Residual stress chevron preloading amplifier for large-stroke stiffness reduction of silicon flexure mechanisms

Residual stresses can be advantageously used to permanently preload flexure micro-mechanisms in order to modify their deflection and stiffness. This paper presents a new preloading chevron mechanism (PCM) used to amplify the preloading effect of thin film residual stress. To evaluate the preloading performances of this structure, the deflection characteristics of buckled beams and flexure linear stages preloaded by a PCM is investigated experimentally. All the mechanisms are manufactured from a monocrystalline silicon substrate using deep reactive ion etching and residual stress is provided by wet thermal oxidation. Measurements show that the deflection magnitude of fixed-fixed oxidized silicon buckled beams can be increased by up to 5 times when a PCM is integrated. The flexure linear stages studied in this research are composed of a parallel leaf spring stage connected to two fixed-guided buckled beams preloaded by a PCM. Depending on the beam dimensions, the stage translational stiffness can be set to a specific value. We designed a near-zero positive stiffness linear stage revealing a measured stiffness reduction of 98%, and a bistable linear stage with a constant negative stiffness region. Thanks to the elevated preloading displacement supplied by the PCM, the operating stroke (actuation region where the stiffness remains constant) is relatively large (more than 0.4 mm travel for 2.59 mm leaf spring length). The analytical and numerical models carried out to design the mechanisms are in good agreement with the experimental data. The results show that the fixed frame stiffness has a significant effect on the preloading performances due to the substantial forces exerted by the PCM. Furthermore, the presented preloading concept, modeling and sizing method could be applied to other compliant mechanism designs, scales and materials, enabling applications in microelectromechanical systems and watchmaking.