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Scanning electron micrographs of the polysilicon MEMS fatigue life characterization resonator fabricated at the MEMSCAP MUMPs foundry (formerly CRONOS http://www.memscap.com/memsrus). (a) Triangular proof mass (thickness ) with interdigited comb drive electrostatical actuator and capacitive displacement sensor comb; (b) notched cantilever beam connecting the resonator mass and anchor (Ref. 1).
Schematic of the “reaction-layer” fatigue mechanism at the notch of the polycrystalline silicon cantilever beam (a). (b) Localized oxide thickening at the notch root. (c) Environmentally assisted crack initiation in the native oxide at the notch root. (d) Additional thickening and cracking of reaction layer. (e) Unstable crack growth in the silicon film (Ref. 1).
Polysilicon curve. Fitted line for previous data from MUMPs run 18 in ambient air compared with fatigue data from MUMPs run 50 in ambient air and in vacuo .
HVTEM images from failed resonator devices. (a) Fatigued in ambient air with thickened oxide layer around the notch root (principal stress at the notch root: ; number of cycles at failure: ). (b) Manually fractured specimen in ambient air: no (local) oxide thickening. Because of sample tilt, some contrast in grains at the edge is visible. Only the top transparent part is amorphous. (c) Device after fatigue attempt in high vacuum and manual fracture: no oxide layer thickening (principal stress at the notch root during fatigue attempt: , number of cycles when stopped: ). Also in this case contrast from grains on the edge is visible because of sample tilt and therefore only the top amorphous layer is oxide.
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