What sets acoustic microscopy apart is its ability to carry out non-destructive inspections of sample surfaces and internal structures, such as those of the ingots used in the manufacture of wafers and MEMS systems. In scenarios where infrared microscopes or X-ray microscopes have reached their limits, ultrasound microscopes can reliably reveal the internal quality of even opaque samples.
- Advantages/Sparing and Accurate
Whereas the sound is propagated in a rectilinear fashion in homogeneous media, it will reveal interfaces between different materials (e.g. inclusions or defects) by reflection, scattering, or absorption of the sound waves. Ultrasound waves can therefore pinpoint acoustic resistances (impedances) within a sample and therefore errors and irregularities with a very high degree of accuracy.
For instance, if ultrasound waves encounter an air inclusion inside a sample, a very bright contrast associated with a phase inversion will be recorded. Other material defects such as microcracks, inclusions, and delamination can also be shown in the same way at resolutions equivalent to that of a light microscope.
- Operation/Focused and Reflecting
All systems of Analytical Systems GmbH operate using the pulse reflection method, also referred to as the pulse-echo method.
At the heart of our ultrasound microscopes is an ultrasound probe consisting of a special acoustic lens (usually a sapphire crystal in cylindrical form) that is connected to a transducer—a piezoelectric crystal that can consist of different materials depending on the frequency range used.
The piezoelectric element in the transducer converts electrical signals into sound waves. The downstream lens focuses and bundles these as acoustic waves on the specimen under investigation. The de-ionized water serves primarily as a coupling medium to transmits the ultrasound waves to the probe. These sound waves are ultimately reflected by the sample back to the ultrasonic transmitter, where they are evaluated by an analog-digital converter (ADC) and suitable software and presented as a gray-level image.
- Data Acquisition/Systematic and Very Precise
The transducer scans the sample line by line (in grids) on the XY plane and on request—in case of samples with bows or protruding elements—in the Z direction and then displays the electromagnetic pulses as pixels with specific gray values.
The information from the individual pixels is used to generate an image showing all the recorded signals. To begin with, the ultrasound reflection at the sample surface is displayed. If the sample is intact, the signal is reflected again a second time at the rear side of the sample. The time difference of arrival of the two signals from the upper and lower side of the sample yields information about its thickness. If the sample contains a defect, this interface—between sample and defect—will also cause sound reflection to occur.
- Frequency & Resolution/Unrivaled and Specific
Acoustic microscopy operates using frequencies that extend to the gigahertz range. As a general rule, the higher the frequency, the greater the achievable resolution and the lower the penetration depth of the sound waves.
Since the attenuation in the coupling media increases quadratically as the frequency rises, the lens must be brought as close as possible to the sample under investigation. This low working distance and the difficulty of creating transducer arrangements offering high local resolution require a type of scanning microscopy in which the sample is investigated pixel by pixel.
The ongoing further development of our high-resolution transducers enables us to improve the measurement accuracy. Our ultrasound microscopes currently offer the highest frequency range up to a maximum of 2,000 MHz, delivering resolutions all the way down to 0.5 µm. Naturally, this depends on the material and the density of the sample to be investigated.
Depending on the operating frequency of the ultrasonic testing probe, the imaging resolutions shown below can be achieved:
- Overview of Scan Modes/Measurements