Terahertz layer thickness measurement in industrial environments

At the Fraunhofer Institute for Industrial Mathematics ITWM, Kaiserslautern, terahertz measurement systems are developed which can be used, among other things, to measure layer thicknesses in an industrial environment. As the latest version of the terahertz layer thickness measurement system, a robot-assisted variant is now available, i.e., a collaborative robot, in short »cobot«, is used for simplified integration into the working environment. The Fraunhofer ITWM system is particularly suitable for the thickness measurement of individual layers within a multilayer system, where the coating can be applied to any material. In addition, moist, sticky, and soft coatings and layers on curved surfaces can also be measured.

Analogous to ultrasonic measurement, the technology sends short terahertz light pulses onto the sample. However, unlike ultrasound, these measurements do not require a coupling medium and therefore work in a non-contact and non-destructive manner. At each interface, a portion of the light is reflected and these reflected signals are registered by the terahertz receiver in a time-resolved manner. The time-resolved echo signals contain the depth information of the sample, from which the layer thicknesses can be determined.

For thin films, the film thickness is determined by comparing the measured and simulated signal waveforms. For the simulation, one needs the material parameters of the coating materials used and the signal course of the incident terahertz pulse. With this knowledge, the thicknesses of the individual layers are varied until an optimum match between measurement and simulation is achieved. Measurement times in the millisecond range per measurement point are possible, since the evaluation of the measurement takes place while the next measurement signal is already being recorded at the same time. The evaluation time for the measurement of up to five layers is thus negligibly short.

The vibrations and oscillations present in many industrial environments are compensated for by specially adapted evaluation software, which thus enables reliable thickness measurements even under adverse conditions.

The terahertz system is of particular interest to manufacturers of automobiles, aircraft and ships. Here, the measurement system can be used for quality control in the final inspection of the painting process. However, due to the large wavelength of the terahertz waves, it is also possible to use the device directly during the painting process for online or inline inspection, since the terahertz waves penetrate the spray mist almost unhindered. Furthermore, the device can be used in all industries where thickness measurement on non-metallic objects is of particular importance. Among these are the manufacturers of foils, bottles and plastic pipes.