Highly piezoelectric co-doped AlN thin films for bulk acoustic wave resonators

In this paper, the piezoelectric properties of Mg and Zr co-doped AlN (MgZr doped AlN) thin films are reported. MgZr doped AlN thin films were prepared on Si (100) substrates with a radio frequency magnetron reactive cosputtering system. The crystal structures and piezoelectric constants d₃₃ of the films were investigated as a function of their concentrations, which was measured by X-ray diffraction and with a piezometer. The d₃₃ of the MgZr doped AlN at total Mg and Zr concentrations of 35 atomic % was about three times larger than that of pure AlN. The experimental results of the crystal structure and d₃₃ as a function of total Mg and Zr concentrations were in very close agreement with first-principle calculations. Finally, thin film bulk acoustic wave resonators (FBARs) that used MgZr doped AlN as a piezoelectric thin film were fabricated and compared with the AlN based FBAR. As a result, the electromechanical coupling coefficient improved from 7.1 to 8.5% with the Mg and Zr concentration at 13atomic % doped into AlN. The results from this study suggest that the MgZr doped AlN films have potential as a piezoelectric thin film for wide band and high frequency RF applications.