Hidekazu Nakanishi

Recent development of temperature compensated SAW Devices

This paper reviews recent progress of temperature compensated surface acoustic wave (SAW) devices for wireless communications. First, temperature compensation techniques based on the SiO2 deposition and the wafer bonding are explained, and their implementation into real devices are discussed. Finally, we will show how high performances have been realized by the use of these technologies.

Miniature Surface Acoustic Wave Duplexer with Wide Duplex Gap on SiO2/Al/LiNbO3 Structure

In this paper, we describe the development of a miniature surface acoustic wave (SAW) duplexer for a Band IV system in the universal mobile telecommunication system (UMTS). We employed the SiO2/Al electrode/5° YX-LiNbO3 structure, which has a high electromechanical coupling coefficient (K2) and a small temperature coefficient of frequency (TCF). It is necessary to suppress the Rayleigh mode spurious response to apply this structure to the duplexer. We clarified experimentally the relationship between the SiO2 shape and the thickness of the Al electrode in the SiO2/Al/LiNbO3 structure to suppress the spurious response. Furthermore, we determined the condition to reduce the bulk radiation for the transmitting band of the Band IV duplexer. The developed SAW duplexer has the optimum structure for the Band IV system. The SAW duplexer exhibits excellent performance for practical use. In addition, the TCF is about -30 ppm/°C. The SAW duplexer is 2.5×2.0×0.5 mm3. Additionally, the SAW duplexer has sufficient power durability owing to the use of multilayer electrodes of AlMgCu/Ti.

High-Electromechanical-Coupling-Coefficient Surface Acoustic Wave Resonator on Ta2O5/Al/LiNbO3 Structure

In this paper, we describe a high-electromechanical-coupling-coefficient (K2) surface acoustic wave (SAW) resonator on a Ta2O5/Al/LiNbO3 structure for wide duplex gap applications. We analyzed the responses of a SAW resonator on a Ta2O5/Al/LiNbO3 structure by finite element method (FEM)/spectrum domain analysis (SDA). We have clarified that the optimum Ta2O5 thickness at which the high-performance SAW resonator without the Rayleigh-mode spurious response could be realized. The results of the simulation were in good agreement with those of the experiment. Also, the Ta2O5 film has the advantage of decreasing the film thickness over the SiO2 film. The developed SAW resonator shows excellent characteristics, namely, a low insertion loss and a high K2 of 23%. By applying the SAW resonator in the filters, a ladder-type filter and a longitudinally-coupled double-mode SAW (DMS) filter with a low insertion loss and a large bandwidth were realized. We have therefore shown the feasibility of applying devices with a wide duplex gap such as Band I, IV, and X duplexers and a wide pass-band filter using the Ta2O5/Al/5°YX-LiNbO3 structure.

Analysis of Rayleigh-Mode Spurious Response Using Finite Element Method/Spectrum Domain Analysis for Surface Acoustic Wave Resonator on Nonflat SiO2/Al/LiNbO3 Structure

Because of their low insertion loss, high out-of-band rejection, and high power durability, miniature surface acoustic wave (SAW) duplexers are widely used in mobile phones. Substrate materials substantially limit and determine the performance of SAW duplexers; for their applications to Band I and Band IV systems with large pass-band widths and wide frequency separations between the transmitting and receiving frequency bands, a larger coupling coefficient (K2) is of primary importance. We have developed a shape-controlled SiO2 film/Al electrode/LiNbO3 substrate structure for their applications. It could lead to a large K2 and suppression of Rayleigh-mode spurious response. In this paper, we report the analysis using finite element method/spectrum domain analysis (FEM/SDA) for the SAW resonator on a nonflat SiO2 film/Al electrode/LiNbO3 structure. It was clarified that the shape-controlled SiO2 was effective in terms of achieving a large K2 for the SAW resonator with suppressed Rayleigh-mode spurious responses and bulk wave radiation. Furthermore, the experiment results showed a good agreement with the analysis results.

Study of Relationship between Layer Profile of SiO2 and Characteristics of Surface Acoustic Wave Devices in SiO2/Al/LiTaO3 System

In this paper, we describe our study of the relationship between the layer profile of SiO2 and the characteristics of surface acoustic wave (SAW) devices in the SiO2/Al/LiTaO3 system. SiO2 coating is a well-known technology to improve the temperature coefficient of frequency (TCF) of a SAW filter, but the characteristics of the SAW filter deteriorate simultaneously. In this study, we focus on the SiO2 layer profile as one of the causes of this deterioration and try to improve the characteristics of the SAW filter with the SiO2/Al/LiTaO3 structure by changing the SiO2 layer profile. The relationship between the layer profile and the characteristics of SAW resonators has been investigated by finite element method (FEM)/spectrum domain analysis (SDA). To confirm the simulation results, we fabricated several SAW resonators with different profiles of the SiO2 layer by changing deposition conditions. As a result, the simulation results were in good agreement with the experimental results, and the enhanced characteristics of the SAW resonator and filter with a low insertion loss, a steep skirt characteristic, and a low TCF of approximately -15 ppm/K have been realized.

Study of Spurious Response near the Fast Shear Wave in SiO2/Al/LiNbO3 Structure

The shear horizontal (SH) mode on the SiO2/Al/LiNbO3 structure is studied because of its sufficient electromechanical coupling factor (K2) and good temperature coefficient of frequency (TCF). The authors proposed a method of suppressing the spurious response of the Rayleigh mode and the transverse mode for narrow duplex gap applications. For the narrow duplex gap application, the SiO2 thickness must be increased to achieve good TCF characteristics. However, another spurious response appears near the fast shear wave with increasing SiO2 thickness. In this paper, we discuss the suppression mechanism of the spurious response near the fast shear wave.

Reduction of Transverse Surface Acoustic Wave Leakage in Resonator on Al/42° YX-LiTaO3 Substrate for High-Frequency Applications

In this paper, we propose a new structure for reducing the extent of transverse surface acoustic wave (SAW) leakage for the SAW resonator on a 42° YX-LiTaO3 substrate. Such leakage occurs from the interdigital region toward the busbar region in the SAW resonators. The new structure has a Ta2O5 film outside the interdigital region. This structure can make the SAW velocity in the busbar region lower than the velocity in the interdigital region. Therefore, the new structure could reduce the extent of leakage, and contribute to confine the SAW energy in the interdigital region. This structure was applied in SAW resonators and ladder-type SAW filters fabricated on a 42° YX-LiTaO3 substrate. The insertion loss could be improved by suppressing transverse SAW leakage. This technique could be applied to the fabrication of the filters and duplexers using leaky SAW on a 42° YX-LiTaO3 substrate, and the SAW devices could exhibit excellent performance.