Silicon crystals do suffer stress

 

Semiconductor material not immune to fatigue as previously thought.

National Institute of Standards and Technology (NIST) researchers have demonstrated a mechanical fatigue process that eventually leads to cracks and breakdown in bulk silicon crystals.

Although the phenomenon has long been thought to be nonexistent, the new discovery has important implications for the design of new silicon-based micro-electromechanical system (MEMS) devices.

Silicon has a wide variety of uses but none as important as its role as the backbone of the semiconductor industry, which makes it one of the world’s most heavily studied materials. One reason for its popularity has been the belief that it is a material that is immune to fatigue. 

Hidden weakness

Conventional tests have validated that it has resisted fatigue from cyclical stresses due to its crystal structure and chemical bonds. However, the recent research into silicon MEMS devices reveals that its mechanical features do seem to develop stress-induced cracks that can lead to its failure.

Reasons behind this occurrence are still up for debate but it could be due to friction or corrosion.

To test the bulk silicon’s ‘toughness,’ the researchers used an alternative method: pressing the top of test crystals with tiny tungsten-carbide spheres about three mm in diameter at pressures below the silicon's breaking point. Simply pressing down hard on the crystal for days at a time caused no detectable cracks - arguing against the corrosion theory.

On the other hand, using half the pressure but cycling the test hundreds of thousands of times revealed a gradually increasing pattern of surface damage at the indentation site – a clear indication of mechanical fatigue.

The next step for the NIST team is to determine whether the same mechanisms operate at the submicromete

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