Stress relaxation and creep analysis of high temperature thin film materials on CTGS substrates

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The understanding of the thermomechanical behaviour of thin films at high temperatures is of fundamental interest in material science and a crucial aspect for improving the operation temperature, reliability and lifetime of modern high-temperature surface acoustic wave (SAW) devices. There is a lack of fundamental knowledge about the processes occurring in high melting BCC metal electrodes for interdigital transducers especially on piezoelectric substrates for high-temperature application above 350 °C and as such is the key challenge faced in SAW technology. The main goal of this project is to study the thermomechanical behaviour of sputter deposited high-temperature-stable metallic thin films of BCC W and Mo on Ca3TaGa3Si2O14-substrates (Catangasite, CTGS). CTGS is a relatively new piezoelectric material of high thermal stability that makes it suitable for innovative high-temperature SAW based temperature sensors. However, there is no fundamental study regarding the thermo-mechanical behaviour of such thin metallic film on CTGS which is essential for understanding the operation or degradation behaviour of such SAW sensors in the high temperature range. For this reason the film stress evolution as well as stress relaxation will be studied on the film-substrate composites by means of a laser-based curvature measuring method (bending-beam) both during the film growth and after the deposition process upon thermal cycling up to a temperature of about 700 °C. A detailed study of the microstructural evolution and its effect on the film stresses is planned owing to concerns regarding damaging processes that can occur in these technologically applicable nanocrystalline materials with average grain size < 100 nm. For this purpose, the microstructure will be corroborated by means of different analytical methods such as thin film X-ray diffraction, scanning and transmission electron microscopy, focussed ion beam technique, atomic force microscopy. The focus of the study is to understand fundamentally and to model the deformation and creep processes in the film-substrate composites in dependence of both deposition parameters or rather intrinsic stress and temperature based on the years of work of W. D. Nix, E. Arzt, C. V. Thompson, H. Gao and others.