Bistability behavior of thermally actuated micro-bridge

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.