Pulsed-DC sputtering of molybdenum bottom electrode and piezoelectric aluminum nitride films for bulk acoustic resonator applications

Various sputtering conditions were employed to explore the feasibility of depositing a suitably textured layer of molybdenum film, as a bottom-electrode of a film bulk acoustic resonator, on a silicon substrate. A fully (110)-textured Mo film, with its full width at half maximum (FWHM) of rocking curve as low as 1.1°, could be made when a 25-nm thick primer layer of aluminum nitride (AlN) film was pre-deposited between Si and Mo. In turn, the degree of the (0002) texture of a subsequently deposited AlN piezoelectric film, about 1.4 μm in thickness, was found to be largely decided by the degree of the (110) texture of the Mo film beneath it. The residual stress of this AlN piezoelectric film also varied virtually according to the texture quality of the underlying Mo film. The optimal process condition resulted in a piezoelectric AlN thin film having a (0002) FWHM as low as 0.98°, and a slightly compressive residual stress of 439 MPa at the same time.