A multi-layered micro-bridge buckles due to residual stresses in the layers of the beam when it is released during fabrication, and the axial load due to this stress exceeds the Euler load. This residual stress renders intrinsic bi-stability behavior to the bridge. In this paper, the effect of axial, rotational stiffness, and residual moment on the buckled shape, snapping, and bi-stability of multi-layered bridge when it is thermally actuated is studied both theoretically, and experimentally. Theoretical analysis, and ANSYS finite element simulation have been carried out to investigate these effects. Deflection versus temperature plots for different axial, rotational stiffness, and residual moment are obtained. The theoretical investigations are applied to bi-morph micro-bridges of 1000um length, and 40um wide made of PECVD silicon dioxide, and epi-taxial silicon, and a tri-layer structure of Poly/SiO2/epi-silicon. The bi-layer structure is fabricated, and its buckled shape is obtained from SEM. Results show that axial, rotational stiffness, and residual moment strongly affect the buckled shape, and bi-stability of the micro-bridge. It is also shown that for thermally actuated micro-bridge, better bi-stability, and snapping characteristics can be obtained when both rotational and axial stiffnesses are reduced, and the residual moment must not exceed a certain threshold value if bi-stability is to be preserved.
In this paper, we present a finite difference based one-dimensional dynamic modeling, which includes electro-thermal coupled with thermo-mechanical behavior of a multi-layered micro-bridge. The electro-thermal model includes the heat transfer from the joule-heated layer to the other layers, and establishes the transient temperature gradient through the thickness of the bridge. The thermal moment and axial load resulting from the transient temperature gradient are used to couple electro-thermal with thermo-mechanical behavior. The dynamic modeling takes into account buckling, and damping effects, asymmetry residual stresses in the layers, and lateral movement at the support ends. The proposed model is applied to a tri-layer micro-bridge of 1000μm length, made of 2μm silicon dioxide sandwiched in between 2μm thick epi-silicon, and 2μm thick poly silicon, with four 400μm long legs, and springs at the four corners the bridge. The beam, and legs are 40μm, and 10μm wide respectively. Results demonstrate the bi-stability of the structure, and a large movement of 40μm between the up and down stable states can easily be obtained. Application of only 21mA electrical current for 15μs to the legs is required to switch buckled-up position to buckled-down position. An additional trapezoidal waveform electrical current of 100mA amplitude for 4μs, and 100μs falling time needs to be applied for the reverse actuation. The switching speed in both cases is less than 500μs.
A novel two-way bistable bimorph bridge actuator for out of plane deflection is reported in this paper. The device has a 1200μm long, 50μm wide and 4μm thick composite bimorph beam consists of PECVD SiO2 and titanium layers. The end supports of the beam consist of 2 pairs of spring and are provided by 2 pairs of long titanium 'legs’ alongside the beam. 10mW and 4mW in the beam and legs for 3ms respectively is needed for 50 micron of out-of-plane deflection travel. By applying the appropriate joule heating sequence to the device, it is possible to snap the buckled beam upward or downward between two equilibrium states. An analytical and simulation models of heat transfer and tunable snapping are developed for the system. This paper presents the working principal, analysis, simulations of the device. The actuator will be used to move a micromirror, located at the centre of beam, for optical switching. This novel mechanism can have useful application in relays, optical switching and threshold sensors.
In this paper , the importance of boundary conditions for two- way bistable thermal snapping action is analysed by Ansys simulations. Several designs are developed and modified into a novel bistable actuator for out of plane deflection. It consists of 4 long epi-silicon single layered legs , a 1200um x 40um x 3um microbeam and 4 spring-supports in which both are polysilicon/oxide/episilicon layered. Buckling of the released structure is achieved by the compressive residual stress remaining in the oxide layer as a result of the processing. The structure switches between two stable equilibrium states by heating the legs and then heating one of the polysilicon layers to produce the thermal moment needed for snapping to occur, hence achieving two-way bistable operation.
A theoretical analysis of initially buckled, and thermally actuated bimorph micro bridge is presented in this paper. The micro bridge is to be buckled by compressive residual stress developed in the beam during fabrication. An analytical model that characterizes the buckling shape is proposed, and used in the analysis. This model considers symmetrical rotational stiffness, and infinite axial stiffness at both ends of the bridge. Deflections versus temperature characteristics for a bimorph micro-bridge of length, 1000µm, thickness, 4µm, with various initial deflections ranging from 5µm to 20µm are obtained, and plotted. The results show that a pin-pin micro bridge (rotational stiffness of zero) exhibits bi-stability at lower snapping temperatures when negative thermal expansion material is used as one of the layers. A snapping temperature of less than 100°C is possible. It is also shown that clamped-clamped (rotational stiffness of infinite) micro bridge does not snap at all, and there is maximum allowable rotational stiffness below which snapping is possible.