Interferometry system for out-of-plane microdisplacement measurement: application to mechanical expertise of scratch drive actuators

The material properties of silicon, as well as the planar and monolithic nature of the microstructures make electrostatic field energy conversion the most suitable driving principle on the micrometer scale. Moreover, compared with most other actuation principles, the scaling of electrostatic forces is particularly suitable for actuator downsizing. In spite of the advantages, it is still difficult to obtain appropriate driving characteristics because of silicon based actuator limitations such as small structural height, micrometer gap requirements and material limitations in the shaping process. Actuators require specific tools to verify that their mechanical properties and motions obey the designer's intent. In this paper capabilities of future direct-drive electrostatic actuators SDA (Scratch Drive Actuators) are investigated through the characterisation of their out-of-plane displacements by interferometry. The actuation involves contact interactions by using flexible polysilicon elementary actuator plate. The region of the physical contact is measured using Twyman-Green interferometer incorporated within a metallurgical microscope. The shapes and out-of-plane displacements of microstructures are extracted from interferograms by temporal phase shift method (TPS). Additionally, the results from interferometric method are compared with numerical simulations given by finite elements software - ANSYS.