AlScN Thin Film Deposition on Quartz for Power Efficient Tuning Forks

This thesis explores the deposition of aluminium scandium nitride (AlScN) thin films on quartz substrates for tuning forks. The study is motivated by the superior piezoelectric properties of AlScN compared to tradition quartz tuning forks. Specifically, this research aims to take advantage of the high electromechanical coupling that AlScN demonstrates to improve the power efficiency of quartz tuning forks. The research methodology involves both analytical modeling and finite element simulations to compare the performance of AlN and AlScN as piezoelectric actuators for quartz tuning forks. The analytical model simplifies the tuning fork to a clamped cantilever and calculates the static and motional capacitances as functions of electrode size. In the model, AlScN shows superior electromechanical coupling. These initial findings are validated through finite element simulations, which further refine the design parameters for optimal performance. The highest figure of merit obtained in the simulation exceeds 22. Fabrication of the tuning forks requires the successful deposition of high-quality AlScN films on quartz. Initial experiments on 4-inch quartz wafers test various deposition parameters. These depositions focus on minimizing filmstress and abnormal grain growth while maximizing film quality. These qualities were measured through bow measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The optimal deposition conditions identified consist of a sputtering power of 1000W, a power bias of 1Wand gas flow rates of 30sccmfor N2 and 20sccm for Ar. These deposition conditions resulted in 30% less stress while maintaining good film quality. The optimized conditions are then used and successfully deposited on custom 127 μm thick quartz chips provided byMicro Crystal AG. Electromechanical coupling is characterized using a high-tone bulk acoustic resonator (HBAR) setup, resulting in effective coupling coefficients up to 25%, confirming that AlScN can be deposited on quartz. Finally, a detailed design for the quartz tuning forks is developed, incorporating the optimal dimensions derived from the finite element simulations. Preliminary fabrication trials are conducted, highlighting challenges such as over-etching during the patterning process. These trials emphasize the need for further optimization in the etching process. In conclusion, this thesis establishes the potential of AlScN thin films to significantly improve the performance of quartz tuning forks, potentially presenting promising applications in MEMS/NEMS devices. The findings provide groundwork for future research to refine the deposition and fabrication process of AlScN actuated quartz tuning forks.