Properties of AlN films grown by two-step deposition and characteristics of AlN-FBAR devices

A two-step deposition method using RF reactive magnetron sputtering is proposed to obtain AlN thin films of high quality for film bulk acoustic resonators (FBARs). The 1^st deposition step is focused on lowering the surface roughness of the AlN films. In the 2^nd deposition step, the RF power and the working pressure are controlled to enhance the c-axis preferred growth of the AlN films. It is seen by monitoring the X-ray diffraction (XRD) spectra that the films deposited by the two-step method reveal an improved c-axis preferred orientation. Atomic force microscopy (AFM) images show that the surface roughness of the films is drastically reduced by adopting two-step deposition. It is also found, from the measurement of the frequency response characteristics of the AlN-FBARs, that the devices based on AlN films prepared using the two-step method reveal better device performance in return loss and electro-mechanical coupling coefficient (k_t²).