橋本 研也

Recently, droplet manipulation working with low driving power was developed by using surface acoustic wave (SAW) technology. The manipulator was driven at a relatively high frequency of 350 MHz to increase the SAW power density. First, a single phase unidirectional transducer (SPUDT) was designed. Because of its low insertion loss of 1.5 dB and high directivity of 10 dB, the SPUDT successfully conveyed a 100-nl water droplet with RF input power of 18 dBm (60 mW). Second, the SPUDT design was extended to a curved transducer to focus and launch the SAW energy into a narrow waveguide. Owing to the increased SAW power density, the required SAW power was reduced to 15 dBm (30 mW) for a 100-nl water droplet and to 12 dBm (15 mW) for a 20-nl water droplet, respectively.

This paper describes the development of extremely wideband and very low-loss RF SAW filters of ladder type for future communication systems. Because of its large electromechanical coupling factor for SH-type SAWs, a Cu-grating/15 degrees YXLiNbO3 structure was employed as a substrate. At first, four-stage ladder type filter was designed and fabricated, where width/length weighted dummy electrodes were used to suppress spurious responses caused by transverse-mode resonances. In addition, a viscous film was coated on the filter surface which is effective in suppress a spurious dip caused by Rayleigh SAWs. As the result, the fractional -3dB bandwidth of 19% and minimum insertion loss of about 0.6 dB were simultaneously achieved around 1 GHz, which basically owes the high electromechanical coupling and high Q factor of SH type SAWs propagating in the structure.

This paper is aimed at demonstrating how the wavenumber domain analysis of two-dimensional images captured by phase-sensitive laser probe systems is effective in selective characterisation of unwanted responses of RF SAW devices. The authors have recently reported the development of a high-speed and phase-sensitive laser probe system. Since the distance between two adjacent points of measurement is constant and calibrated, the 2D Fast Fourier Transform (FFT) and inverse FFT are directly applicable to the measured SAW field for the wavenumber domain analysis. This analysis offers various additional information of spurious signals. Effectiveness of the wavenumber domain analysis is demonstrated through the characterisation of spurious resonance modes and scattered non-guided modes appearing in SAW resonators fabricated on ST quartz substrate.

This paper describes the development of a high-speed laser probing system for Surface acoustic wave (SAW) devices. For the optical detection, the phase-sensitive knife-edge method was employed, by which the system is made very insensitive to low-frequency mechanical vibration. This enables fast stage translation without disturbing the measurement. A fast scanning rate of 240 points/s was realised by continuous stage translation and by successive data acquisitions of instantaneous sampling positions. in addition to the detector output. The position is measured by a high-precision linear-scale installed in the translation stage. Since the measured intervals between two adjacent sampling positions may not be uniform, the measured data are resampled at equal intervals for data processing. By applying the system to the characterisation of spurious resonance modes, it is shown how the present system is effective in designing and developing SAW devices.

This paper discusses a technique to suppress spurious transverse mode responses appearing in ultra-wideband SAW resonators on a Cu-grating/15 degrees YX-LiNhO(3) structure. The basic idea of the technique is inserting length- and width-weighted dummy electrodes between a bus-bar and interdigital electrodes. For practical device design, an analysis was made to show how the profile (field distribution) of both dominant and spurious transverse modes depends on the length and width (equivalent to SAW velocity) of the dummy electrodes. IDT-type SAW resonators were fabricated on a Cu-grating/15 degrees YX-LiNbO3 structure using the length- and width-weighted dummy electrodes. The experimental result was in good agreement with the theoretical analysis and prediction, showing that the proposed technique is most effective in suppressing the spurious responses caused by the transverse modes.

This paper describes the application of resonant single-phase unidirectional transducers (RSPUDTs) to the development of low-loss SAW filters with constant group delay in transition bands as well as in a passband. A low-loss SAW filter with constant group delay was designed and fabricated on a 128 degrees YX-LiNbO3 substrate. Experimental results were in good agreement with the design; the group delay deviation of 20 ns was realized over the range of +/- 8 MHz at the centre frequency of 512 MHz. The minimum insertion loss and -3 dB bandwidth were 4.3 dB and 8.5 MHz, respectively. The application of this technique is also discussed for developing high-performance wideband filters employing slanted interdigital transducers.

One of the authors (KH) previously proposed pitch-modulated IDTs and reflectors for developing low-loss and wideband longitudinally coupled double-mode SAW (DMS) filters. This paper discusses how a wide and flat passband is realised by applying the pitch-modulated IDTs and reflectors to DMS filters. It is shown that the pitch-modulated structure enables to adjust the location of multiple resonance frequencies simultaneously by varying an effective reflective position with frequency. That is, the IDTs are pitch-modulated so that the outer portion has slightly larger pitch than the inside, and the pitch of the outermost reflectors is made largest. Accordingly, higher-order SAW resonances occur between the two pitch-modulated IDTs; the outer portion of each IDT acts as a reflector, while the inner portion is mainly responsible for SAW excitation. Lower-order SAW resonances occur between the two reflectors.

This paper describes the development of a highspeed laser probe system for surface acoustic wave (SAW) devices. A fast scanning rate of 2.5 kS/s is realized by continuous stage translation and successive acquisition of the detector output by a high-speed data-logger. Trigger pulses are generated from the output of a high-precision linear-scale installed in the translation stage and fed to the data-logger for the synchronization with the stage movement. The phase-sensitive knife-edge method is employed for the optical detection. This makes the system very unsusceptible to low-frequency mechanical vibration caused by the fast stage translation. The system is applied for the characterization of spurious resonance modes in SAW devices. In conjunction with skillful use of image processing in wavenumber domain, it is shown how the present system is effective in the diagnosis and development of SAW devices.

This paper discusses, both qualitatively and quantitatively, the operation and the design principle of current surface acoustic wave (SAW) resonators in which the internal reflection within interdigital transducers (IDTs) is not negligible and lower capacitance ratio is necessary. For the purpose, the p-matrix expression is used with the help of the coupling-of-modes theory. The internal reflection causes: deformation of the IDT passband shape, frequency dependence of the effective SAW velocity within IDTs, and suppression of higher-order resonances. It is shown that these features can be effectively used to enhance performances of both one-port SAW resonators and two-port double-mode SAW (DMS) filters. In addition, under proper designs accounting for the internal reflection, most of all structural discontinuities can be removed, and is most preferable for the reduced scattering loss at the discontinuity. Design criteria also are presented for DMS filters of wide bandwidth, and it is demonstrated how device performances are improved by proper design accounting for the criteria.

This paper discusses the application of Love mode propagating on Cu-grating/rotated YX-LiNbO3-substrate structure to the development of ultra-wideband and low-loss RF surface acoustic wave (SAW) filters. Theoretical analysis suggested that high performance resonators with very small capacitance ratio would be realised using Cu gratings with a thickness of several percent of wavelength. It was also suggested that in particular, on 15degreesYX-LiNbO3-substrate structure, the spurious response due to Rayleigh mode could effectively be suppressed. An IDT-type one-port SAW resonator and a ladder-type SAW filter were fabricated on the Cu-grating/15degreesYX-LiNbO3-substrate structure, and their high performances are demonstrated with discussion.

This paper proposes use of pitch-modulated IDTs and reflectors for the realization of low-loss and wideband longitudinally coupled double-mode SAW (DMS) filters. This technique offers drastic improvement of the device performances through the introduction of sufficient number of degrees of freedom in the DMS filter design. Namely, the pass-band becomes wide and flat, and insertion loss can be reduced through the suppression of the bulk acoustic wave (BAW) scattering. First, it is shown how the BAW scattering loss can be reduced by the use of the pitch-modulated structure. The DMS filter with this structure is designed so that the frequency response becomes similar to that of the filter with the conventional un-modulated structure, and device performances are compared both theoretically and experimentally. Then it is demonstrated how the total device performances are improved by the use of this technology: when the device is designed optimally for given specifications. Adding to the reduced bulk wave scattering loss, various distinctive features offer drastic improvement of total device performances.

This paper discusses the application of Love mode propagating on Cu-grating/rotated YX-LiNbO3 -substrate structure to the development of ultra-wideband and low-loss RF SAW filters. Theoretical analysis suggested that high performance resonators with very small capacitance ratio would be realised using Cu gratings of the thickness of several percent of wavelength. It was also suggested that in particular, on 15 degrees YX-LiNbO3-substrate structure, the spurious response due to Rayleigh mode could effectively be suppressed. IDT-type one-port SAW resonators and ladder-type SAW filters were fabricated on the Cu-gi-ating/15 degrees YX-LiNbO3-substrate structure, and their high performances are demonstrated with discussion.

This paper describes micro-fabrication process developed for daily research work on SAW devices in authors' laboratory. The combination of SEM-based electron-beam (EB) lithography and lift-off process installed in a conventional laboratory without any particular cleaning facility is shown to be effective in fabricating well-defined IDT finger patterns with the line width of upto 180 nn in a half day at good yield. For the combination of the EB lithography and lift-off process, it is also shown how a very thin organic anti-static layer works well to avoid charge accumulation on a resist layer, which is easy to occur on insulating piezoelectric substrates and results in the EB diffraction. The present process was applied to the fabrication of SAW devices with various configurations including SHF resonators and single-phase unidirectional transducers. The result concludes how the process is useful for SAW research work in laboratories.

This paper proposes a simple distributed vibration sensor system which employs an optical fibre ring interferometer having an additional sub-loop coupled with a main ring by a 3dB coupler. Comparing multiple responses of interfered beams by the lights travelling n times in the sub-loop with each other, the proposed system can detect the position as well as the amplitude of acoustic vibration occurred in the main ring. The multiple responses are easily extracted in the time domain by driving an optical source with a pulsed current. It is demonstrated theoretically and experimentally that the proposed system is able to detect both the amplitude and position of the acoustic vibration occurred in the main ring with reasonable accuracy.

This paper proposes the application of Cu electrodes to surface acoustic wave single-phase unidirectional transducers (SAW SPUDTs). Since SPUDTs require large and widely variable reflectivity obtained by relatively thin electrodes, Cu with large mass density is one of the candidate materials. First, SAW reflection characteristics of a Cu electrode on 128degreesYX-LiNbO(3) were theoretically investigated. It is shown that Cu electrodes offer much larger reflectivity than Al electrodes. In addition, SAW velocity change with electrode width is quite moderate compared with other materials having large mass density such as Au. An SPUDT was fabricated employing Cu electrodes to discuss how effectively SAW reflectivity contributes, for example, to its unidirectionality. As a result, the obtained directivity for the Cu-SPUDT was about 16 dB, which was 10 dB larger than. that for an SPUDT employing conventional Al electrodes.

We propose a method of analyzing dispersion characteristics of shear-horizontal (SH)-type surface acoustic waves (SAWs) propagating under waveguide structures. SH-type SAWs on 36-42YX-LiTaO3 have widely been used in RF filters of mobile communication systems, and it has strongly been required to clear up loss mechanisms caused by energy leakage to the transverse direction. In this paper, we adopt the orthorhombic system in the following simplified model; the orthorhombic system possesses the slowness very similar to that of 3642YX-LiTaO3, First, elastic coefficients of the orthorhombic system are estimated so that they give the slowness of SH-type SAWs on 36YX-LiTaO3. Secondly, by applying thus determined elastic coefficients to an SAW waveguide structure, simplified dispersion equations in the transverse direction are derived. Solving them, therefore, one can easily characterize the transverse mode behaviors of SH-type SAWs on 36YX-LiTaO3. The method should be effective in a simple analysis of the dispersion characteristics of SH-type SAWs on 36-42YX-LiTaO3 and improving SH-type SAW filter performances.

A two-dimensional theory for surface acoustic waves (SAW) analysis in finite solids is developed for anisotropic materials through the introduction of complex exponentially decaying parameters and expansion functions with the combination of trigonometric and exponential functions. Following the usual procedure for dimension reduction with three components in the basis function and complete coupling of three mechanical displacements due to anisotropy, we now have nine two-dimensional equations for the surface acoustic wave propagation in a finite anisotropic solid, resembling the higher-order Mindlin plate equations for bulk acoustic waves. We also found that these equations give the identical velocity in limiting case where the depth of the solid is infinite. With these equations and corresponding decaying parameters that are material specific and can be extracted through the half-space solutions, we calculated the phase velocity of straight-crested surface acoustic waves in a finite anisotropic strip to demonstrate the applications of these two-dimensional equations and the solution technique. Similar to plate theories, this two-dimensional theory can be used in the surface acoustic wave device modeling for both analytical and numerical solutions with the advantage of simplifying the physical model and reducing the computational difficulty, thus posing great potential in aiding the increasingly complicated but important modeling and analytical efforts.

This paper discusses the application of SAW devices to impedance matching elements in RF circuits. First, the application of L-type cell consisting of two SAW resonators is discussed. It is shown that the properly designed L-type cell enables perfect impedance matching with capacitive loads. However, passband width and/or insertion loss become a trade-off with the impedance matching. The result suggests that this matching function could be installed in ladder-type SAW filters consisting of the cascade-connected L-type cells. Secondly, the application of double-mode SAW (DMS) filters is discussed. The DMS filters also exhibit the impedance matching function although their performance is more sensitive to capacitive loads than that of the L-type cell.

In this paper, we propose a method of analyzing the excitation and detection of leaky modes in a surface acoustic wave (SA V) waveguide structure. These properties could not be directly evaluated by the conventional scalar potential theory and mode expansion. In the present theory, the SAW excitation and detection are taken into account in the scalar potential theory, and then the SAW signal transfer between input and output ports is estimated in a form of the Fourier transform in the wavenumber domain. Finally, the contribution of each mode is estimated by the residue theorem. This method is effective in estimating the contribution of complete spectra, including leaky modes as well as non-leaky guided modes.

In this paper, we propose a method of analyzing the excitation and detection of leaky modes in a surface acoustic wave (SAW) waveguide structure. These properties could not be directly evaluated by the conventional scalar potential theory and mode expansion. In the present theory, the SAW excitation and detection are taken into account in the scalar potential theory, and then the SAW signal transfer between input and output ports is estimated in a form of the Fourier transform in the wavenumber domain. Finally, the contribution of each mode is estimated by the residue theorem. This method is effective in estimating the contribution of complete spectra, including leaky modes as well as non-leaky guided modes.

This paper describes the application of copper (Cu) electrodes to SAW devices employing metal grating reflectors. It is shown both theoretically and experimentally that the metal grating made of Cu electrodes on 128degreesYX - LiNbO(3) gives much larger reflectivity than that made of Al electrodes of the same thicknesses. As one of the applications of Cu electrodes to SAW IF devices, the directivity of Cu-SPUDT is shown experimentally to be 10 dB larger than that of Al-SPUDT. It is also discussed that optimally weighted grating reflectors can be designed by employing Cu electrodes. The result concludes that Cu electrodes on 128degreesYX - LiNbO(3) could further enhance the performance of SAW IF devices employing metal grating reflectors.

This paper discusses the design of wide bandwidth SAW resonators when, the internal reflection within IDTs is not negligible. In the discussion, the p-matrix expression is employed with a help of the coupling-of-modes theory. First, one-port SAW resonators are discussed. It is shown that the deformed IDT passband shape makes the gap between the IDT and reflector unnecessary. This is most preferable for the reduced scattering loss at the discontinuity. Secondly, two-port SAW resonators are discussed. It is shown that although the internal reflection limits the achievable passband width, it is effectively used for controlling the location of the resonance frequencies in the conventional design. The result suggests that the proper handling of the internal reflection would be crucial in realising high performance DMS filters. Finally, design criteria are presented for DMS filters of wide bandwidth, and it is demonstrated how device performances are improved by, proper design taking account of the criteria.

This paper describes a modified sol-gel method to prepare PZT discs of nearly 1μm thickness without repeating any process. The method is based on the subdivision of the PZT precursor into small areas prior to sintering. The subdivision is realised by pouring the precursor solution into a ZnO mold. Ultrasonic transducers were fabricated using the PZT disc and the piezoelectricity of the PZT disc in GHz ranges were discussed.

This paper discusses a method of designing weighted SAW grating reflectors which enables to develop the filters having a small group delay deviation, a flat passband and steep responses in transition bands. Both electrode width- and withdrawal-weightings were adopted to realise weighted reflectors. An SAW delay line employing the optimally designed reflector was fabricated, and the reflector performance was discussed. The group delay deviation of less than 80 ns in the stopband was achieved at the centre frequency of 200 MHz. For practical applications, an SAW IF filter employing the reflectors was fabricated. The reflector response is vividly reflected on filter performance, achieving the group delay deviation of less than 80 ns in the passband.

This paper describes a simple technique for the simulation of devices employing highly piezoelectric shear-horizontal (SH) type surface acoustic waves (SAWs). First, propagation and excitation characteristics of the SH-type SAWs are modelled by the application of Plessky's COM model and its extension by Koskela et al. to the discrete Green function theory[8]. Parameters appeared in the model are determined by the numerical fitting with the rigorous numerical analysis. Then a generalised procedure is presented to derive COM equations of a conventional form from the modelled SH-type SAW propagation and excitation characteristics. Finally, the frequency response of a resonator is discussed based on the derived COM equations, showing how effectively the present technique is applied to device simulation.

This paper describes the deposition of high-quality zinc oxide (ZnO) thin film by reactive de target-facing-type sputtering (DC-TFTS) using an are killer that periodically applies a positive pulse to the negative de discharge current. High-speed deposition of high-quality thin film, which is a merit: of TFTS, is possible because this method can neutralize the electric charge on the target surface without degrading:the plasma convergence. The relationship of the crystallographic and ultrasonic characteristics of the thin films fabricated by this method was studied in detail, and the conditions for optimum sputtering were studied. Based on the conditions obtained, an excellent film was realized with a standard deviation of the c-axis orientation in the X-ray diffraction rocking curve of 0.22 degrees and resistivity of 10(12) Omega cm. We also fabricated a ZnO-glass composite structure surface acoustic wave delay line, evaluated its characteristics in detail, and demonstrated the effectiveness of the proposed method as a ZnO thin-film fabrication technique for high-frequency ultrasonic devices. (C) 2000 Scripta Technica.

Aiming at realising the frequency response with small group delay deviation and wide passband width, we propose miniaturised surface acoustic wave (SAW) filters having folded propagation path and employing a straight edge mirror reflector for SH-type SAWs. In a filter fabricated on a PZT substrate, the group delay deviation of less than +/-15 ns and passband width of 5% were obtained at the centre frequency of 100 MHz. The experimental result and theoretical prediction were in good agreement with each other.

This paper describes the animation of calculated surface acoustic wave fields under a periodic metallic grating. The calculated data of the field distribution are animated by displaying them successively at a specific time interval. This sort of functions helps us to visually understand SAW scattering process and may reveal physics underlying the scattering process. Because of the rapid progress in computer technologies, low-end personal computers are now sufficient to display the animation.

This paper investigates the effects of bulk acoustic wave (BAW) radiation on the admittance of interdigital transducers (IDTs) with significant internal reflections of shear horizontal type (SH-type) leaky surface acoustic waves (SAWs) on 36 degrees YX-LiTaO3 (36-LT). Theoretical analysis is made by using the discrete Green function theory, and synchronous one-port resonators are analyzed. It is shown that the BAW radiation significantly affects the IDT characteristics even for resonators; under certain circumstances, BAWs launched from an IDT are converted into SH-type SAWs by the strong internal reflections, and they interact with the BAWs radiated by the IDT. Then the net amount of the radiated BAW power is highly dependent upon the number of IDT finger pairs. For the precise simulation of devices based on the SH-type SAWs with strong internal reflections, the BAW radiation should carefully be taken into consideration.

This paper describes a modified sol-gel method to prepare PZT discs of nearly 1 mum thickness without repeating any spin-coating and prebaking processes. In the method, PZT precursor-gel discs having a small area are fonned out of precursor-sol prior to sintering. This is carried out by pouring the precursor-sol into a ZnO mold. Ultrasonic transducers were fabricated using the PZT discs and their piezoelectricity in GHz ranges were discussed.

The paper deals with the derivation of simple polynomials characterising surface acoustic wave (SAW) reflection by metallic gratings with various widths and thicknesses. Such polynomial approximation makes it possible to estimate coupling of mode (COM) parameters in markedly reduced computational time. For very sophisticated SAW devices, this is of great use to develop high-speed simulation and design tools based on the COM method. In the present study, it is shown how simply and accurately the SAW reflection by aluminium gratings on 128 degrees YX-LiNbO3 substrates or 36 degrees YX-LiTaO3 substrates is characterised by polynomial approximation.