Novel Thin-Film Piezoelectric Materials for Ultrasound Applications

我们的项目探索了一种设计高频平面微声转换器的新方法。这种装置目前用于移动的通信系统中的信号处理或用作微声传感器。似乎已经达到了普通压电单晶（SiO2、LiNbO 3、LiTaO 3、AlN等）的性能极限。我们的项目采用了一种全新的方法来提高当前微声元件的性能。关键元件是由新型压电材料制成的功能薄膜，用于将高频电信号转换为声学信号。我们的创新在于通过外延生长来控制功能材料的性能，以便针对特定应用优化这些性能。这一方面通过以受控的方式有意地使晶格应变来实现，另一方面通过在膜生长期间有目的地引入缺陷来实现。我们的薄膜是由钾钠铌酸盐（K_xNa_{1-x}NbO_3），这允许压电超声转换制成的。的机电系数是可比的铅基压电化合物。然而，当使用K_xNa_{1-x}NbO_3时，不存在可持续性的问题。与目前的分层超声转换器相比，小于100 nm的极薄膜厚度是一个重要的新奇。特别是，这使得能够通过应变和缺陷工程在大范围内定制功能特性。因此，结构材料特性为下一代超声部件的开发提供了新的自由度。该项目建立在合作者在压电K_xNa_{1-x}NbO_3薄膜生长、结构和功能表征以及微声学元件技术开发方面的专业知识的基础上。通过将基础和应用材料科学与系统设计工程相结合，我们能够探索这种新型材料的潜力，并结合创新的设计策略。然而，开发特定设备不是项目目标。该项目的成功实施在于验证了与已知单晶衬底相比，新型材料系统在高工作频率（> 5 GHz）下的优势。

Our project explores a novel approach for designing high frequency planar microacoustic converters. Such devices are currently used for signal processing in mobile communication systems or as microacoustic sensors. It appears as if the performance limits of common piezoelectric single crystals (SiO2, LiNbO3, LiTaO3, AlN, …) have been reached. Our project employs a radically new approach to enhance the performance of the current microacoustic components. A key element is a functional thin film made from a novel piezoelectric material which is used to convert high frequency electrical into acoustic signals. Our innovation consists in controlling the functional material properties through epitaxial growth in order to optimize these properties with respect to a specificapplication. This is achieved on the one hand by intentionally straining the crystal lattice in a controlled way and, on the other hand, by purposefully introducing defects during film growth. Our thin films are fabricated from potassium-sodium-niobate (K_xNa_{1-x}NbO_3), which allows for piezoelectric ultrasound conversion. The electromechanical coefficients are comparable to lead-based piezoelectric compounds. However, there are no concerns about sustainability when using K_xNa_{1-x}NbO_3. The extremely thin film thickness of less than 100 nm is an essential novelty in comparison with current layered ultrasound converters. In particular, this enables tailoring of functional properties over a large range via strain- and defect engineering. Thus, structural material properties provide a new degree of freedom for the development of next generation ultrasound components. This project builds on the expertise of the collaborators for thin film growth of piezoelectric K_xNa_{1-x}NbO_3, its structural and functional characterization and for technology development for microacoustic components. By combining fundamental and applied material science with system design engineering we are able to explore the potential of this novel material in conjunction with innovative design strategies. However, development of specific devices is not a project goal. The successful execution of the project consists rather in the validation of the advantages of the novel material system at high operation frequencies (> 5 GHz) in comparison with known single-crystal substrates.