A novel bistable two-way actuated out-of-plane electrothermal microbridge

In this paper, we report the design, simulation, fabrication, and testing of a novel electrothermally actuated buckled microbridge for out-of-plane actuation. The new structure consists of a bridge supported by two springs and two "legs" at both supporting ends. The bridge and the spring are trilayer structures: low-stress plasma-enhanced chemical-vapor-deposition (PECVD) oxide (2.5 mu m), high compressive stress PECVD oxide (1 mu m), and phosphorus-doped silicon (2 mu m). The "legs" consist only of 2-mu m-thick phosphorus-doped silicon. The overall size of the actuator is 1200 mu m by 100 mu m excluding the contact pads. Analytical expressions used to determine torsional and axial stiffnesses at the supporting ends are derived. Together with bistability criteria developed by Michael and Kwok, the microbridge structure is designed to exhibit a bistable behavior. It is also designed to have a two-way actuation capability. Electrothermal analysis and thermomechanical finite element ANSYS simulations are performed to determine the switching waveforms for the two-way actuation and verify the bistability. Actuation of the fabricated structure from the buckled-up to the buckled-down states required a 7-V voltage pulse for 1 ms across the "legs," followed by a 5-V pulse for 4 ms across the bridge. A 9-V pulse for 0.5 ms only to "legs" enabled the switching of the bridge from the buckled-down to the buckled-up states. An out-of-plane movement of 31 mu m is demonstrated for a 1200-mu m-long microbridge. Larger movement can be obtained by increasing the microbridge length and the compressive stress of the stressed oxide layer of the bridge.