Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Engineering researchers demystify fatigue failure in polysilicon used in MEMS devices

11.11.2002


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."

Marci E. Hersh | EurekAlert!
Further information:
http://www.cwru.edu/

More articles from Materials Sciences:

nachricht Siberian scientists suggested a new method for synthesizing a promising magnetic material
23.01.2018 | Siberian Federal University

nachricht Complex tessellations, extraordinary materials
23.01.2018 | Technische Universität München

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Optical Nanoscope Allows Imaging of Quantum Dots

Physicists have developed a technique based on optical microscopy that can be used to create images of atoms on the nanoscale. In particular, the new method allows the imaging of quantum dots in a semiconductor chip. Together with colleagues from the University of Bochum, scientists from the University of Basel’s Department of Physics and the Swiss Nanoscience Institute reported the findings in the journal Nature Photonics.

Microscopes allow us to see structures that are otherwise invisible to the human eye. However, conventional optical microscopes cannot be used to image...

Im Focus: Artificial agent designs quantum experiments

On the way to an intelligent laboratory, physicists from Innsbruck and Vienna present an artificial agent that autonomously designs quantum experiments. In initial experiments, the system has independently (re)discovered experimental techniques that are nowadays standard in modern quantum optical laboratories. This shows how machines could play a more creative role in research in the future.

We carry smartphones in our pockets, the streets are dotted with semi-autonomous cars, but in the research laboratory experiments are still being designed by...

Im Focus: Scientists decipher key principle behind reaction of metalloenzymes

So-called pre-distorted states accelerate photochemical reactions too

What enables electrons to be transferred swiftly, for example during photosynthesis? An interdisciplinary team of researchers has worked out the details of how...

Im Focus: The first precise measurement of a single molecule's effective charge

For the first time, scientists have precisely measured the effective electrical charge of a single molecule in solution. This fundamental insight of an SNSF Professor could also pave the way for future medical diagnostics.

Electrical charge is one of the key properties that allows molecules to interact. Life itself depends on this phenomenon: many biological processes involve...

Im Focus: Paradigm shift in Paris: Encouraging an holistic view of laser machining

At the JEC World Composite Show in Paris in March 2018, the Fraunhofer Institute for Laser Technology ILT will be focusing on the latest trends and innovations in laser machining of composites. Among other things, researchers at the booth shared with the Aachen Center for Integrative Lightweight Production (AZL) will demonstrate how lasers can be used for joining, structuring, cutting and drilling composite materials.

No other industry has attracted as much public attention to composite materials as the automotive industry, which along with the aerospace industry is a driver...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

10th International Symposium: “Advanced Battery Power – Kraftwerk Batterie” Münster, 10-11 April 2018

08.01.2018 | Event News

See, understand and experience the work of the future

11.12.2017 | Event News

Innovative strategies to tackle parasitic worms

08.12.2017 | Event News

 
Latest News

Rutgers scientists discover 'Legos of life'

23.01.2018 | Life Sciences

Seabed mining could destroy ecosystems

23.01.2018 | Earth Sciences

Transportable laser

23.01.2018 | Physics and Astronomy

VideoLinks Science & Research
Overview of more VideoLinks >>>