Mechanical Properties of MEMS Materials

Abstract

The performance of micro electronic and mechanical systems (MEMS) strongly depends on the mechanical properties of materials used. The evaluation of the mechanical properties of MEMS materials is indispensable for designing MEMS devices. Accurate values of mechanical properties (elastic properties, internal stress, strength, fatigue) are necessary for obtaining the optimum performances. For an example, elastic properties are necessary in prediction of the amount of deflection from an applied force and material strength sets device operational limits. Also, in view of reliability and life time requirements, mechanical characterization of MEMS materials becomes increasingly important. Small size of MEMS devices often leads to their usage in harsh environments, and good knowledge of mechanical properties may lead to elimination of some of the mechanical failure modes through proper material selection, design, fabrication and packaging processes. As the interest in MEMS grows, the demand for applicable data increases. Reliability, accuracy and repeatability of evaluation methods also became an issue. However, MEMS use materials such as silicon and many other thin films that are not fully characterized regarding their mechanical properties because they had not previously been considered as mechanical materials. The properties of thin films have so far been evaluated mostly to satisfy demands in semiconductor device research, but evaluations were mainly focused on the electrical properties, while investigations of mechanical properties were limited mainly to internal stresses. For that reason, the bulk properties were adopted whenever mechanical properties were needed, but with the growing application of thin films in various mechanical structures grew the need for better understanding of their mechanical and electromechanical properties. Therefore mechanical properties of thin films used in MEMS need to be accurately evaluated – they should be measured at the same scale as microand nanodevices since they differ from bulk material properties. Thin-film and bulk materials usually have different compositions, phase and microstructure and the formation process for thin films must be taken into account (deposition, thermal treatment, implantation and oxidation). Mechanical processing as the processing method for most bulk structures is in case of thin films substituted with photolithography and etching. Also, bulk and thin film have different surface finishing of processed structures. When size effect is concerned, one must have in mind that the ratio of surface area to the volume increases as dimensions of a device decrease. The dimensions of structures in MEMS devices range from submicrometers to millimetres and therefore the size effect in thin films is more sensitive than in bulk materials. Many measurement methods have been developed for evaluation of