Instability in micromachined curved thermal bimorph structures

Abstract

For the micromachined thermal bimorph structure, there is often initial deflection, or so-called geometrical imperfection, which may affect the stability of the structure. Here, finite element simulations and experiments are conducted to study the influences of initial deflection and actuating region on mechanical behaviors of a curved bimorph structure with clamped boundary condition. Devices are fabricated by adjusting the internal stresses of polysilicon layers on bimorph structures to achieve various initial deflections. Various actuation regions are achieved by designing different sizes of top layers covering the bottom layers of the bimorph structures. Stable and unstable regions in terms of two design factors, initial deflection ratio and bimorph ratio, are characterized by simulations and experiments. It is found that the curved bimorph structure is stable when the bottom layer is fully covered with the top layer or the initial deflection is much smaller than the structure thickness. The stable device is found to deflect in one direction only. The bimorph structure becomes unstable while the initial deflection is close to or larger than structure thickness. For unstable curved bimorph structures, we find the snap buckling effect with two-way deflections and a hysteresis loop.