A methodology to predict shock and vibration levels that could lead to the failure of MEMS devices is reported as a function of vibration frequency and shock pulse duration. A combined experimental-analytical approach is developed, maintaining the simplicity and insightfulness of analytical methods without compromising on the accuracy characteristic of experimental methods. The minimum frequency-dependent acceleration that will lead to surfaces coming into contact, for vibration or shock inputs, is determined based on measured mode shapes, damping, resonant frequencies, and an analysis of failure modes, thus defining a safe operating region, without requiring shock or vibration testing. This critical acceleration for failure is a strong function of the drive voltage, and the safe operating region is predicted for transport (unbiased) and operation (biased condition). The model was experimentally validated for over-damped and under-damped modes of a comb-drive-driven silicon-on-insulator-based tunable grating. In-plane and out-of-plane vibration (up to 65 g) and shock (up to 6000 g) tests were performed for biased and unbiased conditions, and very good agreement was found between predicted and observed critical accelerations.
In optical MEMS, the family of diffractive MEMS is interesting for a wide range of applications, such as displays, scanners or switching elements. Their advantages are compactness, potentially high actuation speed and the ability to deflect light at large angles. A deformable diffraction MEMS grating is presented. Electrostatic actuation has experimentally proven a periodicity tuning of 2.5%. The device first resonant mode is expected to be at 28.5 kHz. The present device has been designed as a tuning element in an external cavity mid-infrared quantum-cascade laser.
This work describes a method for tracking the dynamics of electrostatic discharge (ESD) sensitive MEMS structures during ESD events, as well as a model for determining the reduced combdrive snap-in voltage under vibration and shock. We describe our ESD test setup, based on the human body model, and optimized for high impedance devices. A brief description of the MEMS tunable grating, the test structure used here, and its operation is followed by results of the measured complex device dynamics during ESD events. The device fails at a voltage up to four times higher than that required to bring the parts into contact. We then present a model for the snap-in of combfingers under shock and vibration. We combine the results of the analytical model for combdrive snap-in developed here with a shock response model to compute the critical shock acceleration conditions that can result in combdrive snap-in as a function of the operating voltage. We discuss the validity regimes for the combdrive snap-in model and show how restricting the operation voltage below the snap-in voltage is not a sufficient criterion to ensure reliable operation especially in environments with large disturbances.
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