Investigation of Hf doping effects on the performance of AlN-based FBAR

Abstract

In this paper, the effects of Hafnium (Hf) doping on the crystal structures and piezoelectric properties of aluminum nitride (AlN) thin films were reported. Aluminum nitride based (Hf_xAl_1-xN) films with different Hf concentrations were deposited on Si (100) substrates by adjusting the RF power to vary the dopant concentration. The optimum doping ratio and process parameters for optimizing the piezoelectric properties of the proposed films were investigated using various material analysis techniques such as X-ray Photoelectron Spectroscopy (XPS), X-ray diffractometer (XRD), Atomic Force Microscope (AFM), and Piezoresponse Force Microscopy (PFM). The results showed that the best properties of the proposed films were obtained with 12.1 at% Hf doping, and the c/a ratio of the material was 1.561, which confirmed the lattice structure transformation from wurtzite to hexagonal structure. The piezoelectric coefficient d₃₃ was 10.34 p.m./V, 220 % higher than the undoped AlN film. The film bulk acoustic resonators (FBARs) using the proposed film were then fabricated. It was found that the electro-mechanical coupling coefficient ($K_{eff}^2$) was increased to 10.43 % using 12.1 at% Hf-doped AlN piezoelectric film, which is an abrupt increase of the undoped AlN FBAR (with an electro-mechanical coupling coefficient of 7.1 %). It demonstrates the significant potential of using proposed films for broadband applications.