State of the art wireless telecommunication devices are operating in the (low) frequency GHz regime. They pick up broadband electromagnetic signals, which are band-pass filtered to comply with a designated frequency band. This is usually achieved by a filter-stage, based on several highly efficient electroacoustic devices, i.e., bulk-acoustic wave (BAW) filters. The performance of these BAW-filters is influenced by the acoustic losses of its constituent materials. Knowing the acoustic losses of these materials allows filter manufacturers to better simulate and optimize their devices. Here, we present a method for the determination of acoustic losses of individual materials at frequencies around 2GHz. It is based on measuring the quality factor of Lamb-wave resonances in plates with thicknesses of around 1µm with a frequency domain laser-ultrasound setup.In the dispersion curves f(k) of plate-like structures points with zero-group velocity (i.e., with a vanishing slope) occur at certain combinations of frequency and wavenumber. Two types are being distinguished: Points with a vanishing wavenumber (infinite wavelength), called thickness resonances or cut-offs, and such with a finite wavenumber (wavelength) which are usually denoted as zero group velocity resonances (ZGVs). Due to the hampered energy transport, both appear as dominant peaks in local frequency response spectra. The frequency position of the plate-resonances has been successfully used to characterize the thickness and elastic properties of plates. The shape of the resonance-peak is associated with the temporal decay of the resonances, which is composed of a power-law contribution due to transient spatial spreading of the mode, and an exponential decay, associated with the material damping. Thus, the shape (width) of the peaks in the frequency domain can be used to determine the intrinsic acoustic attenuation at a frequency determined by the peak position.Two-approaches for the determination of the longitudinal attenuation coefficient are being presented. One is based on the direct measurement of longitudinal thickness resonances in plates with different thicknesses, i.e., different resonance frequencies. The second uses two ZGV-resonances, which are governed by a mixture of longitudinal and transverse wave motion. A method is shown, to disentangle their damping contributions to obtain pure longitudinal and transvers attenuation coefficients in isotropic materials. For both cases, signal-processing methods for the compensation of the power-law contribution are shown. Experimental results are presented for the materials Aluminum and Tungsten.

Determination of the Acoustic Attenuation in the Ghz-Regime Using Zero-Group Velocity Plate-Resonances

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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