Surface-acoustic-wave (SAW) flow sensor

The use of a surface-acoustic-wave (SAW) device to measure the rate of gas flow is described. A SAW oscillator heated to a suitable temperature above ambient is placed in the path of a flowing gas. Convective cooling caused by the gas flow results in a change in the oscillator frequency. A 73-MHz oscillator fabricated on 128 degrees rotated Y-cut lithium niobate substrate and heated to 55 degrees C above ambient shows a frequency variation greater than 142 kHz for flow-rate variation from 0 to 1000 cm(3)/min. The output of the sensor can be calibrated to provide a measurement of volume flow rate, pressure differential across channel ports, or mass flow rate. High sensitivity, wide dynamic range, and direct digital output are among the attractive features of this sensor. Theoretical expressions for the sensitivity and response time of the sensor are derived. It is shown that by using ultrasonic Lamb waves, propagating in thin membranes, a flow sensor with faster response than a SAW sensor can be realized.     

Application of a surface-acoustic-wave device for measurement of liquid flow rate

The use of a surface-acoustic-wave (SAW) device to measure the flow rate of liquids is described. A delay-line stabilized SAW oscillator heated to a suitable temperature above ambient is cooled by the flowing fluid. This results in a change in the oscillator frequency. The frequency of a 68-MHz oscillator operated at 9 degrees C above ambient is found to vary by more than 40 kHz for variation in water flow rate from 0 to 0.8 ml/min. Attractive features of this device include the ability to measure very low flow rates (less than 0.1 mul/min) and direct digital output. However, since this is a thermal type of flow sensor, the temperature of the fluid will be slightly elevated during its passage through the flow cell. The device should be useful in applications where low flow rates have to be monitored.     

Surface acoustic wave sensors incorporating Langmuir-Blodgett films

Acoustoelectric devices offer many attractive features for applications as physical and chemical sensors. Surface acoustic wave (SAW) oscillators are of particular importance owing to their high sensitivity. This paper describes the use of Langmuir-Blodgett (LB) films as gas absorbent layers on the surface of SAW devices.

Areal densities of standard LB film forming materials were measured and found to agree with those obtained from pressure-area isotherms. Sensors incorporating ω-tricosenoic acid and docosylamine overlayers were examined and their responses to alkanoic acids reported.

The room temperature chemiresponse of a SAW device coated with monolayers of tetra-4-tert butyl silicon phthalocyanine dichloride showed response and recovery times comparable with those reported for other phthalocyanine-based sensors operated at much higher temperatures. The detection limit of the LB film device was found to be 40 ppb NO₂ in dry air at an operating frequency of 98.6 MHz and an ambient temperature of 22°C. The frequency change was shown to be entirely due to the mass of gas absorbed by the film.     

A 300-MHz digitally compensated SAW oscillator

A method for compensating for the inherent temperature sensitivity of surface-acoustic-wave (SAW) oscillators is described. Results for a 300-MHz digitally compensated SAW oscillator (DCSO) show a reduction of temperature-induced frequency variation from ±125 parts per million to ±1.4 parts per million over the temperature range of -23 to 75°C. This is accomplished using simple digital circuitry and microprocessor control. The temperature-sensing scheme, using a SAW structure with two delay paths of different temperature sensitivity on the same AT-cut quartz substrate, virtually eliminates thermal resistance and time-constant problems. Advantages over ovenized systems include fast warmup; reduced size, weight, and power dissipation; low cost potential; and the ability to compensate for other sources of frequency drift     

Theoretical study on SAW characteristics of layered structuresincluding a diamond layer

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus can provide much advantage for fabrication of high frequency SAW devices when it is combined with a piezoelectric thin film. Basic SAW properties of layered structures consisting of a piezoelectric material layer, a diamond layer and a substrate were examined by theoretical calculation. Rayleigh mode SAW's with large SAW velocities up to 12,000 m/s and large electro-mechanical coupling coefficients from 1 to 11% were found to propagate in ZnO/diamond/Si, LiNbO₃/diamond/Si and LiTaO₃/diamond/Si structures. It was also found that a SiO₂/ZnO/diamond/Si structure can realize a zero temperature coefficient of frequency with a high phase velocity of 8,000-9,000 m/s and a large electro-mechanical coupling coefficient of up to 4%     

Suitability of surface acoustic wave oscillators fabricated using low temperature-grown AlN films on GaN/sapphire as UV sensors

Epitaxial AlN films were prepared on GaN/sapphire using a helicon sputtering system at the low temperature of 300 degrees C. Surface acoustic wave (SAW) devices fabricated on AlN/GaN/sapphire exhibited superior characteristics compared with those made on GaN/sapphire. An oscillator using an AlN/GaN/sapphirebased SAW device is presented. The oscillation frequency decreased when the device was illuminated by ultraviolet (UV) radiation, and the downshift of the oscillation frequency increased with the illuminating UV power density. The results showed that the AlN/GaN/sapphire-layered structure SAW oscillators are suitable for visible blind UV detection and opened up the feasibility of developing remote UV sensors for different ranges of wavelengths on the III-nitrides.     

Gas thermal conductivity sensor based on SAW

An uncoated surface acoustic wave (SAW) delay line integrated with a suitable film heater is used as a gas sensitive element based on thermal conductivity change. It detects concentrations from 0.1-100% of H₂, O₂, He, and Ar in N₂ in the temperature range from 20-150°C. The value and sign of the SAW gas response are controlled by substrate material and reference gas     

Deep-UV sensors based on SAW oscillators using low-temperature-grown AlN films on sapphires

High-quality epitaxial AlN films were deposited on sapphire substrates at low growth temperature using a helicon sputtering system. SAW filters fabricated on the AlN films exhibited excellent characteristics, with center frequency of 354.2 MHz, which corresponds to a phase velocity of 5667 m/s. An oscillator fabricated using AlN-based SAW devices is presented and applied to deep-UV light detection. A frequency downshift of about 43 KHz was observed when the surface of SAW device was illuminated by a UV source with dominant wavelength of around 200 nm. The results indicate the feasibility of developing remote sensors for deep-UV measurement using AlN-based SAW oscillators.     

Synthesis and surface acoustic wave properties of AlN films deposited on LiNbO3 substrates

The c-axis-oriented aluminum nitride (AlN) films were deposited on z-cut lithium niobate (LiNbO₃) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AlN film determined by x-ray diffraction (XRD) was found to be dependent on the deposition conditions such as substrate temperature, N₂ concentration, and sputtering pressure. Highly c-axis-oriented AlN films to fabricate the AlN/LiNbO₃-based surface acoustic wave (SAW) devices were obtained under a sputtering pressure of 3.5 mTorr, N₂ concentration of 60%, RF power of 165 W, and substrate temperature of 400°C. A dense pebble-like surface texture of c-axis-oriented AlN film was obtained by scanning electron microscopy (SEM). The phase velocity and the electromechanical coupling coefficient (K²) of SAW were measured to be about 4200 m/s and 1.5%, respectively. The temperature coefficient of frequency (TCF) of SAW was calculated to be about -66 ppm/°C     

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.     

基于COMSOL的声表面波器件仿真

声表面波器件在通信、传感、射频识别等领域有着广泛的应用.以有限元方法为基础,利用有限元软件COMSOL对声表面波器件进行了仿真.从器件的模型建立入手,按由浅入深的顺序对无电极压电基片、压电基片表面沉积叉指换能器、叉指换能器表面溅射薄膜、薄膜上负载液体的4种结构进行了仿真分析.仿真研究表明:叉指换能器的电极效应会产生正、反特征频率,并且两种频率都随着叉指电极的敷金比与高度增加而向低频偏移;薄膜厚度的增加同样会导致器件频率向低频变化;当器件负载液体用于液体密度检测时,可通过器件频率变化对液体密度的灵敏程度来对薄膜厚度进行优化.其研究结果可以为声表面波器件的设计制作提供依据.     

Applications of amorphous magnetic-layers in surface-acoustic-wave devices

This paper reviews the theory and potential applications of magnetically variable delay lines and oscillators which employ a magnetostrictive film on a piezoelectric surface-acoustic-wave (SAW) substrate. Cases analyzed in detail indicate that the delay change arises mainly from a rotation of the magnetic moment from the films' easy axes toward the applied field direction; thus the interaction is essentially nondispersive. Use of amorphous metallic-glass overlays is particularly attractive because their high magnetostriction and low magnetic anisotropy makes a relatively large delay variation possible with a small change in bias field. Since the SAW velocity can be changed only bysim .1% or less with present film technology, applications are restricted to those where only a small frequency or delay adjustment is required. Two prototype examples are considered in detail--a variable delay line for steering an adaptive array antenna and a tunable resonator oscillator capable of tracking high speed Doppler targets. In both of these examples, the magnetic film/SAW substrate geometry is seen to be an attractive alternative to competitive approaches.     

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     

Geometrical factors affecting the CO2 laser chemical vapour deposition of silicon oxide films in parallel configuration

Laser-induced chemical vapour deposition of SiO₂ films in a parallel configuration is a powerful technique for the growth of coatings in some special applications where other conventional low-temperature techniques cannot be applied. A CO₂ laser is more attractive than other lasers for industrial applications since it is less expensive and already widely used in the industry. Growing SiO₂ films is carried out by irradiating a gas mixture composed by SiH₄, N₂O and Ar with a continuous wave CO₂ laser. Energy absorption by the mixture causes a temperature increase in the gas phase which leads to the deposition process. Here we present a study of two important geometrical factors in our experimental set-up: the total flow rate of the reactant gases, and the distance between the laser beam and the substrate surface. Variations in gas flow cause changes in the absorption coefficient of the gas mixture and thus in the gas temperature, which mainly affects the growth rate. The beam-substrate distance influences the gas temperature owing to heat exchange between the gas and the substrate and to the collision rate of the chemical species in their diffusion path towards the substrate surface. Therefore, both the growth rate and the film properties change with this parameter.     

声表面波谐振器型振荡器的研制

本文介绍了金属条射栅双端对声表面波谐振器型的原理和结构特点，给出了一种采用声表面波谐振器稳频的低噪声，高稳定性的振荡器电路设计方案。对影响振荡器频率稳定度的因素进行了分析讨论，并探讨改善声表面波振荡器频率稳定性的方法。该声表面波谐振器的中心频率为１２０ＭＨｚ，无载Ｑｖ，大于２０，０００，插入损耗小于６．０ｄＢ，经测试，秒级频率稳定度为１０＾－１０数量级，在自由室温下的日平均波动为１０＾－６／ｄ数量     

Characterization of surface acoustic wave propagation in multi-layered structures using extended FEM/SDA software

This paper describes the characterization of SAW propagation in layered substrate and overlayered structures. The software based on the finite element method and spectral domain analysis was newly developed and applied to the characterization of SAW propagation under an infinitely-long Al interdigital transducer on a rotated Y-cut LiTaO₃/sapphire substrate. Because of the finite LiTaO₃ thickness, a series of spurious resonances appears. It is shown that the excitation strength of the spurious resonances changes with frequency as well as the rotation angle, which reflects the frequency and rotation angle dependence of the energy leakage. Next, the analysis was carried out for SAWs propagating in a SiO₂ layer/Al IDT/42°YX-LiTaO₃ structure. It is shown that the influence of the SiO₂ layer is significantly dependent on the location where the SiO₂ layer is deposited. In particular, it is shown that when the SiO₂ layer is deposited only on top of the electrodes, the SAW reflectivity increases compared with when the SiO₂ layer is deposited between and on top of electrodes.     

Deposition of ZnO thin films on SiO2/Si substrate with Al2O3 buffer layer by radio frequency magnetron sputtering for high frequency surface acoustic wave devices

ZnO films with c-axis (0002) orientation have been grown on SiO₂/Si substrates with an Al₂O₃ buffer layer by radio frequency magnetron sputtering. Crystalline structures of the films were investigated by X-ray diffraction, atomic force microscopy and scanning electron microscopy. The center frequency of the surface acoustic wave (SAW) device with a 4.8 μm thick Al₂O₃ buffer layer was measured to be about 408 MHz, which was much higher than that (265 MHz) of ZnO/SiO₂/Si structure and approaches that (435 MHz) of ZnO/sapphire. It is a possible way as an alternative for the sapphire substrate for the high frequency SAW device applications, and is also useful to integrate the semiconductor and high frequency SAW devices on the same Si substrate.     

Quantitative surface acoustic wave detection based on colloidal gold nanoparticles and their bioconjugates

The immobilization scheme of monodispersed gold nanoparticles (10-nm diameter) on piezoelectric substrate surfaces using organosilane molecules as cross-linkers has been developed for lithium niobate (LiNbO3) and silicon oxide (SiO2)/gold-covered lithium tantalate (LiTaO3) of Rayleigh and guided shear horizontal- (guided SH) surface acoustic wave (SAW) sensors. In this study, comparative measurements of gold nanoparticle adsorption kinetics using high-resolution field-emission scanning electron microscopy and SAW sensors allow the frequency responses of SAW sensors to be quantitatively correlated with surface densities of adsorbed nanoparticles. Using this approach, gold nanoparticles are used as the "nanosized mass standards" to scale the mass loading in a wide dynamical range. Rayleigh-SAW and guided SH-SAW sensors are employed here to monitor the surface mass changes on the device surfaces in gas and liquid phases, respectively. The mass sensitivity ( approximately 20 Hz.cm2/ng) of Rayleigh-SAW device (fundamental oscillation frequency of 113.3 MHz in air) is more than 2 orders of magnitude higher than that of conventional 9-MHz quartz crystal microbalance sensors. Furthermore, in situ (aqueous solutions), real-time measurements of adsorption kinetics for both citrate-stabilized gold nanoparticles and DNA-gold nanoparticle conjugates are also demonstrated by guided SH-SAW (fundamental oscillation frequency of 121.3 MHz). By comparing frequency shifts between the adsorption cases of gold nanoparticles and DNA-gold nanoparticle conjugates, the average number of bound oligonucleotides per gold nanoparticle can also be determined. The high mass sensitivity ( approximately 6 Hz.cm2/ng) of guided SH-SAW sensors and successful detection of DNA-gold nanoparticle conjugates paves the way for real-time biosensing in liquids using nanoparticle-enhanced SAW devices.     

Analysis of SAW properties of epitaxial ZnO films grown on R-Al2O3 substrates

ZnO thin films with a high piezoelectric coupling coefficient are widely used for high frequency and low loss surface acoustic wave (SAW) devices when the film is deposited on top of a high acoustic velocity substrate, such as diamond or sapphire. The performance of these devices is critically dependent on the quality of the ZnO films as well as of the interface between ZnO and the substrate. In this paper, we report the studies on piezoelectric properties of epitaxial (112¯0) ZnO thin films grown on R-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) technique. The c-axis of the ZnO film is in-plane. The ZnO/R-Al₂O₃ interface is atomically sharp. SAW delay lines, aligned parallel to the c-axis, were used to characterize the surface wave velocity, coupling coefficient, and temperature coefficient of frequency as functions of film thickness to wavelength ratio (h/λ). The acoustic wave properties of the material system were calculated using Adler's matrix method, and the devices were simulated using the quasi-static approximation based on Green's function analysis     

Flux noise and flux creep in YBCO thin films

The authors used a DC SQUID (superconducting quantum interference device) to measure the low-frequency magnetic flux noise produced by thin-film rings of YBa₂Cu₃O_7-δ (YBCO) with various microstructures. Below the transition temperature T _c of the YBCO, the spectral density of the noise scales as 1/f (f is the frequency) from 1 Hz to 1 kHz. This noise generally increases with temperature and vanishes abruptly at T_c . Improvements in crystalline microstructure greatly reduce the magnitude of the noise, which was lowest for a highly orientated sample with its c-axis perpendicular to the substrate. Making a radial cut to interrupt current paths around the sample ring does not significantly affect the magnitude of the noise, demonstrating that the noise arises from a local mechanism such as the thermally activated hopping of flux bundles. Flux creep was observed in one sample cooled in a magnetic field of 1 mT, and the creep rate exhibited a sharp maximum near 80 K. It is concluded that SQUIDs and flux transformers of YBCO must be fabricated from highly orientated films to obtain low noise at low frequencies