Epitaxial and Fiber-Textured Stacks of Al0.7Sc0.3N/Mo/AlN Prepared by Magnetron Sputter Epitaxy for Bulk Acoustic Wave Devices: A Comparison using High Overtone Bulk Acoustic Wave Resonator

Emerging 5G telecommunication has increased the demand for filters with high performance and wide bandwidth. Aluminum scandium nitride (AlScN) bulk acoustic wave resonators offer high quality factor (Q) and effective electromechanical coupling coefficient (k2eff), making them a promising candidate for 5G filters, due to their high piezoelectric coefficient (d33) and intrinsic electromechanical coupling coefficient (k2t). This study compares high overtone bulk acoustic wave resonators (HBAR) fabricated using fiber textured and epitaxial stacks of aluminum nitride (AlN), molybdenum (Mo), and AlScN on silicon (Si) substrates prepared by magnetron sputtering. The mosaicity, crystal texture, thickness, and piezoelectric properties of the sputtered films are studied using X‐ray diffraction (XRD), time‐of‐flight secondary ion mass spectrometry, and Berlincourt piezometry. Additionally, the frequency response of the resonators is studied using a vector network analyzer. The comparison revealed that the HBAR fabricated using epitaxial stack of AlScN/Mo/AlN/Si have a higher k2eff and Q than the fiber‐textured stack. Consequently, the figure of merit (k2eff x Q) calculated at ≈4 GHz indicates a 25% improvement in resonator performance. The study also shows that epi‐AlScN (XRD AlScN 0002 w‐full width at half maximum (FWHM) = 1.91°) grown on epitaxial Mo (XRD Mo 110 w‐FWHM = 0.63°) has superior crystalline quality than its fiber‐textured equivalent.