Yasuo Ebata

Growth and Properties of Li2B4O7 Single Crystal for SAW Device Applications

Large Li2B4O7 single crystals, up to 45 mm in diameter, have been successfully grown along the X- and Z-axis by Czochralski technique. Fundamental phsyical and chemical properties were determined, and basic SAW properties were studied for the various surface orientations and SAW propagation directions from theoretical and experimental points of view. It was found that there were useful substrate orientations for SAW resonators and narrow bandpass SAW filters around the rotated X-cut, Z-propagation, where SAW propagation delay time was a parabolic function of temperature and SAW coupling factor k2 was approximately 1%. When a SAW propagates along the Z-direction on the 28° rotated X-cut substrate, the temperature coefficient of delay time (TCD) is zero at 20°C and the second order TCD is 0.27 ppm/°C2, k2 is 0.8% and SAW velocity vs is 3470 m/s. The bulk wave spurious signal response level is lower by -20 dB than the SAW fundamental response level.

SAW Propagation Characteristics on Li2B4O7

Lithium tetraborate (Li2B4O7) was reported as a new piezoelectric material which has zero temperature coefficient of delay (TCD) and moderately high coupling coefficient (k2). This paper estimates SAW propagation properties (k2 and TCD) on Li2B4O7 over the full cuts and propagation directions. Further, the TCD dependency on aluminum layer is also assessed, considering construction of a SAW device with aluminum electrodes. As a result, it is found that there is a region of zero TCD and high coupling coefficient (1%).

Suppression of bulk-scattering loss in SAW resonator with quasi-constant acoustic reflection periodicity

The author proposes a SAW (surface acoustic wave) resonator structure which suppresses the bulk-scattering loss for a high reflectivity resonator. The approach uses an effective configuration to cancel the scattered radiation bulk waves from the GRs (grating reflectors) and the IDT (interdigital transducer) simultaneously. It consists of an IDT and metal-strip GRs, which are of the same material and which have the same thickness. The distance between the IDT and the GR is set to be a half-wavelength and the pitch for the IDT electrodes is slightly shortened to tune to the resonance condition. The structure realizes quasiconstant acoustic reflection periodicity (QARP) all over the resonator. The QARP structure was shown to suppress the mode conversion loss and to improve the resonator Q-factor. Experimental results provide that the QARP structure led to a two-fold improvement of the Q-factor for the high reflectivity resonator. Furthermore, theoretical analysis also proved that the scattered bulk waves were suppressed to less than 1/100 in the QARP-structured SAW resonator.<<ETX>>

Synthesis and Surface Acoustic Wave Property of Aluminum Nitride Thin Films Fabricated on Silicon and Diamond Substrates Using the Sputtering Method

C-axis oriented aluminum nitride (AlN) thin films with a thickness of 1 µm were prepared by reactive DC magnetron sputtering on polycrystalline diamond substrates at a substrate temperature of 623 K. The average surface roughness (Ra) of the AlN thin films was less than 2 nm obtained by locating the diamond substrates at a position of 100 mm from the aluminum target. The full width at half maximum (FWHM) of the rocking curve for the AlN(002) peak determined by X-ray diffraction analysis was about 0.2°. The surface acoustic wave (SAW) structures were completed by the deposition of aluminum electrodes on the as-deposited AlN surfaces. The SAW characteristics of an interdigital transducer (IDT)/AlN/diamond structure were investigated. The phase velocity and coupling coefficient were 10,120 m/s and 0.3%, respectively.

A numerical method for stress analysis in a SAW resonator

We present an analytical method of stress and strain in a SAW resonator. This can calculate the absolute values of the stress and strain at each precise point over the resonator for any driving condition and frequency. To represent the SAW resonator, we use the Masons equivalent circuit model which has mechanical ports and electrical ports. The acoustic voltage; Vm and current; Im of the mechanical port at the observation point are obtained as a function of the F-matrices and the electrical driving condition as well as driving frequency. Vm and Im are related to all mechanical components and their absolute values of propagating SAW by Partial Wave Theory. Applying this method, we discuss the stress analysis of SAW resonator filters on several substrates. Driving frequency dependency on stress migration is also discussed.

A wide-band balanced SAW filter with longitudinal multi-mode resonator

This paper describes a new balanced SAW filter with wide fractional bandwidth over 5% in 1.8 GHz range. The developed SAW filter is utilizing 5IDT longitudinally coupled multi-mode resonator structure with inverted output IDTs. In order to obtain wide bandwidth, mode coupling conditions were studied. Fabricated SAW filter has been realized with 100 MHz bandwidth and low insertion loss of 2 dB successfully. The package size of the filter is very small such as 2.5/spl times/2.0/spl times/1.0 mm/sup 3/ with flip-chip bonding technology. This SAW filter enables one to eliminate balun coil, which contributes to reduce mobile portable phone size.

SAW Resonator and Resonator Filter on Li2B4O7 Substrate

Lithium tetraborate (Li2B4O7) single crystal has attracted attention as a piezoelectric substrate for SAW devices. This paper focuses on its application to a SAW resonator. Reflection characteristics for aluminum strips and configuration suppressing surface-to-bulk-wave conversion are discussed. Results prove that the strips on Li2B4O7 substrate reflect efficiently, compared with those on the other substrates, and that a newly developed configuration improves the resonator Q at least twice. Further, a low insertion loss of 2.5 dB is achieved for a paging receiver filter.

Mode analysis of longitudinal multi mode SAW resonator filter

This paper describes the mode analysis for a longitudinal multi mode SAW resonator filter having 5 IDTs between the grating reflectors. The mode analysis is carried out at several conditions and frequencies, focusing on trap response. At the trap canceling condition, two resonant modes coincide in frequency and degenerate. Furthermore, at the fundamental resonant mode, SAW distribution at the center of electrode is distorted to be undulatory. In order to unify the SAW distribution of the fundamental mode, electrode pitches are adjusted partially. The developed balanced DCS-Rx filter realized low insertion loss of under 2 dB successfully.

IIDT type SAW filter using acoustic reflection cancel condition with solid IDT

The performance of an IIDT (interdigitated interdigital transducer) filter using the acoustic-reflection cancel condition of the solid IDT is investigated for the substrate A1/128 degrees Y-X LiNbO/sub 3/. The acoustic reflection cancel condition is a combination of 2d/ lambda =0.5 and h/ lambda =0.026 in the solid fingers. Near this condition the solid finger width d and thickness h dependence of the acoustic reflection coefficient is calculated by the finite-element method. The solid-finger width needs to be carefully controlled during the etching process, to avoid filter response distortion caused by the acoustic reflection of the IDT and phase velocity difference between the IDT and the propagation area. A new IIDT-type SAW (surface acoustic wave) filter configuration with a reduced number of fingers is proposed. This filter has a low shape factor (the 1 dB-bandwidth to 6-dB bandwidth ratio is 1.1) and a small ripple pass band.<<ETX>>

Current applications and future trends for SAW in Asia

The authors review SAW (surface acoustic wave) applications which have a growing market and large future potential in Asia. One of the major markets is television-related products, such as stereo sound or bilingual TV sets, CATV (cable television) convertors, satellite TV tuners, and enhanced-definition TV sets. Another market is telecommunication systems, such as pagers, cordless phones, cellular phones, and home security. The latter market is expected to grow rapidly, because of the recent liberalization of the Japanese radio law. Japanese SAW device developments includes not only design technology but also material improvement. Attention is given to SAW substrates and device functions for these applications.<<ETX>>