Optical sensor checks plasma barrier coatings

A new sensor makes it possible to test functional plasma barrier coatings on plastic products at production speed. The sensor uses infrared measurement technology to detect thin coatings with a thickness of less than 10 nm to 200 nm inline.

Plasma coatings are applied to plastic packaging, for example, to protect food. Here, a diffusion-tight layer, for example of silicon oxide (SiOx) or aluminum oxide (AlOx), protects products such as coffee or nuts from harmful external influences or loss of aroma. Comparable thin films are also used in completely different areas - for example in pharmaceutical products, in household appliances, in fuel cells, on vehicle parts and in many other industries. They can be used to optimize wettability, adhesion properties or surface chemistry, or to protect against corrosion.

Until now, there has been no way to test the quality of plasma coatings inline and non-destructively. The state of the art is random quality testing using time-consuming laboratory procedures. The innovative sensor developed by Fraunhofer IPM now enables process-integrated 100 percent individual part testing in the production cycle - even for three-dimensionally shaped, complex surfaces such as those typically used for packaging. The Film-Inspect inspection system was developed by the Fraunhofer Institute for Physical Measurement Techniques IPM in cooperation with plasma system specialist Plasma Electronic GmbH.

The researchers use the infrared optical properties of the coatings for quality control: the chemical bond between atoms can be resonantly excited by infrared light of the appropriate wavelength. The coating thickness can be determined from the intensity of the reflected light. The choice of wavelength depends on the coating material and can be configured to suit the specific material. The compact sensor is only approx. 20 × 40 × 80 mm3 in size and can therefore be easily integrated into the production line. Several sensors can be coupled in production processes and communicate with the system control via Profinet and OPC-UA. A USB interface and evaluation software are available for simpler applications with individual sensors.

An array of eight sensors was successfully integrated and tested in a plasma coating system at project partner Plasma Electronic GmbH. In addition to the application shown there in a batch process, Film-Inspect can also be used in continuous processes. This also makes it possible to monitor a roll-to-roll coating system.

The research and development work was carried out as part of the "O-KUBA - Optical process control for ultra-thin barrier layers" project. O-KUBA is funded by the Baden-Württemberg Ministry of Economic Affairs, Labor and Tourism as part of the Invest BW - Innovation II funding program (BW1_1002/02). The duration was from April 1, 2022 - July 31, 2024, the project partners are Fraunhofer IPM and Plasma Electronic GmbH. (OM-11/24)