High Frequency Scanning Acoustic Microscopy Research Articles

Near-Field Defects Imaging in Thin DLC Coatings Using High-Frequency Scanning Acoustic Microscopy

A high-frequency scanning acoustic microscope (SAM) operating at 1–1.3 GHz was used to investigate subsurface defects in diamond-like carbon (DLC) films that were 2–3 μm thick. Because the wavelength of the longitudinal wave in the film was comparable to the film thickness, the acoustical images obtained were near-field images. To interpret the features in the acoustical images, a multidisciplinary approach was utilized through a combination of SAM with atomic force microscopy (AFM), a focused ion beam (FIB) technique, and optical microscopy. Simulations based on the rigorous theoretical approach better predict the depth of the defect than the ray optical approach. It is shown that scattering of the shear waves by subsurface defects is responsible for the acoustical contrast of the cavity.

Material State Awareness for Composites Part I: Precursor Damage Analysis Using Ultrasonic Guided Coda Wave Interferometry (CWI).

Detection of precursor damage followed by the quantification of the degraded material properties could lead to more accurate progressive failure models for composite materials. However, such information is not readily available. In composite materials, the precursor damages—for example matrix cracking, microcracks, voids, interlaminar pre-delamination crack joining matrix cracks, fiber micro-buckling, local fiber breakage, local debonding, etc.—are insensitive to the low-frequency ultrasonic guided-wave-based online nondestructive evaluation (NDE) or Structural Health Monitoring (SHM) (~100–~500 kHz) systems. Overcoming this barrier, in this article, an online ultrasonic technique is proposed using the coda part of the guided wave signal, which is often neglected. Although the first-arrival wave packets that contain the fundamental guided Lamb wave modes are unaltered, the coda wave packets however carry significant information about the precursor events with predictable phase shifts. The Taylor-series-based modified Coda Wave Interferometry (CWI) technique is proposed to quantify the stretch parameter to compensate the phase shifts in the coda wave as a result of precursor damage in composites. The CWI analysis was performed on five woven composite-fiber-reinforced-laminate specimens, and the precursor events were identified. Next, the precursor damage states were verified using high-frequency Scanning Acoustic Microscopy (SAM) and optical microscopy imaging.

Application of high frequency scanning acoustic microscopy for the failure analysis and reliability assessment of MEMS sensors

We successfully applied high frequency scanning acoustic microscopy (SAM) as a tool for the analysis of MEMS sensors. Using state of the art transducers with frequencies up to 300MHz, we evaluated the achievable resolution and performed case studies: we localized a contamination-induced delamination on the ASIC surface and studied failure modes after mechanical stability tests, showing that a combination of SAM and infrared microscopical evaluation provides information about the course of cracks on a micrometer length scale.

High Frequency Scanning Acoustic Microscopy as Diagnostic Tool in Tissue Science

The objective of this paper is to demonstrate the feasibility of High Frequency Scanning Acoustic Microscopy (HF SAM) as a tool to characterize biological tissues. The HF SAM is shown to provide both imaging and quantitative stiffness measuring abilities. The plant cell wall is used as a test analog to study the influence on stiffness when one important structural component is removed from a complex polymer structure. In particular, the hypothesis of this work was that the biopolymer pectin may have a strong effect on the mechanical properties of primary plant cell walls. The technical approach was to use HF SAM to document the effect of pectinase enzyme treatment to remove pectin from onion primary cell wall. The novelty in this work was to demonstrate the capability of HF SAM in terms of a characteristic curve called the V(Z) signature. The results indicate a significant change in the V(Z) signature with time into the enzyme treatment. Thus the HF SAM method opens the door to a systematically nondestructive study of complex bio-polymer structures.

Inverted high frequency Scanning Acoustic Microscopy inspection of power semiconductor devices

A high frequency Scanning Acoustic Microscopy (SAM) method with inverted inspection direction and its application to power semiconductor devices are described. The method comprises a preparation technique, which in turn allows the inversion of the inspection direction, which means inspection through the die backside and the use of high frequency SAM up to 230MHz, for packaged power semiconductor devices. The improved resolution limit allows the detection of defects, such as metal degradation, bad wirebond adhesion and cracks. Case studies demonstrate the power of this method and advantages over other analysis methods.

Emerging Techniques for 3-D Integrated System-in-Package Failure Diagnostics

Abstract Failure analysis is becoming increasingly difficult with the emergence of 3D integrated packages due to their complex layouts, diverse materials, shrinking dimensions, and tight fits. This article demonstrates several FA techniques, including high-frequency scanning acoustic microscopy, lock-in thermography, and FIB cross-sectioning in combination with plasma ion etching or laser ablation. Detailed case studies show how the various methods can be used to analyze bonding integrity between different materials, chip-to-chip interface structures, buried interconnect defects, and through-silicon vias at either the device or package level.

Transducer designs and simulations for high frequency scanning acoustic microscopy for applications in exploring contrast mechanism and the mechanical properties of biological cells

Scanning acoustic microscopy (SAM) has been extensively accepted and utilized for acoustical cellular and tissue imaging including measurements of the mechanical and elastic properties of biological specimen. SAM provides superb advantages: it is a noninvasive method; it can measure mechanical properties of biological cells or tissues; and fixation/chemical staining is not necessary. The first objective of this research is to develop a program for simulating the images and contrast mechanism obtained by high-frequency SAM. Computer simulation algorithms based on Matlab® were built for simulating the images and contrast mechanism. The mechanical properties of HeLa and MCF-7 cells were computed from the V(z) measurement data. Algorithms for simulating V(z) responses involved calculation of the reflectance function and were created based on ray theory and wave theory. The second objective is to design transducer arrays for SAM. Theoretical simulations based on Field II programs of the high frequency ultrasound array designs were performed to enhance image resolution and volumetric imaging capabilities. The new transducer array design will be state-of-the-art in improving the performance of SAM by electronic scanning and potentially providing a four-dimensional image of the specimen. Phased array beam forming and dynamic apodization and focusing were employed in the simulations.

Localised micro-mechanical stiffening in the ageing aorta

Age-related loss of tissue elasticity is a common cause of human morbidity and arteriosclerosis (vascular stiffening) is associated with the development of both fatal strokes and heart failure. However, in the absence of appropriate micro-mechanical testing methodologies, multiple structural remodelling events have been proposed as the cause of arteriosclerosis. Therefore, using a model of ageing in female sheep aorta (young: <18 months, old: >8 years) we: (i) quantified age-related macro-mechanical stiffness, (ii) localised in situ micro-metre scale changes in acoustic wave speed (a measure of tissue stiffness) and (iii) characterised collagen and elastic fibre remodelling. With age, there was an increase in both macro-mechanical stiffness and mean microscopic wave speed (and hence stiffness; young wave speed: 1701±1ms−1, old wave speed: 1710±1ms−1, p<0.001) which was localized to collagen fibril-rich regions located between large elastic lamellae. These micro-mechanical changes were associated with increases in both collagen and elastic fibre content (collagen tissue area, young: 31±2%, old: 40±4%, p<0.05; elastic fibre tissue area, young: 55±3%, old: 69±4%, p<0.001). Localised collagen fibrosis may therefore play a key role in mediating age-related arteriosclerosis. Furthermore, high frequency scanning acoustic microscopy is capable of co-localising micro-mechanical and micro-structural changes in ageing tissues.

Subsurface damage in optical substrates

The origin, character, analysis and treatment of subsurface damage (SSD) were summarized in this paper. SSD, which was introduced to substrates by manufacture processes, may bring about the decrease of laser-induced damage threshold (LIDT) of substrates and thin films. Nondestructive evaluation (NDE) methods for the measurement of SSD were used extensively because of their conveniences and reliabilities. The principle, experimental setup and some other technological details were given for total internal reflection microscopy (TIRM), high-frequency scanning acoustic microscopy (HFSAM) and laser-modulated scattering (LMS). However, the spatial resolution, probing depth and theoretic models of these NDE methods demanded further studies. Furthermore, effective surface treatments for minimizing or eliminating SSD were also presented in this paper. Both advantages and disadvantages of ion beam etching (IBE) and magnetorheological finishing (MRF) were discussed. Finally, the key problems and research directions of SSD were summarized.

Encapsulation of living cells with polymeric systems

AbstractMicroencapsulation of cells producing recombinant proteins or hormones leads to immunoprotection and immobilization in culture or in vivo. We are investigating three different strategies for the production of calcium cross‐linked alginate beads of a small size with immobilized and immunoprotected mammalian cells: a) the AirJet technology (coaxial gas flow extrusion), b) the vibrating nozzle technology, and c) the JetCutter technology. A alginate/poly‐L‐lysine/alginate complexation was used as the polymeric system. All three methods may be used for production of homogeneous beads with a diameter of approximately 350 μm. While the vibrating nozzle technique was limited to an alginate viscosity of 0.2 Pa·s or less, the AirJet and JetCutter technology were less sensitive to higher viscosities. High frequency Scanning Acoustic Microscopy is used for mechanical characterization of the microspheres as well as for investigation of surface properties.

Investigation of elasto-mechanical properties of alginate microcapsules by scanning acoustic microscopy.

High-frequency scanning acoustic microscopy (SAM) was used for investigation of acoustic impedance and 3D-surface topography of full alginate microspheres that act as model of artificial biological cells. Elasto-mechanical properties of the investigated specimens have been characterized by acoustic impedance. Mean surface impedance of microspheres (diameter: 300 microm) was measured with SAM at 900 MHz with a spatial resolution of 1.5 microm. The sensitivity and reproducibility of SAM had to be increased considerable to receive and quantify signals in the very low impedance region. The multilayer analysis method was used to get quantitative data of acoustic impedance with SAM at a microscopic level. 3D images show details of structure and surface topography. As a reference, bulk measurements were performed on full alginate cylinders. The acoustical impedance and the mechanical stiffness c(11) were obtained from mass density and longitudinal ultrasound velocity at 6 MHz. The impedances received with both methods are in close agreement. The results demonstrate the SAM as a powerful tool for characterizing mechano-elastical parameters as well as surface structure and topography of microspheres with high spatial resolution.

High frequency scanning acoustic microscopy: a novel non-destructive surface analytical tool for assessment of coating-specific elastic moduli and tomographic study of subsurface defects

Acoustic microscopy has been used as a non-destructive analytical tool for characterising various coating/substrate systems. A novel high-frequency instrument especially developed for Materials Science engineers is presented, and its technical and theoretical specifications are described. In the acoustic signature mode (quantitative mode), local elastic moduli are extracted. The numerical results are compared to those obtained by depth-sensing indentation. In the imaging mode (qualitative mode), this technique allows without etching a clear distinction of the surface grain structure from the stress gradients at grain boundaries and from the mechanical anisotropy of differently oriented grains. Moreover, tomographic imaging of subsurface flaws is shown. Hidden scratch-induced cracks are revealed by comparison with standard metallographic microscope observations.

Étude et caractérisation par microscopie acoustique de l’endommagement par fissures

Different non destructive methods of high frequency scanning acoustic microscopy are applied to image both sub-surface and surface microcracks. Spherical acoustic sensors with an aperture of 50° were used working at 50 MHz. and at 570 MHz. Two types of waves generated by the acoustic sensors can be used: Rayleigh surface wave and longitudinal wave. Surface wave (Rayleigh wave) properties allow to detect and quantify surface cracks and subsurface cracks not farther than a wavelength from surface, A fringe pattern can be observed along the cracks wich periodicity is half of the rayleigh wavelength, and a model of the phenomenon can be computed Firstly an experiment of scratching test on thick hard chromium coatings compares optical an acoustic images of microcrack A second experiment on a single cracked glass bar put in evidence like the first one, the capability of the surface wave to detect sub-surface cracks (not deeper than a wavelength), This sample presents a linear and regular single crack profile with an interesting sub-surface diving zone. By decreasing the frequency, acoustic sensors offer to see deeper. The reflection of the longitudinal wave both on the surface of a sample and on a sub-surface crack plane, allows to image inclined deep cracks. The observed fringe patterr, presents a periodicity directly linked to the angle of inclination. We show in this work that acoustic microscopy is an adaptative method to characterize various types of crack using different sensors geometries and frequencies.

Evaluation of diffusion bonds formed between superplastic sheet materials

Diffusion bonds produced in microduplex titanium and stainless steel sheet materials for various bonding conditions have been evaluated using a range of techniques. These include light and scanning electron microscopy (SEM), scanning acoustic microscopy (SAM) and compressive lap shear testing. The potential of other procedures such as ultrasonic inspection and resistivity measurement are also discussed. For imperfect bonds, the bond line in titanium alloys consists of clearly defined interfacial voids separated by metallurgically sound bonded regions, while the unbonded regions in stainless steel often consist of long flat voids in which the opposing surfaces have contacted but not bonded. It was observed that light microscopy and SEM observations provide a convenient and reliable method for the assessment of the bond quality, and in the case of titanium alloys it is possible to obtain quantitative data on the extent of bonding. High frequency SAM also proved to be an effective procedure for qualitative assessment. A linear relationship between the fraction of parent metal strength achieved and bonded area fraction as determined by metallography was observed for titanium alloys.

Scanning Acoustic Microscopy Observations of High‐Temperature Crack‐Bridging Mechanisms in Alumina

High‐purity, fine‐grained alumina showed a strong R‐curve behavior for long cracks propagated at 1200°C. R‐curve behavior was not observed at room temperature. A combined investigation using high‐frequency scanning acoustic microscopy and scanning electron microscopy of crack profiles demonstrated asperity contact in the crack wake at 1200°C. A microcrack zone was not observed. Crack bridging, resulting from intergranular subcritical crack propagation was considered to be responsible for the toughness increase with increased crack length.

Application of ultrasonic microscopy to non-destructive evaluation

In the field of microdefect detection and imaging, in complement to traditional techniques such as X-ray and scanning electron microscopies, there is now a very useful technique for non-destructive evaluation: high frequency scanning acoustic microscopy. A new generation of industrial microscopies has been developed for use in the range from 50 to 100 MHz. Several examples are shown here of scanning electron microscopy used in bulk examination of ceramic or microelectronic materials and metals.