Highly X-axis-oriented tantalum pentoxide (Ta2O5) piezoelectric thin films were deposited on a SiO2 substrate using an RF-magnetron sputtering system with a metal tantalum target and an O2-radical source. The degree of orientation, Rayleigh-type surface acoustic wave properties, and surface morphology were evaluated. The deposition condition with the substrate temperature T S of 700 °C and O2 flow rate of 10 ccm was found to be optimum for obtaining a strongly piezoelectric property. Under the optimum condition, the coupling factor of the oriented Ta2O5 thin film with a normalized thickness h/λ of 0.21 was determined to be 0.88% and was 75% of the reported value. The diffraction angle of the preferential peak under the optimum condition was equal to that of the (200)-plane spacing d (200) in the unit cell of monoclinic Ta2O5. A larger plane spacing with Δd/d (200)=2.9% exists preferentially at T S=600 °C, and the piezoelectricity is considered to be zero or very weak. When T S was higher than 700 °C and the O2 flow rate was more than 8 ccm, a smooth surface with the rms roughness of approximately 8–10 nm was obtained. A correlation was found in which a strongly piezoelectric property was obtained when the thin film had a smooth surface.

Highly X-axis-oriented tantalum pentoxide (Ta 2 O 5 ) piezoelectric thin films were deposited on a SiO 2 substrate using an RF-magnetron sputtering system with a metal tantalum target and an O 2 -radical source. The degree of orientation and the Rayleigh-type surface acoustic wave properties were evaluated. It was found that supplying the RF power to the O 2radical source markedly enhanced the orientation of the film, increased the coupling factor, and reduced the surface roughness. When the substrate temperature was 700˚C and the Ar atmosphere gas flow rates for the two cathodes with/without the target and that of the O 2 -radical source were 30, 3, and 10 ccm, respectively, the coupling factor of the oriented Ta 2 O 5 thin film with a normalized thickness h/λ of 0.135 was measured to be 0.23%.

X-axis-oriented tantalum pentoxide (Ta 2 O 5 ) piezoelectric thin films were deposited on Si substrates using an RF magnetron sputtering system with an LTS cathode and an O 2 -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 K 2 for the Rayleigh-type SAW were evaluated. It was found that the Ta 2 O 5 thin film deposited on the unprocessed Si(100) substrate has a similar piezoelectric property as compared with the Ta 2 O 5 thin film deposited on the SiO 2 substrate. However, the K 2 of the Ta 2 O 5 thin film deposited on a silicon oxide film formed on the Si(100) substrate was smaller than that of the Ta 2 O 5 thin film deposited on the unprocessed Si because the (200) plane spacing slightly increased. Then, a process in which the Ta 2 O 5 thin film itself was used as an etch stop layer was adopted and an FBAR with a Ta 2 O 5 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 t 2 and the admittance ratio were measured to be 7.0% and 3.0 dB, respectively.Keywords-piezoelectric thin film; X-axis-oriented tantalum pentoxide thin film; RF magnetron sputtering, FBAR