Inline inspection of coating thickness on polymers using infrared reflection-absorption spectroscopy

Oxide-coated polymers hold promise as packaging materials that allow closed-loop recycling. In this context, accurate quality control of the coating thickness during the production process is essential. State-of-the-art measurement systems lack the compactness and scalability required for widespread applications inside production equipment. With an adapted spectral measuring range achieved through the implementation of optical filters, we apply infrared reflection-absorption spectroscopy (IRRAS) for inline quality control of nanometer-thin oxide coatings on polymers. For the commonly used SiOx coatings with a thickness of 5 to about 200 nm, our sensor principle enables thickness measurements with a precision of better than 5 nm in two seconds. We show data from an industrial plasma coating system equipped with an array of sensors to measure the SiOx coating thickness on parallelly processed cups during production. The results obtained from the simultaneous measurements of the coated cups effectively demonstrate the inline capability of our sensor.