Strategies to determine coating thickness

Louise Smyth

Inline and in-vacuum process control for thin-film analysis during production is an important factor to improve and stabilise product quality and reduce machine set-up and down times. 

Optical methods

A well-known method to perform process control is the determination of colour values (eg, L*a*b*).

For more complex layer designs, colour variations might no longer be suitable layer thickness indicators. Physical method based calculation on processes can be used to determine the layer thicknesses from the measured spectral data.

Chemometric models for thickness determination

An additional field of application is the process control of wet coatings on diffuse materials such as paper.

Neither colour measurement nor physical thickness determination are sufficient to provide stable process metrology indicators. Therefore, a method based on chemometric models was introduced. This uses a set of calibration and validation samples with known coating thicknesses.

After measuring the samples a multivariate data analysis is used to develop a calibration function while the validation samples are used to quantify the methods accuracy (0.5 g/m²) and reproducibility (0.1 g/m²) for a 3.6 g/m² sample.

All described methods are available within the ThinProcess inline metrology system from Zeiss Spectroscopy.