Thin Film Surface Acoustic Wave Magnetic Field Sensors: Impact of the Thickness of the Magnetostrictive Layer

For the fabrication of surface acoustic wave (SAW) sensors the use of piezoelectric single-crystal substrates (e.g., quartz) is established. Utilizing thin-film technology benefits from larger material flexibility, reduction in chip size, better integrability in standard MEMS and CMOS fabrication technology and thus better scalability. Additionally, it is possible to design multilayer stacks to customise the wave properties to the application purpose. [1] In this context, a piezoelectric AlScN thin film is a promising material, as it combines the advantages from AlN (e.g., high wave velocity, good mechanical and dielectric properties, high thermal conductivity, and high breakdown voltage) with improved electromechanical coupling due to alloying with Sc [2]. AlScN can be fabricated with standard semiconductor technology at reasonable cost on 200 mm silicon wafers.<br/>By combining a SAW sensor with a magnetostrictive film, a magnetic field sensor can be realized that is based on the change of the Young’s modulus (E effect) of magnetostrictive films in the presence of an external magnetic field that can be detected as a phase change of the SAW. [1] The SAW is generated by interdigital transducers on top of the piezoelectric AlScN thin film. By the application of a CMP polished SiO₂ layer on top of the AlScN, the magnetostrictive film quality is improved by reducing the roughness of the surface. The SiO₂ layer also acts as a guiding layer and is used to excite new surface confined SAW modes. High sensitivities of the SAW magnetic field sensor can be obtained by using magnetically soft magnetostrictive films with high magnetostriction like FeCoSiB or FeCoB. [3]<br/>Different layer properties of the sensor stack can be varied (e.g., thickness) and different SAW modes aside from the fundamental Rayleigh mode (e.g., Sezawa mode or shear modes) can be excited in layered thin film structures. Crucial is the thickness of the magnetostrictive FeCoSiB film as it was shown that the sensitivity of the sensor is increasing with the magnetostrictive film thickness [4]. In contrast to the use of bulk piezoelectric substrates like quartz, a post annealing step to induce a uniaxial magnetic anisotropy is possible and the deposition temperature can be elevated to improve the magnetic anisotropy alignment. The influence on the phase shift induced by the applied magnetic field, on the corresponding sensitivity and especially on the frequency dependent noise behaviour of the sensor is investigated and the influence on the different SAW modes compared.<br/>The SAW sensors can be used to measure weak fields at low frequencies as required for biomagnetic applications [5] but is also well suited for current sensing and control applications, such as power electronics [6], benefitting from the large dynamic range (up to 8 orders of magnitude), high bandwidth (&gt; 1 MHz) and the possibility of an isolated measurement. [6]<br/><br/>[1] Meyer, J. M. et al. “Thin-Film-Based SAW Magnetic Field Sensors”. Sensors, 21(24), 8166, 2021.<br/>[2] Fichtner, S. et al. “Identifying and overcoming the interface originating c-axis instability in highly Sc enhanced AlN for piezo-electric micro-electromechanical systems”. J. Appl. Phys., 122, 035301, 2017.<br/>[3] Schell, V. et al. “Exchange biased surface acoustic wave magnetic field sensors”. Sci Rep 13, 8446 2023.<br/>[4] Kittmann, A., et al. “Sensitivity and noise analysis of SAW magnetic field sensors with varied magnetostrictive layer thicknesses”. Sensors and Actuators A: Physical, 311, 111998, 2020.<br/>[5] Kittmann, A.; et al. “Wide band low noise love wave magnetic field sensor system”. Scientific reports, 8 (1), 278, 2018.<br/>[6] Moench, S.; Meyer, J. M. et al. “AlScN-Based SAW Magnetic Field Sensor for Isolated Closed-Loop Hysteretic Current Control of Switched-Mode Power Converters”. IEEE Sensors Letters, 6 (10), 1-4, 2022.