Microelectromechanical systems (MEMS) are integral components of cell phones and telecommunication infrastructure, whose development, production and quality testing are metrologically very challenging due to the high frequencies and small dimensions.
5G and future 6G standards drive the relevant frequency range up to about 20 GHz. We have developed a laser-ultrasonic method (Frequency Domain Laser-UltraSound "FreDomLUS") for spatially resolved characterization of single and multilayer systems with respect to elastic properties, attenuation, thickness and defects.
Figure (A) shows the sample dimensions and the determined damping αL on different single-layer constituent materials (Al and W) used in bulk acoustic wave "BAW" filters, as well as the thermoelastic limit.
For the experiment shown in Figure (B), a sample with a height profile in a checkerboard pattern was fabricated and scanned with FreDomLUS. The thickness variations of 8 nm could be reconstructed successfully and were also comparable to AFM results (Atomic Force Microscopy).
For more details, please see: doi.org/10.1121/10.0017652; ICPPP21 conference presentation.

Elastic waves can be used to determine the thickness and elastic properties of layered structures.
Even if the thickness of a layer is too small for pulse-echo measurements, the thickness or elastic properties of a layer can be calculated from the propagation behavior of surface waves. Depending on the frequency, surface waves penetrate the layer and substrate material below the surface to different depths. Low frequencies correspond to large penetration depths, high frequencies correspond to small penetration depths (see graph).
The different properties of the layer and substrate result in a frequency-dependent phase velocity of the surface wave. This can be measured and numerically modeled, and thus the thickness and elastic properties can be determined.
For more information, see our publication: https://doi.org/10.1115/QNDE2021-74927

The longitudinal and transverse sound velocities allow conclusions to be drawn about the microstructure, for example the degree of recrystallization of metals during thermal treatments.
The thickness of a plate or sheet is also often required. Previous acoustic methods for measuring the speed of sound and the thickness of a plate require knowledge of the other quantity or spatial scanning. The method we have developed can determine these properties in a single measurement.
A specific surface wave and several plate resonances are excited simultaneously in the plate sample by a laser pulse with a periodic line pattern and then analyzed. This allows (with known density) the complete elastic characterization and simultaneous thickness determination of isotropic plates. A patent has been filed in this regard.
Further information can be found in our publication: https://pub.dega-akustik.de/DAGA_2023/data/articles/000177.pdf

Many metals are homogenized by solution annealing as part of the manufacturing process.
After quenching, however, the material is in a supersaturated state, and dissolved alloying constituents deposit over time into precipitate particles, e. g. Guinier-Preston (GP) zones. These have a strong influence on the strength and formability of the material. Using laser ultrasound, the changes in elastic and shear modulus of plates can be measured over many hours with a precision <1%, allowing the progress of precipitation hardening to be monitored with high time resolution.
One possible application is the adjustment of springback compensation depending on the stage of precipitation hardening in which the bent workpiece is in.
For further information, please refer to our publication: https://doi.org/10.1016/j.mtla.2022.101600

Heat treatment of metals is a standard process to bring controlled changes in the microstructure and thereby set the desired mechanical properties in the material.
However, currently used methods for microstructure analysis (micrographs, dilatometry, tensile tests, ...) have the disadvantage that they either cannot be used directly in the process or require special sample geometries. By measuring plate resonances on sheet metal specimens using laser ultrasound, Poisson's ratio and, if the thickness is known, also the longitudinal and transverse sound velocity can be determined without contact and with high time resolution during heat treatment (e.g., in a thermal simulator). The temperature variation of these parameters correlates with changes in the microstructure, allowing phase transitions to be monitored using this method.

Further information can be found in our publication: https://doi.org/10.1016/j.actamat.2022.118097

Optical pump-probe spectroscopy on the picosecond timescale (picosecond laser ultrasound “PLUS”) enables the study of elastic phenomena in the GHz- and even up to the THz-range.
We apply different detection schemes to investigate a large variety of materials from metals, semiconductors and polymers to novel nano- and low dimensional materials.
This gives us access to material intrinsic parameters like acoustic damping, heat diffusion and other dissipation mechanisms, doping profiles, as well as to structural properties like layer thicknesses in the sub-nm to low um range, the adhesion quality of thin coatings and other interface related properties.
Further information about a possible application can be found in https://doi.org/10.1016/j.pacs.2023.100464