Surface Acoustic Wave Interdigital Transducer Research Articles

Rigorous treatment of leaky SAWs and new equivalent circuit representation for interdigital transducers

Leaky surface acoustic wave (SAW) interdigital transducers (LDTs) formed on 36/spl deg/ YX-LiTaO/sub 3/ and 41/spl deg/ and 64/spl deg/ YX-LiNbO/sub 3/ are theoretically analyzed, and a new equivalent circuit representation is proposed. The influence of the attenuation constant due to leakage as well as conductance caused by bulk wave radiation are taken into account. A new treatment based on an integral equation approach provides all circuit parameters. Fundamental experiments show fairly good agreement between theoretical and experimental results, confirming that this treatment provides very accurate tools for designing leaky-SAW devices.

Theoretical determination of equivalent circuit parameters for natural single‐phase unidirectional surface acoustic wave transducers

AbstractThis paper proposes a method for theoretical determination of the circuit parameters in the equivalent circuit for the natural single‐phase unidirectional transducer (NSPUDT), which is one type of unidirectional surface acoustic wave interdigital transducers. The finite element method was used to find the upper and lower edge frequencies of the shorted and open‐ended gratings corresponding to the shorted and open electrical terminal of the NSPUDT, the potential standing wave distribution and the static capacitance per pair of NSPUDT, needed for determination of the circuit parameters. Due to the analysis method, the anisotropy of the substrate, the electrical perturbation effect, the acoustic perturbation effect, and the energy storage effect can be taken into account. Actually, the equivalent circuit parameters of the NSPUDT on an ST‐cut 25° X‐propagation quartz substrate were computed and the reflection and excitation characteristics of the surface wave were derived. From a comparison with the experimental values, the validity and usefulness of the present method is confirmed.

SPICE simulation of surface acoustic wave interdigital transducers

Surface Acoustic Wave (SAW) devices, are not normally amenable to simulation through circuit simulators. In this letter, an electrical macromodel of Mason's Equivalent Circuit for an interdigital transducer (IDT) is proposed which is compatible to a widely used general purpose circuit simulator SPICE endowed with the capability to handle negative capacitances and inductances. Illustrations have been given to demonstrate the simplicity of ascertaining the frequency and time domain characteristics of IDT and amenability to simulate the IDT along with other external circuit elements. >

Surface acoustic wave properties of ZnO films on {001}-cut 〈110〉-propagating GaAs substrates

GaAs has been employed as a material for acoustic charge transport (ACT) devices principally because it is a piezoelectric semiconductor. However, because GaAs is a weakly piezoelectric material, the surface acoustic wave (SAW) interdigital transducer (IDT) drive power required to achieve charge transport is typically about 27 dBm. An enhancement of the piezoelectric coupling could potentially improve device lifetime, reliability, dynamic range, and decrease device power consumption. To this end the use of a ZnO film on top of an ACT-like substrate to enhance the piezoelectric coupling has been investigated. Moreover, the ZnO film structure would also make it possible to monolithically integrate SAW devices and GaAs electronics. In order to provide a basis for the design of such devices, SAW properties are reported, including velocity, effective piezoelectric coupling constant, and attenuation, measured on ZnO films sputtered on {001}-cut 〈110〉-propagating GaAs substrates. The measurements have been performed for the different film thicknesses over the range of 1.6–4 μm and with films of different grain sizes. IDTs operating between 180 and 360 MHz were fabricated, and a knife-edge laser probe was used to measure the SAW propagation. The measured data for the velocity shows good agreement with theoretical values. The quality of the dc triode-sputtered films was superior to that of the rf magnetron-sputtered ones due to higher K2 and less attenuation. The value of K2 for the 1.6-μm-thick dc triode-sputtered one was measured to be 0.75% which is comparable with 1.07% for bulk crystalline ZnO.

Admittance matrix of a surface acoustic wave interdigital transducer

The admittance matrix of a surface acoustic wave interdigital transducer (IDT) is derived from the impulse response model and circuit theory. It is shown, that Y(23)=Y(13) for a symmetrical IDT and Y(23)-Y(13) for an antisymmetrical one. A simple algorithm for the input admittance calculation of apodized IDTs is given. The proposed algorithm is also capable of analyzing withdrawally weighted, withdrawally weighted and apodized, and unapodized IDTs. Calculations of an apodized IDT input admittance are compared with measurements.

Surface Acoustic Wave Transducer in a Layered Structure Composed of Thin Piezoelectric Ceramic Plate and Glass

Operation performance of a surface acoustic wave (SAW) interdigital transducer (IDT) in a layered structure composed of a thin piezoelectric ceramic plate and a glass substrate is described in terms of phase velocity, electromechanical coupling constant and mechanical displacement distributions. The numerical results of SAW phase velocities in the form of a function of the product of frequency and thickness of the piezoelectric ceramic plate are in good agreement with the measured velocities of the constructed layer-structured devices. The first-order SAW mode device has an excellent electromechanical coupling constant of greater than 15%, which is favorable for a SAW device exhibiting a satisfactory performance under single-mode operation in a SAW oscillator construction.

SAW temperature and humidity sensor with high resolution

A surface acoustic wave (SAW) temperature and humidity sensor with high resolution is presented in this paper. The sensor is based upon a SAW delay-line oscillator which is implemented in a 128° rotated Y-Z LiNbO 3 substrate. Two SAW interdigital transducers (IDTs) for the delay line have been developed and analysed. The experimental frequency response of the oscillator is in good agreement with the theoretical analysis. The detection sensitivities of the temperature and humidity sensor for the single SAW delay line are −6 kHz/°C and −3 kHz/Rh%. The frequency response of the sensor is found to decrease nearly linearly with temperature or humidity. The sensitivity of a humidity sensor using temperature compensation is 3 kHz/RH%.

Surface acoustic wave attenuation by localized electrons in a 2DEG at a GaAs/AlGaAs heterojunction

The surface acoustic wave (SAW) transmission has been measured at 4.2 K, as a function of magnetic field up to 5 T, for a GaAs wafer on which a heterojunction has been grown by MBE. Frequencies up to 1920 MHz were generated by interdigital SAW transducers deposited directly on the GaAs surface. At all frequencies, oscillations in the SAW attenuation, analogous to the Shubnikov-de Haas oscillations in sigma xx, can be observed due to piezoelectric coupling to the non-localized electrons of the two-dimensional electron gas (2DEG) of the heterojunction. The authors find, at frequencies of about 900 MHz and above, an additional attenuation at all magnetic fields, which increases with magnetic field. At half-integer filling factor, the increase in attenuation is initially linear with increase of field and then, at higher fields, the attenuation increases as the square root of the field. They interpret this as due to deformation potential coupling to the localized electrons of the 2DEG. A small SAW attenuation is observed at zero magnetic field which may be due to a very low density of three-dimensional electrons in the GaAs buffer layer or remaining in the doped AlGaAs layer.

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.

GHz‐range surface acoustic wave interdigital transducers and applications

AbstractGHz‐range SAW interdigital transducers, unidirectional transducers and SAW filters are described. Also, new fabrication technologies which can achieve higher operating frequencies are presented.

Surface acoustic wave interdigital transducers with narrow electrode gaps

Abstract New surface acoustic wave interdigital transducer (IDT) and unidirectional transducer (UDT) are described. Using lift off-etching method or lift off-anodic oxidation method, we can get narrow gaps, with good insulation, between electrodes. Experimental results show directivities of 6 dB/transducer at 438 MHz and 17 dB/transducer for its second harmonic at 868.5 MHz. Also, minimum insertion loss of 3.6dB for a low loss filter at 440MHz is obtained.

Surface Acoustic Wave Transducers with Narrow Gap Interdigital Electrodes and Low Loss Filters

A new surface acoustic wave interdigital transducer (IDT) and unidirectional transducer (UDT) are described. Using a lift off-etching method or a lift off-anodic oxidation method, controllable gaps between electrodes with good insulation can be obtained. Experimental results show 7.8 dB insertion loss with small ripples of ±1.2 dB for a conventional IDT, and directivities of 6 dB/transducer at 438 MHz for a floating electrode type unidirectional transducer (FEUDT) and 17 dB/transducer for its second harmonic at 868.5 MHz. Also, minimum insertion loss of 3.6 dB for a low loss FEUDT filter at 440 MHz is obtained. These techniques are applied for the separation between electrodes of VLSI.

Fast computation of static capacitance for periodic SAW transducers

An efficient algorithm to compute the static capacitance of periodic SAW (surface acoustic wave) interdigital transducers is proposed. The method is based on a new formulation of the well-known relationship giving the capacitance as a linear combination of the charges induced on an infinite transducer for one-electrode excitation. Besides computational efficiency, an interesting physical interpretation of static capacitance results. The algorithm itself is also tailored to split-finger transducers.

Transfer scattering matrix of non‐uniform surface acoustic wave transducers

This paper is concerned with a general mathematical theory for finding the admittance matrix of a three‐port non‐uniform surface acoustic wave (SAW) network characterized by n unequal hybrid sections. The SAW interdigital transducer and its various circuit model representations are presented in some detail. The Transfer scattering matrix of a transducer consisting of N non‐uniform sections modeled through the hybrid equivalent circuit is discussed. General expression of the scattering matrix elements for a N‐section SAW network is included. Based upon hybrid equivalent circuit model of one electrode section, explicit formulas for the scattering and transfer scattering matrices of a SAW transducer are obtained. Expressions of the transfer scattering matrix elements for the N‐section crossed‐field and in‐line model of SAW transducers are also derived as special cases. The matrix elements are computed in terms of complex frequency and thus allow for transient response determinations. It is shown that the general forms presented here for the matrix elements are suitable for the computer aided design of SAW transducers.

Two-Dimensional Green's Function of a Semi-Infinite Anisotropic Dielectric in the Wavenumber Domain

A closed-form expression for the two-dimensional Green's function of a semi-infinite anisotropic dielectric in the wavenumber space is presented, and then the validity and definiteness of the ob- tained expression for arbitrary values of the wavenumber vector is proven. The derived Green's function is the Fourier transform of the potential response to a point-charge source located on the surface of a constantly stressed semi-infinite anisotropic dielectric. Therefore it is the most significant part in calculating the two-dimensional charge density and field distribution of the surface acoustic wave interdigital transducers in the case of the electrostatic approximation. The inte- grals associated with the inverse Fourier transform of the derived Green's function are discussed.

Investigation of an Equivalent Circuit Including Propagation Loss for an Interdigital Surface-Acoustic-Wave Transducer

An equivalent circuit for an interdigital surface-acoustic-wave transducer (IDT), in which the propagation loss is taken into account, is investigated. From this circuit, the three-port admittance parameter Yij for the IDT having N-pair electrodes is derived in closed form. In the derivation, the image parameter theory is applied. Using this Yij in explicit expression, the driving-point admittance for an IDT is analyzed, and the effect of propagation loss on the radiation admittance is investigated.

The Excitation and Detection of Surface-Generated Bulk Waves

Absrruct-The performance of a conventional pair of surface acoustic wave interdigital transducers (IDT’s) used to generate bulk acoustic waves is analyzed. An IDT located on the upper surface of a thick, rotated Ycut quartz crystal acts as the acoustic equivalent of an array antenna, launchingacollimated beam into the crystal bulk. The beam is reflected off the crystal bottom and redirected towards the upper crystal surface where it is intercepted by a second receiving IDT. Closed-form expressions are derived for the important electrical and acoustical delay line characteristics and comparison is made with experimental measurements. The analyzed characteristics of the reflected bulk wave (RBW) delay line are significantly different from either the SAW or SSBWand include a) a frequency bandwidth directly proportional to transducer length, b) a center frequency determined not by finger-to-finger spacing but rather by the ratio of transducer separation to crystal thickness, c) a nonresonant radiation resistance, d) the capability to synthesize filter designs in which the frequency response is proportional to the correlation of the individual transducer finger weightings, and e) the capability to modify temperature stability by change of transducer separation and crystal thickness. ULK ACOUSTIC wave radiation generated by interdigital B transducers has historically been considered as spurious, often degrading or interfering with the performance of devices based upon surface acoustic wave (SAW) excitation. Within the last several years, however, there has been increased interest in utilizing the bulk waves generated by IDT’s for practical microwave applications. Such surface-generated bulk waves make use of the IDT as an acoustic equivalent of a phasedarray antenna, with the electrode fingers constituting the array elements. The phasing between these elements is controlled by the applied signal frequency thereby allowing the direction of the acoustic beam radiated into the bulk to be scanned.

Low reflectivity surface acoustic wave transducers on GaAs

Experimental results are presented for fabrication and metallization techniques which achieve low reflectivity surface acoustic wave (SAW) interdigital transducer operation on GaAs. The transducer structures considered are single finger pair, quarter wavelength designs with chrome/aluminum/chrome (surface), aluminum (recessed) or gold-germanium/nickel/gold (surface) metallized electrodes which were fabricated directly onto a semi-insulating GaAs substrate. Triple transit signal suppression greater than 53 dB for the recessed aluminum metallization and greater than 62 dB for the Au-Ge/Ni/Au metallization has been achieved for test transducers operating at 159 MHz, compared to typically 30–35 dB for Cr/Al/Cr metallizations. These low reflectivity transducers provide the means for reducing spurious signal responses within monolithic programmable GaAs signal processors and for achieving higher Q performance of SAW resonator filters fabricated on GaAs substrates.

Laboratory Experiments in Surface Acoustic Wave Devices and Their Applications

This project relates to the development of three novel laboratory experiments for electrical engineering seniors enrolled at the Polytechnic Institute of New York introducing them to surface acoustic wave (SAW) devices and their applications. Major developments in this new technology have taken place during the past decade. SAW devices now play a significant role in the signal processing area and industries' strong interest in this new technology is now evident. The first experiment in the series demonstrates the fundamental behavior of SAW devices using the delay line structure. The second experiment relates to the measurement of the complex impedance of the SAW interdigital transducer. The third experiment demonstrates how the SAW device can be used to perform the function of convolution and correlation.

Amplitude weighted surface acoustic wave device

A surface acoustic wave device embodying a novel technique of weighting the tapping amplitude of an interdigital surface acoustic wave transducer, the amplitude weights being implemented by means of planar resistors deposited so that one is connected in series with each sampling electrode of the interdigital transducer.