Characterization of Symmetrical and Asymmetrical Polysilicon Surface Micromachined Electrothermal Actuators

Abstract

Several electrically-driven microactuators have been investigated for positioning individual elements in microelectromechanical systems (MEMS). The most common modes of actuation are electrostatic, magnetostatic, piezoelectric and thermal expansion. Unfortunately, the forces produced by electrostatic and magnetostatic actuators tend to be small, and to achieve large displacements, it is necessary to either apply a large voltage or operate the devices in a resonant mode. On the other hand, piezoelectric and thermal expansion actuators can be configured to produce large forces and large displacements. However, piezoelectric materials are not routinely supported in the fabrication processes offered by commercial MEMS foundries. These limitations have focused attention on thermally-actuated devices for generating the large forces and displacements frequently required to position and assemble complex MEMS. This investigation reports a new polysilicon electrothermal actuator design. In the traditional 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 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. The new double-hot arm 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 electromechanical efficiency by providing a return current conductor that is also mechanically-active. Furthermore, in the new electrothermal actuator design, the rotating cold arm can have a narrower flexure compared to the flexure in the traditional device because it no longer needs to conduct an electrical current. The thinner flexure results in an improvement in mechanical efficiency. This research compares the performance of the single- and double-hot arm electrothermal actuator designs. Force and deflection measurements of both actuator designs as a function of arm length and applied electrical power are presented. The electrothermal actuator designs were accomplished with the MEMSPro ® software program, and they were fabricated using the MEMSCAP Integrated Microsystems Multi-User Microelectromechanical Systems (MEMS) Process ® (MUMPs) foundry at the Microelectronics Center of North Carolina (MCNC). development of integrated circuit microsensors, micromachining techniques applied to laser absorbers, advanced multi-chip module packaging technologies, solid-state gas chromatography systems, microelectromechanical systems