Tokihiro Nishihara

Development of low-loss band-pass filters using SAW resonators for portable telephones

A bandpass filter using surface acoustic wave (SAW) resonators in a ladder circuit structure for portable telephone systems is reported. For filter design, a simulation tool is used to consider the effects of electrodes (their apertures, the number of paired elements, thickness, and bulk wave radiation). Filter input and output impedance conditions are designed by the resonator capacitance to match the line impedance. The insertion loss and stopband rejection values are traded off against each other and are controlled by the ratio of capacitances between the parallel arm and series arm resonators, 800-MHz and 1.9-GHz band filters mounted in 3.8 mm*3.8-mm*1.5-mm ceramic packages were fabricated. They are suitable for surface mount technology (SMT). Typical frequency response characteristics are 2.5-dB maximum insertion loss in the 25-MHz passband and a stopband rejection of 25 dB.<<ETX>>

Development of Piezoelectric Thin Film Resonator and Its Impact on Future Wireless Communication Systems

The bulk acoustic wave filter composed of piezoelectric thin film resonators has many features superior to those of other small filters such as a surface acoustic wave (SAW) filter and a ceramic filter. As it has no fine structure in its electrode design, it has a high Q factor that leads to low-loss and sharp-cut off characteristics and a high power durability particularly in the high-frequency range. Furthermore, it has the potentiality of integrated devices on a Si substrate. In this paper, we review the recent developments of piezoelectric thin film resonator filters in the world, including our development for mobile communication applications. After describing the feature and history of the piezoelectric thin film resonator filters, our technologies are introduced in focusing on the resonator structures, the piezoelectric thin film and electrode film materials, the cavity structures, the filter structure and its design rules and characteristics, comparing with SAW filters. The competition and coexistence between the piezoelectric thin film resonator filters and the SAW filters are also described. In this paper, we describe the development of a piezoelectric thin film resonator from the standpoint of researchers who have a long experience of SAW filter development.

High performance and miniature thin film bulk acoustic wave filters for 5 GHz

Fujitsu has developed high-performance, miniature aluminum nitride (AlN) thin Film Bulk Acoustic Resonators (FBAR) filters for 5-GHz Wireless Local Area Network (WLAN) applications. Filters with higher frequencies of around 5 GHz and wider bandwidths will be needed in future mobile communication systems. Therefore, we developed filters using thin-film Bulk Acoustic Wave (BAW) technology, which shows promise as a possible solution to these higher frequency applications. There are two issues related to the development of FBAR filters. The first has been the need for miniaturization. To achieve this, we developed a new resonator configuration that employs bulk micromachining techniques. The second issue has been to increase the bandwidth. Five-GHz WLAN applications require a 200-MHz bandwidth at 5250 MHz, which corresponds to a fractional bandwidth of 3.8%. However, the piezoelectric coupling coefficient (k/sup 2/) of AlN cannot satisfy this requirement. To achieve wider filter bandwidth, we developed two approaches. They utilize epitaxial AlN film-growth and reactance-controlled flip-chip package design technologies. The packaging technology also enabled us to miniaturize the filters.

New electrode material for low-loss and high-Q FBAR filters

We have developed a new electrode material, suitable for AlN-based FBARs. The necessary conditions for FBAR electrode materials are low density, low resistivity, and high acoustic impedance. We investigated the influence of electrode materials on the resonant characteristics of FBAR in FEM simulations and experiments. We found the properties of ruthenium (Ru) to be suitable for use as electrode materials for AlN-based FBARs. In addition, by smoothing the surface of the electrodes, full width at half-maximum (FWHM) of the X-ray rocking curves of the AlN films is found to be 2.9/spl deg/ on Ru electrodes. The resonant characteristics of FBAR using Ru electrodes were an unloaded resonant quality factor, Q/sub r/, of 3585 and an effective electromechanical coupling coefficient, k/sup 2/, of 6.6% at 5 GHz. Furthermore, we applied these resonators to ladder-type FBAR filters for 5 and 2 GHz. As a result, we achieved low-loss and high-Q FBAR filters with Ru electrodes.

Development of an X-Band Filter Using Air-Gap-Type Film Bulk Acoustic Resonators

We have developed an X-band filter utilizing air-gap-type film bulk acoustic resonators (FBARs). The air-gap structure is simple and cost-effective. Results from both simulations and experiments demonstrate that a dome-shaped air gap was formed between the substrate surface and the bottom electrode and that an air-gap-type FBAR structure was possible. The air gap can be formed on the flat substrate using stress control of piezoelectric and metal films without using a thick sacrificial layer. As a result, the fabricated X-band FBAR operated successfully with a keff2 of 6.30%, a resonance Q of 246, and an antiresonance Q of 462. The fabricated filter had a center frequency of 9.07 GHz, a fractional bandwidth of 3.1% and a minimum insertion loss of 1.7 dB.

A circuit model for nonlinear simulation of radio-frequency filters using bulk acoustic wave resonators

This paper describes a circuit model for the analysis of nonlinearity in the filters based on radio- frequency (RF) bulk acoustic wave (BAW) resonators. The nonlinear output is expressed by a current source connected parallel to the linear resonator. Amplitude of the nonlinear current source is programmed proportional to the product of linear currents flowing in the resonator. Thus, the nonlinear analysis is performed by the common linear analysis, even for complex device structures. The analysis is applied to a ladder-type RF BAW filter, and frequency dependence of the nonlinear output is discussed. Furthermore, this analysis is verified through comparison with experiments.

High-Q Resonators using FBAR/SAW Technology and their Applications

This paper describes FBAR and SAW resonators which have high-Q factor using unique technology, and their applications. A new electrode material suitable for AIN-based FBARs have been developed in order to realize high-Q factor for low loss. The necessary conditions for FBAR electrode materials are low resistivity and high acoustic impedance. It is found the properties of ruthenium (Ru) to be suitable for use as electrode materials for AIN-based FBARs. The resonant characteristics of FBAR using Ru electrodes indicates Q r of 1200 and an effective electromechanical coupling coefficient k 2 of 6.7% at 2 GHz. On the contrary, SAW resonator with narrow finger electrode, which can suppress transverse leakage of SAW, shows Q r of 670 and k 2 of 7.1%. Furthermore, we applied these resonators to ladder-type FBAR filter for 2-GHz WCDMA and SAW duplexer for US-PCS, respectively, and achieved low-loss and step cut-off filters as a result.

SAW duplexer metallizations for high power durability

The improvement on power durability of SAW filters in order to realize a SAW duplexer is described in this study on IDT metallization. Compared with the conventional single layer metallizations such as Al-Cu, the laminated films are found to show the higher power durability. Many kinds of laminated film were tested changing the material of the middle layer for three layered films of which the top and bottom layers were made of Al-Cu or pure Al. Among them, two types of films such as Al-Cu/Cu/Al-Cu and Al/Mg/Al showed the best power durability. Al-Cu/Cu/Al-Cu had a lifetime 250 times longer than that of current Al-Cu single film. Al/Mg/Al further had a longer lifetime than Al-Cu/Cu/Al-Cu. These three layered films are being successfully applied to SAW antenna duplexers of cellular phones, indicating sufficient power durability for practical use.

A design of antenna duplexer using ladder type SAW filters

This paper describes a design technology of a small antenna duplexer, especially package conditions, by using ladder type SAW filters. This requires not only optimizing the filter design but also the electrode materials and the package design which has matching circuitry for the duplexer configuration. The matching circuitry was incorporated into a microstrip structural pattern which is designed onto the surface of a multilayer ceramic package. To avoid the deterioration of in-band VSWR and reflection coefficient of the stop-band after rotating phase caused by the matching pattern, the characteristic impedance of the matching pattern was designed to be 1.06 times Zo. To obtain the high stop-band attenuation and Tx-Rx isolation characteristics, the package parasitic inductance was used by the multilayer structure design of the package.

L and S band low-loss filters using SAW resonators

This paper describes L and S band low-loss filters using one-port SAW resonators in a ladder circuit structure. Four kinds of filters were developed. Three types have a wide frequency band, with fractional bandwidth of about 4%. A 36° y-x LiTaO3 substrate was used for these. The last type of filter has a narrow frequency band, with 0.03% of fractional bandwidth, and used an ST-cut quartz substrate. To design these ladder type SAW filters, several important factors were considered, the static capacitance of IDT, the period of IDT and film thickness. For fabrication, i-line stepper and reactive ion etching was used to delineate fine IDT patterns with line and gap width of 0.4 to 0.7 μm. Minimum insertion losses of 2 dB were obtained for the three wide band filters. The insertion loss of the narrow band filter was 6 dB. The input and output impedance of these filters were 50 Ω. The filters were mounted in 3 mm×3 mm×1 mm or 3.8 mm×3.8 mm×1.5 mm SMT packages