Motoaki Hara

Application of fluorine doped SiO 2 films for temperature compensated SAW devices

In our paper, we investigates acoustic properties including temperature coefficient of elasticity (TCE) of fluorine doped silicon oxide (SiOF) films and proposes their application to the temperature compensation of radio frequency (RF) surface acoustic wave (SAW) devices. From the Fourier transform infrared spectroscopy (FT-IR), SiOF films were expected to possess good TCE properties. Then we fabricated a series of SAW devices using the SiOF-overlay/Cu-grating/LiNbO3-substrate structure, and their device performances were evaluated. The experiments showed that the temperature coefficient of frequency (TCF) increases with the fluorine content r, as we expected from the FT-IR measurement. This means that the Si-O-Si atomic structure measurable by the FT-IR governs the TCE behavior of SiO2-based films even when the dopant is added. In comparison with the pure SiO2 with the film thickness h of 0.3 wavelengths (λ), TCF was improved by 7.7 ppm/°C without deterioration of the effective electromechanical coupling factor K2 when r=3.8 atomic % and h=0.28λ. The F inclusion did not influence the resonator Q factors obviously when r8.8 atomic%.

A design technique to enhance isolation of duplexer in single-ended and differential modes

This paper describes a design technique to enhance isolation of acoustic duplexers in the single-ended and the differential modes for a so-called “inter-stage SAW-less” radio frequency (RF) front-end of cellular phones. The leakage signal from the Tx to the Rx ports existing inside the duplexer, which degrades the isolation characteristics, is cancelled out by utilizing a proposed circuit topology and by performing careful design of the whole duplexer structure. The principle directly enhances the single-ended isolation, but it also contributes to improve the differential mode isolation. The prototypes for the single-ended and differential duplexers exhibit isolation higher than 70 dB and 65 dB, respectively, in the Tx band for UMTS Band 1 application.

A thin-film bulk acoustic resonator and filter with optimal edge shapes for mass production

The manufacturing conditions of a thin-film bulk acoustic resonator (FBAR) filter were investigated to obtain a high Q factor which is stable for mass production. The FBAR consist of patterned electrodes and piezoelectric films. In this study, the influence of edge shape of the films on the anti-resonance characteristics was investigated using a numerical method. Optimized shape was applied to a 2.5-GHz band resonator and filter. As a result, significant improvement of the Q factor and the insertion loss was confirmed.