Surfaces & Coatings

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

In multilayer coatings the individual layers often have individual functions, e.g., protection from mechanical impacts and contamination, corrosion protection, adhesive layers, etc. Therefore, a homogeneous layer structure without defects, inclusions, etc. is essential for the quality and functionality of the product. With OCT (Optical Coherence Tomography) it is possible to measure the thickness of coatings with micrometre precision (even for most multilayer coatings).

In a research cooperation with the Institute of Biomedical Mechatronics (Johannes Kepler University, Linz), the skin structure of a Texan horned lizard was artificially recreated. With its unique skin structure, this animal can move condensed water from the whole body surface directly to the mouth. The successful application of OCT played a major role in the success of this project. As an instrument for imaging and characterization of the surface, OCT contributed to the understanding of functionality of the surface. Applications for similar “functional surfaces” are “Lab-on-chip” applications, wound compresses, textile fibers, and the distribution of lubricant on friction bearings.

Metallic components are often coated dielectrically in order to optimize the operating characteristics (such as tribology).
With Terahertz technology (THz) such coatings can be measured efficiently inline.

The OCT (Optical Coherence Tomography) provides insight into nearly all common polymer materials used in the industrial environment. As a tomographic measurement technique, OCT delivers information about the internal structure in order to detect and characterise cracks, defects, inclusions, pores, etc. OCT not only controls the quality and functionality of the plastic products, but also delivers relevant information to help understand and optimize the manufacturing process.

With laser-based methods surfaces can be structured for different purposes. The OCT technology (Optical Coherence Tomography) is a perfect tool for imaging and measuring such structures in the micrometre range (e.g. microscopic laser drilled holes shown in the picture) and can also be measured online and in real-time during the process.

As illustrated in this image of a coin, the topography or roughness (parameters cf. ISO 25178) of a sample can easily be determined by Optical Coherence Tomography (OCT). The advantage of using OCT, compared to other methods, is the additional possibility of measuring steep edges. The accuracy in the measurement of height profiles is below 1 micron.

In lacquer coating layers (or similar coatings), embedded particles (of different materials) are very important for the functionalization of the coating or for e.g. a perfect automotive-metallic-coating.
With the low-cost OCT method we are able to monitor and characterize the partial embedding, which helps to secure and optimize the production process.

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.

The determination of the hardness penetration depth in thermally hardened components is essential for quality control. State-of-the-art is to cut samples and do etching and hardness measurements. Laser-Ultrasonics opens a non-destructive alternative. Thereby, zones of different microstructures at arbitrary positions can be imaged up to a tomographic representation of the hardness penetration depth.