Mechanical properties of micromachined structures

Abstract

To be able to accurately design structures and make reliability predictions, it is first necessary to know the mechanical properties of the materials that make up the structural components. In the fields of microelectromechanical systems (MEMSmicroelectromechanical system (MEMS)) and nanoelectromechanical systems (NEMSnanoelectromechanical system (NEMS)residual stressstressresidualYoung's moduluspolysilicon (poly-Si)), the devices are necessarily very small. The processing techniques and microstructures of the materials in these devices may differ significantly from bulk structures. Also, the surface area to volume ratio in these structures is much higher than in bulk samples, and so the surface properties become much more important. In short, it cannot be assumed that mechanical properties measured using bulk specimens will apply to the same materials when used in MEMS and NEMS. This chapter will review the techniques that have been used to determine the mechanical properties of micromachined structures, especially residual stress, strength, and Young's modulus. The experimental measurements that have been performed will then be summarized, in particular the values obtained for polycrystalline silicon (polysilicon).