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11 August 2008 Modeling and optical characterization of vibrating micro- and nanostructures
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The lack of commercial equipment for characterization of vibrating micro- and nanostructures has motivated the development of a heterodyne interferometer. The setup is designed to measure phase and absolute amplitude in the entire frequency range 0-1.2 GHz. Its transverse resolution is < 1 μm while the present sensitivity for vibrations is 3 pm/(Hz)1/2. Capacitive micromachined ultrasonic transducers (CMUTs) are being developed for diagnostic imaging of vulnerable plaques in the coronary arteries. The CMUTs have 5.7 μm radii, 100 nm membrane thickness and ~30 MHz center frequency. Arrays of ~7500 CMUTs have been fabricated. Frequency scan measurements along a row of CMUTs reveal a variation in resonance frequency. This may be due to variations of material properties, dimensions such as thickness and transverse dimensions, and other manufacturing variance. The frequency scan revealed the fundamental mode and two closely spaced higher order modes. Modeling of individual CMUT elements was performed using the commercial program COMSOL. A finite element model (FEM) based on symmetry assumptions predicted only one higher order mode. After closer analysis it was found that the symmetry assumptions were insufficient. By using a complete physical model two higher order modes were predicted in agreement with the measurements. Simulations are able to predict transducer characteristics in great detail but are dependent on accurate input parameters. The optical measurements contribute to validate or complement simulations and assumptions they rely on. The heterodyne interferometer is therefore a valuable tool for quality control in the conception, design and manufacturing of new acoustic devices.
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Astrid Aksnes, Erlend Leirset, Hanne Martinussen, and Helge E. Engan "Modeling and optical characterization of vibrating micro- and nanostructures", Proc. SPIE 7063, Interferometry XIV: Techniques and Analysis, 70630H (11 August 2008);

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