Determination of Elastic Modulus of Piezoelectric Aluminum Nitride Coating
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Determination of Elastic Modulus of Piezoelectric Aluminum Nitride Coating Soma S Perooly1, Varun Garg2, Md. H. Rahman3, L.W. Rosenberger3, Gina S.Shreve3 Ronald F. Gibson2 and G.W. Auner1 1. Department of Electrical and Computer Engineering and Smart Sensors and Integrated Microsystems Laboratory, 2. Mechanical Engineering Department and Advanced Composites Research Laboratory, 3. Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202 Abstract: Aluminum nitride (AlN) is a technologically important wide band gap semiconductor and a potential piezoelectric material for Biosensor application [1]. It is a clear candidate for the integration of surface acoustic wave (SAW) devices on chips with silicon-based electronics [2]. AlN may prove to be useful for the integration of mechanical devices and also in the field of packaging of Bio-MEMS devices because of its superior thermal conductivity (130-140 W/mK, room temp-100C) and nontoxicity. In the current work the elastic modulus of AlN coated on Silicon (Si) micro-cantilevers has been estimated, employing the concept of symmetrically laminated beam theory for the first time at the micro level thus avoiding the coupling effects. Previous studies have been done using only simple beam theory. Modal vibration frequencies for microcantilevers without and with the AlN coating were used, along with analytical models and knowledge of the Si modulus, to “back out” the modulus of AlN [3].
Experimental: AlN thin films were deposited on micro- cantilever arrays obtained from Concentris GmbH (Switzerland) by low temperature Plasma Source Molecular Beam Epitaxy (PSMBE) technique. A specially designed fixture made of molybdenum was employed to coat both the top and bottom sides of the cantilevers by inverting the array in between the depositions. A reference piece of Si (100) was placed in the fixture close to the cantilevers to measure the thickness and the crystallinity of the AlN coating. A Polytec MSV-300 Laser Vibrometer system (Fig.1) was used to determine the natural frequencies of the micro cantilevers coated symmetrically with AlN and hence estimate the Young’s modulus. The Doppler frequency shift of a single frequency was used to measure the surface vibration of scattering surfaces. The Polytec MSV-300 Laser Doppler Vibrometer (LDV) can measure vibrations up to 1 MHz with very high accuracy and linear phase response. The uncoated and AlN coated cantilevers were excited using an electromagnetic shaker. The object motion towards the interferometer results in a lower frequency of the carrier signal, while the motion away from the interferometer results in a higher frequency of the carrier signal. The amplitude of the micro-cantilever has its maximum value when the micro-cantilever is vibrating in its natural frequency. X-ray diffraction was used to determine the orientation of the crystalline films. The thickness of the coated film was measured by ellipsometry.
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