Fatigue of Polycrystilline Silicon in MEMS Devices


A clear understanding of the mechanical properties of the structural materials used in MEMS devices is critical in the design and reliability analysis of these devices.  However, it cannot be assumed that macroscale and microscale devices have the same behavior.  Macroscale fatigue mathods cannot accurately predict the behavior of microscale devices because at microscale sizes, the structure size is on the same order of magnitude as the grain size of the material.  Also, grain boundaries, environmental, and facrications effects, which are negligible on the macroscale, may be significant on the microscale.  Although substantial reserach has been devoted to the characterization of the properties of microscale polysilicon structures under static conditions, polysilicon is often used under dynamic loading.  Typical applications where polysilicon structures undergo dynamic loading are suspensions for electrostatic actuators and thermal actuators.  The mechanisms fro crack initiation and propagation under dynamic loading are critical to effectively design long life MEMS structures.

Previous work in determining fatigue of polysilicon was reported by Brown, et al [1]. A notched "in-plane resonant cantilever structure" was used as the test device [1]. Brown found that with these specifmens, fatigue could be realized at resonance with high amplitudes.  He also concluded that testing in high relative himidity conditions accelerates the crack propagations [1].  The method described in this report differs from this previous work by not utilizing an induced stress riser, such as a notch in the specimen.  Any flas or defects will arise during the standard fabrication process just as they are ina ctual MEMS devices.  In addition, the structure is made of polycrystalline silicon as oppsoed to sinlg crystal silicon used by Connally and Brown [2] and Dual et al [3]. The microdevice also has a unique multiaxial loading state induced during cycling.

In previous studies, fatigue experiments were performed using simple devices that cannot resemble fatigue under the multiaxial loading conditions encoutnered during micromachine operation.  However, typical MEMS devices, such as the rotating rings of gyroscopes, oscillating proof masses of accelerometers, and high-resolution microdisplays are subjected to billions of mixed-load cycles and their fatigue behavior under cyclic loading is therefore of paramount importance.  In addition, the use of probe tips or load cells to apply a force on microfabricated test specimens introduces significant errors in force measurements because it is difficult to accurately determine the microscopic displacement of  a macrodevice.  Hence,, it is advantageous to imply on-chip actuation of a microscopic test specimen using electrostatic actuation.  This technique provides greater load control compared to macroscopic methods.

This research has developed an integrated test apparatus that will allow cyclic loading of poly silicon specimens with the ultimate objective to develop a methodology for generating fatigue life curves for MEMS devices.  To achieve this objective, a new design of fatigue micromachines and custom-made equipment for testing at the MEMS level will be employed along with analytical/numerical studies for predicting the actual stress of the fatigued specimens.  A detailed description of a new fatigue test micro device is in Section 2.0. The fabrication of the microdevices is detailed in Section 3.0. Theoretical analysis is presented in Section 4.0. The proposed approaches, testing methodologies and main equipment for performing the fatigue tests are outlined in Section 5.0. Future work is suggested in Section 6.0.

Angela S. Choy
Kyriakos Komvopoulos
Publication date: 
December 31, 1999
Publication type: 
Master's Thesis
Choy, A. S. (1999). Fatigue of Polycrystalline Silicon in MEMS Devices. United States: University of California, Berkeley.

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