High-Q AlN/SiO2 symmetric composite thin film bulk acoustic wave resonators

High-Q, bulk acoustic wave composite resonators based on a symmetric layer sequence of SiO₂-AlN-SiO₂ sandwiched between electrodes have been developed. Acoustic isolation was achieved by means of deep silicon etching to obtain membrane type thin film bulk acoustic wave resonators (TFBARs). Three different device versions were investigated. The SiO₂ film thicknesses were varied (0 nm, 70 nm, 310 nm, and 770 nm) while the piezoelectric AlN film had a constant thickness of 1.2 μm. The sputter-deposited AlN film grown on the amorphous, sputter-deposited SiO₂ layer exhibited a d_33,f of 4.0 pm/V. Experimental results of quality factors (Q) and coupling coefficients (k_t²) are in agreement with finite element calculations. A Q of 2000 is observed for the first harmonic of the 310 nm oxide devices. The most intense resonance of the 770 nm oxide device is the third harmonic reaching Q factors of 1450. The temperature drift reveals the impact of the SiO₂ layers, which is more pronounced on the first harmonic, reducing the TCF to 4 ppm/K for the 3rd harmonic of the 310 nm oxide devices.