Suppression of Stiction in MEMS
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I. INTRODUCTION
Microelectromechanical systems have a wide variety of applications performing basic signal transduction operations as sensors and actuators. Suspended micromachined structures such as plates and beams are commonly used in the manufacturing of pressure [2-5] and acceleration sensors [6,7]. MEMS components have relatively large areas, but very small stiffness. At the same time they are fabricated a few microns off their supporting substrate. The combination of these characteristics makes MEMS devices very susceptible to surface forces [8] which can deflect the suspended members toward the substrate. If the deflection force is sufficiently strong, the elastic member can contact with and permanently adhere to its underlying substrate causing a device failure. This can happen both during the device fabrication and normal use. During processing, adhesion can occur when the suspended member is exposed to an aqueous rinse and dry cycle. Since the device-to-substrate gap is so small, strong attractive capillary forces can develop during the dehydration causing its collapse and subsequent pinning to the substrate [9]. The same collapse can develop when the device is exposed to high humidity conditions leading to capillary condensation [10, 11]. A third yet possible adhesion failure mode develops if the suspended member is placed in contact with its substrate by external forces. This may be done by the deliberate placement of a collapsing force or by accidental shock. The pinning process is hence divided into (a) mechanical collapse and (b) adhesion to the substrate. In practice, either or both mechanisms can be eliminated if stiffness of the microstructure is high enough. The presence of a failure threshold is readily observed experimentally by constructing an array of progressively weaker suspended elements followed by examination of the sticking condition. Figure 1(a) shows a photographs of an array of micromachined polysilicon cantilever beams showing clearly the onset of the pinning condition at a particular beam detachment length. This paper very briefly summarizes these phenomena and reviews techniques used to prevent or eliminate stiction failures. II.
MECHANICAL COLLAPSE BY CAPILLARY FORCE
The behavior of elastic structures under capillary forces such as those developed under fabrication, has been studied in [12]. Here, as an illustrative example, we analyze the simplified lumped elastic structure of Figure 2 in which the plate represents the suspended member surface, the spring its stiffness, and the gap the distance between the member and the substrate. This simplified structure proves useful in understanding the mechanical stability of the suspended member 105 Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
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Fig. 1. (a) SEM of micromachined polysilicon cantilever beams of increasing length. The photograph shows clearly the onset of pinning for beams larger than 34 pm. (b)SEM of array of micromachined polysilicon circular plates. The structures with circula
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