||In this study, a two-step deposition method using RF|
reactive magnetron sputtering is proposed to obtain high quality Piezoelectric ZnO thin films for film bulk acoustic resonators (FBARs). The titanium (Ti) seeding layer and platinum (Pt) bottom electrode were deposited by a dual gun DC sputtering system. The properties of thin film are investigated using the scanning electron
microscope (SEM), Atomic force microscope (AFM) and the
four-point probe method. The results show that the Pt bottom electrode deposited on the Ti seeding layer has lots of favorable characteristics, such as the crystallite size smaller than 10 nm, a surface roughness of 0.69 nm and a sheet resistance of 0.26 Ω/□. The bottom electrode with the low resistance and the low surface roughness contribute markedly to the performances of the FBAR device.
The Ttwo-step deposition method was adopted for ZnO thin
film deposition, The 1st step deposition is focused on lowering the surface roughness of ZnO seeding layer films. The c-axis preferred orientation of ZnO film This is accompanied by the 2nd step deposition where the sputtering parameters O2 concentration arewere controlled to enhance the c-axis preferred growth of ZnO films. The AFM image shows that the surface roughness of the ZnO film is drastically reduced. It is also observed by monitoring the XRD spectra that the films deposited by two-step method reveal the high c-axis preferred orientation.
Finally, the frequency response of ZnO-FBAR device using the two-step sputtering method shows the excellent performance at center frequency of 1.804GHz with the return loss of nearly 60dB and the electro-mechanical coupling coefficient of 3.2%.