ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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Acoustic microscopes are practical tools that have emerged from the laboratory to find useful applications within

industry. They can be applied to a broad range of problems that previously had no solutions, and they have been

especially useful in solving problems with new high-technology materials and components not previously available. The

three acoustic microscope types can be as different from each other as are microradiography and electron microscopy.

This discussion should provide the potential user with an awareness of the techniques and their distinctions in order to

maximize the opportunities for the successful use of acoustic microscopy.

References

A. Korpel, L.W. Kessler, and P.R. Palermo, Acoustic Microscope Operating at 100 MHz, Nature,

Vol 232

(No. 5306), 1970, p 110-111

Product Bulletin, Sonoscan, Inc.

R.A. Lemons and C.F. Quate, Acoustic Microscope--Scanning Version, Appl. Phys. Lett.,

Vol 24 (No. 4),

1974, p 163-165

Acoustic Microscopy

Lawrence W. Kessler, Sonoscan, Inc.

Fundamentals of Acoustic Microscopy Methods

As a general comparison between the methods, the scanning laser acoustic microscope is primarily a transmission mode

instrument that creates true real-time images of a sample throughout its entire thickness (reflection mode is sometimes

employed). In operation, ultrasound is introduced to the bottom surface of the sample by a piezoelectric transducer, and

the transmitted wave is detected on the top side by a rapidly scanning laser beam.

The other two types of microscopes are primarily reflection mode instruments that use a transducer with an acoustic lens

to focus the wave at or below the sample surface. The transducer is mechanically translated (scanned) across the sample

in a raster fashion to create the image. The C-mode scanning acoustic microscope can image several millimeters or more

into most samples and is ideal for analyzing at a specific depth. Because of a very large top surface reflection from the

sample, this type of microscope is not effective in the zone immediately below the surface unless the Rayleigh-wave

mode to scan near-surface regions is used along with wide-aperture transducers. The scanning acoustic microscope uses

this Rayleigh wave mode and is designed for very high resolution images of the surface and near-surface regions of a

sample. Penetration depth is intrinsically limited, however, to only one wavelength of sound because of the geometry of

the lens. For example, at 1 GHz, the penetration limit is about 1 m (40 in.). The C-mode scanning acoustic microscope

is designed for moderate penetration into a sample, and transmission mode imaging is sometimes employed. This

instrument uses a pulse-echo transducer, and the specific depth of view can be electronically gated. More detailed

discussions of each acoustic microscopy technique follow.

SLAM Operating Principles

A collimated plane wave of continuous-wave (CW) ultrasound at frequencies up to several hundred megahertz is

produced by a piezoelectric transducer located beneath the sample, as illustrated in Fig. 1. Because this ultrasound cannot

travel through air (making it an excellent tool for crack, void, and disbond detection), a fluid couplant is used to bring the

ultrasound to the sample. Distilled water, spectrophotometric-grade alcohol, or other more inert fluids can be used,

depending on user concerns for sample contamination. When the ultrasound travels through the sample, the wave is

affected by the homogeneity of the material. Wherever there are anomalies, the ultrasound is differentially attenuated, and

the resulting image reveals characteristic light and dark features, which correspond to the localized acoustic properties of

the sample. Multiple views can be made to determine the specific depth of a defect, as is performed by stereoscopy (Ref

4).

Fig. 1 Simplified view of the methods for producing through transmission and non

collinear reflection mode

acoustic images with the scanning laser acoustic microscope

A laser beam is used as an ultrasound detector by means of sensing the infinitesimal displacements (rippling) at the

surface of the part created by the ultrasound. In typical samples that do not have polished, optically reflective surfaces, a

mirrored plastic block, or coverslip, is placed in close proximity to the surface and is acoustically coupled with fluid. The

laser is focused onto the bottom surface of the coverslip, which has an acoustic pattern that corresponds to the sample

surface. By rapid sweeping of the laser beam, the scanning laser acoustic microscope images are produced in real time

(that is, 30 pictures per second) and are displayed on a high-resolution video monitor. In contrast to other less accurate

uses of the term real time in industry today, the scanning laser acoustic microscope can be used to observe events as they

occur--for example, a crack propagating under an applied stress. The images produced by SLAM are shadowgraph mode

images of structure throughout the thickness of the sample. This provides the distinct advantage of simultaneous viewing

of the entire thickness of the sample, as in x-ray radiography. In situations where it is necessary to focus on one specific

plane, holographic reconstruction of the SLAM data can be employed (Ref 5, 6).

Figure 2 shows a system block diagram for the scanning laser acoustic microscope. In addition to an acoustic image on a

CRT, an optical image is produced by means of the direct scanned laser illumination of the sample surface. For this mode,

the reflective coating on the coverslip is made semi-transparent. The optical image serves as a reference view of the

sample for the operator to consult for landmark information, artifacts, and positioning of the sample to known areas. The

SLAM acoustic images also provide useful and easily interpreted quantitative data about the sample. For example, the

brightness of the image corresponds to the acoustic transmission level. By removing the sample and restoring the image

brightness level with a calibrated electrical attenuator placed between the transducer and its electrical driver, precise

insertion loss data can be obtained (Ref 7, 8). With the acoustic interference mode, the velocity of sound can be measured

in each area of the sample (Ref 9, 10). When these data are used to determine regionally localized acoustic attenuation

loss, modulus of elasticity, and so on, the elastic microstructure can be well characterized.

Fig. 2

Schematic showing principal components of a scanning laser acoustic microscope. The unit employs a

plane wave piezoelectric transducer to generate the ultrasound and a focused laser

beam as a point source

detector of the ultrasonic signal. Acoustic images are produced at a rate of 30 images per second.

The simplest geometries for SLAM imaging are flat plates or disks. However, with proper fixturing, complex shapes and

large samples can also be accommodated. For example, tiny hybrid electronic components, large (254 × 254 mm, or 10 ×

10 in.) metal plates, aircraft turbine blades and ceramic engine cylinder liner tubes have been routinely examined by

SLAM.

C-SAM Operating Principles

The C-mode scanning acoustic microscope is primarily a pulse-echo (reflection-type) microscope that generates images

by mechanically scanning a transducer in a raster pattern over the sample. A focused spot of ultrasound is generated by an

acoustic lens assembly at frequencies typically ranging from 10 to 100 MHz. A schematic diagram is shown in Fig. 3.

Fig. 3 Schematic of the C-mode scanning acoustic microscope. This instrument incorporates a reflection, pulse-

echo technique that employs a focused

transducer lens to generate and receive the ultrasound signals beneath

the surface of the sample.

The ultrasound is brought to the sample by a coupling medium, usually water or an inert fluid. The angle of the rays from

the lens is generally kept small so that the incident ultrasound does not exceed the critical angle of refraction between the

fluid coupling and the solid sample. Note that the focal distance into the sample is shortened considerably by the liquid-

solid refraction. The transducer alternately acts as sender and receiver, being electronically switched between the transmit

and receive modes. A very short acoustic pulse enters the sample, and return echoes are produced at the sample surface

and at specific interfaces within the part. The return times are a function of the distance from the interface to the

transducer. An oscilloscope display of the echo pattern, known as an A-scan, clearly shows these levels and their time-

distance relationships from the sample surface, as illustrated in Fig. 4. This provides a basis for investigating anomalies at

specific levels within a part. An electronic gate selects information from a specific level to be imaged while it excludes all

other echoes. The gated echo brightness modulates a CRT that is synchronized with the transducer position.

Fig. 4 Signal/source interfaces and A-

scan displays for typical sample. (a) Simplified diagram showing pulses of

ultrasound being reflected at three interfaces: front surface, A; material interface, B; and rear int

erface, C.

Main bang is the background electrical noise produced by transducer excitation. (b) Plot of amplitude versus

time as transducer is switched between transmit and receive modes at each interface during a time span of

under 1 s. (c) Signal at interface B gated for a duration of 30 ns

Compared to older conventional C-scan instruments, which produce a black/white output on thermal paper when a signal

exceeds an operator-selected threshold, the output of the C-mode scanning acoustic microscope is displayed in full gray

scale, in which the gray level is proportional to the amplitude of the interface signal (gray-scale digitization is discussed

in the article "Machine Vision and Robotic Evaluation" in this Volume). The gray scale can be converted into false color,

and as shown in Fig. 5, the images can also be color coded with echo polarity information (Ref 11). That is, positive

echoes, which arise from reflection from a higher-impedance interface, are displayed in a gray scale having one color

scheme, while negative echoes, from reflections off of lower-impedance interfaces are displayed in a different color

scheme. This allows quantitative determination of the nature of the interface within the sample. For example, the echo

amplitude from a plastic-ceramic boundary is very similar to that from a plastic-airgap boundary, except that the echoes

are 180° out of phase. Thus, to determine whether or not an epoxy is bonded to a ceramic, echo amplitude analysis alone

is not sufficient. The color-coded enhanced C-mode scanning acoustic microscope is further differentiated from

conventional C-scan equipment by the speed of the scan. Here, the transducer is positioned by a very fast mechanical

scanner that produces images in tens of seconds instead of tens of minutes for typical scan areas to cover the size of an

integrated circuit.

Fig. 5 Schematic and block diagram of the C-

mode scanning acoustic microscope. The instrument employs a

very high speed mechanical scanner and an acoustic impedance polarity detector to produce high-resolution C-

scan images.

With regard to the depth zone within a sample that is accessible by C-SAM techniques, it is well known that the large

echo from a liquid/solid interface (the top surface of the sample) masks the small echoes that may occur near the surface

within the solid material. This characteristic is known as the dead zone, and its size is usually of the order of a few

wavelengths of sound or more. Far below the surface, the maximum depth of penetration is determined by the attenuation

losses in the sample and by the geometric refraction of the acoustic rays, which shorten the lens focus by the solid

material. Therefore, depending on the depth of interest within a sample, a proper transducer and lens must be used for

optimum results.

SAM Operating Principles

The scanning acoustic microscope is primarily a reflection-type microscope that generates very high resolution images of

surface and near-surface features of a sample by mechanically scanning a transducer in a raster pattern over the sample

(Fig. 6 and 7). In the normal mode, an image is generated from echo amplitude data over an x,y scanned field-of-view. As

with SLAM, a transmission interference mode can be configured for velocity of sound measurements. In contrast to C-

SAM, a more highly focused spot of ultrasound is generated by a very wide angle acoustic lens assembly at frequencies

typically ranging from 100 to 2000 MHz. The angle of the sound rays is well beyond the critical cutoff angle, so that there

is essentially no wave propagation into the material. There is a Rayleigh (surface) wave at the interface and an evanescent

wave that reaches to about one wavelength depth below the surface. As in the other techniques, the ultrasound is brought

to the sample by a coupling medium, usually water or an inert fluid. The transducer alternately acts as sender and

receiver, being electronically switched between the transmit and receive modes. However, instead of a short pulse of

acoustic energy, a long pulse of gated radio-frequency (RF) energy is used. No range gating is possible, as in C-SAM,

because of the basic design concept of the SAM system. The returned acoustic signal level is determined by the elastic

properties of the material at the near-surface zone. The returned signal level modulates a CRT, which is synchronized

with the transducer position. In this way, images are produced in a raster scan on the CRT. As with C-SAM, complete

images are produced in about 10 s.

Fig. 6 Schematic of the scanning acoustic microscope lens used for interrogating the surface zone of a sample

Fig. 7 Block diagram of a reflective-

type SAM system that uses mechanical scanning of a highly focused

transducer to investigate the surface zone of a sample at high magnification

With the SAM technique operating at very high frequencies, it is possible to achieve resolution approaching that of a

conventional optical microscope. This technique is employed in much the same way as an optical microscope, with the

important exception that the information obtained relates to the elastic properties of the material. Even higher resolution

than an optical microscope can be obtained by lowering the temperature of operation to near 0 K and using liquid helium

as a coupling fluid. The wavelength in the liquid helium is very short compared to that of water, and submicron resolution

can be obtained (Ref 12).

The SAM technique has also been found to be useful for characterizing the elastic properties of a sample over a

microscopic-size area, which is determined by the focal-spot size of the transducer. In this method, the reflected signal

level is plotted as a function of the distance between the sample and the lens. Because of the leaky surface waves

generated by mode conversion at the liquid/solid interface as the sample is defocused toward the lens, an interference

signal is produced between the mode converted waves and the direct interface reflection. The curve obtained is known as

the V(z) curve; by analyzing the periodicity of the curve, the surface wave velocity can be determined. Furthermore,

defocusing the lens enhances the contrast of surface features that do not otherwise appear in the acoustic image (Ref 13,

14). In addition, by using a cylindrical lens instead of a spherical lens, the anisotropy of materials can be uniquely

characterized (Ref 15).

Comparison of Methods to Optimize Use of Techniques

Figure 8 illustrates the zones of application for all three types of acoustic microscopy techniques. The differences are

substantial with regard to the potential for visualizing features within a sample, and they should be carefully weighed

before a particular method is selected. Table 1 lists some of the major advantages and benefits of the techniques.

However, generalizations are sometimes difficult to make. Proposed or laboratory-demonstrated solutions to some of the

limitations of each technique may already exist. Table 1 was prepared on the basis of instrumentation and techniques that

are commercially available and therefore should be used only as a general guide.

Table 1 Comparison of industrial acoustic microscopy techniques

Parameter SLAM C-SAM

SAM

General

description

Utilizes CW, plane wave

ultrasonic illumination of sample

and scanning focused laser beam

detection of ultrasound;

simultaneous optical images and

acoustic images are produced.

SLAM produces images in real

time, which is the fastest of all

acoustic microscopy techniques.

High-resolution focused-beam C-scan

(a)

Utilizes pulse-echo mode and has full

gray-scale image output. Images are

produced by mechanically scanning the

transducer over the sample area.

Utilizes very highly focused acoustic

lenses to image exterior surfaces by

means of surface acoustic wave

generation. Images are produced by

mechanically scanning the

transducer over the sample area.

SAM produces the highest resolution

of all acoustic microscopy

techniques.

Primary use Nondestructive testing and

materials characterization

Nondestructive testing and materials

characterization

Materials characterization and

research

Frequency

range

10-500 MHz 10-100 MHz

100 MHz-2 GHz

Resolution Resolution limited to the

wavelength of ultrasound within

the sample material

Resolution limited to the wavelength of

ultrasound within the sample material,

multiplied by a factor, typically 2-10, due

to the lens design

Resolution limited to the wavelength

of ultrasound within the coupling

fluid at the surface of the sample

Imaging mode Through transmission

(primarily); off-axis reflection;

orthoscopic view of sample

Reflection (primarily); orthoscopic view

of sample

Reflection (only); orthoscopic view

of sample

Image

information

Image produced is an acoustic

shadowgraph representing the

entire thickness of the sample.

Image is produced at any selected depth

within the sample, but is affected by

wave propagation through all the levels

prior to the focus location.

Image is produced only at the

sample surface, and the depth of the

information extends into the

material a distance of one

wavelength of sound.

Ultrasound

mode

Continuous-wave and

frequency-modulated ultrasound

waves

Short pulses of ultrasound, less than a

few cycles of RF in duration

Gated RF (or tone burst) containing

many cycles (10-100) of RF at

frequency selected

Ultrasound

signal timing

Not applicable due to CW Echoes are spread out in time, and one is

selected and electronically gated for

desired image depth within sample.

Gated at the surface of the sample

where virtually all the ultrasound is

reflected. There are no subsurface

echoes to select.

Transducer

type

Plane wave transducer for

sample illumination and focused

laser beam for detection

Focused transducer used for transmit and

receive mode. The angle of the rays is

less than the critical angles between the

coupling fluid and the sample. This is

known as a low-numerical-aperture

transducer.

Focused transducer in which the

geometric angle of the rays is much

greater than the critical angles in

order to excite surface modes. This

is known as a high-numerical-

aperture transducer.

Image outputs Amplitude mode: Records level

of ultrasound transmission at

each x,y coordinate of the scan

Interference mode: Records

velocity of sound variations

within sample over the field-of-

view at each frequency

Frequency scan mode: Similar to

amplitude mode except that the

insonification frequency is swept

over a range to eliminate speckle

and other artifacts of coherent

imaging

Optical mode: Laser scanned

optical image produced in

synchrony with the acoustic

images

C-mode: Records echo amplitudes at

each x,y coordinate of the scan; image

output on CRT

A-scan: Oscilloscope display of the echo

pattern as a function of time (distance

into sample) at each x,y coordinate. This

can be used to measure the depth of a

feature or the velocity of sound in the

material. The A-scan is used to determine

the setting of the echo gate, which is

critical to the composition and

interpretation of the images.

Normal mode: Records reflected

energy from surface at each x,y

position. Image output on CRT.

V(z), or acoustic material signature

graph, records the change in

reflected signal level at any x,y

coordinate as the z position is varied

(Ref 12, 13, 14). This will

characterize the material by means

of its surface acoustic wave velocity.

Interference mode: Records velocity

of sound variations within sample

over the field-of-view at each

frequency

Depth of

penetration

Penetration is limited by

acoustic attenuation

characteristic of sample.

In addition to attenuation by the sample,

penetration is limited by focal length of

lens and geometric refraction of the rays,

which causes shortening of the focus

position below the surface. There is also

a dead zone just under the surface due to

the large-amplitude front-surface echo,

which masks smaller signals occurring

immediately thereafter. This can be

rectified with a high-numerical-aperture

transducer.

Limited to a distance of one

wavelength of sound below the

surface. There is essentially no wave

propagation into the sample.

Imaging speed True real-time imaging: 30

frames/s; fastest of all acoustic

microscopes

10 s to 30 min per frame; varies greatly

among manufacturers

10 to 20 s/frame

New

developments

Holographic reconstruction of

each plane through the depth

of the sample (Ref 5, 6)

Acoustic impedance polarity detector

to characterize the physical properties

of the echo producing interfaces (Ref

11)

Low-temperature liquid helium

stages for extremely high resolution

images (Ref 15)

(a)

C-scan produces ultrasonic images by mechanically translating a pulsed transducer in an x,y plane above a sample while recording the echo

amplitude within a preset electronic time gate. The transducer may be planar or focused. The frequencies of operation are typically 1-10

MHz, and the data are usually displayed as a binary brightness level on a hard copy output unit, such as thermal paper. A threshold level

selected by the operator determines the transition between what amplitude of echo is displayed as a bright or dark image print.

Fig. 8

Simplified comparison of three acoustic microscopy techniques, particularly their zones of application

(crosshatched area) within a sample. (a) SLAM. (b) SAM. (c) C-SAM

References cited in this section

4. L.W. Kessler and D.E. Yuhas, Acoustic Microscopy--1979, Proc. of IEEE,

Vol 67 (No. 4), April 1979, p

526-536

Z.C. Lin, H. Lee, G. Wade, M.G. Oravecz, and L.W. Kessler, Holographic Image Reconstruction in

Scanning Laser Acoustic Microscopy, Trans. IEEE, Vol UFFC-34 (No. 3), May 1987, p 293-300

B.Y. Yu, M.G. Oravecz, and L.W. Kessler, "Multimedia Holographic Image Reconstruction in a Scanning

Laser Acoustic Microscope," Paper presented at the 16th International Symposium on Acoustical Imaging

(Chicago, IL), Sonoscan, Inc., June 1987; L.W. Kessler, Ed., Acoustical Imaging,

Vol 16, Plenum Press,

1988, p 535-542

7. L.W. Kessler, VHF Ultrasonic Attenuation in Mammalian Tissue, Acoust. Soc. Am.,

Vol 53 (No. 6), 1973, p

1759-1760

8. M.G. Oravecz, Quantitative Scanning Laser Acoustic Microscopy: Attenuation, J. Phys. (Orsay),

Vol 46,

Conf. C10, Supplement 12, Dec 1985, p 751-754

S.A. Goss and W.D. O'Brien, Jr., Direct Ultrasonic Velocity Measurements of Mammalian Collagen

Threads, Acoust. Soc. Am., Vol 65 (No. 2), 1979, p 507-511

10.

M.G. Oravecz and S. Lees, Acoustic Spectral Interferometry: A New Method for Sonic Velocity

Determination, in Acoustical Imaging,

Vol 13, M. Kaveh, R.K. Mueller, and T.F. Greenleaf, Ed., Plenum

Press, 1984, p 397-408

11.

F.J. Cichanski, Method and System for Dual Phase Scanning Acoustic Microscopy, Patent Pending

12.

J.S. Foster and D. Rugar, Low-Temperature Acoustic Microscopy, Trans. IEEE, Vol SU-32, 1985, p 139-