Thermo-mechanical behavior of buckled multi-layered micro-bridges

In this paper, the thermo-mechanical behavior of buckled multi-layered micro-bridge is theoretically analyzed, simulated using ANSYS and experimentally confirmed on a fabricated micro-bridge. The analytical model characterizes its thermo-mechanical behavior by taking into account torsional stiffness, axial stiffness and residual moment at supporting ends of the bridge. The snapping temperatures are obtained from the model and compared with the corresponding ANSYS simulation results. These comparisons were conducted on five composite structures. Four of these are bimorphs consisting of 0.5 μm or 1 μm thick aluminum layer and 3 μm thick PECVD oxide with either 70 MPa or 110 MPa or 220 MPa compressive residual stresses. The fifth structure consists of a tri-morph micro-bridge, which is made of 1 μm thick aluminum, 3 μm thick compressively stressed PECVD oxide and 1.5 μm thick low stress oxide, with various supporting end conditions. Excellent agreement, with an average error of less than 7%, has been obtained between the analytical model and ANSYS simulation. Results show the importance of reducing torsional stiffness of the supporting ends of the bridge if the micro-bridge is to be used for thermal actuation. A micro-bridge device consisting of a tri-layer made of 2.2 μm of phosphorus doped silicon, 1 μm thick compressively stressed oxide and 2.3 μm thick low stress oxide is fabricated, packaged, actuated in situ in a SEM chamber and the mid-point deflection as a function of actuating current is obtained. The measured results are in good agreement with model predictions.