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

Design criteria for acoustic microscope lenses are examined with respect to their intended application. Aside from buffer rod material and F-number, the factors influencing the lens design are the critical angle for surface wave excitation, lens illumination, and leak rate of the surface wave on the sample. It is found that the design criteria are different for surface and subsurface examination and that for different applications and materials, different lenses are required for optimum imaging performance. A formalism for evaluating the performance of an acoustic microscope by considering its response in the time domain, both theoretically and experimentally, is presented. >

Lens design for acoustic microscopy

We have developed an x–y–z scanning stage for mechanically scanned microscopy. The stage is constructed of ‘‘double‐S’’ mode piezoelectric bimorphs. The prototype unit has a deflection sensitivity of 0.3 μm/V and a travel in each of the three axes of ±60 μm. The lowest mechanical resonances of the stage are at 190, 220, and 360 Hz, corresponding to the x, y, and z axes of the stage, respectively. The noise level of the stage, when mounted on an isolation table, is ∼0.1 nm. The performance of the stage can be understood in terms of a simple lumped element model which can be used to optimize such stages for particular applications.

Bimorph‐driven x–y–z translation stage for scanned image microscopy

The principle, design considerations, and operation of a C-mode, 100-MHz ultrasound backscatter microscope are described. Applications of the C-mode microscope, as well as of a new real time B-mode UBM, to the imaging of an in vitro tumor model and ocular tissue are presented. Central to these developments has been the construction of PVDF (polyvinylidene fluoride) transducers with sufficient sensitivity to detect the small backscatter signals from biological materials and bandwidth capable of giving an axial resolution of approximately 10 mu m. The lateral resolution achieved is 17.5 mu m in the C-scanner using an f/1.33 transducer and 36 mu m in the B-scanner, which uses an f/2 transducer. Future developments are also discussed. >

Ultrasound backscatter microscopy

Absfmct-The design and industrial applications of a Quate-Lemons scanning acoustic microscope are described. The microscope is based on a conventional optical microscope, and so comparisons between the two modes are easily made; the instrument is simple to set up and operates routinely in an industrial environment. The basis of the instrument is a miniature lens scanner, little larger than an optical objective lens, which mounts on the turret of an optical microscope. Selection between modes is simply achieved by turret rotation, ensuring good registration between images. An integrated computer system is used to control the instrument, recording images and V(z) curves into a 512point square digital frame store, and this is also used for image processing. The design of the microwave electronics is explained, and emphasis is put on the low frequency performance of the system. This is illustrated with interior images of production components.

http://ieeexplore.ieee.org/iel5/10326/32872/01539670.pdf

R.A. Harvey

Papers

An Acoustic Microscope for Industrial Applications

A Miniature Lens Scanner for Acoustic Microscopy