Acoustic Microscopy

History: Modern technologies require materials that withstand extreme conditions with high reliability. One of the key parameters that have a strong impact on the life expectancy of a material is its microstructure. The acoustic microscope was developed as a tool for studying the internal microstructure of nontransparent solids. In acoustic microscopy, a sample is imaged by ultrasound waves, and the contrast in reflection furnishes a map of the spatial distribution of the mechanical properties. Several books and handbook articles give detailed historical outlines. Briefly, the development of the first high-frequency scanning acoustic microscope was motivated by the idea of using an acoustic field to study the spatial variations of the elastic material properties with nearly optical resolution (The lateral resolution of SAM is dependent on the frequency of the acoustic waves and, at best, is about 0.75 microns). The first experiments date back to the 1940's when high-frequency acoustic images were obtained by the Leningrad scientist Sokolov (Sokolov, 1949). He observed an acoustical image using the tube named after him, in which the acoustic picture was converted into a television display. The first scanning acoustic microscope was created by Lemons and Quate at Stanford University in 1973 (Lemons and Quate, 1974; 1979). It was mechanically driven and operated in the transmission mode. Since then, gradual mechanical and electronic circuit improvements have been made and image recording has been automated. In general, acoustic microscopes now work in the reflection mode.



Sketch of the acoustic microscope.


Principles: Variation of the mechanical properties with depth can be studied by scanning at various defocus values. C-scan acoustical images obtained at different defocus positions were used for the detection of subsurface voids and cracks. Collecting images obtained at various defocus positions allows a three-dimensional image to be constructed, representing the volume of the entire microstructure of the investigated sample. Time-resolved acoustic microscopy adds an additional degree of freedom for quantitative measurement, namely time. In time-resolved acoustic microscopy a short sound pulse is sent toward a sample. The time-of-flight method uses the acoustical contrast to describe the time required for the pulse sent into the sample to return to the acoustic lens. For layered materials the reflected signal represents a train of pulses (A-scan). The first pulse is attributed to the reflection from the liquid/specimen interface. The second pulse appears as a result of reflection from the internal interface. The time delay of the pulses and their amplitudes provides information about the elastic properties and attenuation of sound in the layer. The velocity of the wave can be determined by measuring the time delay of the corresponding pulse. Time resolved images obtained by mechanical scanning along a line are called B-scans.





Visualization of the subsurface structures by acoustic microscopy.
SAM images were taken from the chapter on acoustic microscopy (see below).


Development: Considerable progress in the acoustic microscopy of solid structures has been made since then (Briggs, 1992; 1995; Briggs and Arnold, 1996; Zinin, 2001; Zinin and Weise, 2003).

Developments in the theory of the image formation of subsurface defects (Lobkis et al. 1995) and three-dimensional objects (Zinin, 1997, Zinin and Weise, 2003) provides the basis for images interpretation of subsurface microstructure of solid materials.

Conventionally, SAM images show variations of the amplitude of the acoustical signal. Reinholdtsen and Khuri-Yakub (1991) measured amplitude and phase of the SAM signal at low frequency (3 to 10 MHz) to improve subsurface images. Grill extended this technique to high frequency, (1.2 GHz). This technique permits reconstruction of the surface relief of the sample with submicron resolution (Grill et al., 1996).

An important step has been made in the direction of imaging subsurface structures at high temperatures. Ihara et al.(2000) developed a sound imaging technique to see a small steel object immerged in molten zinc at 600oC.

Recently, a new high-frequency (1 GHz) time-resolved acoustic microscope was developed at the Fraunhofer-Institute for Biomedical Engineering (Lemor, et al.,2004). It is based on an optical microscope from Olympus and it operates in a reflection mode. The new microscope permits fluorescence microscopy and scanning acoustic microscopy at the same time, which are used for synchronous optical and acoustical investigation of cell components with changing cellular stiffness.

Recent developments in acoustic microscopy can be found among books listed below.

Applications

  • Measuring the elastic properties solids and thin films
  • Measurement and visualization of adhesion in layered structures
  • Subsurface imaging defects in coatings
  • Subsurface cracks visualization (Knaus et al., 1995)
  • Characterization of carbon-fiber-reinforced composites (Manghnani et al., 2004)
  • Characterization of cells and biological tissues (Bereiter-Hahn and Blase, 2004)
  • Visualization of stress inside solid materials (Drescherkrasicka and Willis, 1996; Landa, M. and Plesek, 2002).


References

  • Bereiter-Hahn, J., Blase, C., Ultrasonic Characterization of Biological Cells, in T. Kundu ed., Ultrasonic Nondestructive Evaluation: Engineering and Biological Material Characterization, CRC Press, Boca Raton, chapter 11, 722-760 (2004).
  • Briggs, A. Acoustic Microscopy Clarendon Press, Oxford, 1992.
  • Briggs, A. Advances in Acoustic Microscopy, Plenum Press, New York, 1995.
  • Briggs, A, and W. Arnold, Advances in Acoustic Microscopy, Plenum Press, New York, 1996.
  • Drescherkrasicka, E. and J. R. Willis. Mapping stress with ultrasound. Nature 384(6604): 52-55, 1996.
  • Grill, W., K. Hillmann, K. U. Wurtz and J. Wesner. Scanning ultrasonic microscopy with phase contrast. Advances in Acoustic Microscopy. A. Briggs and W. Arnold, Eds. New York, Plenum Press. II: 167-218, 1996.
  • Ihara, I., Jen, C.-K., and Ramos França, D., Ultrasonic imaging, particle detection, and V(z) measurements in molten zinc using focused clad buffer rods, Rev. Sci. Instrum. 71(9), 3579-3586, 2000.
  • Knaus, D., T. Zhai, G. A. D. Briggs, and J. M. Martin, Measuring short cracks by time-resolved acoustic microscopy, in Advances in Acoustic Microscopy, vol. I, A. Briggs, Ed. New York: Plenum Press, 1995, pp. 49-77.
  • Landa, M. and Plesek, J., Contrast enhancement of ultrasonic imaging of internal stresses in materials, Ultrason. 40(1-8), 531-535, 2002.
  • Lemons,R. A. and C. F. Quate. Acoustic microscope-scanning version, Appl. Phys. Lett. 24(2), 163-165, 1974.
  • Lemons,R. A. and C. F. Quate. Acoustic Microscopy, in Physical Acoustics, Mason, W. P. and Thurston, R. N. Academic Press., London, 1979, pp. 1-92.
  • Lemor, R. M., E. C. Weiss, G. Pilarczyk, P. V. Zinin, Measurements of Elastic Properties of Cells Using High-Frequency Time-Resolved Acoustic Microscopy”. in D.E.Yuhas ed., 2003 IEEE Ultrasonic Symposium , IEEE, New York, (2004) p. 752-756.
  • Lobkis, O. I., T, Kundu, and P. V. Zinin. A theoretical analysis of acoustic microscopy of spherical cavities, Wave Motion 21(2), 183-201, 1995.
  • Manghnani, M. H. , P. V. Zinin, Y. Wang, V. Levin, J. Koenig, Characterization of the Fatigue Damage of Advanced Ceramic Composites by Scanning Acoustic Microscopy”. in W. Arnold and S. Hirsekorn eds. Acoustical Imaging, Kluwer Publ., New York, Vol. 27. (2004) pp. 83-90.
  • Reinholdtsen, P. A. and Khuri-Yakub, B. T., Image processing for a scanning acoustic microscope that measures amplitude and phase, IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 38(2), 141-147, 1991.
  • Sokolov, S. , The ultrasonic microscope, Doklady Akademia Nauk SSSR (in Russian) 64, 333-336, 1949.
  • Zinin, P. V. Quantitative Acoustic Microscopy of Solids, in Handbook of Elastic Properties of Solids, Liquids, and Gases. Volume I: Dynamic Methods for Measuring the Elastic Properties of Solids, Levy, M., Bass, H., Stern, R., and Keppens, V. Academic Press, New York, 2001, pp. 187-226.
  • Zinin, P. and W. Weise, Theory and applications of acoustic microscopy, in T. Kundu ed., Ultrasonic Nondestructive Evaluation: Engineering and Biological Material Characterization, CRC Press, Boca Raton, chapter 11, 654-724 (2003).
  • P. Zinin, W. Weise, O. Lobkis, and S. Boseck, The theory of three-dimensional imaging of strong scatterers in scanning acoustic microscopy, Wave Motion, 25(3), 213-236, 1997.

Useful References
  • Cheeke, J. D. N. Fundamentals and applications of ultrasonic waves. Boca Raton, CRC Press, 2002.
  • Kino, G. S. Acoustic waves: Devices, imaging and analog signal processing. Englewood Cliffs, New Jersey., Prentice-Hall, 1987.

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