Ultra-thin oxide films on polymers enable closed-loop recycling of packaging items. Furthermore, plasma coating of polymer products is a state-of-the-art method for achieving indispensable properties such as prevention of oxygen diffusion. In this context, precise quality control of the coating thickness is essential to ensure the final product properties. Existing systems, based on Fourier transform infrared (FTIR) spectrometers lack compactness and scalability. Therefore, we introduce a new measuring system based on infrared reflection absorption spectroscopy (IRRAS) to enable inline quality control of nanometer-thin oxide films.

Our sensor consists of thermal light sources and pyroelectric detectors with optical filtering, for a non-contact measurement of the intensity of the reflected infrared light at the phononic absorption band of the coating material. The appropriate choice of optical filters and layout, such as the angle of incidence has been determined via FTIR measurements and simulated reflection spectra of oxide-coated substrates. The physical dimensions of our sensor are 40 × 19 × 80 mm³. The measured area has a diameter of 2 mm, and the acquisition time is below one second per measurement. The optical elements inside the sensor are specifically designed for the inspection of widely used silicon oxide coatings on polymer substrates. Our sensor is suited for measuring the film thickness from 5 to 200 nm with 10 nm precision. The measured data show a linear correlation between the coating thickness and the intensity of the reflected light.

Based on this sensor principle, we developed a measuring system consisting of multiple sensors for inline quality control of plastic containers, which can be integrated into an industrial coating facility. Due to its compact size, scalability, and flexibility, our measurement system is suited for inline measurements of plasma coatings even in harsh production environments. In this contribution, we show the potential of this technique both for inline control of batch processes and for continuous in-situ monitoring of roll-to-roll processes under vacuum conditions.