A two-beam method for extending the working range of electrostatic parallel-plate micro-actuators

Electrostatic actuation is widely used because of its rapid speed and low power consumption. The parallel-plate actuator is one of the most basic, but also most common electrostatic actuators for Micro Electro Mechanical System (MEMS) applications. The effective working range of parallel-plate actuator is limited to only one-third of the initial gap owing to the pull-in instability. Different strategies have been reported to extend the stable working range, but are mostly focused on improvement of the control circuit. In this paper, we aim to report some progress toward solution of the working range problem from the design of suspension beam. A suspension beam called "two-beam" is presented to achieve parallel-plate actuator with extended working range, but without penalties of complex control circuit and large actuation voltage. A bumper structure is employed in the design of two-beam to change the spring constant near the critical point of pull-in, and a two-layered polysilicon surface process is used to fabricate this two-beam structure. Theoretical calculation and numerical simulation indicate that the working range can be extended depending on the position and depth of the bumper. The fabrication of a 2×2 array of parallel-plate actuators suspended by four sets of two-beam is implemented, and a somewhat larger working range, i.e. about 50% of the initial gap is achieved in the experiment.