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- ItemHigh temperature behavior of rual thin films on piezoelectric CTGS and LGS substrates(Basel : MDPI AG, 2020) Seifert, M.This paper reports on a significant further improvement of the high temperature stability of RuAl thin films (110 nm) on the piezoelectric Ca3TaGa3Si2O14 (CTGS) and La3Ga5SiO14 (LGS) substrates. RuAl thin films with AlN or SiO2 cover layers and barriers to the substrate (each 20 nm), as well as a combination of both were prepared on thermally oxidized Si substrates, which serve as a reference for fundamental studies, and the piezoelectric CTGS, as well as LGS substrates. In somefilms, additional Al layers were added. To study their high temperature stability, the samples were annealed in air and in high vacuum up to 900 °C, and subsequently their cross-sections, phase formation, film chemistry, and electrical resistivity were analyzed. It was shown that on thermally oxidized Si substrates, all films were stable after annealing in air up to 800 °C and in high vacuum up to 900 °C. The high temperature stability of RuAl thin films on CTGS substrates was improved up to 900 °C in high vacuum by the application of a combined AlN/SiO2 barrier layer and up to 800 °C in air using a SiO2 barrier. On LGS, the films were only stable up to 600 °C in air; however, a single SiO2 barrier layer was sufficient to prevent oxidation during annealing at 900 °C in high vacuum.
- ItemDurability of TiAl based surface acoustic wave devices for sensing at intermediate high temperatures(Rio de Janeiro : Elsevier, 2023) Seifert, Marietta; Leszczynska, Barbara; Weser, Robert; Menzel, Siegfried; Gemming, Thomas; Schmidt, HagenTiAl based surface acoustic wave (SAW) devices, which offer a promising cheap and easy to handle wireless sensor solution for intermediate high temperatures up to 600 °C, were prepared and investigated with respect to their durability. To obtain the devices, Ti/Al multilayers were deposited on high-temperature stable piezoelectric catangasite (CTGS) substrates and structured as electrodes via the lift-off technique. AlNO cover layers and barrier layers at the substrate site served as an oxidation protection. The devices were characterized regarding their electrical behavior by ex-situ measurements of their frequency characteristics after heat treatments up to 600 °C in air. In addition, long-term in situ measurements up to 570 °C were performed to analyze a possible drift of the resonant frequency in dependence on the temperature and time. Scanning electron microscopy of the surfaces of the devices and scanning transmission electron microscopy of cross sections of TiAl interdigital transducer electrode fingers and the contact pads were conducted to check the morphology of the electrode metallization and to reveal if degradation or oxidation processes occurred during the heat treatments. The results demonstrated a sufficient high-temperature stability of the TiAl based devices after a first conditioning of system. A linear dependence of the resonant frequency on the temperature of about −37 ppm/K was observed. In summary, the suitability of TiAl based SAW sensors for long-term application at intermediate temperatures was proven.
- ItemXPS chemical state analysis of sputter depth profiling measurements for annealed TiAl-SiO2 and TiAl-W layer stacks(Chichester [u.a.] : Wiley, 2020) Oswald, Steffen; Lattner, Eric; Seifert, MariettaFor the application of surface acoustic wave sensors at high temperatures, both a high-temperature stable piezoelectric substrate and a suitable metallization for the electrodes are needed. Our current attempt is to use TiAl thin films as metallization because this material is also known to be high temperature stable. In this study, Ti/Al multilayers and Ti-Al alloy layers were prepared in combination with an SiO2 cover layer or a W barrier layer at the interface to the substrate (thermally oxidized Si or Ca3TaGa3Si2O14) as an oxidation protection. To form the high-temperature stable γ-TiAl phase and to test the thermal stability of the layer systems, thermal treatments were done in vacuum at several temperatures. We used X-ray photoelectron spectroscopy (XPS) sputter depth-profiling to investigate the film composition and oxidation behavior. In this paper, we demonstrate how the semiautomatic peak fitting can help to extract beside the elemental information also the chemical information from the measured depth profiles. © 2020 The Authors. Surface and Interface Analysis published by John Wiley & Sons Ltd