Design and performance of an electrothermal MEMS microengine capable of bi-directional motion

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Several microactuator technologies have been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion represent the most common modes of microactuator operation. This research focuses on the design and comparative performance evaluation of asymmetrical electrothermal actuators. The motivation is to present a unified description of the behavior of the electrothermal actuator so that it can be adapted to a variety of microsensor and microactuator applications. The MEMS polysilicon surface micromachined electrothermal actuator uses resistive (Joule) heating to generate thermal expansion and movement. In the traditional asymmetrical electrothermal actuator design, the single-hot arm is narrower than the cold arm, and thus, the electrical resistance of the hot arm is greater. When an electrical current passes through the device (both the hot and cold arms), the hot arm is heated to a higher temperature than the cold arm. This temperature differential causes the hot arm to expand along its length, thus forcing the tip of the device to rotate about the flexure. Another variant of the asymmetrical design features a double-hot arm arrangement that eliminates the parasitic electrical resistance of the cold arm. Furthermore, the second hot arm improves electromechanical efficiency by providing a return current conductor that is also mechanically active. In this design, the rotating cold arm can have a narrower flexure compared to the flexure in the traditional single-hot arm device because it no longer needs to conduct an electrical current. The narrower flexure results in an improvement in mechanical efficiency. This research compares the tip deflection performance of the asymmetrical single- and double-hot arm electrothermal actuator designs. Deflection measurements of both actuator designs as a function of arm length and applied electrical power are presented. As a practical application of the electrothermal actuator, the recent realization of a MEMS microengine is described, and evidence of its bi-directional motion is presented. The electrothermal actuator and microengine designs were accomplished with the MEMSPro® CAD software program, and they were fabricated using the MEMSCAP Integrated Microsystems Multi-User Microelectromechanical Systems (MEMS) Process® (MUMPs) foundry.