Search

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 ( $k^{2}$ ) 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.

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.

In this paper, we study a velocity reduction of incredible high performance (I.H.P.) surface acoustic wave (SAW). First, variations of I.H.P. SAW velocity and fractional bandwidth with thickness of Pt layer in Al/Pt electrode are estimated using a finite element method simulator, and specific conditions where the SAW velocity can be reduced maintaining the fractional bandwidth equivalent to that of the conventional structure are determined. Then some one-port resonators under the determined conditions are fabricated and their velocity, fractional bandwidth, quality factor, and temperature stability are evaluated. As the result, it is indicated that, by using Al/Pt electrode and by adjusting the film thicknesses of a multilayer substrate, the velocity of I.H.P. SAW is successfully reduced while keeping other important characteristics.

In this work, a technique to reduce the acoustic velocity of incredible high-performance surface acoustic wave (I.H.P. SAW) using Al/Pt electrodes keeping other important characteristics such as fractional bandwidth and quality factor is proposed. First, using a finite element method simulator, variations of the acoustic velocity and the fractional bandwidth of the I.H.P. SAW with the Pt electrode thickness, thicknesses of LiTaO3 (LT) and SiO2 layers in a substrate, and the cut angle of a LT layer are estimated. And then, to confirm the validity of the estimation results, several one-port SAW resonators with various thickness combinations of Al, Pt, LT, and SiO2 layers are fabricated and their characteristics are evaluated. The evaluation results show that the acoustic velocity of the I.H.P. SAW is successfully reduced keeping the bandwidth, the quality factor, and the temperature characteristics.

Regarding higher frequency above 3 GHz, up until now BAW filters have been believed to be better solution than SAW filters due to their increase of electrodes resistance. This paper describes a new high performance SAW filter surpassing BAW, with 3.5 GHz center frequency employing I.H.P. SAW technology which is new type SAW structure with high Q factor. Experimental studies for one-port resonators in 3.6 GHz show very high Bode-Q max of 2500 and optimum aperture lengths of IDTs. Using the results, a ladder type filter for LTE Band 42 was designed and fabricated. Very low insertion loss of 1.7 dB max has been achieved in the passband of $\mathbf{3.5+/-0.1 \ GHz}$:. Steeper skirt characteristics and adequate attenuation have been also realized.

We discuss transverse modes in a resonator on a multilayer substrate with a thin piezoelectric plate. A generation mechanism of the transverse mode response in I.H.P. SAW (Incredible High-Performance SAW)resonator with a LiTa03 plate is analyzed by using FEM simulations, and it is shown that a slowness curve of the I.H.P. SAW structure is changed significantly compared with a standard 42° Y-X LiTa03 substrate. A new tilted resonator structure is proposed to suppress the transverse mode spurious. It is found that the tilted resonator with a small tilted angle can suppress the spurious response successfully with maintaining a high-Q factor.

Various characteristics for RF filters in mobile terminals are required corresponding to expansion of mobile terminals. I.H.P. SAW (Incredible High-performance SAW) with high Q and temperature stability has been developed to resolve these challenges. In this paper, its high Q mechanism of the new multi-layered structure (LiTaO 3 / SiO 2 / AlN / Si substrate) has been studied numerically in terms of acoustic energy confinement by using wave propagation analysis in real-space and wavenumber domains. One-port resonators employing I.H.P. SAW were fabricated after optimizing the multi-layered structure. Experimental results show higher Bode-Q values over 6000 at 0.9 GHz to 1900 at 3.5GHz successfully, which are over 3 times higher than those of conventional 42YX-LT SAW resonators. Very small TCF of −8 ppm/°C is also realized, and wider bandwidth and superior heat dissipation performance have been achieved. A new Wi-Fi filter with very narrow adjacent gaps and a new Band 25 / 66 / 30 hexaplexer, one of the most difficult multiplexers, have been developed using I.H.P. SAW technology, which show low insertion loss, steep cut-off, deep attenuation and extremely high isolation characteristics.

High Q and low TCF are elemental characteristics for SAW resonator, in order to realize high performance filters and duplexers for new LTE frequency bands. Authors have focused on the acoustic energy confinement in vicinity of substrate surface. A novel multi-layered structure substrate have realized incredibly high performances such as Q factor of 4000 and TCF of 8 ppm/°C compared with conventional structure.

High Q and low TCF are elemental characteristics for SAW resonator, in order to realize high performance filters and duplexers coping with new LTE frequency bands. Authors focus on the acoustic energy confinement in SAW propagation surface and electrical characteristics have been investigated theoretically and experimentally. A new multi-layered structure substrate have realized incredible high performances such as both three times Q and one fifth TCF compared with conventional structure. Band25 duplexer using the new structure has been developed successfully with low loss and good temperature stability.