Surface Acoustic Wave Resonators Research Articles

Sensitized Surface Acoustic Wave (SAW) Sensors for Room-Temperature Gas Detection

IntroductionIdentifying and quantifying gas components in mixtures is an important direction for the development of gas sensors.So far, many types of gas sensors have been developed, which can be mainly classified into semiconductor, catalytic combustion, and electrochemical, solid electrolyte, infrared optical, quartz crystal microbalance type (QCM), surface acoustic wave type (SAW) [1-4], according to the detection mechanism of the sensor.The semiconductor sensor relies on the change of the resistance of the sensitive film, and has nothing to do with the mass of the film. Combining sensor arrays and pattern recognition algorithms, identification of mixed atmospheres has been achieved. Conversely, the QCM sensor depends on the change of the film mass, and has nothing to do with the resistance. Recently, Wang et al. achieved NH3 and H2S sensing based on QCM sensor, and each gas concentration can be quantified individually.[5] The QCM sensor is based on Janus mesoporous carbon/silica films with asymmetric mesostructures and disparate active sites (–NH2 and –COOH groups). However, SAW based gas sensors have mass loading effects, acoustoelectric effects and elastic loading effects.[6]Here, we investigate the NO2 and NH3 detection and mechanisms at room temperature of surface acoustic wave sensors based on Bi2S3 nanobelts. When exposed to NO2, the acoustic and electrical effects dominate. When exposed to NH3, the mass loading effects dominate. Bi2S3-based semiconductor and QCM sensors were used to demonstrate this effect.ExperimentBi2S3 nanobelts were prepared using a hydrothermal method. Briefly, triphenyl bismuth (C18H15Bi, 98%, Aladdin, 0.6 mM) and dibenzyl disulfide(C6H5CH2SSCH2C6H5, 98%, Aladdin, 0.6 mM) were dissolved in 4 ml oleylamine (OLA), and then 0.4 g polyvinyl pyrrolidone (PVP,Aladdin, MW ≈ 24,000-28,000) were dissolved into 26 ml absolute ethanol. After that, these solutions were thoroughly mixed together and transferred into a 50 ml autoclave to react at 180oC for 8 hours. Finally, the product was washed with hexane and dissolved in ethanol at a concentration of 30 mg/mlfor use.In this study, lithium niobate is chosen as the piezoelectric substrate for making the SAW delay-line device. For the sensitive layer deposition, 100 µL of Bi2S3 nanobelts solution was spin-coated (at a speed of 2000 rpm) onto the sensitive area of the SAW resonator as a gas sensing film. The central frequency of SAW delay-line device was measured using a network analyzer.Results and ConclusionsBi2S3/SAW gas sensor showed a response of 5 kHz toward 20 ppm of NO2, and 12 kHz upon 20 ppm NH3 at room temperature, with fast response and recovery speed, respectively. Bi2S3 nanobelts can be used as a sensitive material for high performance NH3 and NO2 gas sensors. The semiconductor type and QCM type gas sensor were used to explain the sensitive mechanism. This provides a reference for us to use different mechanisms to make different types of sensors, and recognize and quantify gas components in mixtures.

Velocity reduction of incredible high-performance surface acoustic wave using heavy metal electrodes

In this work, a technique to reduce the acoustic velocity of incredible high-performance surface acoustic wave (I.H.P. SAW) using Al/Pt electrodes keeping other important characteristics such as fractional bandwidth and quality factor is proposed. First, using a finite element method simulator, variations of the acoustic velocity and the fractional bandwidth of the I.H.P. SAW with the Pt electrode thickness, thicknesses of LiTaO3 (LT) and SiO2 layers in a substrate, and the cut angle of a LT layer are estimated. And then, to confirm the validity of the estimation results, several one-port SAW resonators with various thickness combinations of Al, Pt, LT, and SiO2 layers are fabricated and their characteristics are evaluated. The evaluation results show that the acoustic velocity of the I.H.P. SAW is successfully reduced keeping the bandwidth, the quality factor, and the temperature characteristics.

A novel design method for SAW temperature sensor with monotonic and linear frequency-temperature behavior in wide temperature range

In this work, a novel method was proposed to achieve linear temperature dependent frequency in SAW temperature sensors. The designed SAW temperature sensor can operate in a wide temperature range without the limitation originated from the large second-order frequency-temperature coefficient of traditional SAW resonators. An SAW temperature sensor which consists of two SAW resonators was designed with the proposed method. The frequency characteristics of the SAW resonators were measured from room temperature up to 600 °C. The resonance frequency shows good linearity as the function of the temperature. This novel design method can be widely applied in the SAW temperature sensors based on the piezoelectric substrates with large second-order frequency-temperature coefficient.

High $Q$ Antisymmetric Mode Lithium Niobate MEMS Resonators With Spurious Mitigation

This paper reports on the demonstrations of first-order antisymmetric Lamb wave (A1) mode resonator as a new platform for front-end filtering of the fifth-generation (5G) wireless communication. The sub-6 GHz resonance in this work is achieved by employing the A1 mode in the micromachined Y-cut Lithium Niobate (LiNbO3) thin films. The spurious modes mitigation is achieved by optimizing the distribution of the electric field. The demonstrated figure-of-merit ( $\text {FoM}=Q\cdot k_{t}^{2}$ ) of 435 marks the first time that a new resonator technology with the FoMs exceeds those of surface acoustic wave (SAW) resonators and thin-film bulk acoustic resonators (FBARs) in the sub-6 GHz (1–6 GHz) frequency range. [2019-0241]

Compact and flexible meander antenna for Surface Acoustic Wave sensors

This paper presents an innovative, flexible and biocompatible Ultra High Frequency meander antenna, operating at about 800 MHz, realized by means of sputtering on a Polyethylene Naphthalate substrate, and by means of a multi-material 3D printer. The fabricated antennas were characterized in terms of scattering parameters, showing a good impedance matching and a bandwidth in the order of tens of megahertz, gain and 3D radiation patterns. A numerical model was also introduced to investigate the limits of the proposed technologies in terms of metal thicknesses. The fabricated antenna could be efficiently integrated with Surface Acoustic Wave resonators to realize compact, wireless, wearable and battery-less sensing platforms.

Influence of the anisotropy on the magneto-acoustic response of magnetic surface acoustic wave resonators**Project supported by the National Natural Science Foundation of China (Grant No. 61871081) and the National Key Scientific Instrument and Equipment Development Project of China (Grant No. 51827802).

One-port magnetic surface acoustic wave (MSAW) resonators are fabricated by stacking multilayered (FeCoSiB/SiO2)n films directly on top of interdigital electrodes. It is shown that the magneto-acoustic response of the MSAW resonators critically depends the hysteresis of ΔE effect. For the magnetic multilayer without induced magnetic anisotropy, the resonance frequency (fR) exhibits a butterfly-like dependence on the external field, therefore, enabling bipolar detection of magnetic field smaller than its coercive field. However, for the magnetic multilayers with induced magnetic anisotropy, butterfly-like or loop-like fR–H curves are measured along the interdigtial electrode fingers or the SAW propagation direction, which can be attributed to the competition between the magnetic field-induced anisotropy and the stress-induced or shape anisotropy.

Single-photon quantum regime of artificial radiation pressure on a surface acoustic wave resonator

Electromagnetic fields carry momentum, which upon reflection on matter gives rise to the radiation pressure of photons. The radiation pressure has recently been utilized in cavity optomechanics for controlling mechanical motions of macroscopic objects at the quantum limit. However, because of the weakness of the interaction, attempts so far had to use a strong coherent drive to reach the quantum limit. Therefore, the single-photon quantum regime, where even the presence of a totally off-resonant single photon alters the quantum state of the mechanical mode significantly, is one of the next milestones in cavity optomechanics. Here we demonstrate an artificial realization of the radiation pressure of microwave photons acting on phonons in a surface acoustic wave resonator. The order-of-magnitude enhancement of the interaction strength originates in the well-tailored, strong, second-order nonlinearity of a superconducting Josephson junction circuit. The synthetic radiation pressure interaction adds a key element to the quantum optomechanical toolbox and can be applied to quantum information interfaces between electromagnetic and mechanical degrees of freedom.

Enhanced electromechanical coupling in SAW resonators based on sputtered non-polar Al0.77Sc0.23N 112¯ thin films

Non-polar a-plane Al0.77Sc0.23N 112¯0 thin films were prepared by magnetron sputter epitaxy on r-plane Al2O3(11¯02) substrates. Different substrate off-cut angles were compared, and the off-cut angle of 3° resulted in the best structural quality of the AlScN layer. Structural characterization by x-ray diffraction confirmed that single phase, wurtzite-type, a-plane AlScN 112¯0, surface acoustic wave resonators were fabricated with wavelengths λ = 2–10 μm (central frequency up to 1.7 GHz) with two orthogonal in-plane propagation directions. A strong dependence of electromechanical coupling on the in-plane orientation was observed. Compared to conventional c-plane AlScN based resonators, an increase of 185–1000% in the effective electromechanical coupling was achieved with only a fractional decrease of <10.5% in series resonance frequency.

Ultrasonic Transceiver with a Regular/Periodic 1-3 Piezocomposite Based on the SAW Resonance Mode on Damping Backing

A novel effective vibrational mode was discovered in the conventional transducer with an array of orthogonal (square) regular piezoelectric rods in 1-3 piezocomposite, containing the damping backing and front matching layers. The operational resonance in the structure was determined as the Surface Acoustic Wave (SAW) on the backing boundary excited by the adjacent piezo-rods, with its frequency typically near 3 times lower the fundamental half-lambda conventional piezocomposite resonance. Pulse-echo sensitivity and transmitting sound pressure level (SPL) in air showed that the signal strength is roughly comparable to the industrial similar air transducers at the frequency range 100–700 kHz, where at these frequencies the lateral and longitudinal piezoelement dimensions in the conventional transducer design are typically close to each other causing interference with unwanted coupling modes. As was determined theoretically and proved in experiments, the backing SAW resonance effect in the transducer performance is inherent just to the regular periodic 1-3 piezocomposite structure and does occur neither with randomly located/oriented piezo-rods nor in the homogeneous piezo-plate at least with the same lateral cross-section as the connected to it backing. The purpose of the article is to investigate a newly discovered operational vibrational mode of a SAW type in 1-3 regular piezocomposite, other than piezoelectric resonance. The investigated phenomena can improve the transceiver sensitivity and bandwidth, providing lower drive voltage and smaller and lighter weight ultrasonic transducers. Based on the piezocomposites with thickness’ 1–1.5 mm (rod resonance near 2–3 MHz), pillar width 0.2–0.8 mm, kerf width 0.1–0.4 mm, the transceivers with an operating frequency from 140 kHz to 650 kHz were designed and fabricated with a conventional backing of a mixture of high-density tungsten powder and epoxy and a matching layer of a mixture of low-density glass bubbles and epoxy. Experimental evaluation of their acoustical performance showed expected characteristics suitable for practical applications.

A Hybrid Low-Cost Bandpass Filter With SAW Resonators and External Lumped Inductors Using a Dual-Coupling Scheme

A class of hybrid low-cost bandpass filter (BPF) with only two kinds of circuit components, straightforward design procedure, and miniaturized circuit footprint is presented. The proposed BPF is based on a hybrid module composed of one type of surface-acoustic-wave (SAW) resonator and one external lumped inductor. Compared with traditional SAW-resonator-based BPF devices, the use of a single-SAW-resonator model for its design reduces the cost and complexity by avoiding the need for well-matched SAW-resonator pairs. The engineered filter topology consists of a dual-path coupling scheme, in which one coupling path is simply a series-type SAW resonator and another path is made up of two SAW resonators that are intercoupled by means of an external inductor in the middle. Such coupling architecture generates two additional transmission zeros (TZs) that allow to increase the out-of-band power-rejection levels and enlarge the stopband bandwidth. A circuit analysis based on the even-/odd-mode method is applied to derive its equivalent circuit that can be directly matched to a conventional transversal filter topology. The obtained equivalent circuit readily illustrates the operational mechanisms of the proposed filter principle in terms of reflection zeros and TZs. Subsequently, the quality factors of its even- and odd-mode-subnetwork resonators are analytically derived, and the obtained numerical results determine the tradeoff existing when choosing the external-inductor value among various performance metrics. For experimental validation purposes, two 418-MHz BPF prototypes formed by one and two in-series-cascade-connected second-order filtering units, respectively, are manufactured and tested. The circuit footprints of these filters are largely reduced by using a three-layer lamination technique for their physical implementations. The measured results show sharp-rejection filtering responses and are in close agreement with the theoretical predictions.

Investigation into effect of coupled resonance phenomenon towards sensitivity enhancement of SAW conductivity sensors integrated with ZnO nanorods

In this work, FEM simulation was used to investigate the sensitivity of a one-port surface acoustic wave (SAW) resonator sensor to changes in electrical conductivity of the sensing medium composed of ZnO nanorods offering elastic loading on the surface of the resonator. A system of coupled resonators was formed when the height of the ZnO nanorods attached to the surface of the resonator was adjusted such that the resonant frequency of the nanorod approached the original resonant frequency of the SAW resonator. It was observed that the use of ZnO nanorods of resonant dimensions as sensing medium could enhance the sensitivity of the composite SAW sensor to changes in electrical conductivity of the sensing medium by up to 79 times. The comparatively higher sensitivity of the SAW conductivity sensor utilizing ZnO nanorods at resonant dimensions as sensing medium was attributed to the fact that the system of coupled resonators thus formed operates at a state of high sensitivity to changes induced in piezoelectric stiffening of the substrate during SAW propagation. The observations from FEM simulation conducted in the present work suggests strong prospects for the use of coupled resonance phenomenon at nanoscale for enhancing the sensitivity of conductivity-based SAW gas sensors and UV detectors employing 1-D ZnO nanostructures as sensing medium.

Tunable high-Q SAW resonator loaded on a changing capacitance

A tunable high-Q surface acoustic wave (SAW) resonator in the form of several parallel-connected interdigital transducers loaded on a varying capacitance on lithium niobate substrates was developed and studied. The working frequency range was 90–2450[Formula: see text]MHz. A method of calculating such resonators, considering losses in the metal film as well as losses due to the propagation of SAWs and transformations into bulk waves is proposed. Such a design allows one to obtain a quality factor over 5000 in the frequency range 2400–2483[Formula: see text]MHz. The resonant frequency shifts by 600[Formula: see text]kHz when the capacitance changes by [Formula: see text]% of the value of 21[Formula: see text]pF (or 32[Formula: see text]ppm/pF) and has an almost linear character.

Single‐Crystal Multilayer Nitride, Metal, and Oxide Structures on Engineered Silicon for New‐Generation Radio Frequency Filter Applications

Molecular beam epitaxy is used for growth of structures with ScAlN for radio frequency filter applications. The nitride layers are grown directly on Si substrates for surface acoustic wave resonators, Lamb acoustic wave resonators, and on an epitaxial Mo on Er2O3 buffer layer on Si for film bulk acoustic resonators (FBARs). The crystal structure of the ScAlN layer is defined by Sc concentration. It can vary from wurtzite to hexagonal. Good crystal quality of the Mo layer results in low sheet resistance which is very close to that of the bulk material. Enhanced electroacoustic performance is achieved in fabricated acoustic devices. A Lamb acoustic wave resonator with ScAlN grown directly on Si demonstrates a high coupling factor (4.8%) and figure of merit (Q × kt2) (9.1) at a resonance frequency of 9.02 GHz. A fundamental resonance frequency 4.32 GHz is achieved for an FBAR device fabricated using the structure with the nitride layer on an epitaxial metal electrode. At the 4.32 GHz resonance frequency, the extracted figure of merit of the resonator is 10.6.

Super-resolution measurement method for passive wireless resonant surface acoustic wave sensor

PurposeThis study aims to use resonant surface acoustic wave (SAW) sensors, which have advantages in the harsh application environments, to measure different physical parameters such as temperature, pressure and force. For SAW sensors, the locality in measurement resolution by the effective time is poor, it cannot give the detailed results of SAW echoes.Design/methodology/approachTo promote the application of SAW sensor, this paper proposes a convex program-based super-resolution measurement method to recover the missing spectral line and enhance frequency resolution.FindingsThe proposed method reduces the reliance on effective time and improves the measurement resolution of SAW sensors. The performance was validated by experiments.Originality/valueThe limited resolution capability restricts the benefit of SAW technology in harsh environments. The proposed method shed light on SAW measurement resolution increase, exploiting its full potential and leading to commercial applications.

A Hybrid Transmission-Line/SAW-Resonator Analog Signal-Interference Dual-Band Bandpass Filter

An analog signal-interference dual-narrowband bandpass filter (BPF) by combining transmission lines and surface-acoustic-wave (SAW) resonators is proposed. The filter is based on three-path transversal filtering sections (TFSs) with two embedded one-port SAW resonators. By cascading the TFSs in-series through transmission lines, passbands with enhanced fractional bandwidths (FBWs) (i.e., larger than the electromechanical coupling coefficient $k_{t}^{2}$ of the employed acoustic-wave resonator) can be realized. For the purpose of validation, a two-stage microstrip dual-narrowband BPF with two passbands located at 868.95 and 916.5 MHz has been designed, fabricated, and characterized. It demonstrated an insertion loss (IL) of 0.89 dB at 868.95 MHz, 1.57 dB at 916.5 MHz and a bandwidth (BW) of 3.2 MHz (FBW $=3.21\,\,k_{t}^{2}$ ) at 868.95 MHz, and 2.3 MHz (FBW $=1.92\,\,k_{t}^{2}$ ) at 916.5 MHz.

A Surface Acoustic Wave Ethanol Sensor Based on Uniform ZnO Nanoparticles-reduced Graphene Oxide Composite Film

The uniformity and repeatability of sensing film will greatly affect the sensor-to-sensor reproducibility. In this paper, an ethanol sensor based on a surface acoustic wave (SAW) resonator and uniform ZnO nanoparticles-reduced graphene oxide (rGO) composite film as sensing material was proposed. A uniform ZnO-rGO composite film was firstly formed by vacuum filtration and reduction and then transferred to the SAW ethanol sensor. This film formation and transfer process has very good repeatability and stability, as evidenced by the response of the sensors. Benefitting from the uniform composite film, the ethanol sensor shows advantages of wide response range (0~70 vol%), excellent short-term repeatability, good stability, fast response (23 s) and recovery (18 s). Moreover, a dual-mode detection method was proposed to discriminate different gases through simultaneously detecting the mechanical (resonant frequency, f) and electrical (impedance, Z) responses of the same gas adsorption based proposed sensor. To the best of our knowledge, this is the first report on SAW sensor based on uniform ZnO-rGO composite film formed by vacuum filtration and reduction.

A Sensitive Inexpensive SAW Sensor for Wide Range Humidity Measurement

The dynamic range of humidity measurement is very large. The conventional relative humidity sensor suffers from low dynamic range, particularly below 20%RH and the sensor is also relatively costlier. This paper presents fabrication of inexpensive humidity sensor over wide dynamic range using surface acoustic wave resonators of 433.92 MHz frequency. Three sets, each are having three sensors have been fabricated by varying coating thickness of poly vinyl alcohol (PVA) sensing film. Experiments have been conducted to observe the resonant frequency shift with the variation of humidity in the range of 0 to 99%RH using HP 8753C VNA. All the sensors show significant frequency shift for the full span. The maximum sensitivity of the 5% PVA coated sensor for 0-30%RH is 3.72 kHz/%RH. Performance parameters of the sensors are compared with other polymer SAW sensors. The fabricated sensors compare well with the sensors reported in the literature. Important features of the sensors are wide dynamic range, inexpensive fabrication and high sensitivity.

Extraction of Elastooptic Coefficient of Thin-Film Arsenic Trisulfide Using a Mach–Zehnder Acoustooptic Modulator on Lithium Niobate

An acousto-Optic Modulator (AOM) formed by an Arsenic Trisulfide (As $_2$ S $_3$ ) Mach–Zehnder interferometer (MZI), operating in a push-pull configuration and placed inside a surface acoustic wave (SAW) cavity on a Y-cut Lithium Niobate (LN) wafer is demonstrated. This is the first demonstration of such acousto-optic (AO) modulator in this As $_2$ S $_3$ -LN hybrid platform. In this approach, the high index contrast of As $_2$ S $_3$ waveguides is exploited to attain a high optical confinement. Additionally, the placement of the photonic MZI inside the SAW cavity enhances the AO interaction due to the high quality factor (Q) of the SAW resonator. An analytical expression that describes such enhancement as a function of the elasto-optic (EO) coefficient of As $_2$ S $_3$ is derived in conjunction with COMSOL finite element methods to describe strain in the SAW cavity. By fitting this expression to experimental data, the EO coefficient ( $p_{11}$ and $p_{12}$ , where $p_{11} \approx p_{12}$ ) for As $_2$ S $_3$ is extracted to be 0.29 at the wavelength of 1550 nm. This is the first time that the EO coefficient of thin film As $_2$ S $_3$ is experimentally derived.

Langasite-based surface acoustic wave resonator for acetone vapor sensing

High concentration of acetone sensing is of an increasing interest due to its particular application in health monitoring for medical use as well as chemical industries. This paper proposes an advanced sensor that is based off of a Langasite (LGS) surface acoustic wave (SAW) resonator for acetone vapor sensing/detection. The detection of acetone vapor is based off of the chemo-resistive adsorption of pre-installed air on the LGS resonator surface, which will be significantly reduced via chemical reaction and lead to resonant frequency drift of the resonator. Lab tests of acetone vapor sensing are carried out with various concentration at various operating temperature (50 °C–200 °C). The sensitivity is investigated numerically and experimentally, where a rigorous proportionality between frequency drift and acetone concentration is observed and the temperature effect is further discussed. Compared to the conventional SAW gas sensor, the proposed acetone sensor employs the SAW piezoelectric substrate, LGS, as the sensing material, which not only eliminates the use of extra sensing thin film and consequently reduce the cost of fabrication, but also provides a promising high temperature stability due to the nature of LGS. Therefore, the proposed acetone sensor is of low cost, high sensitivity, and low maintenance, which can be further miniaturized and simplified for MEMS application and real environment acetone sensing in health monitoring.

Cellulose nano-crystals as a sensitive and selective layer for high performance surface acoustic wave HCl gas sensors

We report that cellulose nano-crystals (CNCs) can be used as a sensitive and selective layer in surface acoustic wave (SAW) sensors for in-situ HCl gas detection. CNCs were prepared through directly hydrolysis of cotton fiber and were spin-coated on quartz SAW resonators to form the sensitive layer. The CNCs have been identified to have abundant hydroxyl groups on their surfaces, which can act as the perfect adsorption sites for H2O, which can further act as the active sites for HCl gas adsorption. The absorption of HCl on the CNCs layer, thus leads to an increase of its mass, causing negative responses of the SAW sensors. Ambient humidity and thickness of CNCs layer are found to have significant influences on the responses of the SAW sensor. With an 80-nm-thick CNCs layer, the sensor shows a response of −2 kHz to 1 ppm HCl at 25 °C and relative humidity of 50 % with an excellent selectivity and recovery characteristics.