Coupled Multi-Physics Modeling of Micro-Machined Electro-Thermo-Mechanical Actuator

Electro-thermal actuation mechanism continues to attract attention of MEMS designers as a reliable method of delivering high displacements and high actuation forces [1]. The main purpose of this work is to conduct a coupled multi-physics FEA of a micro-machined electro-thermomechanical actuator (μETMA) in order to determine a nature of a common failure mode, suggest a better fabrication process and compute performance characteristics, such as actuation current, working temperature, actuation force and displacement, characteristic response time, etc. Two designs of μETMA have been considered so far: design for PolyMUMPs and design for MetalMUMPs processes [2]. Comprehensive coupled multi-physics modeling of the device should includ the following phenomena: initial relaxation of the structure due to residual stresses inherent to micro-fabrication methods, computation of electric current through the structure with semiconductor and metallic types of conductivity, Joule’s heating, radiation and natural convection, thermal expansion, modal analysis, air-structure interaction, etc. Current paper presents only several analyses accomplished so far. It has been determined that common mode failure observed with polysilicon devices is caused by run off temperature behavior due to exponential temperature dependence of the conductivity. Introduction Electro-thermal actuation mechanism continues to attract attention of MEMS designers as a reliable method of delivering high displacements and high actuation force [1]. However, design of such devices presents some difficulties due to the fact that several physical phenomena are tightly coupled, and therefore more than ever designer experience and intuition must be supplemented with multi-physics coupled-field analysis. ANSYS Multiphysics affords such kind of modeling and is used throughout this work. Electro-thermo-mechanical actuators are based on simple physical principles: potential difference applied to a conductor causes electric current and Joule heating. Increased temperature gives rise to mechanical deformations. By varying applied potential difference one can control the position of the actuator. Design objective is to increase useful mechanical displacements and minimize energy losses. Typical applications of μETMA include optical and electrical switches, variable optical attenuators, micro-tweezers and micro-manipulators, etc. The problem, which was observed in the laboratory with μETMA fabricated from polysilicon with PolyMUMPs process [2] was that controlling of the actuator position required extremely fine control of the driving voltage, and slight fluctuations in voltage caused distraction of the devices. The objective of this work was to model this phenomenon, determine the cause of it, and rectify the design. Device fabricated by PolyMUMPs process