Characterization of Micro-Acoustic Systems

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:; ICPPP21 conference presentation.

Characterization of coatings with surface waves

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:

Material characterization in the high GHz-range with PLUS

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

Analysis of plate structures: elastic properties and thickness simultaneously

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:

Visualization of multilayer structures

As illustrated in this example of an organic photovoltaic cell, multilayer structures can be measured and analysed with OCT (Optical Coherence Tomography). Organic PV-cells consist of organic semiconducting materials, transparent electrodes, and a protective coating. As a result of this, a homogeneous layer structure without defects, inclusions etc. is relevant for the quality, functionality, and long-lasting performance of the product.

Spatially resolved spectroscopy

Do you want to know the exact local distribution (in micrometer range) of your chemical components? With Mid-Infrared-Microscopy we can chemically characterize and measure materials and cross-sections (e.g. residues or inclusions) with a spatial resolution as small as 5 µm. Quantum Sensing offers interesting opportunities in that area too.