Fabrication of wafer-level Al_1-xSc_xN stack layer for SMR-BAW application

Aluminium nitride (AlN), compared with other piezoelectric materials, has the advantages of high acoustic velocity, low loss and high temperature stability. Meanwhile, it is CMOS compatible, which can be a promising candidate for preparing high-performance piezoelectric acoustic devices. However, AlN thin film suffered from low piezoelectric coefficient and electromechanical coupling coefficient，which was not conducive to the application of high frequency filter. To solve this problem, AlN doped with Sc was proved an effective scheme for significantly increasing its piezoelectric coefficient (d₃₃) and electromechanical coupling coefficient (K_t²).

This report focuses on the key technologies of fabrication of wafer-level Al_1-xSc_xN stack layer for SMR-BAW application. The 8-inch Al_1-xSc_xN ( x ranges from 9.9% to 20.5%) film was sputtered on Si-based stack layers, consisted of SiO₂/W bragger reflector and AlN seed layer. With the increase of Sc doping content, the growth of Al_1-xSc_xN stack will be faced with the risk of crystallization degradation, stress loss and worse roughness. By employing cluster PVD equipped with tailor-made parts and stress uniformity control, we designed, fabricated and characterized high-quality wafer-level Al_1-xSc_xN stack layer which is satisfied with 5G filter applications.