Engineering researchers demystify fatigue failure in polysilicon used in MEMS devices
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 report in the November 8 issue of Science 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. This information, they say, could assist MEMS developers to mitigate fatigue failure in MEMS devices that experience significant mechanical stress during operation.
The Science article (“Fatigue Failure in Polysilicon: It’s Not Due to Simple Stress Corrosion Cracking”) was written by Harold Kahn, Research Associate Professor in the department of materials science and engineering; Roberto Ballarini, Professor in the department of civil engineering and a lead researcher on the project; Arthur Heuer, University Professor and Kyocera Professor of Ceramics in the department of materials science and engineering; and Justin Bellante, a recent BS/MS graduate of materials science and engineering.
Polysilicon, CWRU researchers say, is a manufactured thin film consisting of silicon crystallites that is made in a microfabrication laboratory using chemical vapor deposition. The films are associated with rough surfaces that result from the plasma etching used in the final stages of MEMS processing. The researchers speculate that under compressive loading, these surfaces come into contact, and their wedging action produces microcracks that grow during subsequent tension and compression cycles.
“Over the past few years there has been a debate about the roles that moisture and mechanical stress play in the fatigue failure of polysilicon devices,” said Ballarini. “Some research groups claim that polysilicon fatigue is associated with stress corrosion cracking. This failure mechanism is associated with the propagation of a sharp crack under an applied stress too low for immediate catastrophic failure and in the presence of a corrosive environment like humid air. Our research shows 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 experienced during each cycle. The failure originates from microcracks and those cracks likely originate on the surface of the polysilicon.”
Polysilicon MEMS structures, Heuer explained, contain many raised areas along their surfaces that act as stress concentrators and could result in microcracks when exposed to tensile or compressive stresses. “The microcracks then extend from the surface into the miniaturized structures, weakening the material and causing failure,” he said.
To study the fatigue of polysilicon, Kahn and Bellante used on-chip test structures that rely on electrostatic actuation (the attraction to each other of two plates of opposite electrical charge), rather than an external testing machine.
“By using both DC and AC voltage sources,” Kahn said, “we varied the ratio of compressive to tensile stresses in the cycle, and by using high frequencies, we could subject specimens to more than a billion cycles in less than a day.”
“MEMS, the use of miniaturized devices for high tech products, is becoming more and more popular in modern technology,” said Heuer. “This research tells us to be mindful of the manner in which we create the surfaces of polysilicon chips so that devices that experience significant mechanical stresses like gyroscopes and optical devices can be rendered less susceptible to fatigue failure.”
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