A method of determining acoustical physical constants for piezoelectric materials by line-focus-beam acoustic microscopy

We developed a new method of determining acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of piezoelectric materials with high accuracy. This method acquires velocities of leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface, measured by line-focus-beam (LFB) acoustic microscopy, and bulk velocities of longitudinal and shear waves, measured with planewave transducers replacing the LFB device in the same system, together with the dielectric constants and density measured independently, for a small number of specimens. For LiNbO₃ and LiTaO₃ crystals, we demonstrated that we could accurately determine the constants by choosing proper propagation directions of LSAWs and bulk waves for three principal X-, Y-, and Z-cut specimens and one rotated Y-cut specimen [(104) plate for LiNbO₃ and (012) plate for LiTaO₃]. The accuracy is nearly the same as that for the constants determined only from the bulk wave velocities     

Lens design for acoustic microscopy

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.     

Focusing PVDF transducers for acoustic microscopy

A focusing transducer based on a 9-µm thick PVDF foil was fabricated and tested for performance. The transducer operates in the frequency range of 20–160 MHz. For operation at 78 MHz in water, a lateral resolution of 27.5 µm and a vertical resolution of 35 µm have been observed. Acoustic images of a transistor have been obtained with the focusing PVDF transducer.     

A Lamb wave lens for acoustic microscopy

In a conventional scanning acoustic microscope the excited leaky modes contribute significantly to the high contrast obtained in the images. However, all such modes exist simultaneously, and the interpretation of the images is not straightforward, especially in layered media. A lens geometry is proposed that can be used with acoustic microscopes to image layered solid structures. This lens can focus the acoustic waves in only one of the Lamb wave modes of the layered solid with a high efficiency. V(Z) curves obtained from this lens are more sensitive to material properties compared to that obtained from conventional lens. Measuring the return signal as a function of frequency results in another characteristic curve, V(f). The Lamb wave lens and the associated characterization methods for the layered structures are described. The results presented show that the Lamb wave lens is at least an order of magnitude more sensitive than the conventional lens and can easily differentiate between a good bond and disbond in a layered structure.     

Wideband acoustic microscopy of tissue

A scanning acoustic microscope (SAM) has been used to measure the elastic properties of tissue with a resolution of around 8 mum. This is achieved by broadband excitation of the acoustic lens, and the recording of an undemodulated returning signal. A method of analyzing this information to yield sound velocity, acoustic impedance, section thickness, and acoustic attenuation is described. Results from a sample of skin tissue are presented and compared with data from a computer simulation of the experiment.     

Ultrasonic materials characterization

The National NDT Centre at Harwell has been developing methods for the characterization of materials using ultrasonics. This paper reviews the progress made in applying ultrasonic attenuation measurements to the determination of such quantities as grain size and dislocation content. A method, ultrasonic attenuation spectral analysis, has been developed, which enables the contributions of scattering and absorption to the total attenuation to be separated. The theoretical advances that have been made are also described. Some of the practical applications of the technique are illustrated and future development discussed.     

Detection systems for scanning laser tomoholographic acoustic microscopy

A new microscope, called the scanning laser tomoholographic acoustic microscope, will employ three insonifying transducers to obtain holographic projections from three different directions for use in reconstructing tomograms of microscopic objects. To do this, the detection system should detect with equal sensitivity in all directions of propagation the traveling ultrasonic waves that emerge from the object with the image information. Two such laser-beam detectors, the heterodyne detector and the time-delay interferometric detector, have been examined to find the one best suited for rapid data acquisition and direction-insensitive optical computing. Although each has its own advantages and disadvantages, we find the latter more suitable for our purpose. © 1996 John Wiley & Sons, Inc.     

Extending acoustic microscopy for comprehensive failure analysis applications

In manufacturing of microelectronic components, non-destructive failure analysis methods are important for quality control. These non-destructive methods enable rapid defect localization which then guides micro-structural investigations involving destructive sample preparation. Scanning acoustic microscopy (SAM) is a powerful tool for the inspection of internal structures in optically opaque materials. Depth-specific information can be extracted and applied to create two- and three-dimensional images without the need for time consuming tomographic scan procedures. While traditional SAM imaging of the signal intensity is very valuable, it leaves most of the potential of acoustic microscopy unused. The aim of the current work was to develop comprehensive analysis algorithms to utilize the full potential of SAM and thus to extend the range of its applications. Examples representing different application fields were investigated in the current study. The examples include advanced flip-chip devices, bonded wafer pairs, solder tape connectors of a photovoltaic solar panel and high density chip-to-chip interconnects relevant for 3D integration. Progress achieved during this work can be divided into four categories: Signal Analysis and Parametric Imaging, Signal Analysis and Defect Evaluation, Image Processing and Resolution Enhancement and acoustic GHz microscopy (GHz-SAM). For the first three categories, data acquisition was performed using a commercially available scanning acoustic microscope equipped with several ultrasonic transducers covering the frequency range from 15 to 175 MHz. In the fourth category, data acquisition was performed employing a prototype of a novel acoustic GHz microscopy tool which is currently under development into a commercial system. In the first three categories, recorded acoustic data were subjected to sophisticated algorithms operating in time, frequency and spatial domains for performing signal and image analysis. Acoustic microscopy, combined with such advanced signal and image processing algorithms, proved to be a powerful tool for non-destructive inspection.     

The pyramidal-mirror detector for scanning laser acoustic microscopy

The pyramidal-mirror detector has important advantages over the knife-edge detector currently in use in scanning laser acoustic microscopy. A key element of this new detector is a four-sided pyramid with mirrored surfaces. In the operation of the microscope, the zero-order component of the light of the scanning laser beam that has been reflected from the microscope's coverslip is centered on the tip of the pyramid. Thus, each mirror face reflects one fourth of the light in this component. Making use of an arrangement of four photodetectors, one for each of the four faces of the pyramidal mirror, the light reflected from each face goes to a different photodetector and is processed separately from the light reflected from the other faces. This detector has an almost isotropic transfer function with no negative responses and fourfold symmetry around a null point in the center which coincides with the zero-frequency point in the spatial spectrum. This property makes it possible to detect spatial frequencies in all directions simultaneously with almost equal sensitivity. Oblique insonification, required for best operation with the knife-edge detector because of its region of negative response, is not preferred in the pyramidal-mirror detector and double side-band detection can be employed. Since there is no requirement that a negative response region be avoided, the spatial spectrum of this detector can be more than twice that of the knife-edge detector. The tighter the focusing of the scanning laser beam onto the coverslip, the higher the spatial frequencies detectable in the object. In fact, if the coverslip occupies a position in the very-near field of the object, evanescent-wave detection is possible. Low-frequency, highly penetrable ultrasound can be used and at the same time high spatial frequencies can be detected for obtaining high resolution. © 1999 John Wiley & Sons, Inc. Int J Imaging Syst Technol 10, 323–327, 1999     

A new focusing ultrasonic transducer and two foci acoustic lens for acoustic microscopy

The textured piezoelectric film of a new organic-based material produced by vapor deposition was used as an active element of a focusing ultrasonic transducer. The transducer exhibits near theoretical lateral and axial resolution, unipolar pulse response to a step voltage, and 30 dB insertion losses in an octave frequency band. The transducer is acoustically transparent over a wide frequency range and can be fabricated on a concave spherical surface of the standard acoustic lens. The resulting double transducer and two foci acoustic lens are capable of improving the quality of the surface structure imaging or of being used in continuous wave reflection acoustic microscopy.     

A method for calibrating the line-focus-beam acoustic microscopy system

Absolute accuracy of the line-focus-beam (LFB) acoustic microscopy system is investigated for measurements of the leaky surface acoustic wave (LSAW) velocity and attenuation, and a method of system calibration is proposed. In order to discuss the accuracy, it is necessary to introduce a standard specimen whose bulk acoustic properties, (e.g., the independent elastic constants and density) are measured with high accuracy. Single crystal substrates of gadolinium gallium garnet (GGG) are taken as standard specimens. The LSAW propagation characteristics are measured and compared with the calculated results using the measured bulk acoustic properties. Calibration is demonstrated for the system using two LFB acoustic lens devices with a cylindrical concave surface of 1-mm radius in the frequency range 100 to 300 MHz.     

Thermal effects in scanning acoustic microscopy for fine resolutionapplications

A novel scanning acoustic microscopy technique for achieving high resolution acoustic images by employing thermal effects and image subtraction has been studied and demonstrated. Experiments were performed on a perspex block patterned with a machined grid on the reverse surface, and on a buried channel in similar material. If was found that using the image subtraction technique, short periods of sample heating can lead to a stronger pattern selectivity, because of the strong temperature dependency of the elastic parameters of the polymer. In previous SAM techniques improvement in signal has been achieved through the use of special liquids as acoustic coupling media between the acoustic lens and the sample. The reported technique retains water as the coupling medium and the acoustic impedance matching is performed by varying the elastic parameters of the sample itself through direct heating. The temperature increase in the sample decreases the velocity of propagation of acoustic waves in the solid, and brings the acoustic impedance close to that of water. A theoretical model, including expressions for the acoustic aberrations, depth dependence and acoustic impedance matching has been derived. Examples of the results obtained are presented     

基于频域形态滤波的低速滚动轴承声发射信号降噪新方法

声发射检测技术以其灵敏度高、频响范围宽、信息量大等优点,为机械故障诊断提供了一条新的检测途径,但应用于旋转机械设备时,容易混入各种有色噪声。当噪声频率与声发射信号重叠时,传统的降噪方法难以满足要求。将形态滤波应用到信号频域,可以有效消除有色噪声的干扰。根据声发射频响特性,对频谱进行拟合平滑高斯白噪声的影响,最后重构到时域。仿真和实际低速轴承信号表明此方法具有较好的降噪效果,有利于信号后续的处理和分析。     

Acoustic microscopy of low-ductility materials

Acoustic microscopy offers materials scientists not only high resolution, but also the ability to image sub-surface and surface features by its unique elastic contrast mechanisms. It is a particularly powerful tool for detecting discontinuities such as voids and cracks, and for characterizing interfacial boundaries between different phases. This paper describes the application of acoustic microscopy to two important classes of low-ductility material: engineering ceramics and ceramic-fibre composites. Images of a wide range of near-surface defects are presented for the six ceramics studied, including porosity and microcracks. An application to crack length determination during indentation tests is also discussed. In the composites, there were systematic variations in contrast from the fibre-matrix interface, which appeared to correlate with changes in interfacial strength. Finally, the line-focus microscope was used to demonstrate how the Rayleigh velocity and attenuation can be used to characterize the microstructure of ceramics and ceramic composites.     

Advances in microscopy of materials

Abstract

The field ion microscope is the starting point for this survey. It provided the first photograph of individual atoms. Muller, the pioneer of this technique, went on to develop the atom probe, which allowed individual ions to be identified. An early landmark was the demonstration that grain boundaries are only of atomic thickness. More recently, a Cottrell atmosphere has been directly observed in an Fe–C alloy. The high resolution transmission electron microscope and the scanning transmission electron microscope are also discussed. Work using the latter is leading to improved understanding of grain boundary chemistry. The survey is concluded with an account of the scanning tunnelling microscope: this innovation is revolutionising the study of surfaces. Using the techniques described, atoms can be manipulated separately in a controlled manner and structures can be tailor-made.

MST/1321     

Surface microscopy of porous materials

Atomic force microscopy (AFM) and high-resolution electron microscopy (HREM) are shown to be powerful tools for the investigation of crystal growth processes in microporous materials. Observations at atomic resolution of zeolite surfaces in the absence and presence of adsorbates reveals both surface structure and the mode of interaction of potential templates for crystallisation. A layer-by-layer growth mechanism is observed for many open-framework materials with organic templates acting by keying into exposed surfaces followed by clathration during growth of the subsequent layer. The role of defects in modifying growth processes can be discerned and predictions made for altering conditions of growth to control the defect type and density. Simulation of AFM micrographs permits the relative rates of fundamental growth processes to be ascertained.     

Ultrasonic method for determining the thermal diffusivity of materials

A method is proposed for determining the thermal diffusivity of materials by recording the amplitude variation of resonance vibrations of a disk-shaped specimen of the investigated material subjected to heat flux.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 6, pp. 965–971, June, 1976.