Analysis of Heterostructure Formation of Abnormally Oriented Grains in Sputter-Deposited AlScN Films and Its Impact on Bulk Acoustic Wave Devices

The presence of abnormally oriented grains (AOGs) in sputter-deposited aluminum scandium nitride (AlScN) films significantly degrades their physical properties, compromising the performance of bulk acoustic wave (BAW) devices. This study utilizes first-principles calculations to reveal that in tetrahedral wurtzite AlScN film-doped Sc atoms tend to aggregate at the second nearest-neighbor positions, forming dense ScN octahedral structures. The rock-salt (RS) ScN continued to grow due to further Sc aggregation. However, due to inadequate scandium flux, embryonic RS structures cannot be sustained, resulting in the nucleation of AOGs at the (111) faces of the octahedral ScN structure. Electron microscopy studies indicated that AOGs possess wurtzite structures and originate at tilted grain boundaries. These boundaries were characterized as RS ScN with more Sc atoms. This corroborated the theoretical predictions. BAW resonators and filters fabricated from sputter-deposited AlScN films demonstrate that AOGs degraded the piezoelectricity of AlScN, reducing the resonator’s electromechanical coupling coefficient (K_eff²). Measurements showed that AOG density increased from edge to center of the 8 in. wafer, resulting in a 3% decrease in average K_eff² in the resonators and a 137 MHz decrease in the filter bandwidth at 5 dB.