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-A1N-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 (70nm, 310mn and 770nm) while the piezoelectric A1N film had a constant thickness of 1.2 mu m. The sputter deposited A1N film grown on the amorphous, sputter deposited SiO2 layer exhibited a d(33.f) of 4.0pm/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 310nm oxide devices. The most intense resonance of the 770nm 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.