Fatigue Study of Silicon Using Encapsulated MEMS Resonators
Many silicon based MEMS devices, such as vibratory rate gyroscopes and micro actuators, are exposed to cyclic loadings with a large stress amplitude. As a result, fatigue in single crystal silicon is an important aspect that should be considered to ensure reliability and to improve the performance of those devices by setting a proper safety factor. Howeve, despite the past efforts, the exact mechanicsm of silicon fatigue has not been resolved due to limitations in test environmental control. To date, the role of oxygen and humidity remains controversial.
'Epi-seal' encapsulation developed in our group gives us an oxygen and humidity free test environment that no prior work has been able to accomplish. Previous experiments on 'epi-seal' resonators have demonstrated no measurable frequency change during 10,000 hours at lower stress levels (<150 MPa), thereby ruling out many possible causes of frequency changes that may be observed.
We fabricated wedge-shaped micromechanical resonators in the 'epi-seal' encapsulation to study the fatigue of silicon (Fig. 1-3). A notch on the cantilever beam produces stress concentration, while the amplitude of oscillation is measured by the capacitance change in the comb fingers. The fabricated device is connected to an oscillator crcuit to apply a controlled amount of cyclic loading. A counter records the resonant frequency of the device in real time, which immediately reflects any mass or stiffness change in the test specimens. If any crack initiates through fatigue, the stiffness of the test section is reduced, resulting in resonant frequency decrease.
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Last modified: Wed Jan 5 15:03:15 PST 2011