Abstract text (incl. figure legends and references)

There is a great need in industry for wireless sensors working at high temperatures (above 300 °C) to monitor and control high-temperature processes. Surface acoustic wave (SAW) sensors are promising candidates to realize such devices. A SAW sensor consists basically of a piezoelectric substrate on which metallic electrodes with a defined geometry are prepared. Both materials, the substrate as well as the metallic electrodes, have to be high-temperature stable to enable the application at the aimed high temperatures. Up to now, mainly noble metals, such as Pt-based materials, were reported as a high-temperature stable metallization. However, the strong tendency to agglomeration and the high costs are drawbacks. In former work, we studied the Al-based alloy TiAl as a cost-efficient alternative metallization with promising high-temperature stability up to 600 °C for 10 h by analyzing extended films [1,2].

To study the long term high-temperature stability of TiAl electrodes, TiAl-based SAW sensors were annealed at temperatures up to 600 °C for up to 192 h in air. STEM investigations in combination with EDX and EELS measurements of the electrodes were performed to study their morphology and degradation behavior. To evaluate the influence of the limited dimension of the structured electrodes, the much larger contact pads were analyzed as well.

The samples were realized by e-beam evaporation of alternating thin layers of pure Ti and Al with an individual thickness of 10 nm and a total film thickness of 200 nm on piezoelectric Ca₃TaGa₃Si₂O₁₄ (CTGS) substrates. To prevent a chemical reaction between the metallization and the substrate, a 20 nm thick AlNO barrier layer was deposited on top of the substrate prior to the deposition of the metallization. The structured electrodes were obtained using the lift-off technique. Finally, the whole SAW structure was covered with a 40 nm thick AlNO protection layer.

Figure 1 shows STEM images of the electrodes after the annealing in air at 400 and 500 °C for 192 h and at 600 °C for 24 and 192 h. After annealing at 400 °C, an incomplete interdiffusion of the individual metallic layers was observed. EDX analysis revealed a composition of the layers corresponding to the Ti₃Al and TiAl₂ phase. After annealing at 500 °C, a layered structure was still visible, corresponding to the TiAl and TiAl₂ phase. The annealing at 600 °C for 24 h led to a complete interdiffusion and only minor degradation was observed at the edges of the finger. In contrast to this, annealing at 600 °C for 192 h led to a strong oxidation [3].

The results demonstrated that the TiAl-based SAW sensors are promising devices for a long-term application up to at least 500 °C and allow short- and medium-term application at 600 °C in air.

[1] Seifert M, Lattner E, Menzel SB, Oswald S, Gemming T. Materials. 2020;13(9):2039.

[2] Seifert M, Lattner E, Menzel SB, Oswald S, Gemming T. JMRT 2021;12:2383-95.

[3] Seifert M, Leszczynska B, Menzel S, Gemming T. JMRT 2022;19:989-1002.

The authors gratefully acknowledge funding from German BMWI (grant number 03ET1589 A) and DFG (470028346).

Figure 1: STEM images (predominant element contrast) of the electrodes annealed in air at (a) 400 °C for 192 h, (b) 500 °C for 192 h, (c) 600 °C for 24 h, and (d) 600 °C for 192 h