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.