A scanning acoustic microscope 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.
History
The first scanning acoustic microscope, with a 50 MHz ultrasonic lens, was developed in 1974 by R. A. Lemons and C. F. Quate at the Microwave Laboratory of Stanford University. A few years later, in 1980, the first high-resolution through-transmission SAM was built by R.Gr. Maev and his students at his Laboratory of Biophysical Introscopy of the Russian Academy of Sciences. First commercial SAM ELSAM, with a broad frequency range from 100 MHz up to 1.8 GHz, was built at the Ernst Leitz GmbH by the group led by Martin Hoppe and his consultants Abdullah Atalar, Roman Maev and Andrew Briggs Since then, many improvements to such systems have been made to enhance resolution and accuracy. Most of them were described in detail in the monograph Advanced in Acoustic Microscopy, Ed. by Andrew Briggs, 1992, Oxford University Press and in monograph by Roman Maev, Acoustic Microscopy Fundamentals and Applications, Monograph, Wiley & Son - VCH, 291 pages, August 2008, as well as recently in.
Principles of operation
Scanning acoustic microscopy works by directing focused sound from a transducer at a small point on a target object. Sound hitting the object is either scattered, absorbed, reflected or transmitted. It is possible to detect the scattered pulses travelling in a particular direction. A detected pulse informs of the presence of a boundary or object. The `time of flight' of the pulse is defined as the time taken for it to be emitted by an acoustic source, scattered by an object and received by the detector, which is usually coincident with the source. The time of flight can be used to determine the distance of the inhomogeneity from the source given knowledge of the speed through the medium. Based on the measurement, a value is assigned to the location investigated. The transducer is moved slightly and then insonified again. This process is repeated in a systematic pattern until the entire region of interest has been investigated. Often the values for each point are assembled into an image of the object. The contrast seen in the image is based either on the object's geometry or material composition. The resolution of the image is limited either by the physical scanning resolution or the width of the sound beam.
Applications
- Fast production control - Standards : IPC A610, Mil-Std883, J-Std-035, Esa, etc - Parts sorting - Inspection of solder pads, flip-chip, underfill, die-attach - Sealing joints - Brazed and welded joints - Qualification and fast selection of glues, adhesive, comparative analyses of aging, etc - Inclusions, heterogeneities, porosities, cracks in material
Device testing
SAM is used for counterfeit detection, product reliability testing, process validation, vendor qualification, quality control, failure analysis, research, and development. Detecting discontinuities in silicon is just one of the ways scanning acoustic microscopy is being used for testing in the semiconductor market.
Medicine and biology
SAM can provide data on the elasticity of cells and 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. These studies are particularly valuable in investigating processes such as cell motility. Some work has also been performed to assess penetration depth of particles injected into skin using needle-free injection Another promising direction was initiated by different groups to design and build portable hand-held SAM for subsurface diagnostics of soft and hard tissues and this direction currently in the commercialization process in clinical and cosmetology practice.
