MEMS residual stress characterization: Methodology and perspective

Residual stress characterization in microelectromechanical systems (MEMS) structures is discussed in this chapter. Residual stress characterization in MEMS structures is of inherent importance in various respects. The existence of residual stresses essentially changes the performance and reduces the structural integrity and longevity of MEMS devices. MEMS techniques actually provide a new tool for studying the mechanical properties of materials such as modulus, hardness, and state of stresses. The existence of residual stresses can seriously influence the reliability and dynamical characteristics of devices. Residual stress occurs in materials and mechanical components during manufacturing from many film growth processes. The residual stress characterization techniques related to MEMS can be classified as wafer-level curvature measurement, material-level nondestructive measurement, residual stress measurement using MEMS specimens or structures, and material-level destructive measurement. The most widely applied or acknowledged thin-film stress measurement method is the curvature measurement of beam or plate structures. Bulge test is used to determine the material properties of thin films. The sudden buckling collapse due to excessive compressive stresses has been used to evaluate the lower bound of residual stress level of elastic MEMS structures for many years. Raman spectroscopy allows the identification of the material compositions and yields information about phonon frequencies, energies of electron states and electron-phonon interaction, carrier concentration, impurity content, composition, crystal structure, crystal orientation, temperature, and mechanical strain. The pull-in test method utilizing the nonlinear instability in electrostatic actuation and the famous M-test concept is based on an array of microelectromechanical test structures, fixed beams, and clamped diaphragms of varying dimensions. Indentation testing is a simple method to determine material properties such as Young’s modulus and microhardness, fracture strength, and toughness. Finally, the problems induced by stress gradient and its associated characterizations are also addressed.