SAW Devices Research Articles

Fourier analysis of SAW resonance frequency variations for improved detection

The purpose of this work is to demonstrate a new detection protocol, based on Fourier analysis of the phase shift noise of the SAW oscillator. A SAW device with no chemo-selective layer was used to detect ethanol, methanol and toluene in three concentrations, 6 700, 33 000, 67 000 ppm by measurement of the frequency deviation, and by Fourier analysis of the phase-shift noise. By the latter method, it was possible to detect the presence of the analytes using a blank SAW device in quantities undetectable by the conventional method of frequency deviation.Phase-shift noise Fourier spectra exhibit a series of narrow peaks corresponding to sub-harmonic frequencies of the resonant frequency of the oscillator. Subharmonic peaks are relatively stable in frequency, but have variable amplitudes. Their presence is atributed to carrier-envelope resonance. Following analyte admission, the subharmonic peaks shifted their position in accordance with analyte concentration and type.

Fabrication and characterization of one-port surface acoustic wave resonators on semi-insulating GaN substrates**Project supported by the National Key Research and Development Program of China (Grant No. 2017 YFB0403002), the National Science Fund for Distinguished Young Scholars, China (Grant No. Y3CHC11001), and the National Natural Science Foundation of China (Grant No. 11604368).

One-port surface acoustic wave resonators (SAWRs) are fabricated on semi-insulating high-quality bulk GaN and GaN film substrates, respectively. The semi-insulating GaN substrates are grown by hydride vapor phase epitaxy (HVPE) and doped with Fe. The anisotropy of Rayleigh propagation and the electromechanical coupling coefficient in Fe-doped GaN are investigated. The difference in resonance frequency between the SAWs between [11 0] GaN and [1 00] GaN is about 0.25% for the Rayleigh propagation mode, which is smaller than that of non-intentionally doped GaN film (̃1%) reported in the literature. The electromechanical coupling coefficient of Fe-doped GaN is about 3.03%, which is higher than that of non-intentionally doped GaN film. The one-port SAWR fabricated on an 8- Fe-doped GaN/sapphire substrate has a quality factor of 2050, and that fabricated on Fe-doped bulk GaN has a quality factor as high as 3650. All of our results indicate that high-quality bulk GaN is a very promising material for application in SAW devices.

The investigation of integrated SAW strain sensor based on AlN/TC4 structure

This paper reports a novel strain sensor integrated on metal component. High-performance SAW strain sensor is embed on the layered AlN/TC4(Ti-6Al-4 V) structure by lift-off photolithography techniques, while the AlN film is directly deposited on the TC4 metal beam without bonding materials. The strain characteristics of the integrated sensor have been simulated and experimental investigated. The finite element modeling (FEM) simulation reveals that the strain transfer ratio (STR) of the sensor reaches 0.992 at 20 °C. The experimental results show that the strain sensitivity of the sensor reaches 1.61 ppm/με with a deviation of 0.56%. The strain hysteresis error of the SAW devices is about 0.32% at 25 °C. The temperature behavior in strain sensitivity and hysteresis errors have also been studied. Though negative effects caused by increasing of operation temperature on its strain response, the integrated SAW sensor shows better performance than the reported one in a wide range of operation temperature. The prepared layered SAW sensor integrated with TC4 alloy has great potential applications in high temperature harsh environment.

Modelling of love-type wave propagation in piezomagnetic layer over a lossy viscoelastic substrate: Sturm–Liouville problem

We propose an analytical solution for the propagation of Love-type surface seismic waves in a piezo-composite smart structure. A model describing the Love-type wave propagation in the piezomagnetic waveguide is considered. The model is comprised of piezomagnetic layer lying on a lossy viscoelastic substrate. In this regard a direct Sturm–Liouville problem has been formulated and solved. Complex dispersion equations have been derived for both magnetically open and short cases. For each case we have obtained two nonlinear equations containing two unknowns and various parameters. These nonlinear equations are solved using suitable numerical method implemented in Mathematica 9.0 software. Further, the obtained frequency relations are matched with the classical case of Love wave to validate the problem. The influence of different parameters (thickness of the layer, frequency, piezomagnetic coefficient and elastic parameter) on the phase velocity and attenuation of Love-type waves are discussed and delineated through graphs. The results have immense applications in non-destructive evaluation, designing of viscosity sensors, SAW devices, biosensors and chemosensors.

Influence of the Imperfect Interface on Love-Type Mechanical Wave in a FGPM Layer

In this study, we consider the propagation of the Love-type wave in piezoelectric gradient covering layer on an elastic half-space having an imperfect interface between them. Dispersion relation has been obtained in the form of determinant for both electrically open and short cases. The effects of different material gradient coefficients of functionally graded piezoelectric material (FGPM) and imperfect boundary on the phase velocity of Love-type waves are discussed. Also, the influence of mechanically and electrically imperfect interface on the surface wave phase velocity is obtained and shown graphically. The dispersion curves are plotted and the effects of material properties of both FGPM and orthotropic material are studied. Moreover, dispersion relation of the considered microstructure depends substantially on the material gradient coefficients and width of the guiding plate. Numerical results are highlighted graphically and are validated with existing literature. The present study is the prior attempt to show the interfacial imperfection influence with the considered structure on wave phase velocity. The outcomes are widely applicable and useful for the development and characterization of Love-type mechanical waves in FGPM-layered media, SAW devices and other piezoelectric devices.

Low-loss Floating Electrode Unidirectional Transducer for SAW Sensor

Two types of interdigital transducer (IDT) structures of SAW devices have been investigated; one type consists in a classical single electrode transducer where the unit cell has two electrodes, and the other type has a design of floating electrode unidirectional transducer (FEUDT) where the unit cell has six electrodes per period. By taking the Fourier transform of the corresponding time response, the insertion loss (IL) of the SAW device with FEUDT is calculated as –2.6 (forward) and –20.5 dB (backward). At the same time, the device with conventional IDT exhibits an IL value of –8.4 (forward) and –7.8 dB (backward). It turns out that the design of a novel FEUDT type SAW device can efficiently reduce the insertion loss of the SAW device.

SINGLE-PHASE UNIDIRECTIONAL TRANSDUCERS ANALYSIS AND THEIR APPLICATION TO SURFACE ACOUSTIC WAVE FILTERS DESIGN

The unidirectional interdigital transducer topology features when designing SAW filters are considered. As well there are considered physical properties of various piezo‑crystals with different types of cut that allow to realize unidirectional operation of interdigital transducer. There are analyzed the conditions when the »natural» directivity of interdigital transducer becomes possible, topological variety of such transducers are considered. Various types of topology for transducers with a constant aperture are presented. Influence of the form, topology and dimensions of electrodes on characteristics of devices with directional interdigital transducers is analyzed. Influence of substrates technological parameters and interdigital transducers elements (electrodes material and thickness, substrate crystals types and cuts) on the value of attenuation and level of false signals is considered. Articles of functional electronics and SAW piezoelectric filters are used in various radio systems. Creation of passive wireless monitoring systems with use of SAW devices is considered as a separate direction of research.

Solidly Mounted Anti-Symmetric Lamb-Wave Delay Lines as an Alternate to SAW Devices

This letter reports the first results on a new generation of anti-symmetric lamb-wave devices using a simple solidly mounted platform of AlN/Mo/Si and operating at very-high-to-super-high frequency range. AlN/Mo/Si-based solidly mounted resonators (SMRs) with sub-micrometer interdigitated transducers were fabricated using the electron beam lithography and liftoff process. Plasma etching was used to etch vias in ground pads to get access to the bottom electrode for grounding. By comparing the floating, grounded, and no bottom electrode device performances, it has been observed that the presented SMR design with grounded bottom electrode results in enhancement of the coupling efficiency by 500% compared with conventional SAW devices. In addition, these devices are more mechanically robust and can be monolithically integrated with a CMOS platform without requiring complex packaging processes compared with conventional released lamb-wave resonators.

1P3-4 Study on Generation Mechanisms of Third-order Nonlinear Signals in SAW Devices on the Basis of Simulation

1P3-4 Study on Generation Mechanisms of Third-order Nonlinear Signals in SAW Devices on the Basis of Simulation

Fabrication of ZnO/Si lamb wave acoustic devices

Surface acoustic wave devices have widespread applications in various fields including in communication and wireless sensing devices. However some shortcomings like lower sensitivity and the corrosion of IDT’s limit their application as sensing devices. Lamb wave SAW devices have proved to be promising for sensing applications. In the present work effort has been made to find out the optimum etching temperature in order to get the sufficiently fast etch rate and smooth surface for the fabrication of SiO2/Si membrane for Lamb wave acoustic (LWA) devices. Piezoelectric ZnO thin film deposited using rf sputtering technique has been integrated with the SiO2/Si membrane. The bulk etching of Silicon has been carried out from back side of each LWA device using wet etching. The devices are packaged and frequency response is studied using a network analyzer. The antisymmetric and symmetric modes of the Lamb wave devices have been identified.

Fabrication and characterizations of SAW methane sensor based on cryptophane-E membrane

Since the current mining catalytic combustion-type methane sensor is not a detection device used at normal temperature, it is found to have such problems under high temperature conditions (> 450 °C) as poor thermal stability, frequent calibration, short service life and high power consumption. To this end, this paper has studied a room temperature working device that uses cryptophane-E as gas sensitive material, that is, SAW methane sensor. The “three-step approach” is adopted to prepare cryptophane-E material and characterize HUMR and SEM. COMSOL software is used to simulate the rationality of the parameter design of the delay linetype SAW methane sensor. Electron beam exposure photoresist method is used to prepare interdigital electrode on the quartz substrate and form the delay linetype SAW devices, and sensitive materials are loaded using the dispensing method. A sensor performance test system is set up to measure the sensor’s response characteristics at the concentration of 0–5%CH4 using nitrogen as carrier gas. The study shows that the sensor has good response recovery performance within the test concentration range, and the frequency offset is positively correlated with methane concentration; the sensor’s 90% response time is 50 s, 90% recovery time is 72 s, and the sensor sensitivity is 118.8 Hz ± 5 Hz/(1%CH4). The study provides a reference for the development of a new SAW methane sensor, but the temperature and humidity effects and gas selectivity have yet to be further studied.

Research of micro area piezoelectric properties of AlN films and fabrication of high frequency SAW devices

Aluminum nitride (AlN) thin films were optimized on Pt/Ti/SiO2/Si (100) substrates by magnetron sputtering at different substrate temperatures (room temperature, 400 °C, 500 °C, and 600 °C). The analysis of X-ray diffraction (XRD) and atomic force microscopy (AFM) indicated that the optimal deposition temperature for AlN films with high (002) orientation, smallest value of rocking curve FWHM and superior grain distribution is 500 °C. Micro area piezoresponse force microscopy (PFM) results show that AlN samples deposited under 500 °C have the highest relative piezoelectric coefficient (d⁎33(max) = 0.79) and uniform piezoelectric performance. Moreover, interdigital transducers (IDT) of submicron scale (finger width 300 nm) were designed and fabricated by electron beam lithography (EBL) and lift-off process. Finally, the Pt/AlN/Pt two-port SAW device cell was constructed, with a device frequency up to 4.47 GHz, and an acoustic velocity of deposited AlN of approximately 5300 m/s.

Benefits of Live View of Bond Formation and Alignment for Adhesive Bonding Characterisation

The importance of high accuracy adhesive bonding has grown with mobile phone and virtual reality headset applications and is still widely used in microfluidics, MEMS/MOEMS, capping of cavity structures such BAW and SAW devices and also in wafer level packaging. The well-known advantages of this bonding technique are: relatively low temperature process, no need for voltage or current, possibility of joining any types of material, possible low cost and even, to some extent, more tolerance for particles which presence doesn’t necessarily mean a failure as it would in the unforgiving direct bonding. From our experience though we know that adhesive bonding can be quite problematic in many aspects, starting from application and problems with adhesion to the substrate, through establishing precise lithography parameters – the material needs to be cross linked to a certain level not to flow too much but can’t be overcured as this will prevent bond formation; some materials flow so much that the bond line broadening may not be acceptable and so the bonding pressure/force can be critical; and one of the biggest challenges is maintaining the alignment thorough the process while the adhesive goes into low viscosity state and acts like a lubricant. In the first part of this study we would like to share our work on characterisation of PermiNex 1000 series that can be a simple solution to problems listed above. This material is manufactured and was recommended to us by MicroChem, and with MicroChem support we have developed a void free and high accuracy bonding process using this material. We have performed all work on AML Aligner Wafer Bonder, using in-situ optics and are sharing live view of the alignment and bond formation, highlighting how much easier and more effective qualification of any adhesive bonding process can be, when using this tool. We present our data on <1µm alignment accuracy that we achieved and prove no alignment change while wafers are brought into contact. Figure below shows bond formation while the wafers are in contact and the adhesive is heated up. In the second part of this work we are presenting some data from attempts of BCB bonding as an example for those applications that can’t use the PermiNex because of its optical properties. This shows how much we can learn about any other adhesive by observing its behaviour in-situ during bonding. In our experience, the information obtained from the live view can speed up process development significantly from months to even days. We can learn about the viscosity of the material at different temperatures and this knowledge is needed to be able to align accurately and maintain the alignment until the adhesive is fully cured and also for adjusting the heating rates and hence controlling the cross linking. For example aligning and contacting hot BCB may introduce some misalignment at contact whereas contacting at room temperature can overcome this problem. We can also watch the bond frame broadening and so adjust the bonding pressure and moreover we are able to tell at what point/temperature to apply the force. Finally we are able to observe unwanted voids forming and react to those defects that may be created at any time during the process. This for example may tell us if the adhesives are outgassing and the voids may be trapped in the interface if wafers are contacted too early. Figure 1

Benefits of Live View of Bond Formation and Alignment for Adhesive Bonding Characterisation

Adhesive bonding can be quite problematic in many aspects such as application of the intermediate material and its adhesion to the substrate, establishing precise photolithography or pre-curing parameters, controlling the flow/bond line broadening and maintaining alignment thorough the bonding process.We are sharing our work on characterisation of PermiNex 1005 adhesive that can offer a simple solution to those problems for some applications like capping of BAW and SAW devices or Microfluidic devices (1). Using AML Aligner Wafer Bonder that allows in-situ alignment and live view of bond formation we have established a void free and <1µm accuracy bonding process for PermiNex 1005. We are also presenting data from attempted BCB bonding, as an example of a more challenging material. We are showing how much knowledge the real time information gives and how much easier and more effective it can be to qualify any adhesive bonding process, when using our system.

Finite-element analysis of scattering parameters of surface acoustic wave bandpass filter formed on barium titanate thin film

ABSTRACTThe frequency dependence of scattering parameters of interdigital surface acoustic wave transducers placed on ferroelectric barium titanate (BaTiO3) epitaxial film in c-phase coated over magnesium oxide has been studied using the finite-element method (FEM) approach along with the perfectly matched layer (PML) technique. The interdigital transducer which has a comb-like structure with aluminum electrodes excites the mechanical wave. The distance between the fingers allows tuning the frequency properties of the wave propagation. The magnesium oxide is taken as the substrate. The two-dimensional model of two-port surface acoustic wave filter is created to calculate scattering parameters and to show how to design the fixture in COMSOLTM. Some practical computational challenges of finite element modeling of SAW devices in COMSOLTM are shown. The effect of lattice misfit strain on acoustic properties of heterostructures of BaTiO3 epitaxial film in c-phase at room temperature is discussed in present article for two low-frequency surface acoustic resonances.

Performance of thin AlxOy, SixNy and AlN passivation layers for high temperature SAW device applications

This paper investigates the potential of passivation coatings for surface acoustic wave devices (SAW) based on aluminum nitride as a piezoelectric layer to operate at temperatures up to 1000 °C and in pure oxygen atmosphere. To increase the durability of SAW devices, a thin passivation layer was deposited on top of the structure, consisting of either aluminum nitride, alumina or silicon nitride. To investigate the effectivity of the passivation layer as oxygen diffusion barrier, time resolved X-ray diffraction (XRD) measurements were performed at 1000 °C in pure oxygen atmosphere for up to 24 h. Furthermore, the influence of an additional layer on top of the structure on the SAW performance were examined. Therefore, samples were measured before and after the deposition process. Afterwards, the samples were placed in a furnace in pure oxygen atmosphere to test the robustness of the different layers.

Investigation and improvement of atomization efficiency based on SAW device coated with amorphous fluoropolymer film for olfactory display

This paper reports on a method to realize the instant and high-efficiency atomization for liquid perfumes in an olfactory display system using surface acoustic wave device coated with amorphous fluoropolymer film. The liquid perfumes diluted with ethanol tend to spread into a thin film on LiNbO3 substrate before atomization, which causes low atomization efficiency and serious smell persistence. The proposed method employs a layer of 400 nm thick amorphous fluoropolymer film Cytop on SAW substrate to maintain the droplet’s sphere-like shape due to the excellent hydrophobicity. Amino-based silane coupling agent was used to enhance the adhesion. The difference of atomization process between hydrophilic and hydrophobic surfaces was clearly revealed. With the finer and more stable mists being generated on coated surface, the significant improvement of atomization efficiency was demonstrated. Sensory test results on three liquid perfumes show that the smell persistence was greatly reduced after the coating.

Novel designs of SAW devices for highly sensitive chemical sensors

For the realization of highly sensitive SAW devices, three approaches have been followed. 1) Fabrication of SAW resonators such that the entire energy could be trapped in between the IDTs, to minimizethe losses and increases the sensitivity. 2) Use of high velocity DLC thin film as buffer layer to increase the resonance frequency and hence the sensitivity. 3) Fabrication of the membrane based acoustic devices, where movement of membrane along with the acoustic wave leads to a very high sensitivity. In the present work fabrication of these three type of devices has been presented in detail.

Characteristics of surface acoustic waves in (11 $$\bar 2$$ 2 ¯ 0)ZnO film/R-sapphire substrate structures

(11\(\bar 2\) 0)ZnO film/R-sapphire substrate structure is promising for high frequency acoustic wave devices. The propagation characteristics of SAWs, including the Rayleigh waves along [0001] direction and Love waves along [1ī00] direction, are investigated by using 3 dimensional finite element method (3D-FEM). The phase velocity (v p), electromechanical coupling coefficient (k 2), temperature coefficient of frequency (TCF) and reflection coefficient (r) of Rayleigh wave and Love wave devices are theoretically analyzed. Furthermore, the influences of ZnO films with different crystal orientation on SAW properties are also investigated. The results show that the 1st Rayleigh wave has an exceedingly large k 2 of 4.95% in (90°, 90°, 0°) (11\(\bar 2\) 0)ZnO film/R-sapphire substrate associated with a phase velocity of 5300 m/s; and the 0th Love wave in (0°, 90°, 0°) (11\(\bar 2\) 0)ZnO film/R-sapphire substrate has a maximum k 2 of 3.86% associated with a phase velocity of 3400 m/s. And (11\(\bar 2\) 0)ZnO film/R-sapphire substrate structures can be used to design temperature-compensated and wide-band SAW devices. All of the results indicate that the performances of SAW devices can be optimized by suitably selecting ZnO films with different thickness and crystal orientations deposited on R-sapphire substrates.

Comparison between Ir, Ir0.85Rh0.15 and Ir0.7Rh0.3 thin films as electrodes for surface acoustic waves applications above 800 °C in air atmosphere

In this paper, we investigate the suitability of Ir, Ir0.85Rh0.15 and Ir0.7Rh0.3 thin films as electrodes for surface acoustic waves (SAW devices) applications taking place above 800°C in air atmosphere. As expected, all films oxidize from 800°C in IrO2 or IrxRh1-xO2 phase. The electrical properties of the latter remain compatible with the design of SAW devices, with a specific electrical resistance of 151 and 100μΩ·cm for x=0.7 and x=0.85 respectively. Moreover, we observe that the IrxRh1-xO2 phase is much more stable regarding sublimation effect than the IrO2 phase, highlighting the interest of alloying Ir with Rh for high-temperature applications. SAW devices based on langasite substrate and Ir0.85Rh0.15 electrodes show a very good stability for at least several days at 800°C in air. In the case of Ir0.7Rh0.3 electrodes, this stability is extended to temperatures up to 900°C.