In situ and ex situ stress measurements of YF3 single layer optical coatings deposited by electron beam evaporator

Yttrium fluoride (YF₃) is a material with good potential as a single-layer anti-reflection (AR) coating deposited onto glass substrates. YF₃ is a possible candidate to replace more studied fluorides such as MgF₂ or ThF₄ the latter being radioactive. For thin-film photovoltaic solar cells, depositing such layers could be a cost-effective way of improving the transmission of light to the p–n junction. However, long-term stability of these AR coatings is an important issue to be considered. This paper is concerned with the residual stress of YF₃ single layers deposited onto glass substrates, being a potential failure mechanism. The measured stress values are correlated to the structure of the layers, using ex situ characterization techniques, such as X-ray diffraction (XRD) and atomic force microscopy (AFM), in an attempt to explain the observed changes. Three substrate temperatures were investigated during deposition, namely 25, 115, and 210 °C. Intrinsic stress of up to 197 MPa has been observed in amorphous YF₃ single layers deposited by electron beam evaporation, at ambient temperature. At a substrate temperature of 210 °C, the intrinsic stress decreased to 67 MPa in the YF₃ layer, developing an orthorhombic structure. Adsorptive stress is an important issue encountered in YF₃ layers, directly related to a low packing density. The in situ stress measurements were carried out using a novel optical approach with a laser-fiber system, briefly introduced here. The residual stress values measured with this novel optical system were compared to two ex situ stress measurement techniques. The optical performance of the YF₃ single layers was also assessed using a spectrometer ex situ and in situ by the interferometry capability of the novel in situ monitoring device.