MODELING AND RELIABILITY OF ELECTROTHERMAL MICROMIRRORS
Difference in strains in the layers of a multimorph causes it to curl, thereby leading to transduction. Thermal, piezoelectric, shape-memory alloy and electroactive polymer based multimorph transducers that undergo out-of-plane bending have been widely reported. A thermal multimorph actuator consists of two or more layers with different coefficients of thermal expansion (CTE). Micromirrors actuated by thermal multimorphs provide large scan range at low driving voltage. Up to 600 μm out-of-plane displacement of mirror-plate and full-circumferential scan angle have been reported in literature.
A major contribution of this thesis is the modeling of electrothermal micromirrors for design, optimization and control. Procedure for building compact electrothermomechanical (ETM) models is established and validated against experiments. A key component of ETM model is the thermal model. Thermal models based on finite element (FE) simulations, lumped element method, model order reduction (MOR) and transmission line theory have been developed. The mechanical behavior of a micromirror may be modeled as a mass-spring-damper system. A comprehensive ETM model was implemented in Simulink. Model-based open-loop mirror control for bio-imaging systems has been demonstrated. Another 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.
Most thermal multimorph-actuated MEMS devices reported in literature utilize straight actuator beams which undergo bending deformation. On the other hand, curved multimorph actuators that undergo combined bending and twisting have not been widely investigated prior to this thesis. The small deformation analysis of curved multimorphs is reported for the first time and validated against experiments. Analytical expressions governing curved multimorphs can serve as design equations for novel thermal, piezoelectric, shape-memory alloy, and electroactive polymer based devices. Mirrors actuated by curved multimorphs are fabricated. The unique properties of curved multimorphs are utilized to achieve lower power consumption, higher fill-factor, and lower center-shift compared to previously reported designs.
The major drawbacks of thermal MEMS are high power consumption and slow speed. Several 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 multimorph layers in several devices. The low thermal diffusivity of SiO2 makes the thermal response sluggish. A novel process for fabricating robust electrothermal MEMS with customizable thermal response and power consumption is developed. The process employs Al and W for forming the active layers of the multimorph structure. High temperature polyimide is used for thermal isolation. The mirrors fabricated by the proposed process have improved robustness compared to previous designs and can withstand typical drop heights encountered in hand-held applications.
Another contribution of this thesis is the development of two novel in-plane transducers based on straight thermal multimorph actuators. The proposed designs can produce 100s microns displacement along the substrate surface, which is an order of magnitude greater than previously reported designs. Possible applications include integrated Michelson interferometer, movable MEMS stage and movable micro needles for biomedical applications.