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Abstract on "MODELING AND RELIABILITY OF ELECTROTHERMAL MICROMIRRORS"

  A thermal bimorph actuator consists of two layers with different coefficients of thermal expansion (CTE). Differential expansion or contraction due to temperature change causes a bimorph to bend thereby leading to actuation. Micromirrors actuated by thermal bimorphs provide a large scan range at low driving voltage. Out of plate displacement of the mirror-plate up to 600 μm and full-circumferential scan angle have been reported in literature. However, modeling and reliability have not received sufficient attention. Modeling is essential for design, optimization and control. Comprehensive electrothermomechanical (ETM) model of electrothermal micromirrors have been reported for the static case alone. In this thesis, a procedure for building dynamic ETM models will be developed.

All thermal bimorph actuated MEMS reported in literature utilize straight bimorph beams. The mechanics of curved bimorphs will be analyzed and simulated and micromirrors utilizing curved bimorphs will be fabricated.

Another key contribution of this thesis is the optimization of the inverted-series-connected (ISC) structure which consists of a series connection of two different multimorph structures. Optimization resulted in ten-fold increase in the scan angle of ISC actuator based micromirrors.

The major drawbacks of thermal MEMS are high power consumption and slow speed. Current generation micromirrors utilize SiO₂ thin-film for thermal isolation. This makes the devices highly susceptible to impact failure during handling and packaging. SiO₂ is also used as one of the bimorph layers in current generation devices. The low thermal diffusivity of SiO₂ makes the thermal response sluggish. A process for fabricating robust mirrors with fast thermal response has been proposed. The process employs Al and W for forming the bimorph structure. High temperature polyimide will be used for thermal isolation. The impact failure of devices fabricated using the proposed process will be studied using vibration table and drop tests. Reliability issues encountered during long term device testing will be documented in the final dissertation.