Surface Acoustic Wave Propagation Velocity Research Articles

Enhanced sensitivity of SAW based ammonia sensor employing GO-SnO2 nanocomposites

A GO-SnO2 nanocomposites coated surface acoustic wave (SAW) device is proposed for sensing ammonia with extremely low concentrations. The GO-SnO2 nanocomposites with diameters of ∼4.3 nm was controllably synthesized in a hybrid solution system of oleic acid (OA) and oleylamine. We revealed that the SnO2 nanocrystals were uniformly immobilized on the GO surface and exhibited porous structures through the characterizations of TEM, SEM, XRD, and Raman. The gas adsorption in GO-SnO2 modulates the SAW propagation velocity, and the corresponding changes in phase/frequency can be collected as the sensing signal. A 200 MHz SAW device with the delay-line pattern was prepared by using the lithograph technique on the Y35°X quartz substrate. The GO-SnO2 was deposited on the SAW device by drip-coating, and the device was integrated in the phase detection circuit to build the ammonia sensor and experimentally characterized. Very low detection limit of 40 ppb and a high sensitivity of 0.098 mV/ppb with a fast response of less than 16.4 s were achieved at room temperature (25 °C). In addition, the developed sensing device exhibited excellent selectivity and stability, and the effects of ambient humidity and temperature on the sensing performance were also investigated.

Enhanced Sensitivity of FeGa Thin-Film Coated SAW Current Sensor

A surface acoustic wave (SAW) device is proposed for sensing current by employing the patterned FeGa thin film as the sensitive interface. The layered media structure of FeGa/SiO2/LiNbO3 was established to reveal the working principle of the sensors, and an SAW chip patterned by delay-line and operating at 150 MHz was fabricated photolithographically on 128° YX LiNbO3 substrate. The FeGa thin film with a larger magnetostrictive coefficient was sputtered onto the acoustic propagation path of the SAW chip to build the sensing device. The prepared device was connected into the differential oscillation loop to construct the current sensor. The FeGa thin film produces magnetostrictive strain and so-called ΔE effect at the magnetic field generated by the applied current, which modulates the SAW propagation velocity accordingly. The differential frequency signal was collected to characterize the measurand. Larger sensitivity of 37.9 kHz/A, low hysteresis error of 0.81%, excellent repeatability and stability were achieved in the experiments from the developed sensing device.

Effects of helium production, displacement damage on mechanical properties and surface acoustic wave in austenitic stainless steels and martensitic steel

The effects of helium production, displacement damage on mechanical properties and microstructures were investigated in some specimens such as austenitic stainless steels of SUS304 and SUS316 and a high chromium martensitic steel of HCM12A steels irradiated in a high fluence irradiation facility (HIT) of the University of Tokyo. The simultaneous dual ion (nickel plus helium ions) irradiations were adopted to simulate nuclear irradiation environments such as fast reactor and fusion reactor at HIT to 1 or 10 dpa and to 0, 1, 10, and 100 appm-He at 500, 550, and 600 °C. Thin foils for a transmission electron microscopy (TEM) were prepared with a focused ion beam (FIB) micro-sampling system. After the irradiation, the microstructures were observed by a transmission electron microscope and the irradiation hardening was measured by nano-indentation technique. In this study, the efficient of non-destructive measurement method of surface acoustic wave (SAW) excited by laser beam on the detection of radiation hardening behavior was also examined. Irradiation hardening was observed and the increment was tended to increase with dpa and He/dpa, which was especially effective in the condition of 100 appmHe/dpa even at 550 °C. Non-linear effect on amplitude of the excited SAW was also observed on the ion irradiated materials, and the propagation velocity of SAW was tended to be increased with irradiation dose. It was found that the behaviors of SAWs results were correlated with the changes of mechanical properties.

Finite element simulation of one-port surface acoustic wave resonator with thick interdigital transducer for gas sensing

The paper presents finite element simulation of a surface acoustic wave (SAW) one-port resonator with thick interdigital transducers (IDT) made of nickel metal for sensing dimethyl methylphosphonate (DMMP) gas by placing fluoroalcoholpolysiloxane (SXFA) sensing film in the space between thick IDT fingers. The absorption of DMMP gas changes density and thickness of SXFA film and affect the SAW propagation velocity, hence resonance frequency of the device. The change in resonance frequency is measured as sensor response. In this paper individual effects of change in density and thickness of SXFA film due to absorption of DMMP gas on sensor response are simulated. The results show that the change in thickness of the SXFA film placed in the space between thick IDT fingers of proposed device is dominant in sensor response and the sensitivity of the proposed device (2.1 kHz/mg/m3) is about 3.2 times greater than a conventional device using SXFA coated over the entire top surface of the device.

Imaging surface acoustic wave dynamics in semiconducting polymers by scanning ultrafast electron microscopy

Understanding the mechanical properties of organic semiconductors is essential to their electronic and photovoltaic applications. Despite a large volume of research directed toward elucidating the chemical, physical and electronic properties of these materials, little attention has been directed toward understanding their thermo-mechanical behavior. Here, we report the ultrafast imaging of surface acoustic waves (SAWs) on the surface of the Poly(3-hexylthiophene-2,5-diyl) (P3HT) thin film at the picosecond and nanosecond timescales. We then use these images to measure the propagation velocity of SAWs, which we then employ to determine the Young's modulus of P3HT. We further validate our experimental observation by performing a semi-empirical transient thermoelastic finite element analysis. Our findings demonstrate the potential of ultrafast electron microscopy to not only probe charge carrier dynamics in materials as previously reported, but also to measure their mechanical properties with great accuracy. This is particularly important when in situ characterization of stiffness for thin devices and nanomaterials is required.

Evaluation of annealing and double ion beam irradiation by a laser-induced and laser-detected surface acoustic wave diagnostic system

The effects of annealing and double ion irradiation on nuclear structural materials were investigated using a novel, non-destructive, non-contact diagnostic method. A laser-induced and laser-detected surface acoustic wave (SAW) was adopted as a diagnostic system. The SAWs propagation velocity and the SAWs vibration velocity along the normal direction of the surface were measured to investigate mechanical properties of the substrates. Change of the shear modulus was detected in the annealed substrates. Non-linear effect on amplitude of the excited SAW was observed on the double ion irradiated materials. The potential of the SAW diagnostic system for assessing nuclear structural materials was demonstrated.

Determination of elastic and thermal properties of a thin nanocrystalline diamond coating using all-optical methods

Results are presented on the thermal and elastic properties of a thin, 1.5μm, nanocrystalline diamond coating (NCD), deposited on a silicon substrate by microwave plasma enhanced chemical vapor deposition. A combination of two all-optical measurement techniques, impulsive stimulated thermal scattering and grating induced laser beam deflection, was employed to launch and detect surface acoustic waves (SAWs). The relation between the dispersive propagation velocity of SAWs to the coating-substrate geometry is exploited to determine the elastic properties of the NCD coating. The elastic properties are found to be consistent with literature values. The thermal properties of the coating were determined by monitoring the thermal diffusion induced washing away of the laser induced transient surface temperature grating. The transient thermal grating signals were fitted by the low-frequency limit of a thermoelastic model for a multilayer configuration. Similar to the dispersion of the surface acoustic wave velocity, the characteristic time of the thermal diffusion driven grating decay evolves from a coating-dominated value at short grating spacings towards a substrate-dominated value at grating spacings well exceeding the coating thickness. The grating spacing dependence of the corresponding effective thermal diffusivity was experimentally determined and fitted, leading to a value for the thermal diffusivity of the NCD coating αNCD=8.4−0.1+2.7mm2·s−1, which is an order of magnitude lower than that of the silicon substrate. The low value of the thermal diffusivity is interpreted with a simple touching model.

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 velocity of gold films deposited on silicon substrates at different temperatures

Au thin films have been deposited by DC magnetron sputtering on Si (001) substrates at different substrate temperatures, ranging from 200 K to 450 K. With increasing temperature, the expected crystallinity and morphology of the Au thin film are clearly improved, as shown by x ray diffraction, atomic force microscopy and scanning electron microscopy experiments. Parallel to this, the surface acoustic wave propagation velocity shows a clear enhancement toward the ideal values obtained from numerical simulations of a Au thin film on Si (001) substrate. Moreover, a very thin and slightly rough interlayer between the Si (001) substrate and the Au thin film is developed for temperatures above 350 K. The composition and nature of this interlayer is not known. This interlayer may be responsible for the steep change in the structural and elastic properties of the Au thin films at the higher temperatures and possibly also for an improvement of the adhesion properties of the Au on the Si (001) substrate.

Surface acoustic wave element

A surface acoustic wave element according to the present invention has a structure including a hard layer containing a composition component essentially consisting of at least one of diamond and a diamond-like carbon film [2], a piezoelectric layer formed on the hard layer [4], a silicon dioxide (SiO₂) [5] layer formed on the piezoelectric layer, and electrodes combined with the piezoelectric layer to perform electro-mechanical conversion. The surface acoustic wave element having the above structure according to the present invention has a larger electro-mechanical coupling coefficient and a higher surface acoustic wave propagation velocity than those of a conventional surface acoustic wave element having no silicon dioxide layer, thereby obtaining a surface acoustic wave element operated in a high-frequency range. In particular, the electro-mechanical coupling element is increased. The silicon dioxide (SiO₂) layer is an electric insulator and rarely reacts with moisture or an acid. The silicon dioxide layer can protect the piezoelectric layer and the electrodes from an external environment, thereby providing a surface acoustic wave element having good high-frequency characteristics and a high resistance to environment.

ZnO film for application in surface acoustic wave device

High quality, c-axis oriented zinc oxide (ZnO) thin films were grown on silicon substrate using RF magnetron sputtering. Surface acoustic wave (SAW) devices were fabricated with different thickness of ZnO ranging from 1.2 to 5.5 μmUm and the frequency responses were characterized using a network analyzer. Thick ZnO films produce the strongest transmission and reflection signals from the SAW devices. The SAW propagation velocity is also strongly dependent on ZnO film thickness. The performance of the ZnO SAW devices could be improved with addition of a SiO2layer, in name of reflection signal amplitude and phase velocity of Rayleigh wave.

Study of effect of deposition temperature of AlN films on SAW velocity using Brillouin spectroscopy

The study of the effect of the deposition temperature on the surface acoustic wave (SAW) propagation velocity of aluminum nitride (AlN) films has been made using Brillouin spectroscopy. The AlN films were deposited using magnetron sputtering technique. Acoustical and structural characterizations of the thin films were made to point out the effect of deposition temperature on acoustical properties. The results show a clear dependence of the SAW propagation velocity on the growth temperature of AlN films. The highest value of acoustic velocity was obtained for the film elaborated without heating. Structural characterization of the AlN films obtained at various deposition temperatures was carried out by X-ray diffraction. These analyses pointed out that the deposition temperature influences the standard deviation of (002) AlN film preferred orientation. The growth temperature clearly influences the acoustical and crystalline properties of AlN thin films. The AlN film obtained at lower deposition temperature presents the higher SAW velocity which we have determined by achievement of SAW device based on this AlN film.

Effect of deposition temperature on surface acoustic wave velocity of aluminum nitride films determined by Brillouin spectroscopy

Brillouin spectroscopy has been used to study the effect of the deposition temperature on the surface acoustic wave (SAW) propagation velocity of aluminum nitride (AlN) films. The results show a dependence of the SAW propagation velocity on the growth temperature of AlN films. The highest value of acoustic velocity was obtained for the film elaborated without heating. Structural characterization of the AlN films synthesized at various deposition temperatures was carried out by x-ray diffraction. These analyses pointed out that the deposition temperature influences the standard deviation of (002) AlN film preferred orientation. The growth temperature clearly influences the acoustical and crystalline properties of AlN thin films.

Nanotechnology for SAW devices on AlN epilayers

Hexagonal aluminum nitride (AlN) has revealed as an excellent candidate to be used as substrate for surface acoustic wave (SAW) devices. Its high SAW propagation velocity, electromechanical coupling constant and an almost negligible temperature coefficient of delay (TCD) promise the fabrication of low-cost SAW devices operating in the microwave region of the spectrum, useful under harmful environment conditions of temperature and radiation. In this paper, a revision of the excellent SAW-related characteristics of AlN grown by molecular beam epitaxy on sapphire is presented. Several technological issues related to the fabrication of high frequency (>1 GHz) SAW devices in this semiconductor using an e-beam lithographic system are discussed. On the other hand, design issues like the effect introduced by the generation of bulk acoustic waves (BAW) will be analyzed using a novel method based on the inverse Fourier transform. These approaches will be presented both from theoretical and experimental perspectives.

Passive remote sensing for temperature and pressure using SAW resonator devices

Wireless sensing using passive sensors based on surface acoustic wave (SAW) technology is necessary for many applications. In this paper a new sensor design that uses the differential frequency output from two SAW resonators (SAWR) for temperature and pressure measurement is presented. The sensor, which consists of two SAWRs on the same substrate with different SAW propagation directions, relies on the variations of the SAW propagation velocity to measure environmental quantities such as pressure or temperature. The aim of this approach is to suppress the disturbing influences of the RF link (radio channel) between the interrogation unit and the passive sensor device. In this case, the output signal is a frequency difference and does not depend on changes in the radio channel during the transmission of the sensor signal.