Polysilicon fatigue failure research

The success of many advanced technologies that use devices such as sensors and actuators, including gyroscopes and optical devices, depends on microscopic components called microelectromechanical systems (MEMS) devices made of polycrystalline silicon (polysilicon). Researchers at Case Western Reserve University have reported that miniature micron-sized polysilicon laboratory specimens subjected to cyclic tension/compression loading undergo fatigue, and could ultimately fail as a result of damage produced by the compressive cycles, rather than from moisture-assisted stress corrosion cracking.

Polysilicon is a manufactured thin film consisting of silicon crystallites that is made in a microfabrication laboratory using chemical vapour deposition. The films are associated with rough surfaces that result from the plasma etching used in the final stages of the MEMS processing. The researchers speculate that under compressive loading, the surfaces come into contact and its wedging action produces microcracks that grow during subsequent tension and compression cycles.

The researchers used on-chip test structures that rely on electrostatic actuation rather that an external testing machine. They varied the ratio of compressive to tensile stresses in the cycle, by using both DC and AC voltage sources and high frequencies. The specimens could then be subjected to more than a billion cycles in less than a day.

The research showed that polysilicon under constant stress is not susceptible to stress corrosion , but the fatigue strength is strongly influenced by the ratio of compression to tension stresses experience during each cycle. The failure originates from microcracks and those cracks likely originate on the surface of the polysilicon.