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Theoretical and experimental characterization of the in-plane tip force and deflection achieved with asymmetrical polysilicon electrothermal microactuators

Several microactuator technologies have recently been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion are the most common modes of microactuator operation. This research focuses on the design and experimental characterization of two types of asymmetrical MEMS electrothermal microactuators. The motivation is to present a unified description of the behavior of the electrothermal microactuator so that it can be adapted to a variety of MEMS applications. Both MEMS polysilicon electrothermal microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. In a conventional electrothermal microactuator, the hot arm is positioned parallel to a cold arm, but because the hot arm is narrower than the cold arm, the electrical resistance of the hot arm is higher. When an electric curren passes through the microactuator (through the series connected electrical resistance of the hot and cold arms), the hot arm is heated to a higher temperature than the cold arm. This temperature increase causes the hot arm to expand along its length, thus forcing the tip of the device to rotate about a mechanical flexure element. The new thermal actuator design eliminates the parasitic electrical resistance of the cold arm by incorporating an additional hot arm. The second hot arm results in an improvement in electrical efficiency by providing an active return current path. Additionally, the rotating cold arm can have a narrower flexure than the flexure in a conventional single-hot arm device because it does not have to pass an electric current. The narrower flexure element results in an improvement in mechanical efficiency. Deflection and force measurements of both actuators as a function of applied electrical power are presented.