MEMS structures are built by depositing and etching polysilicon at selected areas using a multi-layer, multi-stage 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 points, known as 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. However at nanoscale the physics of friction is an active area of research.
Many surface interaction models depend on statistical description of the asperity heights for high contact forces, since the real contact area increases with load as asperities are flattened and more come into contact. 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. Forces such as stiction, the force required to get an object moving, result from the interaction between single asperities.
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