ICMCTF2019 Session B1-1-MoM: PVD Coatings and Technologies I

In this work, thin films of CaF₂, MgF₂ and their multilayered stacks have been deposited on microscopy glass substrates by thermal evaporation and their structural, optical and wettability properties have been studied. Four different sets of samples i.e. glass/CaF₂, glass/MgF₂, glass/CaF₂/MgF₂ and glass/CaF₂/MgF₂/CaF₂/MgF₂ were prepared. X-ray diffraction studies revealed that crystalline CaF₂ film grows in glass/CaF₂ and in the 2-layer stacked glass/CaF₂/MgF₂ samples whereas, it grows in the amorphous phase in 4-layered stacked glass/CaF₂/MgF₂/CaF₂/MgF₂ sample. On the other hand, MgF₂ layer in all the samples grows in the amorphous phase. Field emission scanning electron microscopy (FESEM) was used to study the surface morphology and thicknesses of the samples. The surface FESEM image of CaF₂ film shows very small flake-like morphology whereas the MgF₂ film has a smooth morphology due to its amorphous nature. The cross-sectional FESEM images found that the thickness of the pure CaF₂ film (102 nm) is lesser than that of pure MgF₂ film (127 nm). The optical transmittance and reflectance properties were studied by UV-Vis spectroscopy which confirmed that all the films possess good anti-reflecting properties. The average specular reflectance values in the wavelength range: 350-1100 nm are 10.8%, 7.9%, 8.6%, 6.4% and 8.4% for bare glass slide, MgF₂, CaF₂, 2-layer and 4-layer stacked films respectively which confirms that the reflectance decreases with the top coating of the fluoride films. The water contact angle studies were carried out to study the wettability properties of the samples and it is found that the pure CaF₂and MgF₂ films are hydrophobic with an average water contact angle 131±1^o and 98±1^o, respectively. The wettability properties of the 2-layer and 4-layer stacked structures were found to be completely different compared to single layer thin films and showed hydrophilic nature with water contact angles of 20±1^o and 47±1^o respectively with reflectance values that were comparable to those of MgF₂ and CaF₂ films. It is concluded that CaF₂ films have a very good potential to be used as hydrophobic anti-reflecting coatings and stacking with other well known optical material such as MgF₂, can tailor its wettability and anti-reflecting properties.

We reported a high electromechanical coupling coefficient (K²) Surface acoustic wave (SAW) devices on metal / ScAlN/interdigital transducer (IDT)/ LiNbO₃ structure and we used Al/ Ti / Mo as metal layer. The Sc_0.31Al_0.69N films in different thickness (0.5, 1, 1.5, 2 μm ) were deposited on Y-128˚ lithium niobate (LiNbO₃) substrate which possesses high K² by reactive magnetron co-sputtering using Sc and Al as targets. In the previous research, the replacement of Al by Sc increases piezoelectricity because of the phase transition. The Sc_xAl_1-xN films 2D-XRD result showed that high c-axis (002) orientation and 2D-XRD χ angle showed that there is a critical thickness. The (002) plane tilting from a normal direction of LiNbO₃ substrate before 1μm and it growth normal direction after critical thickness. The SEM cross-section result showed that ScAlN films have tilting form substrate and have columnar structures. The SEM top view indicated that spindle-like morphology and grain cover the whole surface when thickness over 1μm. The piezoelectric coefficient (d₃₃) measured the highest value 42.8 pm/V of Sc_0.31Al_0.69N film. The K² values are increasing with the metal layer deposited on. The highest K² value is three time larger than IDT/ LiNbO₃ (4.9%) structure. The metal / ScAlN/ IDT/ LiNbO₃ structure have a great potential in high frequency and high K² SAW devices.