ACOUSTIC MICROSCOPY 1 A review

Since the underlying research and development behind the acoustic microscope has been reviewed in a number of excellent papers (Quate et al.(1979), Ash (1980), Nikoonahad (1983), Wickramasinghe (1984), Atalar and Hoppe (1986)), only a concise account shall be presented here.

The basic idea of an acoustic microscope was first put forward by the Russian scientist S. Ya Sokolov who realised that acoustic waves in water at gigahertz frequencies had wavelengths comparable to that of visible light but due to the lack of technology in generating such high frequencies at that time, he demonstrated a system operating at 1 MHz (Sokolov (1949)). After Sokolov's initial idea, many forms of acoustic microscope imaging have been explored during the last 40 years. A major advance however was the invention of the

60 scanning acoustic microscope (SAM) operating in the reflection mode as illustrated in Figure 2.7 (Lemons and Quate (1974)).

In this configuration, the object is immersed in a liquid (usually water) and placed at the focal plane of the acoustic lens. A piezoelectric transducer bonded to one end of the lens buffer rod converts the transmitted RF pulses into acoustic pulses which are bought to a focus at the focal plane of the lens. These acoustic pulses are incident upon the object and the reflected pulses are collected by the lens and the transducer converts them back into electrical pulses, which after detection, provides a video signal for display purposes. The acoustic pulses reflected by the object are modulated both in phase and amplitude . The lens is scanned over the entire field of view in a raster manner and an image is built up point by point. The lens is mounted on a precision translation stage with 3 degrees of freedom X,Y and Z where X and Y provide horizontal movements for locating the lens on the required field of view to be imaged and the vertical Z movement is used to focus the lens. The electronics of the system are shown in block diagram form in Figure 2.8

61 Mechanical ,*T-scan

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Fig.2.9 A reflection SAM: the basic system

Lens Low noise amplifier To z modulation of TV monitor Video amplifier Isolator Pm switch Pm switch Isolator Sample and hold Timing electronics

62 A CW signal generator acts as the source and frequencies initially used were in the 1 GHz region. The output is pulsed by a series of cascaded PIN switches. On the detection side, a time gate suppresses the echoes from the surface of the lens and other unwanted signals. The signal from the object is subsequently amplified by a high gain amplifier. The baseband pulses are then "square law" detected and then further amplified. The pulses are finally fed into a sample-and-hold or a boxcar integrator circuit at the output which is the last stage before the display.

Another form of acoustic microscopy is the scanning laser acoustic microscope (SLAM) (Kessler and Yuhas (1978)). In the SLAM, the body of the sample under examination is illuminated by an unfocussed beam of acoustic energy and any inhomogeneity in the path of the acoustic beam eg. a defect causes the scattering and interference of the acoustic radiation. The scattering and interference show themselves as ripples on the surface of the sample and is recorded with a rapidly scanned focused laser beam. A diagram of the SLAM is shown as Figure 2

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One major advantage of the SLAM in addition to its real time imaging ability is the fact that the object can be viewed optically while the acoustic image is being obtained as shown in Figure 2.11. The SLAM is basically a near-field shadow imaging system with the definition controlled by the size of the laser beam at the reflecting surface.

LASER BEAM SCANNERS IMAGING OPTICS MIRRORS

M_\

7T

MIRRORED COVERSLIP _SPECIMEN

STAGE ACOUSTIC FREQUENCY GENERATOR PHOTODETECTOR ULTRASONIC TRANSDUCER OPTICAL IMAGE DISPLAY OPTICAL SIGNAL PROCESSOR DEMODULATOR AND PHOTODETECTOR ACOUSTIC SIGNAL PROCESSOR ACOUSTIC IMAGE DISPLAY

Fig. 2.11 The scanning laser acoustic microscope

Pm switch

Compressor Expander

Lens

64 One of the problems associated with imaging arises due to the high impedance discontinuity at the interface between

the coupling fluid and the object under test. This greatly reduces the fraction of acoustic energy transmitted into the solid. Another problem is that the defect to be imaged is usually very close to the surface of the solid and hence the defect pulse arrives very shortly after the interface pulse and hence in order to discriminate against this interface pulse, broadband transducers have to be used that are capable of transmitting very short acoustic pulses. This further reduces the signal/noise ratio in the image.

One of the techniques of improving the signal/noise in such situations has been described ( Nikhoonahad et al. (1985)) which uses coded pulses for transmission followed by a matched filter for reception. The basic idea is shown in Figure 2.12.

An impulse is converted by a SAW delay line designed to expand to a long chirp pulse. At the receiver side, another SAW dispersive delay line this time compresses the long chirps reflected from different planes of the object to narrow pulses at the centre frequency of the SAW filters. The important concept here is that the noise, which is random in nature is not compressed. After time gating and amplification, the signal is converted into an image. Due to the wide input pulses used on transmission, it is possible to supply more energy per pulse using such a system as compared with the conventional SAM. The improvement in the signal/noise ratio can be shown to be the product of the pulse duration time T and the bandwidth B.

Most of the early studies conducted into materials characterisation and imaging used relatively narrowband microscopes at frequencies in the range of 0.75 - 2.0 GHz and in the reflection mode. These images were formed by displaying amplitude variations resulting from interference between narrowband pulses (with carrier frequencies ranging between

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