Effects of bottom electrodes on the orientation of AlN films and the frequency responses of resonators in AlN-based FBARs

Abstract

Effects of bottom electrode materials, included in film bulk acoustic resonators (FBARs), on the orientation of piezoelectric AlN thin films and the frequency response characteristics of AlN-resonators are investigated. AlN films are deposited using RF reactive sputtering on various bottom metals, such as Al, Cu, Ti and Mo. Measurements of X-ray diffraction, field-emission scanning electron microscopy and atomic force microscopy show that the AlN film deposited on the Mo electrode exhibits highly desirable properties, namely, a texture coefficient value of ∼93%, crystallite size of ∼40 nm and surface roughness of ∼8.5 nm. The AlN film deposited on the Mo electrode reveals a relatively dense and well-textured columnar structure with fairly uniform grains, while the films deposited on the other electrode metals exhibit a granular type of structure with mixed small and large grains. FBAR devices employing an Al (top)/AlN/metal (bottom)/Si configuration were also fabricated and their frequency response characteristics ( S 11) were measured. The resonator using the Mo electrode was found to have a superior performance (average return loss ⩾10 dB at 3 GHz). It is concluded that the positive role of the Mo electrode in achieving the high-quality AlN films and the high-performance FBAR devices may be attributed to the smaller lattice mismatch as well as the similarity of the thermal expansion coefficient between the deposited AlN film and the Mo electrode substrate.