Optical characterization of hafnia films deposited by ALD on copper cold-rolled sheets by difference ellipsometry

Hafnium oxide (HfO2) thin films could be potentially used as thermal diffusion barrier coatings for copper absorbers used in solar thermal collectors. Atomic layer deposition (ALD) was used to deposit thin, dense, non-columnar HfO2 films on copper sheets, which are established base materials for solar absorbers. The aim of this work is to find a simple and efficient method for quantitative characterization of thin hafnium oxide films deposited on imperfect rough copper sheets. The difference ellipsometry applied in infrared region was found to be a practical tool for non-destructive characterization of thin films deposited onto metal surfaces even when these surfaces are imperfect. The presented method enables us not only to identify the presence of hafnium oxide but also to perform quantitative characterization, i.e. to determine the thickness of the film. Moreover, a simple method, in which the thickness is determined from the height of the structure in the difference ellipsometric data, is presented. This method is demonstrated on HfO2 film with nominal thickness 60 nm deposited by ALD on copper cold-rolled sheet.