Full-field imaging of gigahertz film bulk acoustic resonator motion

Abstract

A full-field view laser ultrasonic imaging method has been developed that measures acoustic motion at a surface without scanning. Images are recorded at normal video frame rates by using dynamic holography with photorefractive interferometric detection. By extending the approach to ultra high frequencies, an acoustic microscope has been developed that is capable of operation at Gigahertz frequency and micron length scales. Both acoustic amplitude and phase are recorded, allowing full calibration and determination of phases to within a single arbitrary constant. Results are presented of measurements at frequencies of 800-900 MHz, illustrating a multitude of normal mode behavior in electrically driven thin film acoustic resonators. Coupled with microwave electrical impedance measurements, this imaging mode provides an exceptionally fast method for evaluation of electric-to-acoustic coupling of these devices and their performance. Images of 256 240 pixels are recorded at 18 fps rates synchronized to obtain both in-phase and quadrature detection of the acoustic motion. Simple averaging provides sensitivity to the subnanometer level at each pixel calibrated over the image using interferometry. Identification of specific acoustic modes and their relationship to electrical impedance characteristics show the advantages and overall high speed of the technique.

Video Ultrasonics by Pulsed TV Holography: A New Capability for Non-Destructive Testing of Shell Structures

“…In order to allow the comparison, at least one of the wavefronts is frozen and stored in a hologram by some appropriate temporal treatment of the illumination-recording process, and then reconstructed and superimposed to the other wavefronts. Apart from the classical and tedious photographic emulsions (Shiokawa et al 1975;Gagosz 1974, 78-83;Aleksoff 1974, 247-63;Vest 1979, 224-5 and 242-4;Henning and Mewes 1995;Fällströ m et al 1989), real-time recording media like thermoplastic film (Pohl et al 1992;Schroeder and Crostack 1996), bacteriorhodopsin (Blackshire et al 2002;Blackshire and Duncan 2004), or photorefractive crystals (Telschow et al 1999;Telschow et al 2003;Delaye et al 2000;Lemaire et al 2000) have been employed, allowing a much higher refreshing rate (up to tens of Hz for photorefractive media, although at the expense of a large light power demand). , also known as electronic speckle pattern interferometry (ESPI), use the image sensor of the video camera as a recording medium and employ a configuration of image hologram.…”

“…Moreover, it allows extracting the acoustic displacement amplitude and phase and also the instantaneous acoustic displacement field in quasi-real time (several maps per second). This technique was successfully employed to analyze SAW in crystals and metals (Telschow et al 1999;Telschow et al 2003). Another configuration, namely frequency translated holography, combines homodyne phase modulation with spatial filtering to extract the acoustic displacement amplitude, allowing simultaneously to discriminate traveling waves moving in one direction from those moving in the opposite one and from standing wave patterns (Shiokawa et al 1975;Blackshire et al 2002;Blackshire and Duncan 2004).…”

Laser Measurement of SAM Bulk and Surface Wave Amplitudes for Material Microstructure Analysis

“…Note that the SAM is basically designed to visualize the surface and/or the subsurface of microstructure features of the material, but not directly the acoustic waves propagating within the material. However, observing a wave front traveling within the material would be significant for understanding physical phenomena, such as scattering from microstructure features [4,5,6,7]. In addition, visualization of the acoustic wave front can help in measurement of elastic properties and improve quality evaluation of an acoustic lens.…”

Laser Acoustic Imaging of Film Bulk Acoustic Resonator (FBAR) Lateral Mode Dispersion

“…Direct measurements of the acoustic motion of these resonators provide additional characterization of the device's electrical operation. A new method for full-field viewing these acoustic motions at GHz frequencies, without scanning, has been described previously [1][2][3]. This approach performs optical full image detection of the acoustic displacement amplitude and phase motion in a microscopic geometry using photorefractive dynamic holography.…”

“…This approach performs optical full image detection of the acoustic displacement amplitude and phase motion in a microscopic geometry using photorefractive dynamic holography. The apparatus and operation of this approach have been previously described for measuring UHF acoustical motion at a single point [2] and imaging [3,4]. In this paper we describe results and analysis from imaging measurements of the lateral acoustic mode motion in a nearly square FBAR around its series resonance and compare with model predictions.…”