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

了解薄膜在高温下的热机械行为对材料科学具有重要意义，也是提高现代高温声表面波(SAW)器件工作温度、可靠性和寿命的重要方面。对于叉指换能器的高熔点体心立方金属电极，特别是350℃以上高温应用的压电基板，缺乏基本的工艺知识，因此是声表面波技术面临的关键挑战。本项目的主要目的是研究在Ca3TaGa3Si2O14衬底上溅射沉积的高温稳定的bcc W和Mo金属薄膜的热机械行为。CTGS是一种相对较新的高热稳定性压电材料，适用于新型高温声表面波温度传感器。然而，目前还没有关于CTG上这种金属薄膜的热机械行为的基础性研究，这对于理解这种SAW传感器在高温范围内的工作或退化行为是必不可少的。为此，我们将利用激光曲率测量方法(弯曲束)来研究薄膜-衬底复合材料的薄膜应力演化和应力松弛，在薄膜生长过程中以及在热循环至约700°C的沉积过程之后，将计划详细研究微结构演变及其对薄膜应力的影响，原因是担心在这些平均晶粒度为100 nm的技术适用的纳米晶材料中可能发生的损伤过程。为此，将通过薄膜X射线衍射、扫描和透射电子显微镜、聚焦离子束技术、原子力显微镜等不同的分析方法来证实其微观结构。研究的重点是根据W.D.Nix，E.Arzt，C.V.Thompson，H.Gao等人多年的工作，从根本上了解薄膜-基片复合材料的变形和蠕变过程，并模拟薄膜-基片复合材料中的变形和蠕变过程，这些过程依赖于沉积参数，或者更确切地说，取决于本征应力和温度。

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.