Scanning acoustic microscope

The acoustic microscopy is a nondestructive imaging process, the ultrasound of very high frequency used to generate images of the inside of an object. The lateral resolution of detail achieved a classic light microscope, the depth resolution is much better. It is also called ultrasonic microscopy and acoustic microscopy, often with the additional screen ( for example, in raster scanning acoustic microscopy ) in order to describe the operation in detail. The English name for the acoustic microscopy is usually scanning acoustic microscopy and is abbreviated as (SAM ). Commonly but is also the name acoustic micro imaging or shortly AMI.

It is suitable to detect defects and to analyze material characteristics, or changes. Since the method reacts very efficiently on the interfaces between solid and liquid matter and gas, it is widely used in the electronics and semiconductor technology for error analysis ( see Figure 1) to find delaminations, cracks and voids. But even in the material sciences, the acoustic microscopy for the study of metal structures or of ceramics is employed. In the biological and medical research, living cells can be analyzed without embedding, drying or staining.

• 3.1 A-mode
• 3.2 Acoustic depth profile ( B-mode )
• 3.3 Acoustic (horizontal ) Cross section ( C-mode )
• Figure 3.4 Other types

Ultrasound

As a sound is referred to the physical propagation of pressure and density variations in a medium. In homogeneous media, sound propagates in a straight line and can be focused by lenses. In the frequency range between 20 Hz and 20 kHz is called audible sound and thus of different high tones ( see Figure 2). Above it is the ultrasonic range. Well-known examples are, eg, ultrasonic cleaning baths ( frequency 10-30 kHz) or ultrasonography in medical examination (frequency range 1-40 MHz). Acoustic microscopy uses frequencies up to the gigahertz range. With the frequency of the achievable resolution, but also the attenuation increases: while infrasound propagates in the atmosphere over thousands of kilometers, so gases must be highly compressed above a frequency of 10 MHz in order to transmit sound can. In the GHz range, the range is reduced even in condensed matter to well below one millimeter. At the highest frequencies the attenuation of sound waves in liquids is almost as high as that of shear waves can propagate only in solids at low frequencies.

Design and operation of an ultrasonic microscope

The acoustic microscope uses the possibility of the propagation of ultrasound in a solid. For this purpose a short ultrasonic pulse is transmitted into the sample (such as inclusions or voids ) investigates the interactions at the interfaces between different materials. The ultrasonic signal can be reflected, absorbed or scattered in the interior of the sample.

Signal generation and focusing

A transmitter short high-frequency electrical signals are generated and transmitted to the transducer (transducer). The sound transducer is a piezoelectric crystal and is depending on the used frequency range of different materials. The transducer generated from the short electrical signals from the transmitter short pulses with a duration of 20 to 100 ns ( nanoseconds) of high-frequency ultrasound waves and passes it on to the directly connected to the ultrasonic transducer acoustic lens. Thousands of sound pulses per second are emitted thereby.

The bottom of the lens is concave for focusing the ultrasound waves, wherein the radius of curvature may be used, depending on the frequency of less than 100 microns to a few millimeters. A coupling medium (usually water ) transmits ultrasonic waves to the object to be imaged. The waves are reflected at the surface and at interfaces (see types of interactions in the sample). The same ultrasound transducer converts the reflected acoustic waves back into electrical signals which are evaluated in time-resolved by the receiver.

Rasterizes to the ultrasound head with an XY - scanner line by line over the sample, we obtain successively information about the different sample areas and can be calculated therefrom an image. This is often depicted as a gray scale or false-color image ( Fig. 1).

Types of interactions in the sample

Applies the ultrasonic signal to an interface between two different materials, a portion of the ultrasonic signal is reflected, while the remainder is transmitted (Fig. 4a). In a cavity, the ultrasonic waves can not propagate in the original direction. There will be a total reflection of the signal ( Fig. 4b). Structures below the cavity can not be reached and can not be analyzed so. The interface (such as a crack ) is tilted to the direction of signal propagation, as the reflected signal is reflected in a different direction (Fig. 4c). Depending on the inclination of the surface, the reflected signal could possibly no longer be absorbed by the detector. If the sound hits compared with the used wavelength, fine structures, of the sound is scattered in all directions, and thus its intensity greatly attenuated (Fig. 4d).

Figure 4b: Total reflection of ultrasound at a cavity

Figure 4c: Total reflection of ultrasound at an inclined crack

Figure 4d: scattering of the ultrasonic signal of very fine structures

The measured signal

The measurement signal contains information about run-time, amplitude and polarity ( sign) of the reflected sound wave. For evaluating the amplitude of the signal is plotted as a function of time.

The chronologically first signal is from the reflection of sound at the sample surface ( red signal in Figure 5a). Without additional non-homogeneities in the sample, the noise signal is not again reflected at the bottom of the sample ( the green signal in Figure 5a). At a known speed of sound transit time difference of the two signals from the top and bottom of the sample information on the thickness of the sample returns. Located within the sample, a defect, it comes at each interface between two materials to a reflection of the sound (Fig. 5b). This is also evident in the measurement signal ( blue signal in Figure 5b). In the illustrated case there is a reflection from the top and the bottom of the defect. On the basis of the duration may in turn be determined a boundary or defect, the location ( depth).

The amplitude ( signal strength ) provides information on the material properties of the materials involved at the interface (see also theory of sound propagation in a sample ). Larger differences in acoustic impedance produce a stronger reflection and therefore a greater amplitude of the reflected signal.

Going through the sound an interface of a medium of lower density in a denser medium, the shape of the signal corresponds to the originally transmitted signal ( red signal in Figure 5a and 5b). In the transition from a higher density material into a material of lower density, however, it comes to a change of polarity, ie the sign of the signal varies (the green light to the underside of the sample in Figure 5a and 5b). In an enclosure, it is usually ( blue signal in Figure 5b), which originate respectively from the top and from the bottom of the defect to two signals of different polarity.

Modes

In the acoustic microscopy different modes (german fashion ) can be distinguished. The main modes of operation:

• A mode: representation of signal amplitude as a function of time
• B-mode: Acoustic depth profile ( vertical section )
• C-mode: Horizontal acoustic sectional image
• T -Mode: acoustic transmission image
• ToF -Mode: Time of Flight, depicting a Höhen-/Tiefenbildes

A-mode

In the mode A mode ( A = amplitude) of the probe is not moved. One thus obtains information for the position below the transducer. In A-mode is obtained instead of images to display signals as described in the previous section, the measurement signal (Fig. 5a / b ).

Acoustic depth profile ( B-mode )

When using the B- mode ( Brightness = B ) of the transducer is moved along a line across the sample. At each point of the line, a time -resolved measurement signal is sequentially, as described above, was added. The strength of the signal ( amplitude ) is then assigned to different shades of gray. An image that is plotted in the right position of the measuring head and down the running time (depth) corresponds to a vertical slice image or depth profile through the component. This type of imaging is suitable for example to a Bauteilverkippung represent.

Since the signal used for the study contains 1.5 to 3 periods, resulting in vertical section image per interface always several short consecutive lines.

Acoustic (horizontal ) Cross section ( C-mode )

In the image in the C-mode, the transducer is scanned by an xy scanner on the sample surface. For each measuring point, in turn, a time-resolved measurement signal is received. Within this window signal (gate) is set. Only information with maturities within this window are used for illustration (horizontal cross-section). Depending on the location of the windows different depth regions of the sample can be imaged.

The easiest way to explain this is with an example of an electronic component. Fig 6a shows a schematic cross section through an integrated circuit (IC). If one uses for mapping a window in which only the signals are included, originating from the surface of the component ( red signals in Fig 5a/5b ), we obtain an acoustic image of the surface of the IC ( Fig. 6b). This image corresponds largely to the visual impression of the component. To detect the large circular grooves in the surface of the potting compound. The three smaller white structures in the upper part of the image are caused by cavities ( gas bubbles trapped in the grout ) just below the surface.

If you move the window to a different maturity range of the data (eg, to the blue signal in Figure 5b ), a sectional image is produced at a different depth of the component. In Figure 6c is a false-color coding of the maximum signal strength was used. The assignment can be seen in the scale on the left side. The boundary between the casting compound and the silicon chip surface (1) is also displayed on the interface with the base plate ( 2) and the finger-like supply lines ( 4) because of the relatively high signal reflection bright same. Since the leads ( 4) are made of copper, these appear in the image a bit brighter than the interface to the silicon. The red areas in the image ( 3) also correspond to a high reflected signal strength, but with a negative sign of the signal. This corresponds to a total reflection at an interface to a cavity ( delamination ).

The three dark spots in the upper part of the image in Figure 6c are caused by shadowing effects of the signal. These are formed by a total reflection of the signal in the three small cavities below the surface (see Figure 6b). The underlying structures can not be examined. Also in the corners you can see that above the level of investigation arranged inhomogeneities (eg, the depressions in the surface of the potting compound ) can interfere with the image.

Other image types

In addition to the types of functions listed above and entire three-dimensional data sets can be stored and tomographic evaluated. Some acoustic microscopes, it is also possible to arrange a second transducer below the sample and to move parallel to the upper transducer. The resulting transmission image (T mode) shows the absorption and shading of the sound waves in the sample. In simple cases, it represents a negative image of the reflection image

Sample Requirements and preparation

Samples require no special pretreatment prior to the examination, but they should survive at least a brief treatment with water or another liquid without change. The liquid for coupling the acoustic energy necessary since air is a very poor transmitter of sound at high frequencies. The sample can be completely immersed in the measurement in water or are scanned with a thin water jet. Alternatively, alcohols or other liquids may be used so as not to change the sample.

The samples typically have at least one flat surface which is scanned. Above the level to be examined no cracks or voids may be located, as these lead to shadowing of the signal. Inhomogeneities such as fillers or a surface roughness is in the order of the wavelength used, can lead to variations of the signal and hence to problems in the interpretation of results.

Applications

Because of the possibility of non-destructive examination and visual appearance of internal structures of the acoustic microscope is used in the semiconductor industry for quality control and failure analysis. Often it is (for example delaminations, cracks and cavities ) used for the analysis of defects, although an acoustic microscope may also be employed to check the orientation and position of components in the interior of an electronic component. In addition, it is also used for imaging of the printed circuit boards and other assemblies.

In the field of material sciences enable acoustic images represent the microscopic grain structure in metals or ceramics to check for voids or microcracks.

Outside of technical applications, there are other applications in medicine. A major concern of osteological research is the assessment of bone tissue, especially of newly formed bone. Microscopic structural features, as they are obtained by means of acoustic microscopy, to determine the mechanism of the bone.

Many structures of living cells have dimensions in the micrometer range. Small structural elements often differ greatly in their elastic properties. Since the samples are immersed in water and must not be dried or to be stained or subjected to the vacuum, the investigation on the living material is also possible.

Comparison Acoustic Microscopy - Sonography

Although both methods use ultrasound for imaging, there are significant differences. One difference is certainly the frequency and thus the achievable resolution that is significantly higher for the ultrasonic microscope. At the same time the high frequency allowed but only the very investigation of near-surface structures that would be enough for medical sonography in any way.

Another big difference is the type of screening. While mechanically guided over the sample in the acoustic microscope, the transducer, or phased array sector scanner to be used in ultrasound, in which the ultrasound is pivoted from a fixed transducer electronically in different directions. Typical is thus in the ultrasound, an acoustic section in depth, while the acoustic microscopy creates horizontal cuts.

But significant differences also arise because of the materials studied. Since the biological body itself is to a large extent of water, the coupling of ultrasound is here much simpler than that of industrial solids. In addition occur in solids at every sound reflection mode changes of the signal (eg, longitudinally by transverse) that do not occur in soft matter.

Theory of propagation of sound in a sample

Is an important parameter for describing the sound propagation, the acoustic impedance. It is defined by the formula, wherein the density of the material and the speed of sound in the material corresponds. Now propagate acoustic waves starting from a material having the acoustic impedance of another material to the acoustic impedance of such a portion of the signal on the interface is reflected. The proportion of radiation reflected or transmitted radiation is calculated by the following formulas

It follows that whenever reflection occurs when the acoustic impedance of two materials at an interface is different. The reflection is greater, the greater the difference in acoustic impedance of the two materials involved. Sound is particularly well suited for detecting cracks, delaminations and voids, since it leads to a total reflection at the interface between a material and a gas ( acoustic impedance ).

Is the noise from a denser medium to a medium with a lower acoustic impedance on, so there is also a reflection at the interface. Because in this case, the variable is negative, thereby the polarity of the wave changes (negative signal).