A full set of langatate high-temperature acoustic wave constants: elastic, piezoelectric, dielectric constants up to 900°C.

A full set of langatate (LGT) elastic, dielectric, and piezoelectric constants with their respective temperature coefficients up to 900°C is presented, and the relevance of the dielectric and piezoelectric constants and temperature coefficients are discussed with respect to predicted and measured high-temperature SAW propagation properties. The set of constants allows for high-temperature acoustic wave (AW) propagation studies and device design. The dielectric constants and polarization and conductive losses were extracted by impedance spectroscopy of parallel-plate capacitors. The measured dielectric constants at high temperatures were combined with previously measured LGT expansion coefficients and used to determine the elastic and piezoelectric constants using resonant ultrasound spectroscopy (RUS) measurements at temperatures up to 900°C. The extracted LGT piezoelectric constants and temperature coefficients show that e11 and e14 change by up to 62% and 77%, respectively, for the entire 25°C to 900°C range when compared with room-temperature values. The LGT high-temperature constants and temperature coefficients were verified by comparing measured and predicted phase velocities (vp) and temperature coefficients of delay (TCD) of SAW delay lines fabricated along 6 orientations in the LGT plane (90°, 23°, Ψ) up to 900°C. For the 6 tested orientations, the predicted SAW vp agree within 0.2% of the measured vp on average and the calculated TCD is within 9.6 ppm/°C of the measured value on average over the temperature range of 25°C to 900°C. By including the temperature dependence of both dielectric and piezoelectric constants, the average discrepancies between predicted and measured SAW properties were reduced, on average: 77% for vp, 13% for TCD, and 63% for the turn-over temperatures analyzed.

Determination and experimental verification of high-temperature SAW orientations on langatate.

Langatate (LGT) is a member of the langasite family of crystals appropriate for high-temperature frequency control and sensing applications. This paper identifies multiple LGT SAW orientations for use at high temperature, specifically in the 400°C to 900°C range. Orientations with low sensitivity to temperature are desired for frequency control devices and many sensors, conversely large temperature sensitivity is a benefit for temperature sensors. The LGT SAW temperature behavior has been calculated for orientations sweeping the Euler angles (0°, Θ, ψ), (90°, Θ, ψ), and (ψ, 90°, ψ), based on newly identified high-temperature elastic constants and temperature coefficients for this material. The temperature coefficient of delay (TCD) and total frequency change over the temperature range were analyzed from 400°C to 900°C. Multiple SAW orientations were identified with zero-TCD between 400°C and 500°C. Although no orientations that have turn-over temperatures above 500°C were identified, several have low frequency variation with temperature, of the order of -0.8% over the range 400°C to 800°C. Temperature-sensitive orientations with TCD up to 75 ppm/°C at 900°C were identified, with potential for high-temperature sensor applications. The reported predictions are shown to agree with measured behavior of LGT SAW delay lines fabricated along 6 orientations in the (90°, 23°, ψ) plane. In addition, this work demonstrates that concurrently operated LGT SAW devices fabricated on the same wafer provide means of temperature sensing. In particular, the measured frequency difference between delay lines oriented along (90°, 23°, 0°) and (90°, 23°, 48°) has fractional temperature sensitivity that ranges from -172 ppm/°C at 25°C to -205 ppm/°C at 900°C.

Improved SHSAW transduction efficiency using grating and uniform electrode guiding.

Pure shear-horizontal surface acoustic wave (SHSAW) devices have been increasingly considered for liquidphase and biosensing applications because of their ability to operate under liquid-loaded conditions and intrinsic sensitivity to mass, stiffness, viscosity, and electrical perturbations occurring at the device/fluid interface. Typically, the SHSAW is weakly guided by a free surface boundary condition (BC) or may not even exist for some materials and orientations, such as the quartz ST-90° orientation considered in this work. For a surrounding free surface BC, the interdigital transducer (IDT) typically generates strong shear-horizontal bulk acoustic waves (SHBAWs) relative to SHSAW. For that reason, guiding structures, e.g., dense and/or thick electrodes in periodic or uniform configurations, are incorporated into the design and placed between IDTs in delay-line devices to increase the ratio of transduced SHSAW power to IDT input power, ηSHSAW. The degree of ηSHSAW improvement depends on the thickness, composition, and geometry of the guiding structure. In previous work, the authors evaluated ηSHSAW using hybrid finite and boundary element method (FEM/BEM) models, but were limited to cases of stress-free or finite-thickness-grating surrounding surfaces. This work extends the analysis to the important boundary condition case of uniform finite-thickness electrode guiding, which is typically employed in liquid-phase and biosensor applications. To integrate the uniform electrode guiding structure with the SHSAW device analysis, a combined finite-length uniform electrode structure followed by an additional quarter-wavelength electrode was considered. In this work, it is shown that adjusting the quarter-wavelength electrode's film thickness and length allows cancellation of the SHSAW reflection from the edge discontinuity. As a result, the finite-length uniform guiding electrode can be treated as if it extends to infinity, and ηSHSAW can be easily obtained. In addition, the finite thickness of all electrodes is considered in the calculations. To verify the model, an IDT with uniform guiding electrodes was simulated and compared with the experimental results of a fabricated and tested device. The simulations predict SHSAW excitation directivity of 9 dB by the IDT, which is experimentally confirmed to within 0.8 dB.

High-temperature langatate elastic constants and experimental validation up to 900 degrees C.

This paper reports on a set of langatate (LGT) elastic constants extracted from room temperature to 1100 degrees C using resonant ultrasound spectroscopy techniques and an accompanying assessment of these constants at high temperature. The evaluation of the constants employed SAW device measurements from room temperature to 900 degrees C along 6 different LGT wafer orientations. Langatate parallelepipeds and wafers were aligned, cut, ground, and polished, and acoustic wave devices were fabricated at the University of Maine facilities along specific orientations for elastic constant extraction and validation. SAW delay lines were fabricated on LGT wafers prepared at the University of Maine using 100-nm platinumrhodium- zirconia electrodes capable of withstanding temperatures up to 1000 degrees C. The numerical predictions based on the resonant ultrasound spectroscopy high-temperature constants were compared with SAW phase velocity, fractional frequency variation, and temperature coefficients of delay extracted from SAW delay line frequency response measurements. In particular, the difference between measured and predicted fractional frequency variation is less than 2% over the 25 degrees C to 900 degrees C temperature range and within the calculated and measured discrepancies. Multiple temperature-compensated orientations at high temperature were predicted and verified in this paper: 4 of the measured orientations had turnover temperatures (temperature coefficient of delay = 0) between 200 and 420 degrees C, and 2 had turnover temperatures below 100 degrees C. In summary, this work reports on extracted high-temperature elastic constants for LGT up to 1100 degrees C, confirmed the validity of those constants by high-temperature SAW device measurements up to 900 degrees C, and predicted and identified temperature-compensated LGT orientations at high temperature.

Pulse echo and combined resonance techniques: a full set of LGT acoustic wave constants and temperature coefficients.

This work reports on the determination of langatate elastic and piezoelectric constants and their associated temperature coefficients employing 2 independent methods, the pulse echo overlap (PEO) and a combined resonance technique (CRT) to measure bulk acoustic wave (BAW) phase velocities. Details on the measurement techniques are provided and discussed, including the analysis of the couplant material in the PEO technique used to couple signal to the sample, which showed to be an order of magnitude more relevant than the experimental errors involved in the data extraction. At room temperature, elastic and piezoelectric constants were extracted by the PEO and the CRT methods and showed results consistent to within a few percent for the elastic constants. Both raw acquired data and optimized constants, based on minimization routines applied to all the modes involved in the measurements, are provided and discussed. Comparison between the elastic constants and their temperature behavior with the literature reveals the recent efforts toward the consistent growth and characterization of LGT, in spite of significant variations (between 1 and 30%) among the constants extracted by different groups at room temperature. The density, dielectric permittivity constants, and respective temperature coefficients used in this work have also been independently determined based on samples from the same crystal boule. The temperature behavior of the BAW modes was extracted using the CRT technique, which has the advantage of not relying on temperature dependent acoustic couplants. Finally, the extracted temperature coefficients for the elastic and piezoelectric constants between room temperature and 120 degrees C are reported and discussed in this work.

A lateral-field-excited LiTaO3 high-frequency bulk acoustic wave sensor.

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO(3). Prototype LFE LiTaO(3) sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.

SAW parameters on Y-cut langasite structured materials.

This paper presents results and investigations of several new, man-made piezoelectric single crystal, Czochralski-grown substrate materials for surface acoustic waves (SAW) applications. These materials, langanite (LGN), langatate (LGT), Sr3TaGa3Si2O14 (STGS), Sr3NbGa3Si2O14 (SNGS), Ca3TaGa3Si2O14 (CTGS), and Ca3NbGa3Si2O14 (CNGS), have the same structure as langasite (LGS) and are of the same crystal class as quartz. These compounds are denser than quartz, resulting in lower phase velocities. They also have higher coupling. Unlike quartz and lithium niobate, there is no degradation of material properties below the material melting points resulting in the possibility of extreme high-temperature operation (> 1000 degrees C). This paper gives a summary of extracted SAW material parameters for various propagation angles on Y-cut substrates of the six materials. Parameters included are electromechanical coupling, phase velocity, transducer capacitance, metal strip reflectivity, and temperature coefficient of frequency. Using previously published fundamental material constants, extracted parameters are compared with predictions for LGT and LGN. In addition, power flow angle and fractional frequency curvature data are reported for propagation angles on CTGS and CNGS Y-cut substrates that exhibit temperature compensation near room temperature. Detailed descriptions of the SAW parameter extraction techniques are given. A discussion of the results is provided, including a comparison of extracted parameters and an overview of possible SAW applications.

Nucleic acid sensing by regenerable surface-associated isothermal rolling circle amplification.

A novel method for regenerating biosensors has been developed in which the highly specific detection of nucleic acid sequences is carried out using molecular padlock probe (MPP) technology and surface-associated rolling circle amplification (RCA). This technique has a low occurrence of false positive results when compared to polymerase chain reaction, and is an isothermal reaction, which is advantageous in systems requiring low power consumption such as remote field sensing applications. Gold-sputtered 96-well polystyrene microplates and a fluorescent label were used to explore the detection limits of the surface-associated RCA technique, specificity for different MPP, conditions for regeneration of the biomolecular sensing surface, and reproducibility of measurements on regenerated surfaces. The technique was used to create highly selective biomolecular surfaces capable of discriminating between DNA oligonucleotides with sequences identical to RNA from infectious salmon anemia (ISA) and infectious hematopoietic necrosis (IHN) virus. As little as 0.6 fmol of circularized MPP was detectable with this fluorimetric assay. The sensing layers could be reused for at least four cycles of amplification using thermal denaturation, with less than 33% decrease in RCA response over time. Because the nucleic acid product of the test is attached to a surface during amplification, the technique is directly applicable to a variety of existing sensing platforms, including acoustic wave and optical devices.

FEM/BEM impedance and power analysis for measured LGS SH-SAW devices.

Pure shear horizontal piezoelectrically active surface and bulk acoustic waves (SH-SAW and SH-BAW) exist along rotated Y-cuts, Euler angles (0 degrees, theta, 90 degrees), of trigonal class 32 group crystals, which include the LGX family of crystals (langasite, langatate, and langanite). In this paper both SH-SAW and SH-BAW generated by finite-length, interdigital transducers (IDTs) on langasite, Euler angles (0 degrees, 22 degrees, 90 degrees), are simulated using combined finite- and boundary-element methods (FEM/BEM). Aluminum and gold IDT electrodes ranging in thickness from 600 A to 2000 A have been simulated, fabricated, and tested, with both free and metalized surfaces outside the IDT regions considered. Around the device's operating frequency, the percent difference between the calculated IDT impedance magnitude using the FEM/BEM model and the measurements is better than 5% for the different metal layers and thicknesses considered. The proportioning of SH-SAW and SH-BAW power is analyzed as a function of the number of IDT electrodes; type of electrode metal; and relative thickness of the electrode film, h/wavelength, where wavelength is the SH-SAW wavelength. Simulation results show that moderate mechanical loading by gold electrodes increases the proportion of input power converted to SH-SAW. For example, with a split-electrode IDT, comprising 238 electrodes with a relative thickness h/wavelength = 0.63% and surrounded by an infinitesimally thin conducting film, nearly 9% more input power is radiated as SH-SAW when gold instead of aluminum electrodes are used.

Pure SH-SAW propagation, transduction and measurements on KNbO3.

Potassium niobate (KNbO3) supports the electromechanically active pure shear horizontal surface acoustic wave (SH-SAW) mode along Z-axis cylinder orientations, Euler angles (phi, 90 degrees, 0 degrees), in which two uncoupled wave solutions exist: a purely mechanical sagittal Rayleigh SAW and a piezoelectrically stiffened pure SH-SAW. Within this family of cuts, a maximum electromechanical coupling coefficient for the pure SH-SAW, K2 = 53%, is observed along (0 degrees, 90 degrees, 0 degrees). This pure SH-SAW orientation also has the maximum value of electromechanical coupling observed along rotated Y-cut X propagation directions, Euler angles (0 degrees, theta, 0 degrees). The use of the pure SH-SAW mode is attractive for liquid-sensing applications because the SH-SAW is modestly attenuated by the adjacent liquid, unlike the generalized SAW (GSAW), which has particle displacement normal to the surface. This work investigates propagation and excitation properties of the SH-SAW and the shear horizontal bulk acoustic wave (SH-BAW) on single crystal KNbO3, Euler angles (0 degrees, 90 degrees, 0 degrees). Interdigital transducer (IDT) arrays are analyzed using boundary element method (BEM) techniques, addressing IDT properties such as: power partitioning between the SH-SAW and SH-BAW, SH-BAW radiation as a function of wave vector direction and radiation angle, and overall IDT impedance. The percentage of SH-SAW power to total input power is above 98% for IDTs containing 1.5 to 5.5 wavelengths of active electrodes with surrounding metalized regions. For nonmetalized regions outside the IDT, the ratio drops to between 1 and 2%, showing the importance of an energy trapping structure for efficient SH-SAW excitation and propagation along this orientation. Simulated and experimental IDT admittance results are compared, verifying the validity of the analysis performed. The reported measurements on the frequency variation with temperature indicate that the orientation considered is temperature compensated at about 8 degrees C. The surface of the SH-SAW devices fabricated have been loaded with deionized water and showed additional 1.6 dB transmission loss with respect to the unloaded surface, verifying the suitability of the pure SH-SAW mode on KNbO3 for liquid sensor applications.

Platinum and palladium high-temperature transducers on langasite.

There is a pressing need for the fabrication of surface acoustic wave (SAW) devices capable of operating in harsh environments, at elevated temperature and pressure, or under high-power conditions. These SAW devices operate as frequency-control elements, signal-processing filters, and pressure, temperature, and gas sensors. Applications include gas and oil wells, high-power duplexers in communication systems, and automobile and aerospace combustion engines. Under these high-temperature and power-operating conditions, which can reach several hundred degrees Centigrade, the typically fabricated aluminum (A1) thin film interdigital transducer (IDT) fails due to electro and stress migration. This work reports on high temperature SAW transducers that have been designed, fabricated, and tested on langasite (LGS) piezoelectric substrates. Platinum (Pt) and palladium (Pd) (melting points at 1769 degrees C and 1554.9 degrees C, respectively) have been used as thin metallic films for the SAW IDTs fabricated. Zirconium (Zr) was originally used as an adhesion layer on the fabricated SAW transducers to avoid migration into the Pt or Pd metallic films. The piezoelectric LGS crystal, used as the substrate upon which the SAW devices were fabricated, does not exhibit any phase transition up to its melting point at 1470 degrees C. A radio frequency (RF) test and characterization system capable of withstanding 1000 degrees C has been designed and constructed. The LGS SAW devices with Pt and Pd electrodes and the test system have been exposed to temperatures in the range of 250 degrees C to 750 degrees C over periods up to 6 weeks, with the SAW devices showing a reduced degradation better than 7 dB in the magnitude of transmission coefficient, /S21/, with respect to room temperature. These results qualify the Pt and Pd LGS SAW IDTs fabricated for the above listed modern applications in harsh environments.

A lateral field excited liquid acoustic wave sensor.

Lateral field excited (LFE) AT-cut quartz acoustic wave sensors in which the electrodes are located on the reference surface have been fabricated and tested in liquid environments. The sensing surface, which is opposite to the reference surface, is free allowing the electric field of the thickness shear mode (TSM) to penetrate into the liquid. This results in increased sensitivity to both mechanical and electrical property changes of the liquid. In the present paper, several 5-MHz LFE sensors with a range of electrode spacings were exposed to liquid environments in which the viscosity, relative permittivity, and conductivity were varied. The LFE sensors demonstrate sensitivity to viscosity that is more than twice that obtained for the standard quartz crystal microbalance (QCM), and sensitivity to relative permittivity and conductivity about 1.5 times that of the QCM sensors with modified electrodes. The present results clearly indicate that the LFE sensors may have a wide range of liquid phase applications in which sensitivity is crucial.

Pure shear horizontal SAW biosensor on langasite.

The undetected introduction of pathogens into food or water supplies can produce grave consequences in terms of economic loss and human suffering. Sensitive and selective sensors capable of quickly detecting microbial pathogens are urgently needed to limit the effects of bioterrorist incidents, accidents, or pollution. Shear horizontal surface acoustic wave (SH SAW) devices provide an attractive platform for the design of microbial biosensors that function in liquid media, where Rayleigh-type modes are rapidly attenuated. This paper reports on an exploratory SH SAW delay line designed and fabricated on langasite, La3Ga5SiO14 (LGS), along the novel Euler propagation direction (0 degrees, 22 degrees, 90 degrees). A liquid chamber was fabricated and attached to the top surface, and the device was submitted to liquid and biochemical tests. Moderate (6 dB) additional attenuation of the transmission coefficient, /S21/, was consistently observed when the SH SAW delay line was assembled in the test fixture and submitted to the liquid tests, indicating that LGS is an attractive candidate for liquid sensing. Sensor selectivity can be achieved by integrating the LGS SH SAW delay line with a biochemical recognition layer. A test setup was implemented for the characterization of LGS SH SAW-based biosensors. The delay line response to biomolecule binding was shown by detection of sequential binding of proteins to the SH SAW device delay path. The biotinylated sensor was exposed sequentially to biotin-binding deglycosylated avidin, biotin-modified rabbit IgG, and goat anti-rabbit IgG antibody. As each protein was bound to the sensing surface, marked changes in the delay-line phase were recorded. The reported results demonstrate the capability of these devices to act as biochemical detectors in aqueous solutions, and this work represents the first effort using the novel material LGS in SAW-based biosensor technology.

BAW temperature sensitivity and coupling in langanite.

One of the new materials belonging to the trigonal class 32, to which quartz belongs, is langanite (LGN, La3Ga5.5Nb0.5O14). High-quality LGN single crystals are now available, and, although similar in composition and structure to langasite (LGS, La3Ga5SiO14), LGN has smaller thermal expansion coefficients and comparable piezoelectric constants to LGS. These are desirable material properties for both SAW and BAW applications that require low frequency dependence on temperature. This paper examines in detail the LGN characteristics: phase velocity, temperature coefficient of frequency (TCF), electromechanical coupling coefficient, and power flow angle for both singly and doubly rotated plate cuts. Contour plots of these characteristics are constructed, revealing orientation regions where zero TCF and high coupling exist and suggesting potentially interesting cuts for practical BAW device design. Temperature compensated cut regions with coupling coefficients as high as 0.16 are predicted, which is twice the value for AT-cut quartz, along with a temperature compensated cut with cubic behavior around room temperature for one of the sets of material constants used. With such desirable properties, LGN is a promising candidate material for BAW applications requiring low temperature sensitivity with superior bandwidth characteristics due to its values of coupling coefficient larger than quartz. Several other orientations with low TCF and high coupling are also identified.

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.

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.