Mechanical fatigue test under liquid water toward bio-implantable MEMS structures with infinite lifetime

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For those structures in medical MEMS (Microelectromechanical Systems) devices, especially the case of implantable devices, evaluation and improvement of mechanical reliability under humid environment including underwater become an issue of importance. In this study, static strength test and fatigue lifetime test were carried out under wet and humid environment aiming at the establishment of strength design scheme for those silicon MEMS structures in the presence of water molecular and restraint of degradation in living body. For this purpose, static/fatigue testing system coping with liquid water environment was developed. Using this experimental setup, the fracture behavior and mechanical properties of single crystal silicon thin film was investigated under liquid water. Crystal defects in silicon were also observed using EBIC (Electron Beam Induced Current) to survey the correlation between damage accumulation process and environment during the fatigue process. On the other hands, possible deterioration of mechanical properties with proton and hydrogen dissociated from the surface water molecular and trapped in silicon crystal was investigated by using nano-indentation technique. Based on these results, prediction models for the fatigue lifetime in various environments were also surveyed.