Natural Single-phase Unidirectional Transducer Research Articles

Wave Propagation Direction and c-Axis Tilt Angle Influence on the Performance of ScAlN/Sapphire-Based SAW Devices.

Some previously reported surface acoustic wave (SAW) devices using bulk piezoelectric substrates showed higher acoustic power radiated in either forward or backward wave propagation direction depending on their crystal orientations and are called natural single-phase unidirectional transducers (NSPUDT). While these reports were based on bulk piezoelectric substrates, we report directionality in the c-axis tilted 44% scandium doped aluminum nitride thin piezoelectric film-based SAW devices on sapphire. It is worth noting that our observance of directionality is specifically in Sezawa mode. We produced a c-axis tilt up to 5.5° over the single wafer and examined the directionality by comparing the forward and backward insertion loss utilizing split finger electrodes as a receiver. The wave propagation direction and c-axis tilt angle influence on the performance of SAW devices is evaluated. Furthermore, return loss and insertion loss data are presented for various SAW propagation directions and c-axis tilt angles. Finally, the comparison for both acoustic modes, i.e., Rayleigh and Sezawa, is reported.

One-Port SAW Resonators Using Natural SPUDT Substrates

We discuss the design of one-port surface acoustic wave (SAW) resonators using substrates with a partial or total degree of directivity, that is, the natural single-phase unidirectional transducer (N-SPUDT) effect. A general design method gives a resonance when all three frequencies (the required resonance frequency and the Bragg frequencies) are different. A second method has been derived from the resonance condition for a symmetrical substrate. Two further methods incorporate lamda/4 gaps. The capacitance ratio is presented as a function of the phase of the electrode reflection coefficient. The simulations use data for the N-SPUDT orientation of langasite. The reflection coefficient for Al electrodes has been calculated from finite element modeling (FEM) analysis. The approximate perturbation theory is found to agree well for small film thickness (h/lamda < 2%). The phase of the reflection coefficient is typically 150 degrees, not quite the ideal value of 180 degrees. Measurements on resonators using Al and Cu films are presented.

Comparison between ST-cut quartz 25 degrees and -60 degrees NSPUDT propagation directions.

This paper discusses directivity and other propagation properties of natural single phase unidirectional transducer (NSPUDT) propagation directions on ST-cut quartz. The work focuses on the comparison of surface acoustic wave (SAW) directivity and propagation properties between the ST-cut quartz -60 degrees, Euler angles: (0 degrees, 132.75 degrees, -60 degrees), and the ST-cut quartz 25 degrees, Euler angles: (0 degrees, 132.75 degrees, 25 degrees), NSPUDT propagation directions, including predicted and measured directivity responses for both propagation directions. The well-known SAW 25 degrees propagation direction is used for low loss, high performance SAW filter designs for consumer products and communications applications. The ST-cut quartz -60 degrees propagation direction has been predicted to have a reflection coefficient 2.5 times larger than ST-cut quartz 25 degrees. In addition the ST-cut quartz -60 degrees satisfied the NSPUDT 90 degrees reflection coefficient phase condition much more closely, resulting in an improved directivity response. For the delay line structures used in the experiments, the measured directivity is 10.1 dB for the -60 degrees propagation direction. For the same structures, the measured directivity along the 25 degrees propagation direction is about 5.0 dB. The experimental results given in this paper verify that indeed ST-cut quartz -60 degrees has a higher directivity than ST-cut quartz 25 degrees, confirming the theoretical predictions. In addition, this work compares other propagation properties for both directions, namely, phase velocities, electromechanical couplings, temperature coefficients of delay, power flow angles, and metallic strip reflection coefficient amplitudes and phases.

Metal strip reflectivity and NSPUDT orientations in langanite, langasite, and gallium phosphate.

New materials of the trigonal 32 class have received much attention recently, due to their quartz-like temperature behavior and the promise of higher electromechanical coupling coefficients. The magnitude and phase of the reflection coefficient of the metal strips that form the SAW transducers and reflector structures is a critical parameter that allows proper device designs and optimal material surface orientation. This paper describes an investigation of the magnitude and phase of the SAW metal strip reflectivity in some new materials and along new orientations. The materials are langanite, langasite, and gallium phosphate. The results are presented as contour plots of the magnitude and phase of the reflection coefficient. In addition, the phase velocity, temperature coefficient of delay, electromechanical coupling coefficient, and power flow angle are given, thus allowing proper orientation selection for SAW device designs. The results presented highlight the reflection coefficient calculations in the selection of natural single-phase unidirectional transducer orientations (NSPUDT) for these new materials. As a result of the present work, reflection coefficient magnitudes five times larger than those of the well-known quartz NSPUDT orientation (Euler angles: 0 degrees 132.75 degrees 25 degrees) can be obtained with the new materials, for orientations where the optimum NSPUDT reflection coefficient phase criterion is met. The new materials discussed here provide an opportunity to design shorter- and higher-bandwidth devices due to: the increased electromechanical coupling coefficient; the higher reflection coefficient; and the lower phase velocities; while maintaining the excellent temperature insensitivity characteristic of quartz. The orientation regions suggested in this paper for the new materials are thus very promising for low-loss, high-performance consumer and communications SAW designs, such as NSPUDT filters, resonator-based filters, and other devices that can benefit from a high metal strip reflectivity.

Parameters of Coupled-Mode Equations for a Natural Single Phase Unidirectional Transducer on a La3Ga5.5Ta0.5O14 Substrate

The computed parameters of coupled-mode equations are presented for natural single phase unidirectional transducers (NSPUDTs) on La3Ga5.5Ta0.5O14 substrates with Euler angles (0°, 140°, 23°) and (12.7°, 150.1°, 36.3°). The aluminum electrode width and height dependences of the coefficients of coupled-mode equations are investigated in detail. Numerical results show that the amplitude of mutual-coupling coefficient κ12 for an NSPUDT on a (0°, 140°, 23°) La3Ga5.5Ta0.5O14 substrate is larger than that on a (12.7°, 150.1°, 36.3°) La3Ga5.5Ta0.5O14 substrate for the same electrode width and height within the range of 0.5≤w/p and 0.04 ≤h/p. Our results of the insertion loss for a filter which consists of an NSPUDT and two split interdigital transducers on a (0°, 140°, 23°) La3Ga5.5Ta0.5O14 substrate agree well with previous experimental results.

Coefficients of Coupled-Mode Equations for Interdigital Transducers with Solid Electrodes of Gold, Silver, Tantalum, or Tungsten Alloy on a 50°Y-25°X La3Ga5SiO14 Substrate

Computed results of all the coefficients of coupled-mode equations are presented for interdigital transducers with solid electrodes of gold, silver, tantalum, or tungsten alloy on a 50° Y-25° X La3Ga5SiO14 substrate. The electrode thickness and width dependences of the coefficients for tantalum or tungsten electrodes are investigated in detail. Numerical results show that the phase of the mutual-coupling coefficient for natural single phase unidirectional transducers (NSPUDTs) with tantalum or tungsten electrodes do not increase monotonically with increasing electrode height. Our results of the effective velocity and the difference between the phase of the mutual-coupling coefficient and two times the phase of the transduction coefficient agree well with the earlier experimental results.

Parameters of Coupling-of-Modes Equations for a Natural Single-Phase-Unidirectional Transducer on a La3Ga5.5Nb0.5O14 Substrate

Parameters included in coupling-of-modes equations such as coupling coefficients, transduction coefficients and electrostatic capacitances are theoretically determined for a natural single-phase unidirectional transducer (NSPUDT) on a La3Ga5.5Nb0.5O14 substrate with Euler angles of (110°, 145°, 145°) and an electromechanical coupling coefficient larger than quartz. In the hybrid finite-element method (HFEM), used for determining these parameters, all the effects of anisotropy of the substrate, piezoelectric perturbation, mechanical perturbation and energy storage are taken into account. The electrode thickness and width dependence of the parameters and the excitation characteristics of NSPUDT are investigated in detail. The parameters obtained by the present method are compared with those obtained by the perturbation method. Furthermore, numerical results for the frequency characteristics of NSPUDT are compared to the experimental data, and the validity and usefulness of the present method are demonstrated.

Coefficients of Coupled-Mode Equations for a Natural-Single-Phase-Unidirectional Transducer and an Electrode-Width-Difference-Reversal-of-Directivity Transducer on a 50° Y-24° X La3Ga5SiO14 Substrate

Computed results of all the coefficients of coupled-mode equations are presented for a natural-single-phase-unidirectional transducer (NSPUDT) and an electrode-width-difference-reversal-of-directivity transducer on a 50° Y-24° X La3Ga5SiO14 substrate. In order to determine all the coefficients, edge frequencies of a stop-band, standing-wave distributions at these frequencies on a substrate surface, and static capacitances of a transducer are calculated using the hybrid finite-element method. Therefore, all the effects of piezoelectric perturbation, mechanical perturbation, and energy storage due to nonradiated bulk waves are taken into account. The aluminum electrode width dependences of the coefficients are investigated in detail. Numerical results reveal that we can choose pairs of width and height of the active electrodes for a distance of λ/8 with λ being the surface acoustic wave wavelength at the center frequency between the reflection center and the transduction center. Our results of the transducer loss for NSPUDT agree well with previous experimental ones.

Parameters in the Coupling-of-Modes Equations for a Natural Single-Phase Unidirectional Transducer and a Transduction Center Shift Reversal of Directivity Transducer on a La3Ga5SiO14 Substrate

Parameters included in coupling-of-modes equations such as coupling coefficient, transduction coefficient and electrode capacitance are theoretically determined using the finite-element method (FEM) for a natural single-phase unidirectional transducer (NSPUDT) and a transduction center shift reversal of directivity transducer (TCS-RDT) on La3Ga5SiO14 piezoelectric single crystal which has a higher electromechanical coupling coefficient compared to quartz. The electrode thickness dependence of the parameters and the excitation characteristics of NSPUDT and TCS-RDT are investigated in detail. For NSPUDT, the parameters obtained by the present method are compared with those obtained by the perturbation method. Furthermore, numerical results for the frequency characteristics of NSPUDT are compared to the experimental data, and the validity and usefulness of the present method are demonstrated.

Natural Single-Phase Unidirectional Transducer Orientations on Doubly Rotated Y-Cut Langasite Substrates

Natural Single-Phase Unidirectional Transducer Orientations on Doubly Rotated Y-Cut Langasite Substrates

Low-Loss Surface Acoustic Wave Filter on Natural-Single Phase Unidirectional Transducter Orientations of a Li 2B 4O 7 Substrate

Natural-single phase unidirectional transducers (NSPUDT) have attracted much attention for the implementation of low-loss surface acoustic wave (SAW) devices. In this paper, we describe a low-loss SAW filter using NSPUDT orientations of a Li2B4O7 substrate previously reported by the authors. Since the unidirectionality of NSPUDT is inherent to the substrate orientation, it is necessary to make the receiving transducer unidirectional in the opposite direction in order to obtain a low-loss filter using two transducers with reverse directivity. A new transducer configuration to reverse the directivity of an NSPUDT substrate is proposed, which utilizes reflector banks within the ladder transducer. The configuration has the advantage that it can be fabricated through a single level and a single metallization with minimum electrode width of λ/4. The principle of the operation of a reversal of directivity transducer is described. A 200-MHz-band low-loss SAW transversal filter with minimum insertion loss below 3 dB and low passband ripple are demonstrated with a theoretical filter response using coupling-of-modes theory.

Properties of interdigital transducers in relation to the substrate crystal symmetry

Exclusively from the crystal symmetry, we obtain results concerning properties of interdigital transducers radiating Rayleigh waves along high-symmetrical crystal directions. Three cases of high-symmetrical directions are considered: the propagation direction is parallel to a rotation axis, perpendicular to a mirror plane, or parallel to a rotoinversion axis of the substrate crystal. In respect to the symmetry properties of the piezoelectric tensor related to a mirror operation on a plane perpendicular to the propagation direction, the three original cases can be concentrated in three symmetry cases. The crystal symmetry determines the properties of the piezoelectric tensor, and consequently the wave excitation by transducers. These considerations enable one to ascertain the positions of the transduction centers and the influence of internal reflections to the amplitude characteristics. Furthermore, we are able to discuss natural single-phase unidirectional transducer (NSPUDT) effects in layer structures, the old question "crossed-field or in-line model", and the existence of cross-coupling of waves, similar to multistrip couplers, in interdigital transducers (IDTs). The theoretical predictions are compared with our own and other authors' experimental results.

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.

Crystal orientations for natural single phaseunidirectional transducer (NSPUDT) on Li 2 B 4 O 7

New natural single phase unidirectional transducer (NSPUDT) orientations on a lithium tetraborate (Li2B4O7) substrate are reported. The orientations of the Euler angles (0°, 18°,90°) and (0°, 51°, 90°) give an optimum phasing (±45°) between the transduction and reflection centres for an ideal NSPUDT action. The (0°, 78°, 90°) cut with zero temperature coefficient of delay (TCD) gives an NSPUDT with high directivity.

Arbitrarily oriented SAW gratings: network model and the coupling-of-modes description

A network model for surface-acoustic-wave (SAW) structures fabricated along arbitrary and hence nonsymmetric orientations, including natural single-phase unidirectional transducer (NSPUDT) directions, is described. From the predictions of this model for SAW gratings. the frequently used coupling of modes (COM) phenomenological description is evaluated. The network model consists of the usual sequence of mismatched transmission lines with susceptance loading at discontinuities to account for energy storage, plus a new additional reactive element calculated using a perturbation formula, to account for the asymmetry that exists for arbitrary orientations. The circuit elements are determined by geometrical and material parameters. It is demonstrated that the incremental COM description can be derived from the unit-cell-based network model, and analytical formulas quoted in the literature for the COM coefficients are compared with the values calculated from the ABCD network model description. An analytically tractable approximation for the network model is also described, from which rather simple explicit formulas for the COM coefficients that predict their dependence on material parameters and on frequency are derived. The analysis and numerical calculations indicate that these formulas may yield results that are valid over almost 30% relative bandwidths.