Acoustic microscopy is microscopy that employs very high or ultra high frequencyultrasound. Acoustic microscopes operate non-destructively and penetrate most solid materials to make visible images of internal features, including defects such as cracks, delaminations and voids.
The notion of acoustic microscopy dates back to 1936 when S. Ya. Sokolov[1] proposed a device for producing magnified views of structure with 3-GHz sound waves. However, due to technological limitations at the time, no such instrument could be constructed, and it was not until 1959 that Dunn and Fry[2] performed the first acoustic microscopy experiments, though not at very high frequencies.
The scientific literature shows very little progress toward an acoustic microscope following the Dunn and Fry experiments up until about 1970 when two groups of activity emerged, one headed by C.F. Quate (Stanford University) and the other by A. Korpel and L.W. Kessler (Zenith Radio Research Labs). The first efforts to develop an operational acoustic microscope concentrated upon high-frequency adaptations of low-frequency ultrasonic visualization methods. One early system employed Bragg diffraction imaging,[3] which is based upon direct interaction between an acoustic-wave field and a laser light beam. Another example was based on variations of the Pohlman cell.[4] The original device is based upon a suspension of asymmetric particles in a thin fluid layer which, when acted upon by acoustic energy, produce visual reflectivity changes. Cunningham and Quate[5] modified this by suspending tiny latex spheres in a fluid. Acoustic pressure caused population shifts which were visually detectable. Kessler and Sawyer[6] developed a liquid crystal cell that enabled sound to be detected by hydrodynamic orientation of the fluid. In 1973, the Quate group began the development of a concept,[7] which utilized the first scanning acoustic microscope (SAM) with a confocal pair of 50MHz ultrasonic lenses for focusing and detecting the ultrasonic energy. In 1974, this concept was realized by R. A. Lemons and C. F. Quate at the Microwave Laboratory of Stanford University. Advancements of this instrument, a scanning acoustic microscope, have to do with achieving very high resolution, novel modes of imaging, and applications. The SAM was commercially introduced by Leitz Corp and by Olympus Corp. In 1970, the Korpel and Kessler group began to pursue a scanning laser detection system for acoustic microscopy.[8] In 1974, the activity was shifted to another organization under Kessler (Sonoscan Inc), where practical aspects of the instrument were developed. This instrument, the scanning laser acoustic microscope (SLAM), was made commercial available in 1975.[9]
In 1980, first high-resolution (with a frequency up to 500MHz) through-transmission mode SAM was built by Roman Maev and his students at his Laboratory of Biophysical Introscopy of the Russian Academy of Sciences.[10] First commercial SAM ELSAM with the broad frequency range from 100MHz up to ultra high 1.8GHz was built at the Ernst Leitz GmbH (Wetzlar, Germany) by the group led by Martin Hoppe and his consultants Abdullah Atalar (Stanford Univ.,USA), Roman Maev(Russian Academy of Sciences, Russia).
At the same time, in 1984, Kessler's group completed development of the C-SAM concept instrument[11] which operated in the reflection mode, as well as the through-transmission (only) mode of the SLAM. Using the same transducer to pulse ultrasound and receive the return echoes meant that the acoustic image could easily be constrained to a depth of interest. This design was the precursor of essentially all of the acoustic microscopes in use today, and was the development that made possible numerous later advances such as cross-sectional acoustic imaging, three-dimensional acoustic imaging, and others.
Since then, many improvements to Acoustic Microscopy Systems have been made to enhance resolution, image quality and accuracy.[12][13][14]
Types of acoustic microscopes
In the half-century since the first experiments directly leading to the development of acoustic microscopes, at least three basic types of acoustic microscope have been developed. These are the scanning acoustic microscope (SAM), confocal scanning acoustic microscope (CSAM), and C-mode scanning acoustic microscope (C-SAM).[15]
More recently acoustic microscopes based around picosecond ultrasonics systems have demonstrated acoustic imaging in cells using sub-optical wavelengths working with ultrasonic frequencies into the multi-GHz. Since the vast majority of acoustic microscopes in use today are C-SAM type instruments, this discussion will be limited to these instruments.[16]
Behavior of ultrasound in materials
Ultrasound is broadly defined as any sound having a frequency above 20kHz, which is approximately the highest frequency that can be detected by the human ear. However, the acoustic microscopes emit ultrasound ranging from 5MHz to beyond 400MHz so that micrometre size resolution can be achieved. The ultrasound that penetrates a sample may be scattered, absorbed or reflected by the internal features or the material itself. These actions are analogous to the behavior of light. Ultrasound that is reflected from an internal feature, or (in some applications) that has traveled through the entire thickness of the sample, is used to make acoustic images.
Sample types and preparation
Samples need no special treatment before acoustic imaging, but they should be able to withstand at least brief exposure to water or to another fluid, since air is a very poor transmitter of high frequency acoustic energy from the transducer. The sample may be completely immersed in the water, or scanned with a narrow stream of water. Alternately, alcohols and other fluids can be used so as to not contaminate the sample. Samples typically have at least one flat surface that can be scanned, although cylindrical and spherical samples can also be scanned with the proper fixtures. In the following paragraphs, the sample being described is a plastic-encapsulated integrated circuit.
Ultrasonic frequencies
The ultrasonic frequencies pulsed into samples by the transducers of acoustic microscopes range from a low of 10MHz (rarely, 5MHz) to a high of 400MHz or more. Across this spectrum of frequencies there is a trade-off of penetration and resolution. Ultrasound at low frequencies such as 10MHz penetrates deeper into materials than ultrasound at higher frequencies, but the spatial resolution of the acoustic image is less. On the other hand, ultrasound at very high frequencies do not penetrate deeply, but provide acoustic images having very high resolution. The frequency chosen to image a particular sample will depend on the geometry of the part and on the materials involved.
The acoustic image of the plastic-encapsulated IC below was made using a 30MHz transducer because this frequency provides a good compromise between penetration and image resolution.
Scanning process
In the acoustic image ultrasound was pulsed through the black mold compound (plastic), and reflected from the interface between the overlying mold compound and the top surface of the silicon die, the top surface of the die paddle, delaminations (red) on top of the die paddle, and the outer portion (lead fingers) of the lead frame.Side-view diagram
The ultrasonic transducer raster-scans the top surface of the sample. Several thousand pulses enter the sample each second. Each pulse may be scattered or absorbed in passing through homogeneous parts of the sample. At material interfaces, a portion of the pulse is reflected back to the transducer, where it is received and its amplitude recorded.
The portion of the pulse that is reflected is determined by the acoustic impedance, Z, of the each material that meets at the interface. The acoustic impedance of a given material is the material's density multiplied by the speed of ultrasound in that material. When a pulse of ultrasound encounters an interface between two materials, the degree of ultrasonic reflection from that interface is governed by this formula:
where R is the fraction of reflection, and z1 and z2 are the acoustic impedances of the two materials, analogous to refractive index in light propagation.
If both materials are typical solids, the degree of reflection will be moderate, and a significant portion of the pulse will travel deeper into the sample, where it may be in part reflected by deeper material interfaces. If one of the materials is a gas such as air – as in the case with delaminations, cracks and voids – the degree of reflection at the solid-to-gas interface is near 100%, the amplitude of the reflected pulse is very high, and practically none of the pulse travels deeper into the sample.
Gating of the return echoes
A pulse of ultrasound from the transducer travel nanoseconds or microseconds to reach an internal interface and are reflected back to the transducer. If there are several internal interfaces at different depths, the echoes arrive at the transducer at different times. Planar acoustic images do not often use all return echoes from all depths to make the visible acoustic image. Instead, a time window is created that accepts only those return echoes from the depth of interest. This process is known as "gating" the return echoes.
In the plastic-encapsulated IC, gating was on a depth that included the silicon die, the die paddle and the lead frame.Still scanning the top of the sample, the gating of the return echoes was then changed to include only the plastic encapsulant (mold compound) above the die. The resulting acoustic image is shown above. It shows the structure of the particle-filled plastic mold compound, as well as the circular mold marks at the top surface of the component. The small white features are voids (trapped bubbles) in the mold compound. (These voids are also visible in the previous image as dark acoustic shadows.)Gating was then changed to include only depth of the die attach material that attaches the silicon die to the die paddle. The die, the die paddle, and other features above and below the die attach depth are ignored. In the resulting acoustic, shown above slightly magnified, the red areas are voids (defects) in the die attach material.
Finally, the plastic-encapsulated IC was flipped over and imaged from the back side. The return echoes were gated on the depth where the backside mold compound interfaces with the back side of the die paddle. The small black dots in the acoustic image above are small voids (trapped bubbles) in the mold compound.
Other image types
The acoustic images shown above are all planar images, so named because they make visible a horizontal plane within the sample. The acoustic data received in the return echo signals can also be used to make other types of images, including three-dimensional images, cross-sectional images, and thru-scan images.
Range of applications
The samples imaged by acoustic microscopes are typically assemblies of one or more solid materials that have at least one surface that is either flat or regularly curved. The depth of interest may involve an internal bond between materials, or a depth at which a defect may occur in a homogeneous material. In addition, samples may be characterized without imaging to determine, e.g., their acoustic impedance.
Because of their ability to find visualize features non-destructively, acoustic microscopes are widely used in the production of electronic components and assemblies for quality control, reliability and failure analysis. Usually the interest is in finding and analyzing internal defects such as delaminations, cracks and voids, although an acoustic microscope may also be used simply to verify (by material characterization or imaging, or both) that a given part or a given material meets specifications or, in some instances, is not counterfeit.[17] Acoustic microscopes are also used to image printed circuit boards[18] and other assemblies.
There are in addition numerous applications outside of electronics. In many industries, products that involve tubing, ceramic materials, composite materials or various types of bonded joints, including adhesive layers and different welds may be imaged acoustically.
The assembly of numerous medical products uses acoustic microscopes to investigate internal bonds and features. For example, a polymer film may be imaged to examine its bond to a multi-channel plastic plate used in blood analysis. SAM can provide data on the elasticity of cells and both, hard and soft tissues, which can give useful information on the physical forces holding structures in a particular shape and the mechanics of structures such as the cytoskeleton.[3][4] These studies are particularly valuable in investigating processes such as cell motility.[5][6]
Another promising direction was initiated by different groups in the world to design and build portable hand-held SAM for subsurface 3D imaging and diagnostics of soft and hard tissues[16][19] and this direction currently is successfully developing with the goal to implement those methods into the clinical and cosmetology practice.
Also during last decade was expressed an interest to apply acoustic microscopy methods for 3D non-invasive inspection of the paint layers of painted art and other art and culture heritage objects.[20][21]
Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye. There are three well-known branches of microscopy: optical, electron, and scanning probe microscopy, along with the emerging field of X-ray microscopy.
Ultrasound is sound with frequencies greater than 20 kilohertz. This frequency is the approximate upper audible limit of human hearing in healthy young adults. The physical principles of acoustic waves apply to any frequency range, including ultrasound. Ultrasonic devices operate with frequencies from 20 kHz up to several gigahertz.
Medical ultrasound includes diagnostic techniques using ultrasound, as well as therapeutic applications of ultrasound. In diagnosis, it is used to create an image of internal body structures such as tendons, muscles, joints, blood vessels, and internal organs, to measure some characteristics or to generate an informative audible sound. The usage of ultrasound to produce visual images for medicine is called medical ultrasonography or simply sonography, or echography. The practice of examining pregnant women using ultrasound is called obstetric ultrasonography, and was an early development of clinical ultrasonography. The machine used is called an ultrasound machine, a sonograph or an echograph. The visual image formed using this technique is called an ultrasonogram, a sonogram or an echogram.
Laser-ultrasonics uses lasers to generate and detect ultrasonic waves. It is a non-contact technique used to measure materials thickness, detect flaws and carry out materials characterization. The basic components of a laser-ultrasonic system are a generation laser, a detection laser and a detector.
Photoacoustic imaging or optoacoustic imaging is a biomedical imaging modality based on the photoacoustic effect. Non-ionizing laser pulses are delivered into biological tissues and part of the energy will be absorbed and converted into heat, leading to transient thermoelastic expansion and thus wideband ultrasonic emission. The generated ultrasonic waves are detected by ultrasonic transducers and then analyzed to produce images. It is known that optical absorption is closely associated with physiological properties, such as hemoglobin concentration and oxygen saturation. As a result, the magnitude of the ultrasonic emission, which is proportional to the local energy deposition, reveals physiologically specific optical absorption contrast. 2D or 3D images of the targeted areas can then be formed.
Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.
Dr. Mehrdad Nikoonahad is an Iranian-American electrical engineer, technologist, innovator and entrepreneur.
Characterization, when used in materials science, refers to the broad and general process by which a material's structure and properties are probed and measured. It is a fundamental process in the field of materials science, without which no scientific understanding of engineering materials could be ascertained. The scope of the term often differs; some definitions limit the term's use to techniques which study the microscopic structure and properties of materials, while others use the term to refer to any materials analysis process including macroscopic techniques such as mechanical testing, thermal analysis and density calculation. The scale of the structures observed in materials characterization ranges from angstroms, such as in the imaging of individual atoms and chemical bonds, up to centimeters, such as in the imaging of coarse grain structures in metals.
A scanning acoustic microscope (SAM) is a device which uses focused sound to investigate, measure, or image an object. It is commonly used in failure analysis and non-destructive evaluation. It also has applications in biological and medical research. The semiconductor industry has found the SAM useful in detecting voids, cracks, and delaminations within microelectronic packages.
Ultrasonic transducers and ultrasonic sensors are devices that generate or sense ultrasound energy. They can be divided into three broad categories: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasound, receivers convert ultrasound into electrical signals, and transceivers can both transmit and receive ultrasound.
Thermoacoustic imaging was originally proposed by Theodore Bowen in 1981 as a strategy for studying the absorption properties of human tissue using virtually any kind of electromagnetic radiation. But Alexander Graham Bell first reported the physical principle upon which thermoacoustic imaging is based a century earlier. He observed that audible sound could be created by illuminating an intermittent beam of sunlight onto a rubber sheet. Shortly after Bowen's work was published, other researchers proposed methodology for thermoacoustic imaging using microwaves. In 1994 researchers used an infrared laser to produce the first thermoacoustic images of near-infrared optical absorption in a tissue-mimicking phantom, albeit in two dimensions (2D). In 1995 other researchers formulated a general reconstruction algorithm by which 2D thermoacoustic images could be computed from their "projections," i.e. thermoacoustic computed tomography (TCT). By 1998 researchers at Indiana University Medical Center extended TCT to 3D and employed pulsed microwaves to produce the first fully three-dimensional (3D) thermoacoustic images of biologic tissue [an excised lamb kidney ]. The following year they created the first fully 3D thermoacoustic images of cancer in the human breast, again using pulsed microwaves. Since that time, thermoacoustic imaging has gained widespread popularity in research institutions worldwide. As of 2008, three companies were developing commercial thermoacoustic imaging systems – Seno Medical, Endra, Inc. and OptoSonics, Inc.
Non-contact ultrasound (NCU) is a method of non-destructive testing where ultrasound is generated and used to test materials without the generating sensor making direct or indirect contact with the test material or test subject. Historically this has been difficult to do, as a typical transducer is very inefficient in air. Therefore, most conventional ultrasound methods require the use of some type of acoustic coupling medium in order to efficiently transmit the energy from the sensor to the test material. Couplant materials can range from gels or jets of water to direct solder bonds. However, in non-contact ultrasound, ambient air is the only acoustic coupling medium.
In the field of industrial ultrasonic testing, ultrasonic thickness measurement (UTM) is a method of performing non-destructive measurement (gauging) of the local thickness of a solid element based on the time taken by the ultrasound wave to return to the surface. This type of measurement is typically performed with an ultrasonic thickness gauge.
Roman Grigorievich Maev , is a Canadian professor of physics at the University of Windsor, distinguished university professor, the Foreign Member of the Russian Academy of Sciences (RAS) (2019), full professor in physics (2005), Dr. Sc. (2002), Ph. D. (1973). Dr. Maev is the founding director of the Institute for Diagnostic Imaging Research at the University of Windsor.
Atomic force acoustic microscopy (AFAM) is a type of scanning probe microscopy (SPM). It is a combination of acoustics and atomic force microscopy. The principal difference between AFAM and other forms of SPM is the addition of a transducer at the bottom of the sample which induces longitudinal out-of-plane vibrations in the specimen. These vibrations are sensed by a cantilever and tip called a probe. The figure shown here is the clear schematic of AFAM principle here B is the magnified version of the tip and sample placed on the transducer and tip having some optical coating generally gold coating to reflect the laser light on to the photodiode.
Scanning near-field ultrasound holography (SNFUH) is a method for performing nondestructive nano-scale high-resolution imaging of buried and embedded structures. SNFUH is critical for analysis of materials, structures and phenomena as they continue to shrink at the micro/nano scale. SNFUH is a type of scanning probe microscopy (SPM) technique that provides depth information as well as spatial resolution at the 10 to 100 nm scale.
Reflectometry is a general term for the use of the reflection of waves or pulses at surfaces and interfaces to detect or characterize objects, sometimes to detect anomalies as in fault detection and medical diagnosis.
Super-resolution photoacoustic imaging is a set of techniques used to enhance spatial resolution in photoacoustic imaging. Specifically, these techniques primarily break the optical diffraction limit of the photoacoustic imaging system. It can be achieved in a variety of mechanisms, such as blind structured illumination, multi-speckle illumination, or photo-imprint photoacoustic microscopy in Figure 1.
Photoacoustic microscopy is an imaging method based on the photoacoustic effect and is a subset of photoacoustic tomography. Photoacoustic microscopy takes advantage of the local temperature rise that occurs as a result of light absorption in tissue. Using a nanosecond pulsed laser beam, tissues undergo thermoelastic expansion, resulting in the release of a wide-band acoustic wave that can be detected using a high-frequency ultrasound transducer. Since ultrasonic scattering in tissue is weaker than optical scattering, photoacoustic microscopy is capable of achieving high-resolution images at greater depths than conventional microscopy methods. Furthermore, photoacoustic microscopy is especially useful in the field of biomedical imaging due to its scalability. By adjusting the optical and acoustic foci, lateral resolution may be optimized for the desired imaging depth.
A specific branch of contrast-enhanced ultrasound, acoustic angiography is a minimally invasive and non-ionizing medical imaging technique used to visualize vasculature. Acoustic angiography was first developed by the Dayton Laboratory at North Carolina State University and provides a safe, portable, and inexpensive alternative to the most common methods of angiography such as Magnetic Resonance Angiography and Computed Tomography Angiography. Although ultrasound does not traditionally exhibit the high resolution of MRI or CT, high-frequency ultrasound (HFU) achieves relatively high resolution by sacrificing some penetration depth. HFU typically uses waves between 20 and 100 MHz and achieves resolution of 16-80μm at depths of 3-12mm. Although HFU has exhibited adequate resolution to monitor things like tumor growth in the skin layers, on its own it lacks the depth and contrast necessary for imaging blood vessels. Acoustic angiography overcomes the weaknesses of HFU by combining contrast-enhanced ultrasound with the use of a dual-element ultrasound transducer to achieve high resolution visualization of blood vessels at relatively deep penetration levels.
1 2 Korpel, A. (1966). "Visualization of the Cross Section of a Sound Beam by Bragg Diffraction of Light". Applied Physics Letters. 9 (12): 425–427. Bibcode:1966ApPhL...9..425K. doi:10.1063/1.1754639.
1 2 R. Pohlman, "Material illumination by means of acoustic optical imagery," Z. Phys., 1133 697, 1939. See also Z. Angew. Phys., vol. 1, p. 181, 1948.
1 2 J. A. Cunningham and C. F. Quate, "Acoustic interference in solids and holographic imaging," in Acoustical Holography, vol. 4, G. Wade, Ed., New York: Plenum, 1972, pp. 667–685.
↑ A. Korpel and L. W. Kessler, "Comparison of methods of acoustic microscopy," in Acoustical Holography, vol. 3 by A. F. Metherell, Ed., New York: Plenum, 1971, pp. 23–43.
↑ R. Gr. Maev, Principles and Future of Acoustic Microscopy, Proceedings of the Joint Soviet-West Germany International Symposium on Microscope Photometry and Acoustic Microscopy in Science, Moscow, Russia, 1-12, 1985.
↑ "Acoustic Imaging and Acoustic Microscopes From Sonoscan Inc." Sonoscan. 11 July 2008
↑ Maev, Roman (2013). Advances in Acoustic Microscopy and High Resolution Ultrasonic Imaging: From Principles to New Applications. Wiley-VCH. ISBN978-3-527-41056-9.
↑ Kessler, L.W., "Acoustic Microscopy", Metals Handbook, Vol. 17 – Nondestructive Evaluation and Quality Control, ASM International, 1989, pp. 465–482.
1 2 R.Gr. Maev, Editor and Co-author, Advances in Acoustic Microscopy and High Resolution Ultrasonic Imaging: From Principles to New Applications, Monograph, 14 Chapters, 400 pages, Wiley & Son - VCH, April 2013
↑ Vogt, M., and Ermert, H., "Limited-angle spatial compounding imaging of skin with high-frequency ultrasound", IEEE Trans. Ultrason., Ferroelectr. Freq. Control, 55 (9), 1975 –1983 (2011).
↑ Georgios Karagiannis, Dimitrios Alexiadis, Argirios Damtsios, Georgios Sergiadis and Christos Salpistis, 3D non-destructive "sampling" of art objects, IEEE instrumentation and measurements, vol.60, issue 9, Pages 1-28, September 2011.
↑ D. Thickett, C.S. Cheung, H. Liang, J. Twydle, R.Gr. Maev, D. Gavrilov, Using non-invasive non-destructive techniques to monitor cultural heritage objects, Insight Magazine, 59 (5): 230–234, 2017
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