ABSTRACT
In response to the increased mobile data traffic, there is a growing need for more low-loss RF band filters with steep frequency characteristics, and high-quality ( Q )-factor and low-temperature coefficient of frequency (TCF) resonators are required to achieve this. We previously reported that for a surface acoustic wave (SAW) resonator on a three-layer structure, which is composed of a thin LiTaO3 (LT) plate whose orientation is 50° rotated YX propagation, SiO2 layer, and AlN layer on a Si substrate, a Q-factor several times higher than that of an SAW resonator on a standard 42° rotated YX propagation LiTaO3 (42YX-LT) substrate could be obtained. In this study, we investigated this layer structure and found that a two-layer structure, in which the AlN layer is removed, achieves a high Q -factor. Numerical analyses using a finite element method showed that the acoustic wave energy can be confined to the surface of the two-layer substrate, and the TCF and electromechanical coupling coefficient ( k2 ) were improved by optimizing the thickness of each layer. We fabricated and evaluated prototype one-port resonators with the two-layer structure and the standard 42YX-LT SAW substrate with resonant frequencies from 0.95 to 3.6 GHz. An improvement of the Q-factor of 3 to 4 times compared with that of the resonator with standard 42YX-LT substrate was observed for the two-layer structure, which means that a reduction of complexity of the layer structure could be obtained without performance loss. The two-layer structure was applied to a 2.4-GHz band Wi-Fi filter to achieve high performances such as low-loss, better steepness, and high attenuation.
ABSTRACT
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.
