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)_xAl_(1-x)N thin films, where the dopant concentration x is in the range of 0-0.13. The (Mg,Hf)_xAl_(1-x)N 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 d_(33) 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 d_(33) of the (Mg,Hf)_xAl_(1-x)N thin films increases as the concentration x increases. The d_(33) of the (Mg,Hf)_xAl_(1-x)N and our previously reported (Mg,Zr)_xAl_(1-x)N 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 k~2 of fabricated FBARs increased in proportion to the concentration x. These improved k~2 are the direct consequence of the increased piezoelectric constant e_(33) in conjunction with the decreased elastic constant C_(33). These behaviors of the (Mg,Hf)_xAl_(1-x)N are consistent with those of Sc_xAl_(1-x)N, indicating that the Mg-Hf co-doping and Sc doping have the same effect as improving of the piezoelectric properties and k~2 of AlN.