J.V. Knuuttila

Scanning Michelson interferometer for imaging surface acoustic wave fields

A scanning homodyne Michelson interferometer is constructed for two-dimensional imaging of high-frequency surface acoustic wave (SAW) fields in SAW devices. The interferometer possesses a sensitivity of ~10(-5)nm/ radicalHz , and it is capable of directly measuring SAWs with frequencies ranging from 0.5 MHz up to 1 GHz. The fast scheme used for locating the optimum operation point of the interferometer facilitates high measuring speeds, up to 50,000 points/h. The measured field image has a lateral resolution of better than 1 mu;m . The fully optical noninvasive scanning system can be applied to SAW device development and research, providing information on acoustic wave distribution that cannot be obtained by merely electrical measurements.

Estimating materials parameters in thin-film BAW resonators using measured dispersion curves

The dispersion curves of Lamb-wave modes propagating along a multilayer structure are important for the operation of thin-film bulk acoustic wave (BAW) devices. For instance, the behavior of the side resonances that may contaminate the electrical response of a thin-film BAW resonator depends on the dispersion relation of the layer stack. Because the dispersion behavior depends on the materials parameters (and thicknesses) of the layers in the structure, measurement of the dispersion curves provides a tool for determining the materials parameters of thin films. We have determined the dispersion curves for a multilayer structure through measuring the mechanical displacement profiles over the top electrode of a thin-film BAW resonator at several frequencies using a homodyne Michelson laser interferometer. The layer thicknesses are obtained using scanning electron microscope (SEM) measurements. In the numerical computation of the dispersion curves, the piezoelectricity and full anisotropy of the materials are taken into account. The materials parameters of the piezoelectric layer are determined through fitting the measured and computed dispersion curves.

Phase-sensitive absolute-amplitude measurements of surface waves using heterodyne interferometry

Phase-sensitive absolute-amplitude measurements of surface waves using heterodyne interferometry. We describe a novel heterodyne interferometer for scanning surface-acoustic waves at high frequencies, up to 6 GHz. The heterodyne operation facilitates the measurement of the absolute amplitude of the surface waves without calibration. It simultaneously also enables measuring the phase. The amplitude and phase information allows a precise characterization of the surface acoustic waves. Moreover, the wave motion can be visualized in the form of animations to gain insight. The interferometer has been tested by measuring a 167 MHz SPUDT device and a 374 MHz fan-shaped SAW filter.

Acoustic loss mechanisms in leaky SAW resonators on lithium tantalate

We discuss acoustic losses in synchronous leaky surface-acoustic wave resonators on rotated Y-cut lithium tantalate substrates. Laser probe measurements and theoretical methods are employed to estimate the radiation of leaky waves into the busbars of the resonator and the excitation of bulk-acoustic waves. We find that the escaping waves lead to a significant increase in the conductance, typically in the vicinity of the resonance and in the stopband, but that they do not explain the experimentally observed deterioration of the electric response at the antiresonance. At frequencies above the stopband the generation of fast shear bulk-acoustic waves is the dominant loss mechanism.Discusses acoustic losses in synchronous leaky surface acoustic wave (LSAW) resonators on rotated Y-cut lithium tantalate (LiTaO/sub 3/) substrates. Laser probe measurements and theoretical models are employed to identify and characterize the radiation of leaky waves into the busbars of the resonator and the excitation of bulk acoustic waves. Escaping LSAWs lead to a significant increase in the conductance, typically occurring in the vicinity of the resonance and in the stopband, but they do not explain the experimentally observed deterioration of the electrical response at the antiresonance. At frequencies above the stopband, the generation of fast shear bulk acoustic waves is the dominant loss mechanism.

Bulk-acoustic waves radiated from low-loss surface-acoustic-wave resonators

The bulk-acoustic conductance in low-loss surface-acoustic-wave filters utilizing leaky surface-acoustic waves is significant for the device operation. Here we directly measure the bulk acoustic wave radiation pattern on the backside of the piezoelectric substrate with the help of a scanning laser-interferometer probe. For the case studied, a leaky surface-acoustic wave resonator on 36°YX–LiTaO3, a numerical calculation is carried out and the different bulk-wave modes arriving at the substrate bottom are identified by comparing the measured and computed energy-flow angles. The results are expected to lead to improved models for describing the operation of low-loss surface-acoustic-wave filters.

Asymmetric acoustic radiation in leaky SAW resonators on lithium tantalate

We discuss an acoustic loss mechanism in leaky surface-acoustic wave resonators on 36°YX-cut lithium tantalate substrate. Our recent acoustic field scans performed with an optical Michelson interferometer revealed a spatially asymmetric acoustic field atop the busbars of a resonator, giving rise to acoustic beams which escape the resonator area and lead to undesired losses. Here, we link the phenomenon with the inherent crystalline anisotropy of the substrate crystal: the shape of the slowness curves and the asymmetry of the polarization for the leaky surface-acoustic waves propagating at an angle with respect to the crystal X-axis.

Mechanism for acoustic leakage in surface- acoustic wave resonators on rotated Y-cut lithium tantalate substrate

We discuss an acoustic loss mechanism in surface-acoustic wave resonators on 36° YX-cut lithium tantalate substrate. Recent acoustic field scans performed with an optical Michelson interferometer reveal a spatially asymmetric acoustic field atop the busbars of a resonator, giving rise to acoustic beams which escape the resonator area and lead to undesired losses. Here, we link the phenomenon with the inherent crystalline anisotropy of the substrate: the shape of the slowness curves and the asymmetry of the polarization for leaky surface-acoustic waves, propagating at an angle with respect to the crystal X-axis.

Imaging surface-acoustic fields in a longitudinal leaky wave resonator

Acoustic wave fields in a surface-acoustic-wave resonator employing the longitudinal leaky wave mode have been imaged using a scanning Michelson laser interferometer. The synchronous one-port resonator is fabricated on YZ-cut lithium niobate. The vibration amplitude component perpendicular to the surface has been measured at several frequencies around the fundamental-mode resonance frequency of 1.54 GHz and around the Rayleigh-wave resonance frequency of 0.82 GHz. The longitudinal beating pattern, typically observed in the resonators utilizing Rayleigh waves, is not observed in the longitudinal leaky surface acoustic-wave resonator within the measured frequency range.

Laser interferometric measurement of Lamb wave dispersion and extraction of material parameters in FBARs

Laser interferometric measurement and Fourier transformation techniques have been used to extract the dispersion characteristics of a mirror (SMR) type thin film bulk acoustic wave resonator (FBAR). A numerically robust matrix method incorporating anisotropic materials and piezoelectricity has been used to calculate the dispersion curves based on information of the layer thicknesses and material parameters of individual layers. A fit has been performed between the measured and simulated curves thereby allowing the extraction of the material parameters of the ZnO piezolayer. We present the set of material parameters resulting from the fitting.

Recent advances in laser-interferometric investigations of SAW devices

Several improvements for our Michelson laser interferometer have been implemented. High frequency RF leakage has been suppressed to allow measurements at 1 GHz frequencies. Fast automatic computer-controlled focusing and high-precision XY-translation system provide two-dimensional scans with resolution better than one micrometer and with measuring speeds up to 7000 points/hour. At each probe point the interferometer can detect vibrations normal to the surface down to amplitudes on the order of an Angstrom. These advances, combined with the long working distance of the optical system, enable an efficient scanning of commercial SAW devices with speed and precision.