Acoustic microscopy

Fig. 1: Acoustic image of an integrated circuit with material peeling inside (red).

The acoustic microscopy is a non-destructive , an imaging method , the ultrasonic very high frequency used to generate images of the inside of an object. The lateral detail resolution achieves that of a classic light microscope , the depth resolution is much better. It is also called ultrasound microscopy or acousto microscopy , often with the addition of scanning (e.g. in scanning ultrasound microscopy) to describe how it works. The English term for acoustic microscopy is mostly scanning acoustic microscopy and is abbreviated to (SAM) . The term acoustic micro imaging or AMI for short is also common .

It is suitable for detecting defects and analyzing material properties or changes. Since the process reacts particularly efficiently to interfaces between solid or liquid matter and gas, it is often used in electronics and semiconductor technology for error analysis (see Fig. 1) in order to find detachments, cracks and cavities. But acoustic microscopy is also used in materials science to examine metal structures or ceramics. In biological and medical research, living cells can be examined without embedding, drying or staining.

Ultrasonic

Fig. 2: Acoustic frequency spectrum

In physical terms , sound is the propagation of pressure and density fluctuations in a medium. In homogeneous media, sound propagates straight forward and can be carried lenses focus . In the frequency range between 20 Hz and 20 kHz, one speaks of audible sound and thus of different high tones (see Fig. 2). The ultrasound area lies above this . Known application examples are z. B. Ultrasonic cleaning baths (frequency 10–30 kHz) or sonography in medical examinations (frequency range 1–40 MHz). Acoustic microscopy uses frequencies in the gigahertz range. The achievable resolution increases with the frequency, but so does the attenuation : While infrasound spreads over thousands of kilometers in the atmosphere, gases above a frequency of 10 MHz have to be strongly compressed in order to be able to transmit sound. In the GHz range, the range drops to well below a millimeter, even in condensed matter . At the highest frequencies, the attenuation of sound waves in liquids is almost as high as that of shear waves , which can only propagate in solids at low frequencies.

Structure and functionality of an ultrasonic microscope

The acoustic microscope uses the possibility of ultrasound propagation in a solid body. For this purpose, a short ultrasonic pulse is sent into the sample and the interaction at interfaces between different materials (e.g. inclusions or defects) is examined. The ultrasonic signal can be reflected, scattered or absorbed inside the sample.

Fig. 3: How an acoustic microscope works

Signal generation and focusing

Short electrical high-frequency signals are generated by a transmitter and passed on to the sound converter (transducer). The sound transducer is a piezoelectric crystal and consists of different materials depending on the frequency range used. The sound transducer uses the short electrical signals from the transmitter to generate short pulses with a duration of 20 to 100 ns (nanoseconds) from high-frequency ultrasonic waves and forwards them to the acoustic lens connected directly to the ultrasonic transducer. Several thousand sound impulses are emitted per second.

The underside of the lens is concave in order to focus the ultrasonic waves , whereby the radius of curvature can be from less than 100 µm to a few millimeters, depending on the frequency used. A coupling medium (usually water) transmits the ultrasonic waves to the object to be imaged. The waves are reflected on the surface and on internal interfaces (see section Types of interaction in the sample ). The same ultrasonic transducer converts the reflected acoustic waves back into electrical signals, which are evaluated by the receiver in a time-resolved manner.

If the ultrasound head is scanned line by line over the sample with an XY scanner, information about the various sample areas is obtained one after the other and an image can be calculated from this. This is often shown as a grayscale or false color image (Fig. 1).

Types of interaction in the sample

If the ultrasonic signal hits an interface between two different materials, part of the ultrasonic signal is reflected, while the rest is let through (Fig. 4a). In the case of a cavity, the ultrasonic waves cannot propagate any further in the original direction. There is a total reflection of the signal (Fig. 4b). Structures below the cavity are not reached and can therefore not be analyzed. If the interface (e.g. a crack) is tilted to the direction of signal propagation, the reflected signal is reflected back in a different direction (Fig. 4c). Depending on the inclination of the surface, the reflected signal might no longer be picked up by the detector. If the sound hits fine structures compared to the wavelength used, the sound is scattered in all directions and its intensity is thus greatly weakened (Fig. 4d).

Fig. 4a: Interaction of ultrasound at an interface
Fig. 4b: Total reflection of ultrasound on a cavity
Fig. 4c: Total reflection of ultrasound on an oblique crack
Fig. 4d: Scattering of the ultrasonic signal on very fine structures

The measurement signal

Fig. 5a: Origin of the measurement signal in acoustic microscopy (without defect).

Fig. 5b: Origin of the measurement signal in acoustic microscopy (with defect).

The measurement signal contains information on the transit time , amplitude and polarity (sign) of the reflected sound wave. For the evaluation, the amplitude of the signal is plotted as a function of time.

The first signal in time comes from the reflection of the sound on the sample surface (red signal in Fig. 5a). Without additional inhomogeneities in the sample, the sound signal is only reflected again on the underside of the sample (green signal in Fig. 5a). If the speed of sound is known, the transit time difference between the two signals from the top and bottom of the sample provides information about the thickness of the sample. If there is a defect within the sample, the sound is reflected at every interface between two materials (Fig. 5b). This is also evident in the measurement signal (blue signal in Fig. 5b). In the case shown, there is a reflection on the top and bottom of the defect. The position (depth) of an interface or defect can in turn be determined on the basis of the transit time.

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

If the sound passes through an interface between a medium of lower density and a denser medium, the form of the signal corresponds to the originally transmitted signal (red signal in Fig. 5a or 5b). When changing from a material of higher density to a material of lower density, on the other hand, there is a change in polarity, i. H. the sign of the signal changes (green signal on the underside of the sample in Fig. 5a or 5b). In the case of inclusion, there are usually two signals of different polarity (blue signal in Fig. 5b), each coming from the top and bottom of the defect.

Operating modes

In acoustic microscopy, a distinction is made between different modes of operation. The most important operating modes are:

A-mode: Representation of the signal amplitude as a function of time
B-mode: acoustic depth profile (vertical section)
C-Mode: Horizontal acoustic cross section
T-mode: acoustic transmission image
ToF-Mode: Time of Flight, representation of a height / depth image

A-mode

In A-mode (A = amplitude), the transducer is not moved. Information is thus obtained for the position below the transducer. In A-mode, no images are obtained, but a signal display as described in the previous section The measurement signal (Fig. 5a / b).

Acoustic depth profile (B-mode)

When using the B mode (B = brightness), the transducer is moved along a line over the sample. At each point of this line, as described above, a time-resolved measurement signal is recorded one after the other. The strength of the signal (amplitude) is then assigned to different gray levels. An image in which the position of the measuring head is plotted to the right and the transit time (depth) downwards corresponds to a vertical sectional image or depth profile through the component. This type of image generation is suitable, for example, to depict component tilting.

Since the signal used for the investigation contains 1.5 to 3 periods, there are always several lines following each other in the vertical sectional image per interface.

Acoustic (horizontal) cross-section (C-mode)

Fig. 6a: Cross section through an electronic component

Fig. 6b: acoustic image of the surface of the IC

Fig.6c: acoustic image with material detachment (red)

When imaging in C-mode, the transducer is scanned over the sample surface with an xy scanner. A time-resolved measurement signal is recorded for each measurement point. A gate is set within this signal. Only information with transit times within this window is used for the illustration (horizontal slice image). Depending on the position of the window, different depth ranges 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 you use a window that only contains the signals that come from the surface of the component (red signals in Fig. 5a / 5b), you get an acoustic image of the surface of the IC (Fig. 6b). This image largely corresponds to the visual impression of the component. The large circular depressions in the surface of the potting compound can be seen. The three smaller white structures in the upper part of the picture are caused by cavities (gas bubbles trapped in the potting compound) just below the surface.

If you move the window to a different transit time range of the measurement signal (e.g. around the blue signal in Fig. 5b), a sectional image is generated at a different depth of the component. In Fig. 6c, a false color coding of the maximum signal strength was used for this. The assignment can be seen in the scale on the left edge of the picture. The interface between the potting compound and the silicon chip surface (1) appears bright due to the relatively high signal reflection. The same applies to the interface to the base plate (2) and the finger-like leads (4). Since the leads (4) are made of copper, they appear a little lighter in the picture than the interface to the silicon. The red areas in 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 with a cavity (material detachment).

The three dark spots in the upper part of the picture in Fig. 6c are caused by shadowing of the signal. These are caused by a total reflection of the signal at the three small cavities below the surface (see Fig. 6b). The underlying structures can no longer be examined. In the corners, too, it can be seen that inhomogeneities arranged above the examination plane (e.g. the depressions in the surface of the potting compound) can disturb the image.

Other types of images

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

Rehearsal requirements and preparation

Samples do not require any special pre-treatment prior to examination, but they should at least survive a brief treatment with water or another liquid without any change. The liquid is necessary for coupling in the acoustic energy, since air is a very poor transmitter for sound with high frequencies. The sample can be completely immersed in water during the measurement or scanned with a narrow water jet. Alternatively, alcohols or other liquids can be used so as not to alter the sample.

The samples usually have at least one flat surface that is scanned. There must be no cracks or cavities above the plane to be examined, as these lead to a shadowing of the signal. Inhomogeneities such as B. fillers or a surface roughness that is in the order of magnitude of the wavelength used can lead to scattering of the signal and thus to problems in interpreting the results.

Applications

Due to the possibility of the non-destructive examination and visual representation of internal structures, the acoustic microscope is used in the semiconductor industry for quality control and error analysis. It is often used to analyze defects (e.g. detachments, cracks and voids), although an acoustic microscope can also be used to check the location and position of the components used inside an electronic component. It is also used to depict printed circuit boards and other assemblies.

In the field of materials science, acoustic images allow the microscopic structure of metals to be shown or ceramics to be checked for cavities or microcracks.

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

Many living cell structures have dimensions in the micrometer range. Small structural elements often differ greatly in their elastic properties. Since the samples are embedded in water and do not have to be dried, stained or exposed to vacuum, the examination on living material is possible.

Comparison of acoustic microscopy and sonography

Although both methods use ultrasound for imaging, there are clear differences. One difference is certainly the frequency and thus the achievable resolution, which is significantly higher with the ultrasonic microscope. At the same time, however, the high frequency only allows the examination of structures very close to the surface, which would in no way be sufficient for medical sonography .

Another big difference is the type of screening. While in acoustic microscopy the transducer is moved mechanically over the sample, sector scanners or phased arrays are used in sonography, in which the ultrasound is swiveled electronically in different directions by a fixed transducer. In sonography, an acoustic cut is typical in depth, while acoustic microscopy creates horizontal cuts.

However, there are also clear differences due to the materials examined. Since the biological body itself consists to a large extent of water, the coupling of ultrasound is much easier here than with technical solids. In addition, with every sound reflection in solids, mode conversions of the signal occur (e.g. longitudinal to transverse ), which do not occur with soft matter.

Theory of sound propagation in a sample

Acoustic impedance ${\ displaystyle Z_ {F}}$
material	10 ⁶ kg / (m ² s)
air	0.00
Water (20 ° C)	1.48
Epoxy resin	3.12
Resin for ICs	6.76
Glass	15.04
aluminum	16.90
silicon	20.04
Al ₂ O ₃	39.56
copper	41.83
gold	62.53

Acoustic impedance is an important parameter for describing the propagation of sound . It is defined by the formula , where corresponds to the density of the material and the speed of sound in this material. If sound waves now propagate from one material with the acoustic impedance into another material with the acoustic impedance , part of the signal is reflected at the interface. The proportion of the reflected radiation or the transmitted radiation is calculated using the following formulas ${\ displaystyle Z}$ ${\ displaystyle Z_ {F} = \ rho \ cdot v}$ ${\ displaystyle \ rho}$ ${\ displaystyle v}$ ${\ displaystyle Z_ {1}}$ ${\ displaystyle Z_ {2}}$ ${\ displaystyle R}$ ${\ displaystyle T}$

{\ displaystyle R = {\ frac {Z_ {2} -Z_ {1}} {Z_ {1} + Z_ {2}}} \ qquad {\ text {or}} \ qquad T = {\ frac {2Z_ {2}} {Z_ {1} + Z_ {2}}}.}

This means that a reflection always occurs when the acoustic impedance of two materials differs at an interface. The greater the difference in the acoustic impedance of the two materials involved, the stronger the reflection. Sound is particularly well suited to detect cracks, detachments and cavities, since the transition between a material and a gas (acoustic impedance ) leads to total reflection. ${\ displaystyle Z_ {2} = 0}$

If the sound passes from a denser medium into a medium with a lower acoustic impedance, there is also a reflection at the interface. Since is in this case , the variable becomes negative, which changes the polarity of the wave (negative signal). ${\ displaystyle Z_ {1}> Z_ {2}}$ ${\ displaystyle R}$

Web links

Commons : Acoustic Microscopy - collection of images, videos and audio files

Acoustic microscopy basics. Sonoscan, accessed on September 13, 2009 (English, manufacturer's methoddescription).
SAM working principle. KSI Kraemer Sonic Industries, accessed on January 22, 2014 (method description of a manufacturer).
How an ultrasonic scanning microscope works. PVA TePla Analytical Systems GmbH, accessed on January 16, 2016 (methoddescription ofa manufacturer).
Thomas Rapp: Ultrasonic microscope. In: ELO The Magazine. Retrieved September 13, 2009 (Instructions for Building a Simple Ultrasonic Microscope (Part 1)).
Thomas Rapp: xy scanner. In: ELO The Magazine. Retrieved September 13, 2009 (How to Build a Simple Ultrasound Microscope (Part 2)).
Acoustic microscopy introduction at the University of Leipzig.
Operating modes of acoustic microscopy. Institute for Acoustic Microscopy Dr. Krämer GmbH, accessed on November 30, 2009 (operating modes / scan modes and their applications).

Individual evidence

↑ Ursula Christine Winkler-Budenhofer, Scanning Acoustic Microscopy for the Assessment of Newly Formed Bones (PDF; 3.2 MB), dissertation on the acquisition of a doctorate in medicine, University of Munich, 2007

↑ ^a ^b ^c Volker Deutsch, Michael Platte, Manfred Vogt: Ultrasonic testing: Basics and industrial applications, Verlag Springer, 1997, ISBN 3-540-62072-9 , page 146, (preview google-books )

[Winkler-1] Ursula Christine Winkler-Budenhofer, Scanning Acoustic Microscopy for the Assessment of Newly Formed Bones (PDF; 3.2 MB), dissertation on the acquisition of a doctorate in medicine, University of Munich, 2007

[Deutsch-2] Volker Deutsch, Michael Platte, Manfred Vogt: Ultrasonic testing: Basics and industrial applications, Verlag Springer, 1997, ISBN 3-540-62072-9 , page 146, (preview google-books )