The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Laser-Interferometric Analysis of Surface Acoustic Wave Resonators

Jouni Knuuttila

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics for public examination and debate in Auditorium TU1 at Helsinki University of Technology (Espoo, Finland) on the 3rd of June, 2005, at 12 noon.

Overview in PDF format (ISBN 951-22-7699-2)   [2922 KB]
Dissertation is also available in print (ISBN 951-22-7698-4)

Abstract

The research work described in this Thesis concentrates on studying surface acoustic wave (SAW) resonators, in particular resonators which utilize the leaky surface acoustic wave (LSAW) mode. Such resonators constitute building blocks for radio-frequency SAW bandpass filters, which are widely employed in modern cordless and cellular telecommunication systems. The number of radio frequency SAW filters produced presently exceeds 3 billion per year.

The work is carried out with an optical Michelson laser interferometer developed at the Materials Physics Laboratory specifically for the purpose of studying SAW components. In the course of this work the interferometer was equipped with a high-speed photodetector and state-of-the-art detection electronics, enabling the measurement of surface vibrations at frequencies as high as 2 GHz with amplitudes on the order of a few picometers. Furthermore, the setup was equipped with high-precision motorized scanning stages and computer control in order to facilitate automatically performed two-dimensional scans with a large number of scanning points and measuring speeds up to 50 000 points per hour. The optical setup features a spatial resolution better than one micrometer, enabling measurement of surface waves with wavelengths down to 2 micrometers. The interferometer can be used for analysis of surface acoustic wave devices as well as for thin-film bulk acoustic wave resonators and radio-frequency microelectromechanical systems (RF-MEMS).

Laser-interferometric measurements were performed on LSAW resonators and filters on rotated Y-cut lithium tantalate (LiTaO3). As a result, an unexpected acoustic field distribution was observed. Further measurements and simulations showed that the observed field distributions resulted from LSAWs escaping outside the resonator into the busbars. This acoustic loss mechanism can significantly degrade the performance of an LSAW filter. The obtained results have been acknowledged by SAW filter manufacturers in Japan and in Europe. In addition, measurements of bulk acoustic wave (BAW) radiation from LSAW resonators were carried out. Such radiation is inherent for LSAW resonators. Theoretical models and numerical simulations characterizing the phenomenon exist but very few direct measurements have been reported. Here, direct measurement results of BAW radiation fields generated by an LSAW resonator on LiTaO3 are reported revealing both fast shear and slow shear bulk waves. Furthermore, two coupling mechanisms, backscattering and direct excitation, were identified. Such information can be used in the development of more accurate simulation models.

This thesis consists of an overview and of the following 7 publications:

  1. J. V. Knuuttila, P. T. Tikka, and M. M. Salomaa. 2000. Scanning Michelson interferometer for imaging surface acoustic wave fields. Optics Letters 25, pages 613-615. © 2000 Optical Society of America (OSA). By permission.
  2. J. V. Knuuttila, P. T. Tikka, C. S. Hartmann, V. P. Plessky, and M. M. Salomaa. 1999. Anomalous asymmetric acoustic radiation in low-loss SAW filters. Electronics Letters 35, pages 1115-1116. © 1999 IEE. By permission.
  3. J. Koskela, J. V. Knuuttila, P. T. Tikka, C. S. Hartmann, V. P. Plessky, and M. M. Salomaa. 1999. Mechanism for acoustic leakage in surface-acoustic wave resonators on rotated Y-cut lithium tantalate substrate. Applied Physics Letters 75, pages 2683-2685. © 1999 American Institute of Physics. By permission.
  4. T. Makkonen, S. Kondratiev, V. P. Plessky, T. Thorvaldsson, J. Koskela, J. V. Knuuttila, and M. M. Salomaa. 2001. Surface acoustic wave impedance element ISM duplexer: modeling and optical analysis. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 48, pages 652-665. © 2001 IEEE. By permission.
  5. J. Koskela, J. V. Knuuttila, T. Makkonen, V. P. Plessky, and M. M. Salomaa. 2001. Acoustic loss mechanisms in leaky SAW resonators on lithium tantalate. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 48, pages 1517-1526. © 2001 IEEE. By permission.
  6. J. V. Knuuttila, J. Saarinen, C. S. Hartmann, V. P. Plessky, and M. M. Salomaa. 2001. Measurement of BAW radiation from low-loss LSAW resonators. Electronics Letters 37, pages 1055-1056. © 2001 IEE. By permission.
  7. J. V. Knuuttila, J. J. Vartiainen, J. Koskela, V. P. Plessky, C. S. Hartmann, and M. M. Salomaa. 2004. Bulk-acoustic waves radiated from low-loss surface-acoustic-wave resonators. Applied Physics Letters 84, pages 1579-1581. © 2004 American Institute of Physics. By permission.

Errata of publication 3

Keywords: leaky surface acoustic waves, bulk acoustic waves, LSAW resonator, laser interferometry, acoustic losses

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© 2005 Helsinki University of Technology

Last update 2011-05-26