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Silicon fatigue: not a myth

RESEARCH UPDATE

In work with ramifi cations for MEMS (microelectromechanical systems), researchers at NIST (National Institute of Standards and Technology) claim to have proved that, contrary to conventional wisdom, bulk silicon crystals are vulnerable to fatigue from cyclic stresses.

The scientists used 3-mmdiameter tungsten-carbide spheres to apply pressure to the surfaces of test crystals. Simply pressing, even for days at a time, caused no discernible damage. But cycling the test hundreds of thousands of times, even at low pressure, resulted in a gradually worsening damage pattern.

NIST claims that this clear evidence of mechanical stress resolves a debate about cracks, which scientists observed in some MEMS structures, by ruling out a competitive theory that pointed to chemical corrosion as the culprit. The team proposes that its test found damage that conventional tensile-strength tests miss because it induced shear stress—causing crystal planes to slide against each other. And shear stress, the team notes with concern, is not uncommon in real-world applications. The next step for the research team is to scale down the testing from the current scale of hundreds of microns to the submicron level.

National Institute of Standards and Technology, www.nist.gov