Theoretical Studies on the SAW Properties of LGS at High-Temperature in Optimal Cuts

Surface acoustic wave (SAW) devices play a significant role in signal processing, frequency control and sensing applications. In general, they cannot operate at elevated temperature. The new crystal of langasite provides possibility for surface acoustic wave devices applied at high temperature. This paper studies the SAW properties of single and doubly rotated cuts of LGS at high temperature. The calculated SAW properties of LGS are analyzed in space with aid of contour plots, and two promising SAW orientation regions at high temperature are presented. For some typical cuts and propagating directions within the two optimal regions, their SAW characteristics vs temperature are discussed. Based on the experimental evidence in some literatures, the reliability of calculated results is verified.     

Investigation on SAW properties of LGS and optimal cuts for high-temperature applications

A promising perspective for surface acoustic wave (SAW) device applications at high temperature has been opened by langasite (LGS). The SAW properties of LGS in singly and doubly rotated cuts at 250 degrees C are investigated. Three noticeable regions for SAW-cut orientations and propagation directions at high temperature are put forward and are defined by Euler angles [0 degrees, 20 degrees --> 50 degrees, 35 degrees --> 45 degrees], [0 degrees, 85 degrees --> 110 degrees, 0 degrees --> 5 degrees], and [0 degrees, 138 degrees --> 145 degrees, 20 degrees --> 23 degrees], respectively. The first region includes zero or comparatively reduced temperature coefficient of delay (TCD) (<2 ppm) and smaller electromechanical coupling factor (K2) (0.2%-0.35%); the second one exhibits higher K2 (0.35%-0.45%) and moderate TCD (<5 ppm); and the highest K2 (>0.45%) and larger TCD (25-30 ppm) characterize the last region. For some typical orientations within the above-mentioned three regions, the temperature dependency of SAW characteristics (up to 1000 degrees C) is discussed. The second region [0 degrees, 85 degrees --> 110 degrees, 0 degrees --> 5 degrees], especially the orientation [0 degrees, 90 degrees, 0 degrees], has better comprehensive characteristics of SAW and is more suitable for high-temperature applications. Therefore, we should give a top priority to the orientation [0 degrees, 90 degrees, 0 degrees] in the design of SAW devices operating at high temperature. Comparison between published experimental results and numerical predictions based on LGS constants and their temperature coefficients available in the literature reveals that the theoretical results of TCD under 250 degrees C are in agreement with the experimental ones (the relative error of TCD is within 10%).     

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.     

新型压电晶体锗酸镓锶与硅酸镓镧声表面波传播特性的研究

新型压电晶体Sr₃Ga₂Ge₄O₁₄(SGG)在声表面波传感器方面有潜在的应用前景. 根据最新的关于锗酸镓锶的特性报道, 理论计算了SGG在X切、Y切和Z切上的声表面波(SAW)传播特性(相速度、机电耦合系数、能流角). 与类质同构体LGS相比较, SGG晶体表现出了更优异的机电特性.     

Investigation on recent quartz-like materials for SAW applications

Recent progress in growing and characterizing quartz-like materials of the trigonal system class 32 has been reported by several groups. The promising perspective for bulk acoustic wave frequency control applications indicates the potentiality of employing these materials for SAW applications as well. This paper reports results of investigations focused on SAW orientations of langasite (LGS), gallium phosphate (GaPO(4)), and langanite (LGN), both singly and doubly rotated cuts. Among the characteristics explored, major attention is paid to the temperature coefficient of delay (TCD), the electromechanical coupling coefficient (K(2)), and the power flow angle (PFA). Contour graphs are plotted based on our calculated results and show the regions in space in which low TCD and high K(2 ) can be obtained; they also exhibit the associated PFA and phase velocity characteristics. The influence of different sets of material constants is addressed. The spatial investigation performed shows that there are promising orientation regions in these materials at which zero or reduced TCD (<10 ppm/ degrees C) and PFA are obtained. Additional attractive characteristics for SAW applications have been observed: values of K(2) a few times higher than the K(2) of quartz ST-X, thus finding applications in larger bandwidth devices; variation of the TCD with respect to temperature, which is comparable to the variation found for quartz ST-X and less than that for zero TCD Li (2)B(4)O(7) cuts like 45 degrees X-Z and (0 degrees 78 degrees 90 degrees ); and phase velocity values circa 13 to 26% smaller than the phase velocity of quartz ST-X thus allowing a reduction in size for intermediate frequency device applications.     

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.     

一种可同时测量温度和压力的新型SAW传感器

针对汽车轮胎压力监测系统（TPMS）的典型应用,提出了一种基于声表面波（SAW）的新型传感器.这用SAW延迟线理论,温度和压力对于传感器的影响能够通过射频回波信号的变化反映出来.通过在数据处理中引入权重因子,实现对于温度和压力的准确测量.在一定压力（0-200 kPa）和温度（20-100℃）范围内的测试结果表明,该传感器能够同时准确测量温度和压力.SAW传感器对温度的测量精度可以达到0.05℃,对压力的测量精度可以达到7.2 kPa.在对于可靠性和耐久性具有特殊要求的汽车轮胎压力监测等领域,该传感器简单的结构和无线无源的测量方式,进一步提高了其实用性和推广价值.     

Influence of metal thickness on phase velocity and thermal sensitivity of SAW devices

Most surface acoustic wave (SAW) devices exhibit a very small sensitivity to thermal effects. However, even on intrinsically compensated crystal cuts, the deposition of metal strips at the surface (transducers or reflectors) induces important changes in the thermoelastic properties of the device. A theoretical approach based on the Sinha-Tiersten perturbation method is proposed to model the influence of metallization on SAW properties on (ST, X) quartz, namely the temperature stability of the phase velocity of Rayleigh waves. Because this perturbation method only gives access to the first-order temperature coefficient of frequency (TCF), it is combined with a conventional calculation of the second-order TCF to predict the evolution of the turnover temperature. The proposed calculation also requires temperature derivatives of the elastic constants of the metal, which can be calculated for different materials. Finally, theoretical results are compared with experimental data measured on SAW devices on (ST, X) quartz, using aluminum gratings     

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.     

X-ray topography analysis of acoustic wave fields in the SAW-resonator structures

The formation of fields of standing surface acoustic waves (SAW) in LiNbO3 and La3Ga5SiO14 (LGS) crystals was studied by high-resolution topography method on a laboratory X-ray source. The fields of standing SAW were formed using SAW-resonator structures consisting of interdigital transducer (IDT) and reflecting gratings. The SAW amplitudes and power flow angles were measured by X-ray topography, diffraction in acoustic beam was visualized, and the SAW interaction with the crystal structure defects was studied.     

Optimal selection of piezoelectric substrates and crystal cuts for SAW-based pressure and temperature sensors

In this paper, the perturbation method is used to study the velocity shift of surface acoustic waves (SAW) caused by surface pressure and temperature variations of piezoelectric substrates. Effects of pressures and temperatures on elastic, piezoelectric, and dielectric constants of piezoelectric substrates are fully considered as well as the initial stresses and boundary conditions. First, frequency pressure/temperature coefficients are introduced to reflect the relationship between the SAW resonant frequency and the pressure/temperature of the piezoelectric substrates. Second, delay pressure/temperature coefficients are introduced to reflect the relationship among the SAW delay time/phase and SAW delay line-based sensors' pressure and temperature. An objective function for performance evaluation of piezoelectric substrates is then defined in terms of their effective SAW coupling coefficients, power flow angles (PFA), acoustic propagation losses, and pressure and temperature coefficients. Finally, optimal selections of piezo-electric substrates and crystal cuts for SAW-based pressure, temperature, and pressure/temperature sensors are derived by calculating the corresponding objective function values among the range of X-cut, Y-cut, Z-cut, and rotated Y-cut quartz, lithium niobate, and lithium tantalate crystals in different propagation directions.     

Stress-sensitivity mapping for surface acoustic waves on quartz

A model is presented, relating the velocity shifts of surface acoustic waves (SAW) to the six tensor components of quasistatic stresses. Stress sensitivity is then defined through six independent coefficients, whatever the origin of the stress (direct external forces, thermoelastic stresses) might be. These coefficients, depending on crystal anisotropy, are computed for different cut angles and propagation directions of quartz crystal, and represented as a contour-line mapping. The determination of SAW quartz cuts compensated for both planar isotropic stresses and first-order temperature effects make it possible to define a family of quartz cuts with potentially low stress and temperature sensitivities for oscillator applications.     

硅酸镓镧单晶材料的声表面波标签研究

声表面波射频识别标签在射频标签领域获得广泛关注,其中对标签基底材料的研究也成了研究热点。文章旨在研究使用硅酸镓镧单晶材料为压电基底的声表面波标签的标签特性。对使用硅酸镓镧单晶材料为压电基底的声表面波标签进行频域和时域分析,并结合有限元分析方法,对标签的特征频率、叉指换能器(Interdigital Transducer, IDT)的反射系数、叉指电极金属化比、金属电极厚度以及标签回波特性进行研究分析,提取了耦合模COM(Coupled-mode)模型参数。分析结果表明了压电效应是声表面波的谐振与反谐振频率存在的根源,验证了脉冲幅度编码方式,并为使用硅酸镓镧材料作为压电基底的声表面波标签的制作提供了仿真实验依据。     

Temperature coefficients of SAW delay and velocity for Y-cut and rotated LiNbO(3)

Measurements that indicate that the surface-acoustic-wave (SAW) temperature coefficients of delay and velocity over approximately room temperature to +100 degrees C for the popular cuts of lithium niobate are presented. These values of delay coefficient are close to the value previously given for Y-cut LiNbO(3) but are significantly different from values given for the rotated cuts.     

Optimal cuts of langasite, La3Ga5SiO14 for SAW devices

The results of a theoretical and experimental investigation of the SAW propagation characteristics in an optimal region of langasite defined by the Euler angles φ from -15° to +10°, &thetas; from 120° to 165°, and ψ from 20° to 45° are presented. Based on temperature coefficients of the elastic constants derived from experimental data, some optimal orientations of langasite characterized by high electromechanical coupling factor (k²), zero power flow angle (PFA) and low or zero temperature coefficient of frequency (TCF) were found. The SAW velocity in the region of interest is highly anisotropic; this results in a significant amount of diffraction, which must be taken into account in the search for orientations useful for SAW devices. An orientation having simultaneously zero PFA, zero TCF, negligible diffraction, and relatively high piezoelectric coupling has been found and verified experimentally. The experimental results are in excellent agreement with the calculated SAW characteristics. The frequency response of a SAW device fabricated on the optimal cut of langasite is presented and demonstrates that high performance SAW filters can be realized on this optimal cut of langasite     

Surface acoustic wave gas sensor of the sorption type sensitive to the thermal properties of gases

The basic principles of a new surface acoustic wave (SAW) gas sensor are described. Being essentially a sensor of the sorption type, the proposed device possesses certain features of the thermometric SAW sensors and is not only sensitive to the vapors of volatile substances, but capable of detecting gases by their thermal properties as well. In contrast to the known thermometric SAW sensors, the proposed sensor is characterized by high temperature stability and fast response. A variant of the sensor based on a LiNbO₃ SAW delay line is described and some results of the test for detecting propane-butane mixtures are presented.     

Sputtering ZnO films on langasite and its SAW properties

C-axis-oriented ZnO films were sputtered on Langasite substrate (LGS, La(3)Ga(5)SiO(14)). The crystalline structure of the films was determined by grazing incident angle X-ray diffraction, the surface microstructure of films was investigated by scanning electron microscopy and atomic force microscopy, the atom composition ratio O/Zn of films was determined by energy dispersive X-ray spectroscopy, and the resistivity of films was determined by the four-point probe instrument. The measurement results showed those films prepared were all polycrystalline hexagonal ZnO films. By analyzing the microstructure of the ZnO films, those prepared at the oxygen flow rate (O(2)/O(2)+Ar) of 20%, the RF power of 200 W, and the substrate temperature of 200 degrees C had the best performance: highly c-axis-oriented microstructures, dense surface morphology, and the atom composition ratio 1.02. The measured scattering parameters of the SAW device fabricated on the composite substrate (ZnO/LGS) with film thickness 1.76 microm showed an average shifted velocity around 2741 m/s at 57.1 MHz and a electromagnetic coupling coefficient greater than 1%.     

Fabrication of high frequency ZnO thin film SAW devices on silicon substrate with a diamond-like carbon buffer layer using RF magnetron sputtering

Diamond-like carbon (DLC) film is a promising candidate for surface acoustic wave (SAW) device applications because of its higher acoustic velocity. A zinc oxide (ZnO) thin film has been deposited on DLC film/Si substrate by RF magnetron sputtering; the optimized parameters for the ZnO sputtering are RF power density of 0.55 W/cm², substrate temperature of 380 °C, gas flow ratio (Ar/O₂) of 5/1 and total sputter pressure of 1.33 Pa. The results showed that when the thickness of the ZnO thin films was decreased, the phase velocity of the SAW devices increased significantly.     

Temperature-stable double SAW resonators

The temperature stability of SAW resonators on quartz can be enhanced by means of double resonators. The turnover temperatures of the double resonators' components, called single resonators, are positioned above and below room temperature. As a consequence, the temperature coefficients of frequency of the 1st order (TCF1) have opposite signs at room temperature, leading to the vanishing TCF1 of the double resonators. Frequently, different turnover temperatures are adjusted by different propagation directions on an ST cut of quartz. An overview of known and new methods for compensating the temperature coefficient of frequency of the 2nd order (TCF2) of two-port and one-port SAW double resonators is given. A concept by means of which temperature-stable circuits of single resonators are found is described. Two types of temperature-stable double resonators found by applying that concept are treated in detail: 1) a two-port resonator composed of two cascaded two-port resonators and a coupling inductance, and 2) a one-port resonator comprising a series connection of one-port resonators with an inductance in parallel with each single resonator. The substrates are 35.5 degrees rotY cuts of quartz. In both cases, the shift of resonance frequency within the temperature range from -30 degrees C to 70 degrees C is smaller than 20 ppm.