Surface Acoustic Wave Delay Line Research Articles

High-sensitivity humidity sensors with ZnO nanorods based two-port surface acoustic wave delay line

Abstract This paper describes the fabrication and characterization of zinc oxide (ZnO) nanorods based surface acoustic wave (SAW) humidity sensors. In this work, the ZnO nanorods were synthesized and coated on nanocrystalline ZnO/AlN/Si-layered structures. SAW properties (e.g., resonance frequency) over a relative humidity (RH) ranged from 10% to 90% at 25 °C were investigated. The surface morphologies of these SAW structures (AlN/Si, nanocrystalline ZnO/AlN/Si and ZnO nanorods/nanocrystalline ZnO/AlN/Si) were also evaluated. Experimental results showed that the performance of SAW structures was slightly inferior compared with that of nanocrystalline ZnO/AlN/Si-layered structures. However, the sensor deposited ZnO nanorods showed the largest resonance frequency shift of approximately 750 kHz due to the high surface-to-volume ratio of the ZnO nanorod sensing layer.

Polarized recombination of acoustically transported carriers in GaAs nanowires

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires deposited on a SAW delay line on a LiNbO3 crystal. The carriers generated in the nanowire by a focused light spot are acoustically transferred to a second location where they recombine. We show that the recombination of the transported carriers occurs in a zinc blende section on top of the predominant wurtzite nanowire. This allows contactless control of the linear polarized emission by SAWs which is governed by the crystal structure. Additional polarization-resolved photoluminescence measurements were performed to investigate spin conservation during transport.

Development of a SnO2/CuO-coated surface acoustic wave-based H2S sensor with switch-like response and recovery

A new surface acoustic wave (SAW) sensor with switch-like response and recovery was developed for H2S gas sensing, in which, as a sensitive interface material toward H2S, a SnO2/CuO composite film was deposited onto a 147MHz SAW delay line on 36° YXLiTaO3 substrate by the RF magnetron sputtering technique, and annealed at 450°C for 2.5h in the dry air. The adsorption of H2S by the sensitive coating modulates the phase velocity of the acoustic wave due to the mass loading and acoustoelectric effect, thus, the target specific H2S gas can be evaluated by recording the frequency shift of the SAW device. The temperature effect on sensor response characteristics was investigated in temperature range of 70–205°C, and the optimal working temperature of 160°C was extracted experimentally, at which, fast response time (55s) and recovery time (45s), and larger frequency response of 230kHz were observed from the fabricated SAW sensor toward H2S with concentration of 20ppm. Moreover, high sensitivity of 16.9kHzppm−1, excellent selectivity, and superior repeatability were observed.

Simplified Model for FEM Simulation of SAW Delay Line Sensor

Surface acoustic wave (SAW) devices can be modeled using various methods such as equivalent circuit model, coupling of modes method and finite element method (FEM). FEM simulation of SAW devices serves to optimize SAW structures and allows getting insight physics of propagation of SAW over the substrate. In most of the earlier work on FEM simulation of SAW sensors, entire device structure is considered for the simulation and the number of degrees of freedom (DOF) to solve is significantly high. Thus it is important to identify valid approximation and simplified models to reduce the computational cost. In this paper a 2D-finite FEM simulation of SAW delay line model using COMSOL Multiphysics (commercial FEM software) is discussed. We have modeled a Rayleigh wave type SAW device choosing YZ Lithium Niobate as the substrate. The major contribution of this work is the application of periodic boundaries to the transmitter interdigital transducer (IDT) to reduce the number of degrees of freedom of a SAW delay line model solution. The simulation methodology and time domain response of the model are discussed.

Surface Acoustic Wave (SAW) Vibration Sensors

In the paper a feasibility study on the use of surface acoustic wave (SAW) vibration sensors for electronic warning systems is presented. The system is assembled from concatenated SAW vibration sensors based on a SAW delay line manufactured on a surface of a piezoelectric plate. Vibrations of the plate are transformed into electric signals that allow identification of the sensor and localization of a threat. The theoretical study of sensor vibrations leads us to the simple isotropic model with one degree of freedom. This model allowed an explicit description of the sensor plate movement and identification of the vibrating sensor. Analysis of frequency response of the ST-cut quartz sensor plate and a damping speed of its impulse response has been conducted. The analysis above was the basis to determine the ranges of parameters for vibrating plates to be useful in electronic warning systems. Generally, operation of electronic warning systems with SAW vibration sensors is based on the analysis of signal phase changes at the working frequency of delay line after being transmitted via two circuits of concatenated four-terminal networks. Frequencies of phase changes are equal to resonance frequencies of vibrating plates of sensors. The amplitude of these phase changes is proportional to the amplitude of vibrations of a sensor plate. Both pieces of information may be sent and recorded jointly by a simple electrical unit.

Acoustically Driven Photon Antibunching in Nanowires

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited carriers, as well as to spatially control exciton recombination in GaAs-based nanowires (NWs) on a subns time scale. The experiments are carried out in core-shell NWs transferred to a SAW delay line on a LiNbO(3) crystal. Carriers generated in the NW by a focused laser spot are acoustically transferred to a second location, leading to the remote emission of subns light pulses synchronized with the SAW phase. The dynamics of the carrier transport, investigated using spatially and time-resolved photoluminescence, is well-reproduced by computer simulations. The high-frequency contactless manipulation of carriers by SAWs opens new perspectives for applications of NWs in opto-electronic devices operating at gigahertz frequencies. The potential of this approach is demonstrated by the realization of a high-frequency source of antibunched photons based on the acoustic transport of electrons and holes in (In,Ga)As NWs.

Development of a New Surface Acoustic Wave Based Gyroscope on a X-112°Y LiTaO3 Substrate

A new micro gyroscope based on the surface acoustic wave (SAW) gyroscopic effect was developed. The SAW gyroscopic effect is investigated by applying the surface effective permittivity method in the regime of small ratios of the rotation velocity and the frequency of the SAW. The theoretical analysis indicates that the larger velocity shift was observed from the rotated X-112°Y LiTaO3 substrate. Then, two SAW delay lines with reverse direction and an operation frequency of 160 MHz are fabricated on a same X-112°Y LiTaO3 chip as the feedback of two SAW oscillators, which act as the sensor element. The single-phase unidirectional transducer (SPUDT) and combed transducers were used to structure the delay lines to improve the frequency stability of the oscillator. The rotation of a piezoelectric medium gives rise to a shift of the propagation velocity of SAW due to the Coriolis force, resulting in the frequency shift of the SAW device, and hence, the evaluation of the sensor performance. Meanwhile, the differential structure was performed to double the sensitivity and compensate for the temperature effects. Using a precise rate table, the performance of the fabricated SAW gyroscope was evaluated experimentally. A sensitivity of 1.332 Hz deg−1 s at angular rates of up to 1,000 deg s−1 and good linearity are observed.

Surface Acoustic Wave Delay Line for Bond Rupture Biosensors

This paper presents the analysis, design, and manufacture of a Surface Acoustic Wave (SAW) delay line for use as a bond-rupture biosensor. Bond-rupture biosensors use acoustic energy to probe the bond strength between the sensor substrate and the analyte. The method of detection of the analyte and its rupture differs depending on the sensor platform. SAW devices are a promising avenue for bond-rupture detection because they can be integrated into optical sensors such as Surface Plasmon Resonance (SPR) as well as the in-built electroacoustic transducers. The prototype device discussed in this paper utilizes the SAW delay line approach, where the analyte changes the properties of the delay path, and the energy of the surface acoustic waves then removes the particles. Design constraints arise from both the manufacturing process and intended application. The developed device is reusable, durable, and able withstand cleaning. Using established Impulse Response models, the SAW device is designed for this application. Experimental results are compared with the simulation predictions and future improvements are considered.

Surface acoustic wave hydrogen sensors based on ZnO nanoparticles incorporated with a Pt catalyst

This paper describes the fabrication and characterization of surface acoustic wave (SAW) hydrogen sensors using ZnO nanoparticles incorporated with a Pt catalyst for hydrogen detection at room temperature. The Pt/ZnO nanoparticle sensing layer was deposited onto SAW delay line sensor-based interdigitated electrodes (IDTs)/aluminum nitride (AlN)/silicon (Si) structure. The average ZnO crystallite sizes varied from 30 nm to 40 nm and were synthesized by a sol–gel method and deposited onto SAW sensors by spin coating. The Pt-like catalyst was deposited onto ZnO nanoparticles by RF sputtering for 2 s. The effect of ZnO crystalline quality, crystallite size and humidity on hydrogen sensing performance was evaluated. Moreover, the sensitivity of SAW delay line sensors as they depend on conventional structures and layered structures were also characterized in this work. The SAW delay line sensors using AlN/Si structures have fundamental frequency of 129.28 MHz for uncoated, 128.85 MHz and 126.93 MHz for coated Pt/ZnO sensing layer corresponding to the conventional and layered structures, respectively. The layered SAW structure using Pt/ZnO nanoparticles as sensing layer show frequency shift of 55 kHz in 1% H 2 concentration at room temperature with good repeatability and stability.

Electronic Warning System Based on Surface Acoustic Wave Vibration Sensors

In the paper feasibility analysis of the surface acoustic wave based electronic warning system was presented. As a transducer in the system the surface acoustic wave delay line was proposed. It is fabricated in standard single mask process as a pair of interdigital transducers placed on the top of piezoelectric crystal substrate. Transducer is a plate with rectangular cross-section and it converts all environmental vibration to electric signal. From electronic point of view it was considered as a two-port network. Necessary condition for proper sensor operation is that the input signal frequency from external generator should be close to central frequency of delay line. Principle of operation of the sensor is that the measured phase shift of high frequency signal has frequency equal resonant frequency of vibrating plate and its amplitude is proportional to vibration amplitude of transducer. The environmental vibrations are defined by amplitude of the output signal and the threat location is pointed by resonant frequency of excited transducer. The described above principles of sensor operation allow us designing electronic warning system consisting of many sensors with different resonant frequencies of transducers. Output signals from precisely located sensors contain all necessary information about the whole warning system. This information is easy to transmit and detect by simple electronic circuits based on phase detector and proper data acquisition system, for which necessary conditions are presented.

Surface Acoustic Wave Vibration Sensor Electronic System

The paper presents surface acoustic wave vibration sensor electronic system. The system cooperates with surface acoustic wave delay line. The sensor has been designed for electronic warning systems. The sensor is a four-terminal network. In the system, there is a cascade connection of sensors by means of coaxial cable. Test signal and constant supply voltage are jointly sent between sensors. Separating the signals, their summation and amplification of test signal as well as matching the line to impedance 50 › are performed by surface acoustic wave vibration sensor electronic system. The electronic system devices and the making of them are discussed. System development of such a sensor is presented and its design is analysed. Experimental examination results of surface acoustic wave vibration sensor are presented. Parasitic eects occurring in vibration sensor electronic system operation connected with electromagnetic coupling and ground current coupling are discussed.

Frequency hopping due to acousto-electric interaction in ZnO based surface acoustic wave oscillator

A 36 MHz surface acoustic wave delay line based oscillator has been used to study the effect of acousto-electric interaction due to photo generated charge carriers in rf sputtered ZnO film under UV illumination (λ = 365 nm, 20–100 μW/cm2). Design aspects for developing a delay line based SAW oscillator are specified. The observed linear downshift in frequency (2.2 to 19.0 kHz) with varying UV intensity (20–100 μW/cm2) is related to the fractional velocity change due to acousto-electric interaction. UV illumination level of 100 μW/cm2 leads to a characteristic frequency hopping behavior arising due to a change in the oscillation criteria, and is attributed to the complex interplay between the increased attenuation and velocity shift.

Investigations of several langasite cuts by high temperature in situ characterizations of surface acoustic waves delay lines

In this paper, we investigate the thermal frequency behavior of the surface acoustic waves (0°, 138.5°, 26.7°), (0°, 22°, 31°) and surface transverse waves (STW) (0°, 22°, 90°) langasite (LGS) cuts by RF characterization means performed on delay lines up to 900°C. Above 700°C, results are altered by degradation phenomena of the Pt electrodes. We observed a strong impact of the operating frequency on the thermal frequency shifting of devices. Around the 434 MHz-ISM band, the (0°, 22°, 90°) STW cut offers similar thermal behavior as the one of the wellknown (0°,138.5°, 26.7°) Rayleigh cut. Moreover, associated to the (0°, 22°, 31°) cut, the STW one could allow for differential measurements on one unique substrate.

Surface Acoustic Wave Sensors of Delay Lines Based on MEMS

A high frequency Surface Acoustic Wave (SAW) filter which is the key point of the SAW sensor is made by MEMS technology. The SAW filter used for sensors needs to have the factors of high frequency, low loss and high quality factor. In order to satisfy the performance of the factors, the SAW delay lines of Electrode Wave Control Single Phase Unidirectional Transducer (EWC/SPUDT) are used in the filter. This transducer can be considered as the networks of connection of some units of IDTs and the loss of device can be effectively reduced. Based on the Coupling of Modes (COM) theory and equivalent circuit modal, the mixed matrix (P matrix) of the transducers can be deduced, and further, the response of the delay lines can be obtained. According to the calculated results, a mask layout is made. Then the SAW delay line is completed and found to agree with the theoretical simulation. Its center frequency is 503 MHz and insertion loss is low enough for use of sensors.

Performance of Non-Contact Linear Actuators Driven by Surface Acoustic Waves

A new kind of non-contact linear actuator (motor) driven by surface acoustic waves (SAWs) is presented, in which the stators are made from SAW delay lines using 128° YX-LiNbO3 substrates. A fluid layer is introduced between the slider and the stator of the actuator, and the slider is a circular aluminum disk suspended on the surface of the liquid (water) layer. As the SAW is excited on the stator, the SAW is converted to a leaky wave in the interface of the stator and the liquid, and then propagates into the liquid. Owing to the nonlinear effect of wave propagation, acoustic streaming is generated, which pushes the slider to move. By the experiments, the relations between the slider velocity and the experimental parameters, such as the exciting voltage of the SAWs, the thickness and the kinematic viscosity of the liquid layer, are obtained.

Surface acoustic wave devices based on AlN/sapphire structure for high temperature applications

AlN/sapphire layered structure has been investigated as a potential substrate for surface acoustic wave (SAW) devices operating at high temperatures up to 950 °C under air atmosphere. Frequency characterizations of the SAW delay lines based on this structure indicate a slight increase of 2 dB in the insertion losses after annealing for 30 min at 900 °C. Scanning electron and atomic force microscopy as well as x-ray diffraction measurements suggest that theses losses are due to the deterioration of the Pt/Ta electrodes and to a slight oxidation of the AlN film.

Impact of Photocapacitance on Phase Response of GaN/Sapphire SAW UV Sensor

The response of GaN/sapphire SAW delay line to the focused ultraviolet (UV) illumination was investigated at the SAW frequency 306 MHz and optical wavelength 375 nm. The UV irradiation of an interdigital transducer (IDT) led to 4 to 5 times larger phase change of transmitted signal than the irradiation of the surface acoustic wave (SAW) propagation path between the IDTs at the same UV light spot size of 2.4 mm. The UV-induced phase change has been attributed to the variation in IDT impedance due to photo-capacitive effect. Simulation of the SAW sensor phase response on the basis of this model is in good agreement with experimental results.

Voltage tunable surface acoustic wave phase shifter on AlGaN/GaN

A voltage tunable surface acoustic wave (SAW) phase shifter on a two-dimensional electron gas (2DEG) AlGaN/GaN heterostructure has been developed. The acoustoelectric interaction is field-effect modulated by means of an insulated gate placed in the acoustic path of a SAW delay line. The phase velocity under the gate is tuned by modifying the conductance of the 2DEG with a dc bias, while the insulating layer prevents the screening of the SAW piezoelectric fields by the gate. The device is compatible with the nitride transistor technology for signal processing and frequency control applications.

Assessment of langatate material constants and temperature coefficients using SAW delay line measurements

This paper reports on the assessment of langatate (LGT) acoustic material constants and temperature coefficients by surface acoustic wave (SAW) delay line measurements up to 130 degrees C. Based upon a full set of material constants recently reported by the authors, 7 orientations in the LGT plane with Euler angles (90 degrees, 23 degrees, Psi) were identified for testing. Each of the 7 selected orientations exhibited calculated coupling coefficients (K(2)) between 0.2% and 0.75% and also showed a large range of predicted temperature coefficient of delay (TCD) values around room temperature. Additionally, methods for estimating the uncertainty in predicted SAW propagation properties were developed and applied to SAW phase velocity and temperature coefficient of delay calculations. Starting from a purchased LGT boule, the SAW wafers used in this work were aligned, cut, ground, and polished at University of Maine facilities, followed by device fabrication and testing. Using repeated measurements of 2 devices on separate wafers for each of the 7 orientations, the room temperature SAW phase velocities were extracted with a precision of 0.1% and found to be in agreement with the predicted values. The normalized frequency change and the temperature coefficient of delay for all 7 orientations agreed with predictions within the uncertainty of the measurement and the predictions over the entire 120 degrees C temperature range measured. Two orientations, with Euler angles (90 degrees, 23 degrees, 123 degrees) and (90 degrees, 23 degrees, 119 degrees), were found to have high predicted coupling for LGT (K(2) > 0.5%) and were shown experimentally to exhibit temperature compensation in the vicinity of room temperature, with turnover temperatures at 50 and 60 degrees C, respectively.

Development of SAW based gyroscope with high shock and thermal stability

A novel surface acoustic wave (SAW)-based gyroscope with an 80 MHz central frequency was developed on a 128° YX LiNbO 3 piezoelectric substrate. The developed sensor was composed of a SAW resonator, metallic dots, and two SAW delay lines. A SAW resonator was employed to generate a stable standing wave with a large amplitude, metallic dots were used to induce a Coriolis force and to form a secondary SAW, and two delay lines were formed to extract the Coriolis effect by comparing the resonance frequencies between these two delay lines. Coupling of modes (COM) modeling was conducted to determine the optimal device parameters prior to fabrication. According to the simulation results, the device was fabricated and then measured on a rate table. When the device was subjected to an angular rotation, resonant frequency differences between the two oscillators were observed because of the secondary wave, generated by the Coriolis force, perturbed the propagation of the SAW in the sense element. Depending on the angular velocity, the difference of the resonance frequency was linearly modulated. The obtained sensitivity was approximately 172 Hz deg −1 s −1 at an angular rate range of 0–500 deg/s. Device performances depending on different mass weights and temperatures were also characterized. Good thermal and shock stabilities were observed during the evaluation process.