In-plane tip deflection and force achieved with asymmetrical polysilicon electrothermal microactuators

Abstract

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 current 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 ‘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 manifests improved mechanical efficiency. Deflection and force measurements of both actuators as a function of applied electrical power have been presented in this work.