Analysis of Heterostructure\nFormation of Abnormally\nOriented Grains in Sputter-Deposited AlScN Films and Its Impact on\nBulk Acoustic Wave Devices

The presence of abnormally oriented grains (AOGs) in\nsputter-deposited\naluminum scandium nitride (AlScN) films significantly degrades their\nphysical properties, compromising the performance of bulk acoustic\nwave (BAW) devices. This study utilizes first-principles calculations\nto reveal that in tetrahedral wurtzite AlScN film-doped Sc atoms tend\nto aggregate at the second nearest-neighbor positions, forming dense\nScN octahedral structures. The rock-salt (RS) ScN continued to grow\ndue to further Sc aggregation. However, due to inadequate scandium\nflux, embryonic RS structures cannot be sustained, resulting in the\nnucleation of AOGs at the (111) faces of the octahedral ScN structure.\nElectron microscopy studies indicated that AOGs possess wurtzite structures\nand originate at tilted grain boundaries. These boundaries were characterized\nas RS ScN with more Sc atoms. This corroborated the theoretical predictions.\nBAW resonators and filters fabricated from sputter-deposited AlScN\nfilms demonstrate that AOGs degraded the piezoelectricity of AlScN,\nreducing the resonator’s electromechanical coupling coefficient\n(<i>K</i><sub>eff</sub><sup>2</sup>). Measurements showed that AOG density increased from edge\nto center of the 8 in. wafer, resulting in a 3% decrease in average <i>K</i><sub>eff</sub><sup>2</sup> in the resonators and a 137 MHz decrease in the filter bandwidth\nat 5 dB.