Optical monitoring of thin film properties using in-situ measurement of transmittance and reflectance

Having a good reproducibility and uniformity of the coating properties is a mutual challenge for all coating processes. To face this challenge adequately, it is not only necessary to have accurate control of the coating process but also to have the capability to monitor the optical properties of the coating layers during or directly after deposition. Especially in sensitive multi-layer products produced by large area coating technology, small uniformity variations may give rise to a variation in the visual appearance or other deviations from the product requirements. It becomes necessary to monitor the individual layer thicknesses, requiring sensitive and accurate optical measurement techniques that offer nanoscale precision over large areas. This demand for sensitivity and accuracy puts a strain on the limits of existing in-line measurement capability. The objective of this paper is to discuss some of the measurement problems and give practical solutions to improve accuracy and repeatability of in-situ transmittance and reflectance measurements for optical monitoring of thin film properties.