The electrothermomechanical characteristics of an electrically-heated polycrystallinesilicon microactuator are explored. Using finite-difference techniques, an electrothermal model based on the balance of heat dissipation and heat losses is developed. For accurate simulation, the relevant temperature dependent properties from the microactuator material are included in the model. The electrothermal model accurately predicts the steady-state power required to hold position, and the energy consumed during the thermal transient. Thermomechanical models use the predictions of temperature from the electrothermal solution to calculate displacement and force from pseudo-rigid-body approximations and commercial finite-element code. The models are verified by comparing experimental data to simulation results of a single leg-pair on a particular configuration of the device.
The particular microactuator studied is called a Thermomechanical In-plane Microactuator, or TIM, and was fabricated with surface micromachining technology. A TIM requires a single releasable structural layer, is extremely flexible in design, and can operate with simple drive and control circuitry. The TIM produces linear motion of a center shuttle when slender legs on either side move the shuttle as a result of constrained thermal expansion.
In a single example, when the current through a leg with dimensions 250×3×3.5 µm^3 and suspended 2 µm off the substrate is sufficient to maintain an average temperature of 615 C in air and vacuum environments, model simulated temperatures along the leg have a peak of 860 C in air and 1100 C in vacuum. The final measured and predicted displacement is 14 µm. In air, the power predicted by the model needed to maintain this average temperature profile is 95 mW while consuming 16.4 µJ in 0.22 ms to reach 90 percent of the final average temperature. In a vacuum, only 6.4 mW are required to maintain the same average temperature with 97.6 µJ consumed in 18.5 ms. Simulation results suggest that short-duration high-current pulses can improve the transient response and energy consumed in a vacuum when steady-state temperatures are not required. For a TIM leg with the dimensions above, the maximum measured force is approximately 47 µN per leg-pair when enough current is provided to move the TIM 8 µm as a result of ohmic heating and thermal expansion.
College and Department
Ira A. Fulton College of Engineering and Technology; Mechanical Engineering
BYU ScholarsArchive Citation
Lott, Christian D., "Electrothermomechanical Modeling of a Surface-micromachined Linear Displacement Microactuator" (2005). All Theses and Dissertations. 306.
microelectromechanical systems, MEMS, thermal actuator, modeling