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Acoustic Micro Imaging instruments (acoustic microscopes)
used for nondestructive inspection of samples uses sound waves
to penetrate samples while operating with the pulse-reflection
method. There is a special acoustic objective centerpiece
in the microscope, which produces, transmits, and receives
short ultrasonic sound pulses of a high penetration rate.
The acoustic lens converts the high frequency electromagnetic
vibrations which are propagated as a parallel plane wave field
inside the lens. The cavity focuses the acoustic ultrasound
field on the sample through the coupling medium (which is
typically water). The same acoustic lens then receives the
returning sound pulses which are reflected from the sample.
The transducer transforms the ultrasonic pulses into electromagnetic
pulses which are displayed as pixels with defined gray values
on the acoustic microscopes monitor. To produce the image
the acoustic microscope’s objective lens scans the sample
line by line and transmitting that information back on the
system’s monitor.
The time to build up a complete image on the acoustic microscope
depends on the scan rate and the selected image resolution.
For a given deflection of the acoustic objective at 50MHz,
it takes about 10 seconds to produce an image of 512 x 512
pixels.
The unique property of acoustic microscopy or ultrasonic microscopy
is the ability to image the interaction of acoustic waves
with the elastic properties of a specimen with comparable
resolution of optical light microscopy. In many applications
of acoustic microscopy, the acoustic microscope is used to
image the interior of an opaque material. In such cases somewhat
lower frequencies 10-400 MHz are used to achieve greater penetration
into the sample. Uses of this type often include examination
of packaging materials to ensure integrity, especially in
high value-added industries. Additional popular applications
include examining for microscopic voids, delaminations, improper
bond layers between structures as well as inspection of wafer
bonding, microscopic cracks and many other defects.
More rigid specimens, including most metals, semiconductors
and ceramics, a very dominant role in the contrast can be
played by Rayleigh waves in or near the surface of the sample.
If the specimen examined in the acoustic microscope has a
surface layer, then the propagation of the Rayleigh waves
is sensitive to the disturbing action of the layer. If the
specimen is anisotropic, then there will be dependence on
the orientation of the surface and the direction of propagation
in the layer. If there are microscopic surface cracks or grain
boundaries, then there will be a strong contrast when these
effects scatter the Rayleigh waves. In this type of use with
ultrasonic microscopes, ultra-high frequencies up to 800-2000MHz
are required.
Several transducers used on the acoustic imaging microscope
cover a large range of frequencies and lens designs for different
applications with semiconductors, ceramics, metals, plastics,
rubber and other samples. Different transducers offer either
higher penetration into the sample or increased resolution.
The acoustic microscope’s return echoes from each scan
position are analyzed for amplitude, time of flight and polarity.
Thus, the samples size, depth, location and other measurements
can be made particularly on the defect in question. This process
used in acoustic microscopy is simple, fast, and accurate.
Moreover, the ultrasonic microscopy sound waves do not change
or alter the sample in any way.
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Graphical presentation of
working method
A-scan signal of a single
point position
Schematic of a meander
scan over the specimen
area
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