AVS 47th International Symposium
    MEMS Wednesday Sessions
       Session MM-WeA

Paper MM-WeA7
A Novel Anti-Stiction Method Using the Harmonic Excitation on the Microstructure

Wednesday, October 4, 2000, 4:00 pm, Room 309

Session: MEMS Processing
Presenter: W.P. Lai, National Tsing Hua University, Taiwan
Authors: W.P. Lai, National Tsing Hua University, Taiwan
W. Fang, National Tsing Hua University, Taiwan
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The sticking problem due to the reaction mechanism between rinse liquid and microstructures becomes a serious issue in micromachining fabrication processes. A novel anti-stiction method using the harmonic excitation on the microstructure is presented in this research. The proposed method had been studied through the analytical and experimental approaches. Consequently, the yield of the micromachining processes can be significantly improved by this technique. We exploit a simplified model consisting of a single degree of freedom mass-spring-damper system to simulate the drying process of the microstructure. In this model, the equivalent spring effect mainly comes from the Laplace pressure@footnote 1@ of the liquid film and the equivalent damping effect @footnote 2@ is due to the squeeze@footnote 3@ of the liquid film. Based on the analysis, the dynamic response of the microstructure including the resonant frequency and the damping characteristic of the drying system were predicted. Analysis results displayed that the system is initially underdamped, then is critically damped, and finally is overdamped when the drying time increased. Hence the cantilever beam could be separated with the liquid film by the harmonic excitation at the underdamped condition. In application of the proposed anti-stiction technique, we fabricated silicon oxide cantilevers on (111) silicon wafer. The micromachined cantilever was 0.7 µm thick, 16 µm wide, and 120 µm long, and the gap between the beam and the substrate was 4 µm. The sample was excited by a PZT actuator at various frequencies. After excited by a 250 KHz harmonic load for 10~110 seconds, the beam was released from the substrate. However, the beams were sticking to the substrate permanently if they were not driven by the harmonic excitation. The application of this technique is also demonstrated by the experiment. Theoretical predictions of the dynamic behavior of microstructure during drying process agree well with experimental results. In short, the proposed technique effectively enhances yield rate of the microstructure without additional masks and complicated process. Although the application of the proposed approach is limited to the overdamped system, it provides an additional option to prevent stiction problem. @FootnoteText@@footnote 1@Takeshi Abe, W. C. Messner, and M. L. Reed, 'Effects of elevated temperature treatments in microstructure release procedures' Journal of Microelectromechanical System, vol. 4, pp. 66-74, 1995. @footnote 2@ H. Hosaka, K. Itao, and S. Kuroda, 'Damping characteristics of beam-shaped micro-oscillators' Sensors and Actuators A, vol. 49, pp. 87-95, 1995. @footnote 3@ J. J. Blech 'On isothermal squeeze films' ASME Journal of Lubrication Technology, vol. 105, pp. 615-620, 1983.