Optimal cut for leaky SAW on LiTaO3 for high performance resonators and filters

The paper describes how the characteristics of leaky surface acoustic wave (LSAW) propagation depend on the thickness of Al grating electrodes on rotated Y-X LiTaO3. It is shown that the propagation loss arising from leaky nature changes parabolically with both the grating electrode thickness and rotation angle and becomes zero when electrode thickness and rotation angle are properly determined. This means that even when thick grating electrodes are needed in device design, zero propagation loss is always realized by properly determining the rotation angle. When the grating electrode thickness is 0.07 to 0.1 in wavelength for example, LSAWs on 40-42 degrees Y-X LiTaO3 give zero propagation loss without deteriorating other characteristics. Ladder-type filters for the 800-MHz range were fabricated, which essentially need thick Al grating electrodes of about 0.1 wavelength thickness. As predicted by theoretical calculation, experimental results showed that if the rotation angle is increased to circa 420 from a conventional value of 36 degrees, the insertion loss and shape factor are markedly improved compared with devices based on 36 degrees Y-X LiTaO3. This is essentially a result of the minimized propagation loss.     

Formation and properties of proton-exchanged and annealed LiNbO3 waveguides for surface acoustic wave

The proton-exchanged (PE) and annealed PE (APE) z-cut LiNbO₃ waveguides were fabricated using H₄P₂O₇. The positive strain, c-axis lattice constant change (Δc/c), was calculated to be about +0.43%, which was almost independent of the exchanged conditions. The penetration depth of H measured by secondary ion mass spectrometry (SIMS) exhibited a step-like profile, which was assumed to be equal to the waveguide depth (d). The surface acoustic wave (SAW) properties of PE and APE z-cut LiNbO₃ samples were investigated. The phase velocity (V_p) and electromechanical coupling coefficient (K ²) of PE samples were significantly decreased by the increase of kd, where k was the wavenumber (2π/λ). The insertion loss (IL) of PE samples was increased by the increase of kd and became nearly constant at kd>0.064. The temperature coefficient of frequency (TCF) of PE samples allowed an apparent increase with kd, reaching a maximum at kd=0.292, then slightly decreased at higher kd. The effects of annealing resulted in a restoration of V_p and an improvement of IL     

Effect of a ferroelectric inversion layer on the temperature characteristics of SH-Type surface acoustic waves on 36 degrees Y-X LiTaO (3) substrates

The temperature coefficient of delay of the SH-type surface acoustic wave on 36 degrees Y-X LiTaO(3) substrates with a ferroelectric inversion layer is theoretically analyzed and it is shown that the temperature characteristics can be improved by an electric-field short-circuiting effect of the domain boundary. The experimental results of the temperature coefficient of delay agree well with the theoretical. The minimum value of measured temperature coefficients of delay is 12.6 ppm/ degrees C for the metallized surface case, which is about one-third of that in a 36 degrees Y-X substrate with no inversion layer. Experimental results on the phase velocity and the electromechanical coupling factor of the SH-type surface acoustic wave are also presented and compared with the theoretical.     

Proton-exchanged Z-cut LiNbO3 waveguides for surfaceacoustic waves

Proton-exchanged (PE) waveguides in Z-cut LiNbO₃ have been fabricated using benzoic acid. Secondary-ion mass spectrometry (SIMS) measurements show that the distribution of hydrogen in the PE Z-cut LiNbO₃ samples exhibits a step-like profile with the diffusion constant D₀ and the activation energy Q of about 2.82×10⁸ μm²/h and 87.76 kJ/mol, respectively. On the other hand, the important parameters for the design of surface acoustic wave (SAW) devices are measured and discussed. The results show that the phase velocity and electromechanical coupling coefficient decrease with the increase of kd, where k is the wavenumber and d is the waveguide depth. The variation of insertion loss becomes saturated at about kd=0.068 with a maximum increase of about 4~5 dB. The temperature coefficient of delay calculated from the frequency change of the output of SAW delay line shows an evident increase in the PE layer. Moreover, the effects of postannealing can result in a restoration of the decreased velocity and an improvement of the insertion loss     

Surface acoustic wave properties of proton-exchanged LiNbO3 waveguides with SiO2 film

Surface acoustic wave (SAW) properties of proton-exchanged (PE) z-cut lithium niobate (LiNbO3) waveguides with silicon dioxide (SiO2) film layers were investigated using octanoic acid. The distribution of hydrogen measured by secondary ion mass spectrometry (SIMS) showed a step-like profile, which was assumed to be equal to the waveguide depth (d). The SiO2 film was deposited on z-cut LiNbO3 waveguide by radio frequency (rf) magnetron sputtering. We investigated the important parameters for the design of SAW devices such as phase velocity (Vp), insertion loss (IL) and temperature coefficient of frequency (TCF) by a network analyzer using thin-film aluminum interdigital transducer electrodes on the upper SiO2 film surface. The experimental results showed that the Vp of SAW decreased slightly with the increase of h/lambda, where h was the thickness of SiO2 films and lambda was the wavelength. The IL of SAW increased with increased h/lambda. The TCF of SAW calculated from the frequency change of the output of SAW delay line showed an evident decrease with the increase of h/lambda. The TCF for PE z-cut LiNbO3 was measured to be about -54.72 ppm/degreees C at h/lambda = 0.08. It revealed that the SiO2 films could compensate and improve the temperature stability as compared with the TCF of SAW on PE samples without SiO2 film.     

Evaluation and selection of LiNbO(3) and LiTaO(3) substrates for SAW devices by the LFB ultrasonic material characterization system

This paper demonstrates the evaluation and selection of commercially available LiNbO(3) and LiTaO(3) single crystals and wafers for surface acoustic wave (SAW) devices using the line-focus-beam ultrasonic material characterization (LFB-UMC) system. This system enables measuring leaky-SAW (LSAW) propagation characteristics precisely and efficiently for a number of specimens. The wafers are prepared from the top, middle, and bottom parts of four 128 degrees YX LiNbO(3) and seven X-112 degrees Y LiTaO(3) single crystals. For both series of crystals, the measured LSAW velocities increase from top to bottom in the crystals and with the increasing crystal growth number. The velocity changes for all wafers are 0.036% for 128 degrees YX LiNbO(3) and 0.035% for X-112 degrees Y LiTaO(3), corresponding to changes of 0.038 mol% and 0.075 mol% in Li(2)O concentration, respectively. Moreover, the inhomogeneity in the first X-112 degrees Y LiTaO(3) single crystal caused by some undesirable wafer fabrication processes can be detected precisely, although it is difficult for the conventional methods to obtain such information.     

Annealing behavior of barriers in ion-implanted LiNbO3 and LiTaO3 planar waveguides

We formed planar waveguides in LiNbO(3) and LiTaO(3) crystals by megaelectron volt He-ion implantation. The dark modes of both waveguides are measured and their refractive-index profiles are described according to the parameterized index profile reconstruction method. The extraordinary indices of both ion-implanted waveguides exhibit quite different profiles. We compare the thermal stability of barriers in ion-implanted LiNbO(3) and LiTaO(3) waveguides by annealing at different temperatures. The results show that the barrier in a LiTaO(3) planar waveguide has higher thermal stability than that in a LiNbO(3) waveguide. The experiments also show that annealing at a temperature higher than 400 degrees C results in recrystallization of the barrier.     

Planar LiTaO(3) waveguides fabricated by proton exchange in concentrated and diluted pyrophosphoric acid with annealing

Annealed proton-exchanged z-cut LiTaO(3) planar waveguides fabricated with pyrophosphoric acid have been characterized. For proton exchange, the extraordinary index increase Δ n(e) ranged from 0.0119 to 0.0141, depending on the exchange temperature. The effective diffusion coefficient D(e) ranged from 0.1325 μm(2)/h at 240°C to 0.545 μm(2)/h at 280 °C. Single-mode propagation losses were α = 0.7 dB/cm. Compared with benzoic acid, pyrophosphoric acid produces waveguides with a higher Δ n(e) and a lower propagation loss. For proton exchange in lithium phosphate-diluted pyrophosphoric acid, a lower Δ n(e) was obtained, but D(e) and propagation losses were not reduced. After proton exchange, the waveguides were annealed. The surface index initially increased, peaked, and then decreased. The waveguide depth d and the surface index n(s) were measured at regular intervals. Figures and empirical formulas relating the waveguide depth d and the surface index increase Δ n(s) to the anneal time, anneal temperatures, and the waveguide depth after proton exchange are given. An example is given in which, the desired waveguide parameters d and Δ n(s), the fabrication conditions could be calculated with the previously derived formulas. Propagation losses decreased to 0.4 dB/cm after prolonged annealing.     

Calibration of Curie temperatures for LiTaO/sub 3/ single crystals by the LFB ultrasonic material characterization system

The line-focus-beam ultrasonic material characterization (LFB-UMC) system is applied to a standardized comparison and evaluation of the Curie temperatures, T/sub C/, exclusively used in evaluating the chemical compositions of commercial LiTaO/sub 3/ crystals by measuring the velocities of Rayleigh-type leaky surface acoustic waves (LSAWs), V/sub LSAW/. We measured V/sub LSAW/ and T/sub C/ (standardized) under the same T/sub C/ measurement conditions for 36/spl deg/Y X-LiTaO/sub 3/ single-crystal wafers produced by four manufacturers and related the results to the T/sub C/ (individual) measured by the individual manufacturers. The relationships between V/sub LSAW/ and T/sub C/ (individual) varied from one company to another, and a single straight line of the proportional relationship between V/sub LSAW/ and T/sub C/ (standardized) was obtained for all wafers regardless of the manufacturer. These experimental results clarify that the problem associated with T/sub C/ measurements lies in the measurement conditions and the absolute accuracy of the measurement instruments. Measurements of the center frequencies of SH-type surface acoustic wave (SAW) filter devices are compared with V/sub LSAW/ measurements. A method of calibrating T/sub C/ using this ultrasonic system is proposed to establish standardized specifications of SAW-device crystal wafers.     

Conductance measurements on a leaky SAW harmonic one-port resonator

Conductance measurements are reported on a leaky SAW (LSAW) harmonic one-port resonator on a 64 degrees Y-X LiNbO(3) substrate. This employed a short three-finger IDT for fundamental and second harmonic operation together with long reflection gratings. Conductances were measured with and without the end gratings. From an analysis of the measurements, it was deduced that, for optimum second harmonic performance, the grating stop-band frequency should be higher than the IDT unperturbed center frequency. This result is in contrast to fundamental frequency resonator designs in which the end grating stop-band frequency is placed below the IDT center frequency for optimum performance.     

Theoretical studies on leaky-SAW properties influenced by layers on anisotropic piezoelectric crystals

Theoretical studies on the behavior of leaky-SAW (LSAW) properties in layered structures were performed. For these calculations rotYX LiTaO (3) and rotYX LiNbO(3) LSAW crystal cuts were used, assuming different layer materials. For LSAWs both the velocity and the inherent loss due to bulk wave emission into the substrate are strongly influenced by distinct layer parameters. As a result, these layer properties like elastic constants or thickness have shown a strong influence on the crystal cut angle of minimum LSAW loss. Moreover, for soft and stiff layer materials, a different shift of the LSAW loss minimum can occur. Therefore, using double-layer structures, the shift of the LSAW loss minimum can be influenced by appropriate chosen layers and ratios.     

Determination of the true congruent composition for LiTaO3 single crystals using the LFB ultrasonic material characterization system.

The true congruent composition for LiTaO3 single crystals was determined by measuring the velocities of leaky surface acoustic waves (LSAWs) with the line-focus-beam ultrasonic material characterization (LFB-UMC) system for two 42 degrees YX-LiTa3s crystal ingots. The congruent composition determined here was 48.460 Li2O-mol%, corresponding to the LSAW velocity (42 degrees YX-LiTaO3) of 3125.3 m/s, and the absolute relationship between the LSAW velocity and chemical composition was obtained. Simulations on the variation of the melt and crystal compositions in a mass production of 100 crystals were conducted as a function of the composition of the starting material around the congruent composition. The result showed that the distributions of the melt and crystal compositions within and among the crystals varied largely with the material composition, providing the relationship of the material composition with the maximum composition variation for the 100 crystals. Based on these results, we verified the relationships between the tolerance of the material composition variation and the tolerances for the SH-type SAW velocity, LSAW velocity, and Curie temperature. The material composition needs to be constrained to within +/- 0.007 Li2O-mol% around the congruent composition to mass-produce the crystals with reliable homogeneity, satisfying the tolerance of +/- 0.01% in the SAW velocity. Furthermore, a guideline for the specification of reliable piezoelectric SAW-device wafer substrates was presented with the accurate interrelationships among the chemical composition ratio, LSAW velocity, and Curie temperature.     

Side radiation of Rayleigh waves from synchronous SAW resonators

Surface acoustic wave (SAW) resonators on lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are investigated. The amplitude of the acoustic fields in the resonators are measured using a scanning laser interferometer. The amplitude profiles of the surface vibrations reveal the presence of distinct acoustic beams radiated from the transducer region of the SAW resonators and propagating with low attenuation. We suggest that this radiation is generated by the charges accumulating at the tips of the finger electrodes. The periodic system of sources, namely oscillating charges at the fingertips, generates Rayleigh-wave beams in the perpendicular and oblique directions. Green's function theory is used to calculate the coupling strength and slowness of the Rayleigh waves on 42 degrees Y-cut LiTaO3 and Y-cut LiNbO3 substrates as a function of the propagation direction. Furthermore, the propagation angles of the Rayleigh-wave beams as a function of frequency are calculated. The computed angles are compared with the measured ones for both the LiTaO3 and LiNbO3 substrates.     

Standardized evaluation of chemical compositions of LiTaO3 single crystals for SAW devices using the LFB ultrasonic material characterization system

The line-focus-beam ultrasonic material characterization (LIFB-UMC) system is applied to compare and evaluate tolerances provided independently for the Curie temperature T/sub C/ and lattice constant /spl alpha/ to evaluate commercial LiTaO/sub 3/ single crystals by measuring the Rayleigh-type leaky surface acoustic wave (LSAW) velocities V/sub LSAW/. The relationships between VLSAW, and T/sub C/ and /spl alpha/ measured by individual manufacturers were obtained experimentally using 42/spl deg/YX-LiTaO/sub 3/ wafers as specimens from three crystal manufacturers. In addition, the relationship between VLSAW and SH-type SAW velocities V/sub SAW/ that are actually used for the SAW device wafers was obtained through calculations, using the chemical composition dependences of the acoustical physical constants for LiTaO/sub 3/ crystals reported previously. The result of a comparison between the T/sub C/ tolerance of /spl plusmn/3/spl deg/C and the /spl alpha/ tolerance of /spl plusmn/0.00002 nm through the common scale of VLSAW or VSAW demonstrated that the /spl alpha/ tolerance is 1.6 times larger than the T/sub C/ tolerance. Furthermore, we performed a standardized comparison of statistical data of T/sub C/ and /spl alpha/ for LiTaO/sub 3/ crystals grown by two manufacturers during 1999 and 2000, using VLSAW. The results clarified the differences of the average chemical compositions and of the chemical composition distributions among the crystal ingots between the two manufacturers. A guideline for the standardized evaluation procedure has been established for the SAW-device wafer specifications by the LFB-UMC system.     

Equivalent networks for SAW gratings

An equivalent-network approach is described for the analysis of surface-acoustic-wave gratings. Circuit parameters can be theoretically determined by applying the finite-element method to an infinite array. In this approach, all of 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 groove and metallic gratings. Both short and open circuited metallic gratings are treated. For grooves on isotropic and Y-Z LiNbO(3) substrates, the dependence of reflection characteristics on groove depth is investigated. For aluminum strips on X-112 degrees Y LiTaO(3) 34 degrees Y-X quartz, Y-Z LiNbO(3), and 128 degrees Y -X LiNbO(3) substrates, the dependence on metallization ratio is investigated in detail.     

Collinear guided wave to leaky wave acoustooptic interactions in proton-exchanged LiNbO(3) waveguides

The results of a detailed theoretical study on collinear guided wave to leaky wave acoustooptic (AO) interactions in proton-exchanged LiNbO(3) (PE:LiNbO(3)) planar waveguides are presented. The guided-to-leaky mode conversion for an input optical beam at the wavelength of 632.8 nm by the induced diffraction grating from a collinear surface acoustic wave (SAW) is analyzed using a generalized multimode formulation of the coupled mode theory. Mode conversion efficiency and AO bandwidth have been calculated as functions of acoustic frequency, interaction length, guiding layer thickness, and acoustic drive power density for three cuts of the LiNbO(3) substrate. High performance configurations that are desirable for application to demultiplexing and switching in optical communication systems are identified, and the corresponding channel capacity and frequency resolution are determined. For example, it was shown that the X-cut configuration features the highest mode conversion efficiency. However, a relatively small AO bandwidth is associated with this configuration. Both high mode conversion efficiency and large AO bandwidth can be accomplished at the guiding layer thickness of 1.0 mum. A TM(o)-->TE(nu) mode conversion efficiency as high as 42% together with an AO bandwidth of approximately 70 MHz can be achieved in the Z-cut waveguide at the guiding layer thickness of 1.0 mum, acoustic drive power density of 50 mW/mm, interaction length of 40 mm, and acoustic frequency of 460 MHz. The corresponding channel capacity and frequency resolution are 745 and 0.09 MHz, respectively. Measured mode conversion efficiencies as high as 90 and 78% obtained at the acoustic frequencies of 107 and 367 MHz using the X-cut substrate and the Y-propagation SAW have verified the theoretical prediction on the mode conversion efficiencies.     

A low-loss SAW-TV-IF filter with an extended impedance matching range

A novel low-loss SAW (surface acoustic wave) filter for an intermediate frequency (IF) circuit in a color TV receiver has been developed. It consists of an apodized bidirectional and an unapodized group-type unidirectional transducer. The unidirectional transducer is designed to use different numbers of finger pairs in sending and reflecting electrodes for extension of the impedance-matching range. A thin-film capacitor for use as a phase shifter is monolithically fabricated on a 128 degrees Y-X LiNbO(3) substrate. A low insertion loss (11.3 dB) and impedance matching without adjustment are achieved at the same time without increasing the device chip size or number of electrical parts.     

Characterization of 36°YX-LiTaO3 wafers byline-focus-beam acoustic microscopy

Application of line-focus-beam (LFB) acoustic microscopy is extended to characterization of substrates for SH-type SAW devices. Theoretical and experimental studies on a wave mode for characterization are carried out on 36°Y-cut LiTaO₃ wafers. A Rayleigh-type mode of leaky surface acoustic waves (LSAWs) must be employed instead of an SH-type mode of leaky pseudo-surface waves (LPSAWs). Experimental results show that the LSAW propagation should be directed along the X-axis because the LSAW velocities are more sensitive to chemical composition and elastic inhomogeneities. The relations among the LSAW velocities, densities, and Curie temperatures are determined. The LSAW velocity increases linearly at the rate of 0.52 m/s/°C with the Curie temperature. A chemical composition change of 0.03 Li₂ O-mol%, corresponding to temperature resolution of better than 0.3°C, is easily detected by the velocity measurements. Elastic inhomogeneities due to residual multi-domains, produced during the poling process during wafer fabrication, are interpreted quantitatively by this ultrasonic technology     

Longitudinal-mode leaky SAW resonator filters on 64° Y-Xlithium niobate

Y-parameter modeling is applied to obtain the frequency response of a first/third dual-mode longitudinally-coupled leaky-SAW (LSAW) resonator-filter on 64°C Y-X LiNbO₃. Conventional driving-point and transfer admittance relationships for SAW filters are modified to incorporate the effects of LSAW reflection and bulk-wave scattering in the interdigital transducers and are expanded to include reflection gratings with shorted-metal strips. As inferred from experiment, the grating parameters incorporate a phase-shift term to relate the anomalous dispersion of shorted gratings, as well as a modified phase-slope parameter between input and output interdigital transducers (IDT's). Good agreement is obtained with experimental results published for 836.5 MHz two- and four-pole structures in the maximum-bandwidth configuration     

Accurate measurements of the acoustical physical constants of LiNbO (3) and LiTaO(3) single crystals

The acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of commercial surface acoustic wave (SAW)-grade LiNbO(3) and LiTaO(3) single crystals were determined by measuring the bulk acoustic wave velocities, dielectric constants, and densities of many plate specimens prepared from the ingots. The maximum probable error in each constant was examined by considering the dependence of each constant on the measured acoustic velocities. By comparing the measured values of longitudinal velocities that were not used to determine the constants with the calculated values using the previously mentioned constants, we found that the differences between the measured and calculated values were 1 m/s or less for both LiNbO(3) and LiTaO(3) crystals. These results suggest that the acoustical physical constants determined in this paper can give the values of bulk acoustic wave velocities with four significant digits.