Terahertz measuring method

Measuring organic and dielectric single and multilayer systems.

The terahertz measuring method enables the coating thickness measurement as well as the material analysis of a wide range of organic and dielectric materials. Up to seven layers can be penetrated with terahertz waves – contactless, completely non-destructive and non-ionizing.

This is how terahertz measuring works.

There are many techniques that use electromagnetic waves in the terahertz frequency range. One such technique is the Terahertz Time Domain Spectroscopy (TDS) – an established method for material analysis that uses extremely short pulsed terahertz waves in a broad frequency range from 0.1 to 6 terahertz. When the terahertz waves hit a non-conductive or weakly conductive material, they penetrate it and are partially reflected. If the materials are applied as multilayer systems on a base material, such as lacquers on car bodies or foils on carrier materials, the terahertz waves are partially reflected at the interfaces of the individual layers.

These reflected "echo" pulses are detected with specific time differences, which allows measuring the transit time of the reflected signal. This allows the distances between the interfaces – i.e. the thickness of each layer – to be determined very precisely and without contact. Thus, Terahertz Time Domain Spectroscopy can detect the thickness of each layer in a multilayer system separately in one measuring.

Other parameters such as homogeneity, porosity, conductivity and mobility of free charge carriers (2DEG) can also be determined. The properties of the substrate have no influence on the measurement. In addition, this technique screens out incoherent radiation caused, for example, by room temperature or ambient light.

Schematic representation of the basic principle of terahertz time domain spectroscopy

This schematic diagram illustrates the basic principle of Terahertz Time Domain Spectroscopy.

  • Ultra-short pulsed terahertz waves strike the material under investigation, penetrate it and are partially reflected at the interfaces of the layers.
  • The different reflections are detected at different times, which provides information about the distances and thus about the layer thicknesses.

Terahertz measurement technology is characterized by its high precision: Layer thicknesses of > 10 μm can be determined on a measuring surface of less than 2 mm. Compared to the magnetic induction method, which have a similar resolution, terahertz measuring offers 10 times better repeatability with 1 ‰.

Since organic or dielectric layers, such as lacquers or paints, are at least partially transparent to terahertz waves, they do not affect the materials. Measuring is completely non-destructive. In contrast to magnetic induction and ultrasonic measuring, the terahertz method works completely contactless with several centimeters working distance. Therefore, even moist and soft layers can be measured without any problems.

Terahertz waves are in the far-infrared range, that means they have less energy than visible light or X-rays. Therefore they are non-ionizing and harmless. Terahertz instruments can be operated openly and do not require radiation protection.

Where is this process used?

The terahertz technology can be used for numerous applications in many different industries, for example in sectors such as automotive, semiconductor manufacturing and wafer testing, battery manufacturing, multilayer plastic extrusion/lamination, aerospace, fuel cells, photovoltaics, chemical, paints and lacquers, pharmaceutical, medical and many more.

  • Coating thickness measurement of organic and dielectric (non-conductive or low conductive) single and multilayer systems on plastic or metal substrates
  • Coating thickness measurement of dry and wet, hard and soft, smooth or rough coatings
  • Non-contact conductivity measurement (such as solar cells, wafer 2DEG, graphene)
  • Quality control and non-destructive testing (NDT), imaging through the material, detection of hidden defects and inclusions, spectroscopic measuring
  • Material characterization and development
  • Measuring of radar relevant properties, such as radar transmission and radar reflection

What factors can influence the measurement?

In terahertz measurement technology, several factors can affect the accuracy and reliability of the results.

  • Application on various materials and surfaces

      Different materials have different absorption and reflection properties in the terahertz frequency range. The composition, density, conductivity, surface roughness and transparency of the material can affect the measurement with terahertz waves. Furthermore, the surface shape can have an impact on measuring. Therefore, specific material properties should be taken into account.

  • Movement of the object to be measured

      If the object to be measured is on a moving carrier such as an assembly line, the measuring can be influenced by the speed of the assembly line. The movement of the object causes the terahertz waves to penetrate different areas of the object during the measuring. This can lead to blurring or distortion of the measurement data. Our innovative intrinsic vibration compensation minimizes this influence.

  • Influences due to the temperature

      Terahertz waves are absorbed and scattered by matter, and the absorption and scattering properties depend on temperature. If the object to be measured does not have a unifrom temperature, this can lead to changes in the intensity and distribution of the reflection signals that are detected. To achieve accurate measurement results, the temperature of the object to be measured should be taken into account and corrected if necessary.

  • Air quality

      Moisture, particles, dust and contaminants in the air can absorb or scatter the terahertz waves, resulting in signal loss and distortion. Especially in environments with high air pollution, such as industrial areas, these effects can be amplified. Helmut Fischer's Clean-Trace technology guarantees stable and reproducible measurement conditions and avoids blurring in the measurement results.

  • Layer thickness

      The total layer thickness of the measurement object is also decisive for the measurement. If the material is too thick, the terahertz waves cannot penetrate it completely. As a result, the reflected signals can no longer be detected and the measurement is incomplete.

      At the same time, it is challenging to measure very thin layers of a few µm. The reflected terahertz signals from very thin layers have small time intervals. To distinguish these "echoes" from each other, the measurement system needs a high temporal resolution. This in turn requires a high bandwidth, as required by the Nyquist-Shannon sampling theorem. However, this bandwidth cannot be infinitely high.

      In addition, the echoes can be overlaid by background noise, which can affect the measurement accuracy. Therefore, it must be investigated in each individual case where the individual upper and lower limits lie in the layer thickness analysis.

Which standard is applied here?

Non-conductive coatings – Non-destructive measurement of coating thickness – Terahertz time-domain measuring method according to DIN 50996
Terahertz systems – Terminology according to VDI/VDE 5590 Sheet 1
Terahertz systems – Time-domain spectrometers (TDS systems) according to VDI/VDE 5590 Sheet 2