Fast Characterization of Silicon Membrane Structures by Laser-Doppler Vibrometry
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1052-DD01-06
Fast Characterization of Silicon Membrane Structures by Laser-Doppler Vibrometry Ronny Gerbach, Matthias Ebert, and Joerg Bagdahn Components in Microelectronics, Microsystems and Photovoltaics, Fraunhofer Institute for Mechanics of Materials, Walter-Huelse-Str. 1, Halle, 06120, Germany ABSTRACT Micromechanical structures were investigated nondestructively via laser-Dopplervibrometry to determine defect structures. Silicon membrane structures were characterized by their measured resonant frequencies and mode shapes. The influence of defects on the micromechanical structures on the measured dynamic properties is shown. Defective samples were indentified on the basis of the ratios of measured resonant frequencies and the quantified comparison of mode shapes without an identification of unknown parameters. The investigations showed that a fast determination of defect structures is possible by measuring dynamic properties. INTRODUCTION The nondestructive characterization of microelectromechanical systems (MEMS) is a key element for the monitoring of manufacturing processes of MEMS. It is necessary to describe all components of these systems including electronic and non-electronic components. There are a lot of well established measuring techniques for the characterizing of the electrical components. In contrast, there are few nondestructive methods the testing of MEMS. Optical measurement techniques have gained great importance. They allow a fast and nondestructive characterization of mechanical components of MEMS. They should follow general requirements such as being a reliable measurement without a changing of the behavior of the structure [1]. Some measuring techniques have already been developed for these tasks [1-3]. Laser-Doppler vibrometry is a well established method for the characterization of micromechanical structures on basis of their dynamic properties. Different investigations were already presented results for the identification of unknown properties [4-7]. Validation experiments were performed to verify identified values with an error less than 2% [7]. In the following paper, different approaches were presented for the determination of defect structures without an identification of parameters. The approaches introduced here are based on the use of measured resonant frequencies and mode shapes. PRINCIPLES OF THE METHOD Mechanical structures can be analyzed on basis of their dynamic properties. Dynamic properties can be characterized in the time and frequency domain. Investigations in the time domain comprise the behavior of the mechanical structure under a pulsative or homogeneous actuation. A characterization of micromechanical structures in the frequency domain can occur by examining the frequency spectrum generating to a wide-band excitation and also by their resonant frequencies and mode shapes. Micromechanical structures vibrate at high frequencies with small amplitudes because of the small size of the mechanical structures which leads to a
high signal to noise ratio in the time domain
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