12E-5 Fabrication and Properties of High-Q AlN/SiO2 Composite TFBAR´s for above IC Oscillators

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 TFBAR´s. Three different device versions were investigated. The SiO₂ film thicknesses was varied (70 nm, 310 nm and 770 nm) while the piezoelectric A1N film had a constant thickness of 1.2 mum. 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_t ²) 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.