Deposition of highly oriented Ta2O5 piezoelectric thin films on silicon for fabricating film bulk acoustic resonator structure by RF magnetron sputtering

X-axis-oriented tantalum pentoxide (Ta₂O₅) piezoelectric thin films were deposited on Si substrates using an RF magnetron sputtering system with an LTS cathode and an O₂-radical source with the aim of obtaining an FBAR structure. First, to clarify the fabrication condition necessary for obtaining a strongly piezoelectric property, the degree of orientation and the K² for the Rayleigh-type SAW were evaluated. It was found that the Ta₂O₅ thin film deposited on the unprocessed Si(100) substrate has a similar piezoelectric property as compared with the Ta₂O₅ thin film deposited on the SiO₂ substrate. However, the K² of the Ta₂O₅ thin film deposited on a silicon oxide film formed on the Si(100) substrate was smaller than that of the Ta₂O₅ thin film deposited on the unprocessed Si because the (200) plane spacing slightly increased. Then, a process in which the Ta₂O₅ thin film itself was used as an etch stop layer was adopted and an FBAR with a Ta₂O₅ thin film/Si substrate structure was fabricated. The resonance response corresponding to a longitudinal bulk wave was observed at 1.7 GHz for the sample with a film thickness of 1.4 μm. The coupling factor k_t² and the admittance ratio were measured to be 7.0% and 3.0 dB, respectively.