Authors: S. Deladi, G. Krijnen and M. Elwenspoek
Affilation: University of Twente, Netherlands
Pages: 400 - 403
Keywords: electrothermal actuator, MEMS, FEM simulation
The demand for powerful actuators, which can be embedded into MEMS devices draws attention to electrothermal actuators, although there can be drawbacks such as the heat created during actuation. The main advantage is the low actuation voltage in comparison with electrostatic actuators, and an appropriate design allows good temperature control over the entire structure. In this work the authors present novel electrothermal actuators, which have been simulated and tested successfully. The goal of the coupled electrothermal in-plane actuator design (figure 1) was to increase the force by adding the individual contributions of the actuators [2,3], however the increased rigidity of the structure leads to smaller in-plane displacement (7.43 µm) than the individual ones. The electric circuits of the actuator are obtained by local B diffusion into the polycrystalline Si. Novel out-of-plane actuators have been developed for tribological purposes, mainly to overcome adhesion between smooth surfaces and to quantify pull-off forces. The authors reported a computational model for adhesion between arbitrary rough surfaces , which has been the input for the requirements of the novel out-of-plane actuator. The out-of-plane motion (up to 7-8 µm) is obtained by thermal expansion mismatch of the used materials (polycrystalline Si, silicon oxide, SiRN). The novelty of the actuator is the resistive heating of the structure, which creates only local heating compared with previously reported global heating [4,5]. The SiRN capping protects the silicon oxide (figure 2.c.) during sacrificial etching. Combined in-and out-of-plane motion can be obtained with the actuator shown in figure 3, as a result the tip of the actuator moves on a 3D curve. The in-plane motion due to differential expansion of the legs and the out-of-plane motion due to thermal expansion mismatch of the materials are obtained simultaneously, the difference in magnitude depends on the geometrical characteristics of the actuator. All three actuators have been embedded into a fabrication process that allows their combination, consequently in-and out-of-plane motions can be provided for complex MEMS. The development of novel electrothermal actuators comprises: design, FEM simulation, redesign, FEM simulation, fabrication and testing. The difference between the performance of the simulated (ANSYS/Multiphysics) and tested actuators is below 10%, which can be explained by the convection and radiation between surfaces-air and surfaces-surfaces respectively. The releasing of the actuators is not critical due to their relative large output force. The above-described actuators are fully controllable in each intermediate position.