Final report : compliant thermo-mechanical MEMS actuators, LDRD #52553.

Abstract

Thermal actuators have proven to be a robust actuation method in surface-micromachined MEMS processes. Their higher output force and lower input voltage make them an attractive alternative to more traditional electrostatic actuation methods. A predictive model of thermal actuator behavior has been developed and validated that can be used as a design tool to customize the performance of an actuator to a specific application. This tool has also been used to better understand thermal actuator reliability by comparing the maximum actuator temperature to the measured lifetime.Modeling thermal actuator behavior requires the use of two sequentially coupled models, the first to predict the temperature increase of the actuator due to the applied current and the second to model the mechanical response of the structure due to the increase in temperature. These two models have been developed using Matlab for the thermal response and ANSYS for the structural response. Both models have been shown to agree well with experimental data. 3In a parallel effort, the reliability and failure mechanisms of thermal actuators have been studied. Their response to electrical overstress and electrostatic discharge has been measured and a study has been performed to determine actuator lifetime at various temperatures and operating conditions. The results from this study have been used to determine a maximum reliable operating temperature that, when used in conjunction with the predictive model, enables us to design in reliability and customize the performance of an actuator at the design stage. AcknowledgmentThe authors would like to thank all of the staff in the MDL for fabrication and release/dry/coat support through out this project. We would also like to thank Ken Pohl, Mark Jenkins and David Luck for their assistance in testing and characterization, Mike Rye for TEM sample preparation, and Hoshang (Amir) Shahvar and Ted Parson for their work in getting SHiMMeR operational and configured for this experiment. Also, thanks to Sean Kearney and Leslie Phinney for their work in collecting the Raman temperature data.4

Design of a MEMS-based motion stage based on a lever mechanism for generating large displacements and forces

“…An additional design constraint is a thermal melting-down condition, because the thermal actuator is made of silicon and is limited by the material properties of silicon; the temperature over its maximum endurable limit causes serious structural damage or permanent deformation. For this reason, the temper ature rise in a thermal actuator should be limited to 550 °C [24] for a reliable operation.…”

“…For these reasons, an actuator and a displacement amplifying mechanism are designed and analyzed. The commonly used actuating mechanisms in MEMS are electrostatic [19][20][21], magnetic [4,22,23] and thermal [24,25]. Magnetic and electrostatic actuators are well known for their stable response at high frequency range, but their μN level force makes it difficult to provide an adequate force in some applications.…”

Electrothermomechanical modeling of out-of-plane deformation in single-stepped beams actuated by resistive heating

“…Polysilicon films fabricated by the PolyMUMPs ® process behave like a brittle material at temperatures up to ~540 °C [45] with fracture strength of σ = − 1.2 1.7 GPa Y [46,47]. Polysilicon starts to glow at 500-600 °C [48,49], with an average threshold value of 550 °C [50,51]. The mechanical behavior of polysilicon transitions from that of a brittle material to ductile and its strength drops to σ ~0.5 GPa Y at 670 °C [51], and further down to σ ~89 MPa Y at 800 °C [52].…”

Iterative Learning Control for V-Shaped Electrothermal Microactuator

“…The most appropriately conventional control methods for VEM are open loop, such as the publications presented in [10][11][12]. They are simple in their layout and hence very economical, stable due to their simplicity, and easier to construct.…”