High-Performance SAW Resonator on New Multilayered Substrate Using LiTaO3 Crystal.

To develop the high-performance filters and duplexers required for recent long-term evolution frequency bands in mobile handsets, a surface acoustic wave (SAW) resonator is needed that has a higher quality (Q) and a lower temperature coefficient of frequency (TCF). To achieve this, the authors focused on acoustic energy confinement in the depth direction for a rotated Y-X LiTaO3 (LT) substrate. Characteristics of multilayered substrates with low-impedance and high-impedance layers under LT layer were studied numerically in terms of acoustic energy distribution, phase velocity, coupling coefficient, and temperature characteristics employing a finite-element method simulation. After several calculations, a novel multilayered structure was developed that uses SiO2 for a low-impedance layer and AlN for a high-impedance layer under the thin LT layer. A one-port resonator using the new substrate was fabricated, and its experimental results showed that the developed resonator had a Bode-Q over 4000 and TCF of -8 ppm/°C, which are four times higher than and one-fifth as small as those of a conventional 4° YX-LT SAW resonator, respectively. By applying this technology, a band 25 duplexer with very narrow duplex gap was successfully developed, which shows extremely low insertion loss, steep cutoff characteristics, and stable temperature characteristics.

Small and low-loss IF SAW filters using zinc oxide film on quartz substrate.

It was previously reported that a Rayleigh wave propagating on a zinc oxide film (ZnO)/ST-cut 35 degrees X propagation quartz substrate structure has the characteristics of an excellent temperature coefficient of frequency (TCF) and a large electromechanical coupling factor k(s). This substrate was applied to various intermediate-frequency (IF) stage filters. During the filter development, it was clarified that a spurious response due to the Love wave was generated. In this study, a new quartz substrate has been developed with a specific cut and propagation angle, that has the same values of the TCF and the coupling factor as the above-mentioned ones. In addition, it does not have the spurious response due to the Love wave. The combination of this specific-cut-angle quartz and ZnO film has been applied to IF filters for wideband code division multiple access (W-CDMA) and narrow-band CDMA (N-CDMA) systems. The insertion losses of their IF filters were 3-5 dB better and their TCF was superior (deltaf/f = 0.37 ppm/degrees C: one-third) compared with the conventional surface acoustic wave (SAW) filters.