Jan-Peter Urbach, Thomas Schütte, PLASUS GmbH, Mering, Germany
In-situ layer thickness can be determined from the broad band spectral reflectivity of the growing film. While this technique is fairly common in many vacuum coating applications, its accuracy relies heavily on the correct refractive indices of the materials involved.
Typically the refractive indices are determined from ex-situ measurements on witness samples coated in the production process. Such measurements however may deviate from the in-situ values for various reasons. The refractive index may be different because the ambient conditions during the ex-situ measurement (temperature, humidity) differ from the process conditions. Furthermore the process condition may vary from batch to batch, e.g. because of the buildup of coating material on the chamber walls.
A more accurate value for the in-situ refractive index can be obtained by combining the reflectivity analysis with real-time data from plasma monitoring based on broad band emission spectroscopic. Changes in the plasma composition are detected instantly and can be used to adjust the value of the refractive index used for reflectivity modelling. This approach has been implemented in the EMICON system and applied to a variety of typical industrial coating process covering PECVD for solar cell production, nanocrystalline diamond coatings and amorphous carbon films (aC:H) for tribological applications.
In this presentation, application examples are presented that show how intentional as well as unwanted process changes do impact the precision of the film thickness measurement and how this can be avoided by suitable combination of the different in-situ data sources.