ABSTRACT

In this chapter, we demonstrate a low-cost and multifunctional interferometric platform based on a Twyman-Green interferometer (TGI) [1], as shown in Section 10.2, for testing microelectromechanical systems/microoptoelectromechanical systems (MEMS/MOEMS). This technique may be used for relatively smooth surfaces to measure the 3-D map of out-of-plane displacements. It provides both micromechanical and material properties of buckled membranes, quasi-static actuator behavior, and vibrational analysis of microdevices. In Section 10.3, specific metrology procedures have been demonstrated to determine the residual stress of silicon membranes prestressed by silicon oxinitride (SiO

N

) grown by plasma-enhanced chemical vapor deposition (PECVD) [2]. The

compressive stress produced by the deposition of SiO

N

films generates a measurable initial bending of the bimorph membrane, corresponding to an out-of-plane displacement, measured by interferometry. To measure the Young’s modulus and hardness of SiO

N

films, we use the nanoindentation technique. The interpolation of mechanical parameters in case of compressively prestressed membranes is limited because of the absence of validated mechanical models. In order to improve this situation, the extraction of residual stress is obtained by linking the interferometry and nanoindentation data with an analytical “stress function” calculated by finite elements method (FEM). Because material properties of SiO

N

films depend strongly on details of the PECVD process, the relationship between the micromechanical properties, physicochemical characteristics, and, finally, the optical properties of SiO

N

thin films are established carefully. The design of MEMS actuators requires precise prediction of displacements. Although simulation

capabilities are advancing rapidly, many devices exhibit nonlinear effects such as electrostatic attraction or coupling of deformation modes, all of which must be accurately accounted for in the simulations. Even if the models for these effects have been developed, they are not sufficient for

a priori

prediction of actual performances. Thus, calibration and design verification require high-precision absolute measurements of displacements of structural parts. In Section 10.4, the polysilicon scratch drive actuator (SDA) is used as an example structure for demonstrating applications of interferometry for the determination of actuator performance [3]. The actuation involves contact interactions performed by the flexible polysilicon actuator plate. A better understanding of the driving and stiction mechanisms is given precisely by the length of such contact interactions that could lead to the optimization of SDA design. Until now, only in-plane characterizations have been available through microscope visualization and camera video recording allowing linear speed and driving force of actuators to be measured. However, stiction mechanisms are strongly tied to the out-of-plane actuator behavior and, particularly, the actuator/substrate contact zone. Interferometric technique measures complete deflection curves of the electrostatically actuated SDA plate. The experimental results obtained by this technique are compared with numerical data calculated by FEM.