Inline quality control of nanometer-thick oxide coatings on polymer substrates using quantum cascade laser spectroscopy

Nanometer-thick oxide coatings on polymers improve barrier performance against oxygen and water vapor, enabling closed-loop recycling for food or pharmaceutical packaging, because this allows monomaterials to be used instead of complex plastic composites. Non-destructive, inline thickness monitoring of the coatings on webs or parts is essential for industrial quality control but challenging during motion. The infrared absorption properties of oxides can be used to determine layer thickness. However, for very thin coatings, the resulting signal is weak, making measurements on moving objects, especially thin polymer films, challenging. In this contribution, we present quantum cascade laser (QCL)-based spectroscopy and QCL-based ellipsometry in the infrared spectral range on silicon oxide (SiOx) layers and show that spectroscopy in a transflection arrangement meets requirements for inline inspection. For comparison, ellipsometry is employed as a well-established method that enables thickness measurements with an accuracy of ±1 nm. While it provides excellent precision under controlled conditions, its performance can be affected by variations in the angle of incidence, for example, due to surface unevenness. Therefore, we introduce an infrared transflection method that is more robust, using a micro-opto-electro-mechanical external-cavity quantum cascade laser (MOEMS EC-QCL) with a spectral scan rate up to 2 kHz and a spectral range of 970 cm-1 to 1300 cm-1 (7.7µm to 10.3µm wavelength), covering the entire SiOx phonon absorption band at a spectral resolution of 1.5 cm-1. From the transflection spectra acquired on opaque PP substrates, the thickness of sub-100 nm SiOx coatings can be determined with an accuracy of ±2 nm. Additionally, the spectral information can be utilized to obtain more detailed material information and to remove background effects. We demonstrate that this method even works for the particularly challenging measurement on moving polymers, making it suitable for roll-to-roll applications.