Novel Thin-Film Piezoelectric Materials for Ultrasound Applications

• Applicants: Professor Dr.-Ing. Gerhard Fischerauer; Dr. Peter Gaal; Dr. Jutta Schwarzkopf
• Subject Area: Synthesis and Properties of Functional Materials ; Microsystems
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 512808298

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.

• DFG Programme: Research Grants