Surface Acoustic Wave Reflective Delay Research Articles

A surface acoustic wave hydrogen sensor with tin doped indium oxide layers for intermediate temperatures

High temperature surface acoustic wave (SAW) gas sensors with conducting sensing layers require tuning of the sheet conductivity for optimal response. Conducting metal oxides are attractive sensing materials for their tunable electronic properties and high thermal stability, amongst others. Here, we have investigated the application of indium oxide (IO) and indium tin oxide (ITO) films on langasite (LGS)-based SAW reflective delay line sensor devices for monitoring hydrogen at 350 °C. Specifically, we modeled the effect of the IO and ITO sensing layer thickness on the wave velocity, attenuation, and effective electromechanical coefficient. This was followed by an experimental demonstration of tuning of the ITO film sheet conductivity by controlling the dopant concentration and yielding an improvement in the sensor sensitivity. The current study provides a pathway towards the development of conductivity-based sensing layers for high temperature SAW gas sensors with improved sensitivity.

An 860 MHz Wireless Surface Acoustic Wave Sensor With a Metal-Organic Framework Sensing Layer for CO2 and CH4

Wireless and passive surface acoustic wave (SAW) devices with nanoporous metal-organic framework (MOF) sensing layers are attractive gas sensors for applications in many fields such as energy industries and air pollution control. Here, we report on enhancing the sensitivity and detection limit of zeolitic imidazolate framework-8 (ZIF-8) MOF-coated SAW reflective delay line mass sensors by increasing the operating frequency for sensitive detection of carbon dioxide (CO2) and methane (CH4) at ambient conditions. In particular, we show at least four times higher sensitivity of an 860 MHz (4-{\mu}m periodicity) SAW reflective delay line coated with a 240 nm thick ZIF-8 compared to the sensitivity of a 430 MHz (8-{\mu}m periodicity) otherwise identical sensor device to the targeted gases. The detection limits of the higher frequency wireless devices for CO2 and CH4 were estimated to be 0.91 vol-% and 7.01 vol-%, respectively. The enhanced sensitivity for higher frequency devices is explained in terms of the frequency dependent acoustic wave energy confinement.

Development of chipless and wireless underground temperature sensor system based on magnetic antennas and SAW sensor

A chipless and wireless underground sensor system was developed based on two magnetic coil antennas with magnetic cores and a surface acoustic wave (SAW) resonator sensor to monitor temperature variations around buried utilities from the ground. A ∼250 MHz alternating current from the magnetic antenna generates a SAW along the piezoelectric substrate, and the returned SAW energy owing to the reflection bars on the sensor is reconverted to magnetic flux by the sensor’s interdigital transducer (IDT) and subsequently transmitted to a reader via magnetic antenna. By observing changes in the center frequency of the SAW sensor with temperature, we were able to monitor the underground temperature variations in real time. The developed magnetic coil antenna has a large diameter Ni0.8Fe0.2 core wound by Cu coil, and a convex shape at the core end to converge the magnetic flux and enhance the readout distance. Two types of temperature sensors, a one-port SAW resonator and two-port SAW reflective delay line, were fabricated on a 128° YX LiNbO3 and their characteristics were compared in terms of soil temperature. SAW generation by magnetic antenna followed by SAW propagation along the piezoelectric substrate were each confirmed by the AC voltages derived at the output IDT on the SAW sensor, and the amplitude of the SAW was greatly dependent on the current applied to the coil. In soil testing, long readout distance between the reader and underground SAW sensor was observed. The temperature sensors provided stable performance in terms of underground temperature changes, soil type, and soil compactness at that readout distance. The resultant sensitivity and linearity for the SAW resonator temperature sensor was 0.3 MHz/ºC and 0.96, respectively. COMSOL and coupling of Mode (COM) modeling were also performed to find the optimal sensor system parameters and predict the results in advance.

Reader Architectures for Wireless Surface Acoustic Wave Sensors.

Wireless surface acoustic wave (SAW) sensors have some unique features that make them promising for industrial metrology. Their decisive advantage lies in their purely passive operation and the wireless readout capability allowing the installation also at particularly inaccessible locations. Furthermore, they are small, low-cost and rugged components on highly stable substrate materials and thus particularly suited for harsh environments. Nevertheless, a sensor itself does not carry out any measurement but always requires a suitable excitation and interrogation circuit: a reader. A variety of different architectures have been presented and investigated up to now. This review paper gives a comprehensive survey of the present state of reader architectures such as time domain sampling (TDS), frequency domain sampling (FDS) and hybrid concepts for both SAW resonators and reflective SAW delay line sensors. Furthermore, critical performance parameters such as measurement accuracy, dynamic range, update rate, and hardware costs of the state of the art in science and industry are presented, compared and discussed.

Development of chip-less and wireless neural probe functioning stimulation and reading in a single device

A chip-less, wireless neural probe system was developed for stimulation of neurons and for reading of neural communications in the brain. The developed neural probe system is composed of a one-port surface acoustic wave (SAW) reflective delay line, neural firing-dependent ferroelectric capacitor, two antennas, and a network analyzer for reading of neural communications, as well as Schottky diode, static capacitor, and sharp metal tip for stimulation of neurons. The one-port SAW reflective delay line replaces existing wireless transceiver system composed of ~5000 electronic components and makes chip-less, wireless interrogation system possible. The probe for reading of neural signals employs PVDF ferroelectric capacitor which changes polarization and volume depending on neural firings. A 4.3nH inductor was also connected between one-port SAW reflective delay line and reading probe to obtain a large linearity and high sensitivity through impedance matching effect. As electrical pulses imitating neural signals were applied to the reading probe at a 0.9% saline solution, amplitude changes in the reflection peaks were observed depending on the magnitude of applied electrical pulses. Good linearity and sensitivity were observed at the amplitude variations in terms of electrical pulses. The probe system for stimulation of neurons consists of Schottky diode, capacitor, and sharp metal probe. By modulating RF input power and the number of cycle from interrogator, stimulation amplitude and duration time of the resultant output pulses are manipulated. Coupling-of-mode (COM) modeling was also performed to predict device performances and to find optimal device parameters.

Maximum measurement range and accuracy of SAW reflective delay line sensors.

In a surface acoustic wave (SAW) wireless sensor with a reflective delay line structure, three reflectors are often used to eliminate 2π ambiguity of phase measurement. The maximum range of the measured parameter and the maximum accuracy have recently been attracting much research attention. In this paper, an analytical formula for all the factors influencing the measurement range and accuracy of the delay line SAW sensor are deduced for the first time. The factors include: the sensor sensitivity, the topology of the delay line, the available wireless bandwidth and the allowed maximum phase measuring error of the reading system, which is easier to retrieve and more fully describes the possible noises than SNR. Additionally, many designers believe that increasing the reflector could improve accuracy continuously or realize multi-resolution measurement. However, they ignore some certain criteria that the reflector location must satisfy. The reachable maximum accuracy by every increase of a reflector is also presented. A SAW temperature sensor system using 128° YX-LiNbO3 is designed to verify the above theoretical analysis.

Wireless neural probes based on one-port SAW delay line and neural firing-dependent varicap diode

A battery-free, wireless neural probe system was developed for reading neural signals in the brain by using a one-port surface acoustic wave (SAW) reflective delay line, neural firing-dependent varicap diode, two antennas, and a network analyzer as measurement unit. The one-port SAW reflective delay line replaces existing complex wireless transceiver system composed of ∼5000 electronic components and makes battery-free, wireless measurements possible. The varicap diode interconnected with sharp metal shank via operational amplifier (op-amp) was electrically linked to the corresponding split-type reflectors on a one-port SAW reflective delay line. A 4.3nH inductor was also placed in between the split-type reflector on one-port SAW reflective delay line and varicap diode to obtain a large linearity and high sensitivity through impedance matching effect. As electrical pulses imitating neural signals were applied to the sharp metal shanks, overall impedance perturbations between the split-type reflector and external varicap diode under reverse bias were observed, giving rise to amplitude changes in the reflection peaks in the time domain depending on the magnitude of the electrical pulses. Good linearity and sensitivity were observed at the amplitude variations in terms of electrical pulses. Coupling-of-modes (COM) modeling and impedance matching simulations were also performed to predict device performances and compare experimental results.

Chipless wireless neural probes based on one-port surface acoustic wave delay line and neural-firing-dependent capacitors

A wireless, battery-free neural probe system was developed for reading neural signals in the brain using a one-port surface acoustic wave (SAW) reflective delay line, neural-firing-dependent capacitive electrodes, two antennas, and a network analyzer as a measurement unit. The one-port SAW reflective delay line supersedes the existing complex wireless transceiver system composed of a few hundreds of transistors and a heavy rechargeable battery and makes battery-free, wireless measurements possible. The multicapacitive electrodes placed on a sharp shank were electrically connected to the corresponding interdigital transducer (IDT)-type reflectors on a one-port SAW reflective delay line. Each electrode on the sharp shank was made using a copolymer poly(vinylidene–fluoride–trifluoroethylene) (PVDF–TrFE) ferroelectric material sandwiched between two metals. As electrical pulses were applied to the capacitive electrode, overall impedance perturbations between the IDT and the external capacitive electrode system were observed, resulting in amplitude changes in the reflection peaks in the time domain depending on the magnitude of the electrical pulses. Good linearity and sensitivity were observed at the amplitude variations in terms of applied electrical pulses. Coupling-of-modes (COM) modeling and impedance matching simulations were also performed to predict device performances and compare experimental results.

Detection of ppb ozone using a dispersive surface acoustic wave reflective delay line with integrated reference signal

We report the characterization of a dispersive surface acoustic wave (SAW) delay line for the detection of ozone in air at parts-per-billion (ppb) concentrations. Augmenting recent developments in wireless SAW tag technology, we fabricated 435MHz SAW devices and prepared them with polybutadiene coatings to serve as ozone-specific gas sensors. We measured the frequency dependent one-port electromagnetic scattering parameter, S11(f), at regular time intervals as the gas stream was adjusted using an EPA primary standard ozone calibrator. The temperature and relative humidity were also recorded using an independent commercial sensor. The data allowed us to estimate the sensitivity to ozone and to temperature, and we estimated other parameters useful in personal dosimetry: saturation dose, shelf life, and limit of detection. Finally, the built-in time multiplexed response provided reference data for first order temperature compensation, yielding a chemical limit of detection of 63ppb-min ozone.

Towards optimised wireless Love wave biosensor with high sensitivity

A Love wave biosensor, which is composed of a one-port surface acoustic wave reflective delay line on a piezoelectric substrate, a thin overlayer (waveguide layer) on top of the substrate, and a sensitive film that responds only to a specific cell, was optimally designed on a 41° YX LiNbO3 substrate and then fabricated according to the extracted design parameters. Based on multilayer theory, polymethylmethacrylate waveguide thickness was optimised. For derivation of the coupling of mode parameters, the periodic using the periodic finite-element method/boundary element method modelling was utilised. Optimal interdigital transducers and reflectors' features were determined to realise high-quality reflection peaks. The experimentally measured reflection coefficient S11 showed good agreement with simulated results. The evaluated sensitivity was 11.5 deg/µg/ml in terms of anti-DNP immunoglobulin G absorption.

A novel shock and heat tolerant gyrosensor utilizing a one-port surface acoustic wave reflective delay line

A surface acoustic wave (SAW)-based gyroscope with an 80 MHz central frequency was fabricated on a 128° YX LiNbO3 piezoelectric substrate. The fabricated gyroscope is composed of a SAW resonator, metallic dots and a SAW reflective delay line. The SAW resonator, which is activated by a voltage-controlled oscillator, generates a stable standing wave with a large amplitude at an 80 MHz resonant frequency, and the metallic dots induce a Coriolis force and generate a secondary SAW in the direction orthogonal to the propagating standing wave. The SAW reflective delay line is employed to measure the Coriolis effect by analyzing the deviations in the resonant frequency of the SAW reflective delay line. A combined finite element method/boundary element method was utilized to extract the optimal device parameters prior to fabrication. The device was fabricated according to the modeling results and then measured on a rate table. When the device was subjected to an angular rotation, a secondary SAW from the vibrating metallic dots was generated owing to the Coriolis force, resulting in a perturbation of the propagating SAW in the SAW reflective delay line. Depending on the angular velocity, the reflection peak of SAW reflective delay line was changed linearly, and this change was measured by the network analyzer. The measured results matched the modeling results well. The obtained sensitivity was approximately 1.23 deg/(deg/s) in an angular rate range of 0–2000 deg s−1. Good thermal and shock stabilities were observed during the evaluation process proving the shock and heat robustness of the fabricated SAW gyroscope.

Development of SAW-based multi-gas sensor for simultaneous detection of CO 2 and NO 2

A 440 MHz wireless and passive surface acoustic wave (SAW)-based multi-gas sensor integrated with a temperature sensor was developed on a 41° YX LiNbO 3 piezoelectric substrate for the simultaneous detection of CO 2, NO 2, and temperature. The developed sensor was composed of a SAW reflective delay lines structured by an interdigital transducer (IDT), ten reflectors, a CO 2 sensitive film (Teflon AF 2400), and a NO 2 sensitive film (indium tin oxide). Teflon AF 2400 was used for the CO 2 sensitive film because it provides a high CO 2 solubility, with good permeability and selectivity. For the NO 2 sensitive film, indium tin oxide (ITO) was used. Coupling of mode (COM) modeling was conducted to determine the optimal device parameters prior to fabrication. Using the parameters determined by the simulation results, the device was fabricated and then wirelessly measured using a network analyzer. The measured reflective coefficient S 11 in the time domain showed high signal/noise (S/N) ratio, small signal attenuation, and few spurious peaks. The time positions of the reflection peaks were well matched with the predicted values from the simulation. High sensitivity and selectivity were observed at each target gas testing. The obtained sensitivity was 2.12°/ppm for CO 2 and 51.5°/ppm for NO 2, respectively. With the integrated temperature sensor, temperature compensation was also performed during gas sensitivity evaluation process.

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A 440-MHz wireless and passive surface acoustic wave (SAW) microsensor was developed for simultaneous measurement of CO2 and relative humidity (RH). The developed SAW microsensor was composed of SAW reflective delay lines structured by an interdigital transducer (IDT) and several shorted grating reflectors, CO2, and water vapor-sensitive films. A theoretical model was established to predict the response mechanism of the polymer-coated SAW gas sensor. Infusion of CO2 into the chamber and humidity changes induced large phase shifts in the reflection peaks. Good linearity and repeatability were observed in the CO2 concentration of 0~400 ppm and 20~80% RH. The obtained sensitivity was ~2° ppm−1 for CO2 detection and 7.45°/%RH for humidity sensing. Temperature and humidity effects were also investigated during the sensitivity evaluation process.

Sensitivity Improvement of Wireless Pressure Sensor by Incorporating a Saw Reflective Delay Line

Abstract this paper presents a wireless surface acoustic wave (SAW) pressure sensor on 41oYX LiNbO3 for tire pressure monitoring system (TPMS) application, in which a reflective delay line composed of an interdigital transducer (IDT) and several reflectors was used as the sensor element. Using the coupling of modes (COM), the SAW reflective delay line was simulated, and the optimal design parameters were determined. The fabricated 2.4GHz SAW sensor was wirelessly characterized by the network analyzer. Sharp reflection peaks, few spurious signals, and relatively high signal-to-noise (S/N) ratio were observed. High sensitivity of 2.9 deg/kPa and good linearity were observed.

A novel wireless, passive CO2 sensor incorporating a surface acoustic wave reflective delay line

A 440 MHz wireless and passive surface acoustic wave (SAW) chemical sensor was developed forCO2 detection. The developed SAW gas sensor is composed of single phaseunidirectional transducers (SPUDTs), three shorted grating reflectors, andCO2-sensitivepolymer film on 41° YX LiNbO3 substrate. Coupling of modes (COM) modeling was used to find optimal designparameters. Using the extracted design parameters, the SAW device wasfabricated. Teflon AF 2400 was used as the sensitive film because it provides highCO2 solubility, permeability and selectivity. In wireless device testing using anetwork analyzer, four sharp reflection peaks with high signal-to-noise(S/N) ratio, small signal attenuation, and few spurious peaks were observedin the time domain. The time positions of the reflection peaks werewell matched with the predicted values from the simulation. Infusion ofCO2 into thechamber induced large phase shifts of the reflection peaks. Good linearity and repeatability were observedfor a CO2 concentration of 0–450 ppm. The obtained sensitivity was1.98° ppm−1. Temperature and humidity effects were also investigated during the sensitivity evaluationprocess.

A wireless pressure-measurement system using a SAW hybrid sensor

A pressure-measurement system based on surface-acoustic-wave (SAW) sensors is presented in this paper. Since SAW sensors are powered by the energy of the RF field, no battery is required, which is a major drawback of conventional microcontroller-based telemetry systems. A successful combination of a SAW reflective delay line with a high-Q capacitive pressure sensor is shown, With a new way of matching the sensor impedance to the SAW reflector impedance, both a high signal-to-noise ratio and a high signal dynamic are achieved, which supports accurate signal evaluation. As an example of realization, the prototype of the pressure sensor unit is presented.