Dopant concentration dependence of electromechanical coupling coefficients of co-doped AlN thin films for BAW devices

摘要

In this paper, we present piezoelectric co-doped (Mg,Hf)xAl1-xN thin films, where the dopant concentration x is in the range of 0-0.13. The (Mg,Hf)xAl1-xN thin films were prepared on Si (100) substrates with a dual magnetron with AC power applied between two ring targets. The compositions of the targets were pure Al and Al-Mg-Hf metals. The composition of the films was controlled by adjusting a DC bias power between the two ring targets. The crystal structures and piezoelectric coefficients d33 of the films were investigated by X-ray diffraction (XRD) and with a piezometer. XRD measurement showed that the increases of concentration x did not affect the c-axis orientation of AlN and induced a decrease of lattice constant ratio c/a. The d33 of the (Mg,Hf)xAl1-xN thin films increases as the concentration x increases. The d33 of the (Mg,Hf)xAl1-xN and our previously reported (Mg,Zr)xAl1-xN have the same linear relationship in respect to the change of the c/a. Furthermore, the film bulk acoustic resonators (FBARs) using these films were fabricated and characterized as a function of the concentration x. The electromechanical coupling coefficients k2 of fabricated FBARs increased in proportion to the concentration x. These improved k2 are the direct consequence of the increased piezoelectric constant e33 in conjunction with the decreased elastic constant C33. These behaviors of the (Mg,Hf)xAl1-xN are consistent with those of ScxAl1-xN, indicating that the Mg-Hf co-doping and Sc doping have the same effect as improving of the piezoelectric properties and k2 of AlN.