Equivalent networks for SAW interdigital transducers

An equivalent network approach is described for the analysis of surface-acoustic-wave interdigital transducers. Circuit parameters can he theoretically determined by applying the finite-element method to an infinite array. In this approach, all the effects of piezoelectric perturbation, mechanical perturbation, and energy storage are taken into account, To show the validity and usefulness of this approach, examples are computed for both single- and double-electrode interdigital transducers and one-port resonators. For single- and double-electrode interdigital transducers on 128°Y-X LiNbO₃, X-112°Y LiTaO₃,₄₅°X-Z Li₂B₄O₇, and ST-X quartz substrates, the dependence of excitation characteristics on electrode thickness and metallization ratio is investigated in detail. For a 128°Y-XLiNbO ₃ substrate, frequency responses of various one-port resonators are also investigated     

3-D electrostatic hybrid element model for SAW interdigital transducers

In this work, the singular behavior of charges at corners and edges on the metallized areas in SAW transducers are investigated. In particular, it is demonstrated that a tensor product of the commonly used Tchebychev bases overestimates the singularities at corners, and, hence, it cannot be used in a proper boundary element method formulation. On the other hand, it is shown that a simple finite element method-like approach is impractical due to the enormous number of unknowns required to model the electrode's large length-to-width ratio. These considerations are then used for defining a hybrid element model, which combines Tchebychev and linear polynomials over differently meshed domains. Such an approach is shown to suitably account for charge singularities while greatly reducing the number of unknowns. Results are obtained for isotropic and anisotropic substrates for non-periodic configurations.     

SAW filters including one-focus slanted finger interdigital transducers

The present paper describes a new filter type, slanted finger interdigital transducer (SFIT) that allows fast analysis as a precondition for fast optimization. Therefore, filter structure and analysis are especially suitable for optimizing unidirectional SFIT (USFIT) filters. For analysis, a SFIT filter is usually divided into many narrow channels. Every channel is considered to be a subfilter and is analyzed separately. Therefore, the total filter analysis is very time consuming. For reducing computing time, we suggest a one-focus structure. In contrast to conventional SFIT filters, not only the finger and gap widths but also the spaces between transducers of different channels differ by one and the same scaling factor. As a consequence, all prolonged finger edges of both transducers intersect in one point called focus. As a result, the parameters of all subfilters can be calculated from the parameters of only one subfilter by simple frequency scaling. Consequently, the total time for analysis is essentially reduced. However, one-focus SFIT filters with continuous finger edges show a deep minimum within the passband. This problem can be overcome by using stepped one-focus structures.     

Design techniques for SAW filters using slanted finger interdigital transducers

This paper describes useful design techniques for wide-band linear phase SAW filters using slanted finger interdigital transducers (SFITs): to increase stop band attenuation, to flatten the pass band response, and to draw SFIT patterns for photo-masks. In order to increase stop band attenuation, a withdrawal-weighted SFIT is employed. A combination of a withdrawal-weighted SFIT and an unweighted SFIT provides good stop band responses, even though the number of finger pairs is small. In order to achieve a desired amplitude response in the pass band, an aperture-weighting technique for divided channels of SFITs is presented. A design method for drawing SFIT patterns for photo-masks is also described. A linear-phase SAW filter using SFITs with a 50% fractional bandwidth is designed and fabricated on 128 degrees y-x LiNbO(3). A sidelobe suppression of 40 dB and a 3 dB to 40 dB shape factor of 1.12 were realized. Good agreement between theoretical and experimental results is obtained.     

Longitudinal leaky SAW resonators and filters on YZ-LiNbO3.

The high-phase velocity (above 6100 m/s in an aluminum (Al) grating on lithium niobate (LiNbOs)) of the longitudinal leaky surface acoustic wave (SAW) (LLSAW) mode makes it attractive for application in high-frequency SAW ladder filters in the 2-5 GHz range. We investigate the dependence of one-port synchronous LLSAW resonator performance on YZ-LiNbO3 on the metallization thickness and metallization ratio, both experimentally and theoretically. Our results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness, with the optimal thickness that produces the highest Q values being about 8%. The optimal thickness increases with the metallization ratio. The observed behavior is interpreted with the help of simulations using a combined finite element method (FEM)/boundary element method (BEM) technique. As an application, bandpass filters have been fabricated in the 2.8 GHz frequency regime, based on LLSAWs. The synchronous resonators constituting the ladder filters operate in the fundamental mode. The filters feature low insertion losses below 3 dB and wide relative passbands of 4.5-5%.     

Automatic computer-aided design of SAW filters using slanted finger interdigital transducers

This paper describes a design procedure for surface acoustic wave (SAW) filters using slanted finger interdigital transducers (SFIT) that are suitable for mid-band or wideband applications. The SFITs cannot represent the impulse response directly, in contrast to apodized IDTs. A design method for SFITs based on a building-block approach in the frequency domain is described. An automatic computer-aided design tool for SFIT filters has been achieved. The SFIT filters can be designed using a withdrawal weighting for stop-band responses, an aperture weighting for pass-band amplitude responses, and a distance weighting for pass-band phase responses. In addition, a SFIT pattern for photo mask can be automatically designed using this tool. Using this tool, an SFIT filter with a relative bandwidth of 15% was designed on an x-cut 112y-direction LiTaO(3) substrate.     

Optimization of broadband withdrawal weighted interdigital transducers for high selective SAW filters

The problem under discussion is the design of highly selective broadband surface acoustic wave (SAW) interdigital transducers (IDT) with uniform electrodes. In most SAW filters, such transducers are used with apodized IDTs or instead of them. The proposed optimization algorithms are intended for improvement of IDT selectivity by means of a withdrawal weighting (WW) technique. Unlike the familiar methods of WW transducer optimization, these algorithms choose the best IDT structure on the basis of how well it meets the specifications, not in the time but in frequency domain directly. This approach is more effective for broadband WW transducers. A number of SAW filters have been designed using the described algorithms. Their experimental characteristics follow: bandwidths of 0.5 to 15%, stopband rejection of 40 to 50 dB, 3 dB/40 dB shape factors of 1.07 to 1.3.     

Longitudinal leaky SAW resonators and filters on YZ-LiNbO/sub 3/

The high-phase velocity (above 6100 m/s in and aluminum (Al) grating on lithium niobate (LiNbO/sub 3/)) of the longitudinal leaky surface acoustic wave (SAW) (LLSAW) mode makes it attractive for application in high-frequency SAW ladder filters in the 2-5 GHz range. We investigate the dependence of one-port synchronous LLSAW resonator performance or YZ-LiNbO/sub 3/ on the metallization thickness and metallization ratio, both experimentally and theoretically. Our results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness, with the optimal thickness that produces the highest Q values being about 8%. The optimal thickness increases with the metallization ratio. The observed behavior is interpreted with the help of simulations using a combined finite element method (FEM)/boundary element method (BEM) technique. As an application, bandpass filters have been fabricated in the 2.8 GHz frequency regime, based on LL-SAWs. The synchronous resonators constituting the ladder filters operate in the fundamental mode. The filters feature low insertion losses below 3 dB and wide relative passbands of 4.5-5%.     

Design technique for nonlinear phase SAW filters using slanted finger interdigital transducers

This paper describes a useful design technique to achieve a nonlinear phase SAW filter using slanted finger interdigital transducers (SFITs) or tapered interdigital transducers which are suitable for wide-band filters in intermediate frequency stages. A required nonlinear phase response in the passband can be obtained by changing center-to-center distances between input and output SFITs along an axis perpendicular to the SAW propagation axis. The design is based on a building-block approach in the frequency domain. A nonlinear phase SAW filter with a center frequency of 70 MHz and a fractional bandwidth of about 10% is demonstrated on x-cut 112.2 degrees y-propagating LiTaO(3 ). Because the substrate has a power flow angle of 1.55 degrees, the SFIT pattern is tilted along that angle. Good agreement between theoretical and experimental results is obtained.     

A theoretical and experimental study of low-loss SAW filters with interdigitated interdigital transducers

A study of low-loss surface-acoustic-wave (SAW) interdigitated interdigital transducer (IIDT) structures is presented. A model based on mixed scattering matrices was used in the analysis. It is shown that the inherent comb frequency response of the IIDT can be smoothed by slanting the transducers in the structure. Alternatively, the combs can be enhanced to produce a compact low-loss feedback element for potential application in multimode oscillators. Experimental verification of the analysis is provided by a 3:2 slanted-finger IIDT filter with a 10% relative bandwidth of 5.9-dB insertion loss, and a comb filter with 13.4-dB insertion loss and comb spacing of about 3 MHz. The measured responses agree quite well with the theory.     

New electrode separation technology using anodic oxidation and application to SAW interdigital transducers

By anodic oxidation of the edges of the portions of the Al film under photoresist, controllable gaps between electrodes with good insulation can be obtained. These techniques are applied to realize a surface-acoustic-wave (SAW) narrow gap interdigital transducer (NG-IDT) and narrow gap unidirectional transducer (NG-UDT). The experimental result shows 7.2 dB insertion loss with amplitude ripples of +/-1.2 dB for a conventional NG-IDT. Directives of 3-dB/transducer at 440 MHz (fundamental) and 13-dB transducer at 870 MHz (second-harmonic operation) for a new floating electrode type unidirectional transducer (NG-FEUDT) are demonstrated. Also a three transducer low loss filter using a combination of NG-IDT and NG-FEUDT exhibits 3.46 dB insertion loss at 894 MHz (second-harmonic operation, electrode width of 1.7 mum) with sidelobe suppression greater than 35 dB.     

Behavior of platinum/tantalum as interdigital transducers for SAW devices in high-temperature environments

In this paper, we report on the use of tantalum as adhesion layer for platinum electrodes used in high-temperature SAW devices based on langasite substrates (LGS). Tantalum exhibits a great adhesive strength and a very low mobility through the Pt film, ensuring a device lifetime at 900°C of about one hour in an air atmosphere and at least 20 h under vacuum. The latter is limited by morphological modifications of platinum, starting with the apparition of crystallites on the surface, followed by important terracing and breaking of the film continuity. Secondary neutral mass spectroscopy (SNMS), Auger electron spectroscopy (AES), X-ray diffraction (XRD) measurements, and comparison with iridium-based electrodes allowed us to show that this deterioration is likely intrinsic to platinum film, consisting of agglomeration phenomena. Finally, based on these results, we present a solution that could significantly enhance the lifetime of Pt-based IDTs placed in high-temperature conditions.     

The electrostatic problem for the SAW interdigital transducers inan external electric field. II. Periodic structures

For pt.I see ibid., vol.43, no.6, pp.1150-9 (1996). An exact solution of the electrostatic problem for calculating the surface charge and electric field distributions in an arbitrary periodic interdigital transducer (IDT) is given using the results of our companion paper. An arbitrary external electric field may be specified along the electrode structure with the unit cell containing one electrode, or several electrodes, of different widths. The potentials of the electrodes that may be specified are also arbitrary. It is shown that in the case without an external field, the solution includes all the known results as special cases. The case of shorted electrodes in the external electric field is investigated in detail. The surface charge and electric field distributions are calculated for a spatially harmonic external field with an arbitrary wavenumber. The results of the calculations are represented graphically for various ratios between the period of the electrode structure and the wavelength of the external field for the case of a unit cell containing one or two electrodes of different widths     

Characterization of material versus temperature using P(VDF-TrFE)SAW transducers

This work approaches the elastic properties of non-piezoelectric materials and reports their temperature dependence for a duraluminum substrate. The method is based on the propagation time measurement of ultrasonic Rayleigh waves and attenuation of electrical voltage between the emitter and receiver. Transducers made of P(VDF-TrFE) polymer film are used for Surface Acoustic Waves (SAW) generation and detection     

A simple equivalent circuit for interdigital transducers based on the coupled-mode approach

The coupled-mode approach is a powerful tool for analyzing surface acoustic wave (SAW) periodic structures such as reflectors and interdigital transducers. The relations among the terminal quantities at two acoustical ports and one electrical port of an interdigital transducer (IDT) are derived from coupled-mode equations. A simple distributed-parameter equivalent circuit representing the entire IDT is proposed. A few examples of applications of this equivalent circuit to analysis of SAW devices are presented.     

The electrostatic problem for the SAW interdigital transducers inan external electric field. I. A general solution for a limited numberof electrodes

An exact solution for finding the surface charge and electric field distributions in interdigital transducers (IDTs) with a limited number N of electrodes is given. It is based on the Keldysh-Sedov solution to the mixed boundary problem of the analytic function theory. The IDT electrodes are placed on the plane boundary between two anisotropic dielectric media. The external electric field may arbitrarily vary along the structure. The solution contains N constants which may be found from the electrodes' connection conditions. For determining these constants a linear set of algebraic equations is obtained. The coefficients of this system are written in explicit form. The capacitance coefficients for a system of electrodes of different widths are obtained. For illustration purposes systems with one and two electrodes are considered in greater detail. In these cases the external electric field is assumed to vary harmonically along the structure with an arbitrary wavenumber. For one electrode the Fourier transform of the charge distribution is obtained in terms of the Bessel functions. For two electrodes of different widths a simple expression for the capacitance is found. The charge and electric field distributions are represented graphically for several wavenumbers and geometrical sizes of the electrode system. Section I contains a survey, including Russian literature, which is not well known in the west     

Optimization of broadband uniform beam profile interdigital transducers weighted by assignment of electrode polarities

This paper considers techniques for optimizing surface acoustic wave (SAW) interdigital transducers (IDTs) with uniform electrode length (no apodization). Unlike other optimization techniques developed for this type of IDT, the algorithms proposed here choose the best electrode structure on the basis of how well it meets the specifications, not in the time domain, but in the frequency domain directly. In the first step of the optimization, a periodic transducer is synthesized. The weighting technique for providing a desired frequency response involves assigning polarities to the individual electrodes. The next step of the IDT selectivity improvement is the optimization of the electrode positions. In addition, the second algorithm can synthesize specified nonsymmetric frequency responses. A set of IDTs and SAW filters with bandwidths of 1 to 14% has been designed using this algorithm     

Design considerations on surface acoustic wave resonators with significant internal reflection in interdigital transducers

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.     

Precision surface-acoustic-wave (SAW) oscillators

The evolution of SAW oscillator technology over the past 17 years is described and a review of the current state of the art for high-performance SAW oscillators is presented. This review draws heavily upon the authors' own experience and efforts, which have focused upon the development of a wide variety of SAW oscillators in response to numerous high-performance military system requirements.     

Surface-acoustic-wave (SAW) flow sensor

The use of a surface-acoustic-wave (SAW) device to measure the rate of gas flow is described. A SAW oscillator heated to a suitable temperature above ambient is placed in the path of a flowing gas. Convective cooling caused by the gas flow results in a change in the oscillator frequency. A 73-MHz oscillator fabricated on 128 degrees rotated Y-cut lithium niobate substrate and heated to 55 degrees C above ambient shows a frequency variation greater than 142 kHz for flow-rate variation from 0 to 1000 cm(3)/min. The output of the sensor can be calibrated to provide a measurement of volume flow rate, pressure differential across channel ports, or mass flow rate. High sensitivity, wide dynamic range, and direct digital output are among the attractive features of this sensor. Theoretical expressions for the sensitivity and response time of the sensor are derived. It is shown that by using ultrasonic Lamb waves, propagating in thin membranes, a flow sensor with faster response than a SAW sensor can be realized.