The Fabrication of Thin-Film Bulk Acoustic Wave Resonators Employing a ZnO/Si Composite Diaphragm Structure Using Porous Silicon Layer Etching

Thin-film bulk acoustic wave resonators (FBAR's) are used in monolithic microwave integrated circuits (MMIC's) for semiconductor devices. FBAR's are more attractive than surface acoustic wave resonators since they have the advantages of small size, low cost, and mass-production ability. In this letter, an FBAR with an air-gap is fabricated by a surface micromachining technique which utilizes porous silicon layer (PSL) etching. This FBAR has a forward reflection coefficient of -18.912 dB when the thickness of the ZnO thin film measures 1 μm. The FBAR is composed of a piezoelectric zinc oxide (ZnO) thin film and top and bottom electrode thin films of Au(1000 Å)/Ni-Cr(50 Å). The ZnO thin film is deposited by RF magnetron sputtering. This fabrication process is compatible with conventional IC processes, thereby enabling the development of monolithic-integrated FBAR's on Si or GaAs substrates.