Acoustic microscopy enables one to image the interaction of acoustic waves with the elastic properties of a specimen with microscopic resolution. A lens with good focusing properties on the axis can be used for both transmitting and receiving the signal, and an image is formed by scanning the lens mechanically over the specimen. In pressurised superfluid helium with nonlinear coupling to harmonics a resolution of 15 nm has been achieved, but for routine use 2 GHz is the highest practical frequency, which offers a resolution of about 0.7 mu m. The information is contained in the way that the acoustic wave is reflected from the specimen. For subsurface imaging, especially in polymer based materials such as composites and electronic packaging, the enhanced depth resolution of a confocal imaging system can be exploited to give good contrast from the plane of interest even when the specimen contains many scatters. In higher stiffness specimens, including most metals, semiconductors and ceramics, a dominant role in the contrast can be played by Rayleigh waves in the surface. If the specimen has a surface layer, then the propagation of the Rayleigh waves is sensitive to the perturbing action of the layer. If the specimen is anisotropic, then there will be a dependence on the orientation of the surface and the direction of propagation in it. If there are surface cracks or boundaries, then there will be strong contrast when they scatter the Rayleigh waves. Detailed theory is available to relate the elastic properties of the surface to the contrast, and these enable informed interpretation of the acoustic images to be made, and also provide a foundation for more quantitative acoustic microscopy.

http://iopscience.iop.org/article/10.1088/0034-4885/55/7/001/pdf

Acoustic microscopy-a summary

The Lamb wave lens is a type of lens that can replace the conventional lens of an acoustic microscope for some applications. When it insonifies a layered solid structure, it excites a single mode in the structure, provided that the frequency is properly adjusted. Measuring the return signal as a function of frequency results in a characteristic curve showing the excited modes. Since the number of excited modes, the frequencies at which they are excited, and the efficiency of excitation are highly dependent on the elastic and physical parameters of the layered solid, a characterization method emerges. The authors describe this characterization method for the layered structures using the Lamb wave lens. Theoretical and experimental results are presented. >

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Characterization of layered materials by the Lamb wave lens

Deep subsurface high resolution shear wave imaging using a conventional acoustic microscope and lens is presented. The technique is based on mode conversion phenomena which occur when a focused acoustic beam is incident from water at a material surface. A theoretical framework for the interpretation of the experimental results is discussed. Experimental results are presented using a 50 MHz acoustic lens with a 28 o aperture and 11.5 mm working distance in water

Shear wave imaging for deep nondestructive evaluation

Measuring elastic properties of bones and silicon from V(z) curve generated by multiply reflected signals.

The purpose of the first Lemons-Quate acoustic microscope(1) was to image the surfaces of materials or biological cells with a high resolution Unfortunately, competition with the optical microscope was only partially successful due to the high degree of absorption in the liquid-coupling medium at high frequencies Increasing the resolution beyond optical limits was possible with the use of hot water(2) or cryogenic liquids,(3) at the cost of operational difficulty and system complexity Meanwhile it was shown that the acoustic microscope can generate information that has no counterpart in the optical world(4) The presence of leaky waves resulted in an interference mechanism known as V(z) curves The V(z) method involves recording the reflected signal amplitude from an acoustic lens as a function of distance between the lens and the object This recorded signal is shown to depend on elastic parameters of the object material After underlying processes are well understood, new lens geometries or signal-processing electronics are designed to emphasize the advantage of the acoustic lens In any case, the aim has been to increase the quantitative characterization ability of the microscope

/pdf/lens-geometries-for-quantitative-acoustic-microscopy-2ungi1o2gb.pdf

Lens Geometries for Quantitative Acoustic Microscopy

A conventional scanning acoustic microscope lens excites all the possible modes of acoustic waves in the solid structure under examination. The excited leaky modes contribute significantly to the high contrast obtained in the images, but because all such modes exist simultaneously, the interpretation of the images is not straightforward. A new lens geometry that can be used with acoustic microscopes to image layered solid structures is proposed. This new lens can efficiently focus the acoustic waves in only one of the Lamb wave modes of the layered solid. The images obtained are easy to interpret, and the subsurface sensitivity is high. >

/pdf/a-high-efficiency-lamb-wave-lens-for-subsurface-imaging-4x39zqsgmk.pdf

H. Koymen

Papers

A high efficiency Lamb wave lens for subsurface imaging