MEMS are increasingly being considered for applications that involve immersion in liquids. However, there
are very little reliability data for MEMS structures in liquids environments. In this study, an apparatus was developed
which enables the investigation of fatigue failure of MEMS in liquids. MEMS cantilever beams were mounted on a PZT
piezoelectric actuator and immersed in a liquid. A laser is reflected off the tip of the vibrating cantilever and onto a
position-sensing photo-diode device (PSD) to obtain position data. From this data resonance frequency can be extracted
for long-term monitoring. Cantilevers are resonated for at least 108
cycles. This apparatus allows for the testing of many
combinations of materials and environments. For this study, the fatigue performance in liquid of silicon nitride
cantilever beams was evaluated and compared to single crystal silicon cantilever beams. Tests were conducted in
deionized water and a saline solution. Silicon nitride exhibited no long-term degradation of resonance frequency within
measurement limits in air, DI water, and saline environments. Silicon exhibited a steady decrease in resonance. Results
showed that this method could be extended to conduct reliability studies on other MEMS materials.
Reliable operation of MEMS in liquid environments is an important design requirement for numerous MEMS devices in chemical, pharmaceutical, biomedical, consumer product, and defense industries. In this paper, reliability and long-term performance of microcantilevers in liquid environments is investigated. Single crystal silicon microcantilevers are subjected to long-term cyclic actuation (≈108 -109 cycles) in enclosures filled with two different liquids- de-ionized water and saline solution. Additionally, silicon microcantilevers are actuated in air to enable comparison of experimental data in air and liquids. The microcantilevers have an electroplated Permalloy layer and are magnetically actuated. The resonance frequency of the microcantilevers is periodically monitored to track changes in stiffness and mechanical performance. The microcantilevers are subjected to peak stresses ranging from 0-10 MPa, which are typical for MEMS applications like AFM tips, and resonating sensors. Since the peak stresses are small compared to the tensile strength of silicon (1-3 GPa), complete structural fatigue failure is neither expected nor observed. However, operational failures characterized by a gradual decrease in resonance frequency of the microcantilevers are observed in saline solution. Changes in resonance frequency of microcantilevers actuated in air and water were negligible to within the limits of experimental accuracy. These results demonstrate that an understanding of MEMS reliability in air cannot necessarily be extended to explain and predict device reliability in liquid environments.
KEYWORDS: Microelectromechanical systems, Actuators, Reliability, Interferometry, Power supplies, Oxides, Mirrors, Switches, Space based lasers, Prototyping
Reliability of MEMS is a major concern for the commercialization of laboratory prototypes. Surface adhesion or stiction strongly affects the reliability of MEMS devices which have sliding or rubbing contacts. Determination of adhesion energies, adhesion forces, and pull-off forces are important for predicting stiction in MEMS. We present an experimental technique to estimate the pull-off forces for MEMS surfaces. Polysilicon microcantilevers were electrostatically actuated using gradually varying voltages. A hysteresis was observed in the voltage at which the tip of the cantilevers made and broke contact with the substrate. Pull-off forces were estimated from the hysteresis in the voltage values using a strain energy formulation. The pull-off forces for microcantilevers dried out of isopropyl alcohol and repaired using laser irradiation were estimated to be in the range of 45-121 nN. The role of adhered length, variable external loading, and actuating signal on in-use stiction is also investigated. From our experimental results, we demonstrate an empirical approach to predict in-use stiction of microcantilevers.
°Microelectromechanical systems (MEMS) have enormous potential to contribute in diverse fields such as automotive, health care, aerospace, consumer products, and biotechnology, but successful commercial applications of MEMS are still small in number. Reliability of MEMS is a major impediment to the commercialization of laboratory prototypes. Due to the multitude of MEMS applications and the numerous processing and packaging steps, MEMS are exposed to a variety of environmental conditions, making the prediction of operational reliability difficult. In this paper, we investigate the effects of operating temperature on the in-use adhesive failure of electrostatically actuated MEMS microcantilevers coated with octadecyltrichlorosilane (OTS) films. The cantilevers are subjected to repeated temperature cycles and electrostatically actuated at temperatures between 25°C and 300°C in ambient air. The experimental results indicate that temperature cycling of the OTS coated cantilevers in air reduces the sticking probability of the microcantilevers. The sticking probability of OTS coated cantilevers was highest during heating, which decreased during cooling, and was lowest during reheating. Modifications to the OTS release method to increase its yield are also discussed.
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