Full-wave analysis of piezoelectric boundary waves propagating along metallic grating sandwiched between two semi-infinite layers

This paper describes full-wave analysis of piezoelectric boundary acoustic waves (PBAWs) propagating along a metallic grating sandwiched between 2 semi-infinite layers. In the analysis, the finite element method (FEM) is used for the grating region while the spectral domain analysis (SDA) is applied for an isotropic overlay region as well as a piezoelectric substrate region. The combination of the FEM and SDA makes the numerical analysis very fast and precise. As an example, the analysis was made on the PBAWs propagating in an SiO₂ overlay/ Cu grating/rotated Y-cut LiNbO₃ structure. It is shown that both the shear-horizontal (SH) type and Rayleigh-type PBAWs are supported in the structure, and that their velocities are very close to each other. Thus spurious responses due to the Rayleigh-type PBAW should completely be suppressed for device implementation. Discussions are made in detail on the influence of Cu grating thickness, substrate rotation angle, and metallization ratio on excitation and propagation characteristics of the SH- and Rayleigh-type PBAWs.     

Propagation of boundary acoustic waves along a ZnO layer between two materials

Theoretical and experimental results on boundary acoustic waves propagated along a ZnO layer sandwiched between two materials are presented. It is shown that boundary acoustic waves can exist only when the material constants of the three materials satisfy the particular conditions obtained here. Experiments on SiO(2)/ZnO/SiO(2 ) were performed to verify the theoretical prediction of the existence of boundary waves. Boundary waves were excited and received by interdigital transducers and propagated along the ZnO layer. Propagation loss was practically the same as for Rayleigh waves, indicating a proper mode of the system. The results suggest that future SAW (surface acoustic wave) devices can be made without any package.     

Change in piezoelectric boundary acoustic wave characteristics with overlay and metal grating materials

This paper describes how the characteristics of shear-horizontal type piezoelectric boundary acoustic waves (PBAWs) change with combination of different overlay and metal grating materials. It is shown that PBAWs are supported in various structures provided that highly piezoelectric material(s) are employed as structural member(s). For verification, numerical simulation of different material combinations is done. The results are in good agreement with the qualitative prediction. That is, large electromechanical coupling factor K² is obtainable when materials having small mass densities shear modulus c₄₄ and shear velocity VBS; and materials having extremely large shear modulus c₄₄ are chosen, respectively, for overlay and metallic grating. When YX-LiNbO₃ is assumed as a substrate, for example, the best choice seems to be SiO₂ and Au for overlay and metallic grating, respectively. Although metals with extremely large ρ and c₄₄ such as W and Ta offer large K², they may not be acceptable for practical PBAW applications because of their large electric resistivity.     

Propagation of longitudinal leaky surface waves under periodic SiO (2)/Al structure on Li(2)B(4)O(7) substrate

The properties of longitudinal leaky surface waves (LLSW) under a periodic SiO(2)/Al structure on Li(2)B(4)O (7) (LBO) substrate, were investigated theoretically and experimentally, in order to improve the high propagation losses of LLSWs under a periodic Al grating with the normalized thickness over 2%. In the theoretical analysis, the previously presented method based on the boundary integral equations for the periodic metal grating structure on the substrate was extended to include the dielectric layer. In the experiments, devices with Al electrodes recessed into a SiO(2) groove on LBO were fabricated, and the propagation losses of them were estimated. As a result, it was shown that, when the surface of the structure was flattened, the propagation losses were sufficiently low and the first Bragg stopband width decreased.     

Transparent nanostructured coatings with UV-shielding and superhydrophobicity properties

Visible light transparent, UV-shielding and superhydrophobic nanostructured coatings have been successfully fabricated through a facile layer-by-layer deposition of TiO(2) and SiO(2) nanoparticles. The coatings are composed of an underlying UV-shielding TiO(2) layer and a top fully covered protective SiO(2) layer. The resulting coatings can block 100% of UVB and UVC and almost 85% of UVA. The fabricated surfaces have contact angles exceeding 165° after coating with organic PTES (1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane) molecules. The transparent superhydrophobic surfaces exhibit extremely strong UV stability. All coatings retain the initial UV-shielding and superhydrophobic properties even after exposure to 275 nm UV light with a light intensity of 75 mW cm(-2) for 12 h.     

A Physical Layer Algorithm for Estimation of Number of Tags in UHF RFID Anti-Collision Design

A priori knowledge of the number of tags is crucial for anti-collision protocols in slotted UHF RFID systems. The number of tags is used to decide optimal frame length in dynamic frame slotted ALOHA (DFSA) and to adjust access probability in random access protocols. Conventional researches estimate the number of tags in MAC layer based on statistics of empty slots, collided slots and successful slots. Usually, a collision detection algorithm is employed to determine types of time slots. Only three types are distinguished because of lack of ability to detect the number of tags in single time slot. In this paper, a physical layer algorithm is proposed to detect the number of tags in a collided slot. Mean shift algorithm is utilized, and some properties of backscatter signals are investigated. Simulation results verify the effectiveness of the proposed solution in terms of low estimation error with a high SNR range, outperforming the existing MAC layer approaches.     

Formulation and validation of Berenger's PML absorbing boundary forthe FDTD simulation of acoustic scattering

Berenger's perfectly matched layer (PML) absorbing boundary condition for electromagnetic (EM) waves is derived to absorb 2-D and 3-D acoustic waves in finite difference time domain (FDTD) simulation of acoustic wave propagation and scattering. A PML medium suitable for acoustic waves is constructed. Plane wave propagation in the PML medium is solved for both 2-D and 3-D cases and explicit FDTD boundary conditions are derived. The equations show that a matched PML medium is a perfect simulation of free space in that a plane wave does not change its direction of propagation or its speed when it propagates from free space into a matched PML medium. FDTD simulation of a pulsed point source propagating in two dimensions is carried out to test the performance of the PML boundary for acoustic waves. Results show that an eight layer PML boundary condition reduces the reflected error 40 dB over Mur's second order boundary condition     

The effect of an SiO(2) buffer layer on the SAW properties of ZnO/SiO(2)/GaAs structure

The effect of an SiO(2) buffer layer on the surface acoustic wave (SAW) properties of ZnO/SiO(2)/GaAs structure is examined. Both theoretical and experimental results show that the coupling coefficient is increased appreciably by providing an SiO(2 ) film between the ZnO film and the GaAs substrate. Adding an SiO (2) film is also beneficial to the promotion of quality of ZnO thin film. The results could be useful for the further development of monolithic SAW devices.     

Air-hydrogen fuel cell with two-level slotted silicon-based electrode

The design of a two-level slotted silicon-based electrode and a technology of its manufacturing by anisotropic etching of (110)-oriented Si wafers are described. The electrode contains two layers, one with wide channels and another with narrow slots. The first layer provides gas transport in an air-hydrogen fuel cell (FC), while the second layer can perform the functions of (i) a catalyst carrier and (ii) a gas-diffusion layer. It is shown that, by filling the narrow-slot layer with a carbon-platinum catalytic ink, it is possible to obtain a higher specific power on the anode due to an increase in the effective area as compared to that of a flat catalyst carrier. The operation of the two-level slotted electrode as a gas-diffusion layer was studied for an FC operating in a free-breathing cathode regime. This FC is characterized by increased power compared to that with a carbon paper and a single-level slotted Si electrode.     

Transformation of acoustic waves in periodic metal grating sandwiched between piezoelectric and dielectric

The mechanism of SAW transformation with variation of film thickness is investigated in a piezoelectric substrate with a metal grating overlaid by a dielectric film, via simulation and visualization of the acoustic fields. By way of example, two orientations of lithium niobate substrates are analyzed, YX-LN and 128°YX-LN, with a Cu grating and an isotropic silica glass overlay. The motions, which follow the wave propagation in the sagittal plane, are visualized within two periods of the grating, with added contour plots showing the shear horizontal displacements. The continuous transformation of the wave's nature is investigated for each wave propagating in the analyzed material structures when the film thickness is increased from zero to a few wavelengths. The examples of the SAW transformation into boundary waves and into plate modes of different polarization have been found and investigated. The behavior of the SAW characteristics in the grating is correlated with transformation of the wave structure with increasing overlay thickness.     

c-Axis zig-zag ZnO film ultrasonic transducers for designing longitudinal and shear wave resonant frequencies and modes

A method for designing frequencies and modes in ultrasonic transducers above the very-high-frequency (VHF) range is required for ultrasonic non-destructive evaluation and acoustic mass sensors. To obtain the desired longitudinal and shear wave conversion loss characteristics in the transducer, we propose the use of a c-axis zig-zag structure consisting of multilayered c-axis 23° tilted ZnO piezoelectric films. In this structure, every layer has the same thickness, and the c-axis tilt directions in odd and even layers are symmetric with respect to the film surface normal. c-axis zig-zag crystal growth was achieved by using a SiO(2) low-temperature buffer layer. The frequency characteristics of the multilayered transducer were predicted using a transmission line model based on Mason's equivalent circuit. We experimentally demonstrated two types of transducers: those exciting longitudinal and shear waves simultaneously at the same frequency, and those exciting shear waves with suppressed longitudinal waves.     

Excitation and propagation of shear-horizontal-type surface and bulk acoustic waves

This paper reviews the basic properties of shear-horizontal (SH)-type surface acoustic waves (SAWs) and bulk acoustic waves (BAWs). As one of the simplest cases, the structure supporting Bleustein-Gulyaev-Shimizu waves is considered, and their excitation and propagation are discussed from various view points. First, the formalism based on the complex integral theory is presented, where the surface is assumed to be covered with an infinitesimally thin metallic film, and it is shown how the excitation and propagation of SH-type waves are affected by the surface perturbation. Then, the analysis is extended to a periodic grating structure, and the behavior of SH-type SAWs under the grating structure is discussed. Finally, the origin of the leaky nature is explained     

Effect of An Initial Stress on SH-Type GuidedWaves Propagating in a Piezoelectric Layer Bonded on A Piezomagnetic Substrate

Propagation of SH-type guided waves in a layered structure with an invariant initial stress is studied, where a piezoelectric thin layer is perfectly bonded on a piezomagnetic substrate. Both the layer and the substrate possess transversely isotropic property. The dispersion relations of SH waves are obtained for four kinds of different electro-magnetic boundary conditions. The effects of initial stress, thickness ratio and electro-magnetic boundary conditions on the propagation behaviors are analyzed in detail. The numerical results show that: 1) The positive initial stresses make the phase velocity increasing, while the negative initial stresses decrease the phase velocity; 2) The smaller the thickness ratio of a piezoelectric layer to a piezomagnetic substrate, the larger the phase velocity of SH-type guided wave propagating in the corresponding layered structure; 3) The electrical boundary conditions play a dominant role in the propagating characteristics. Moreover the phase velocities for the electrically shorted surface are smaller than that for the open case. The obtained results are useful for understanding and design of the electromagnetic acoustic wave and microwave devices.     

Controlled two-dimensional pattern of spontaneously aligned carbon nanotubes

We report a simple solution process to form controlled patterns of aligned single-walled carbon nanotubes on solid substrates. The essential step of the process is to deposit a dilute solution of DNA-wrapped carbon nanotubes (DNA-CNTs) on a SiO(2) surface covered with a thin hydrophobic layer. This leads to deposition of fully aligned CNTs. The alignment pattern can be controlled by metal electrodes in the deposition region and can be quantitatively modeled by the behavior of a quasi-two-dimensional DNA-CNT nematic phase near the solution/SiO(2) interface. These results point to the possibility of rational design and economical fabrication of CNT alignment patterns on solid substrates.     

Graphene growth at the interface between Ni catalyst layer and SiO2/Si substrate

Graphene was synthesized deliberately at the interface between Ni film and SiO2/Si substrate as well as on top surface of Ni film using chemical vapor deposition (CVD) which is suitable for large-scale and low-cost synthesis of graphene. The carbon atom injected at the top surface of Ni film can penetrate and reach to the Ni/SiO2 interface for the formation of graphene. Once we have the graphene in between Ni film and SiO2/Si substrate, the substrate spontaneously provides insulating SiO2 layer and we may easily get graphene/SiO2/Si structure simply by discarding Ni film. This growth of graphene at the interface can exclude graphene transfer step for electronic application. Raman spectroscopy and optical microscopy show that graphene was successfully synthesized at the back of Ni film and the coverage of graphene varies with temperature and time of synthesis. The coverage of graphene at the interface depends on the amount of carbon atoms diffused into the back of Ni film.     

Analysis of SAW propagation in gratings on ZnO/diamond substrates

The space harmonic method is used to analyze surface acoustic wave (SAW) propagation under an infinite periodic metal grating on ZnO/diamond composite layered substrates. Dispersion properties for shorted and open gratings are derived as a function of the thickness of the grating electrodes. From these dispersion relations, the coupling of modes (COM) parameters are derived. Energy profiles inside ZnO/diamond show that the energies contained in each of the ZnO and diamond layers are of the same order when the thickness of the ZnO layer is P/pi (P = grating period) and that the energy is contained within two wavelengths below the ZnO/diamond interface.     

Flow around transverse slots

An inspection analysis of a plane two-scale problem on a stationary laminar flow around an array of slots has been carried out on the assumption that the internal scale, representing the spacing of the slots, is small compared to the external scale of the global problem. Topological and asymptotical classifications of the structure of a boundary layer have been developed with consideration of the rate of the boundary-layer suction and the spacing of the slots. The dependence of the multizone structure of a boundary layer with discontinuously distributed suction on the width of the slot and the rate of the suction was investigated. A mathematical model of a nonviscous compressible-gas flow in the weak-interaction and strong-interaction regimes is proposed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 3, pp. 480–488, May–June, 2008.     

Influence of viscosity on the motion of quasi-particles associated with surface acoustic waves

Selecting this case as a tractable example, this work constructs an original quasi-particle mechanics associated with the dissipative continuum wave solution of so-called Bleustein–Gulyaev (BG) surface acoustic waves (SAWs). These propagate along an elastic semi-infinite space of appropriate crystalline symmetry allowing for the existence of a pure shear-horizontal (SH) displacement but with small viscosity treated as a perturbation. This association is built through the consideration of the canonical conservation (or non-conservation) of energy and wave momentum integrated over a material volume representative of the wave process in the presence of viscous dissipation. At the considered order of approximation, the resulting point mechanics is that of a quasi-particle moving at constant velocity (equal to that of the non-viscous case), but the motion is no longer inertial – due to viscosity – exhibiting a varying “mass” in time, in fact decreasing during the progress of the wave through the viscous material. This “mass” is characteristic of the amplitude of the wave signal, of the behavior of the wave in depth in the substrate, of the present electromechanical coupling, and of the boundary conditions assumed at the top surface, here enriched by the viscous effect. The mechanics obtained reminds us of the Leibnizian “vis-viva”, when the energy variation is considered.     

Micromachined thin film plate acoustic resonators utilizing the lowest order symmetric lamb wave mode

Thin film integrated circuits compatible resonant structures using the lowest order symmetric Lamb wave propagating in thin aluminum nitride (AlN) film membranes have been studied. The 2-mum thick, highly c-oriented AlN piezoelectric films have been grown on silicon by pulsed, direct-current magnetron reactive sputter deposition. The films were deposited at room temperature and had typical full-width, half-maximum value of the rocking curve of about 2 degrees. Thin film plate acoustic resonators were designed and micromachined using low resolution photolithography and deep silicon etching. Plate waves, having a 12-mum wavelength, were excited by means of both interdigital (IDT) and longitudinal wave transducers using lateral field excitation (LW-LFE), and reflected by periodical aluminum-strip gratings deposited on top of the membrane. The existence of a frequency stopband and strong grating reflectivity have been theoretically predicted and experimentally observed. One-port resonator designs having varying cavity lengths and transducer topology were fabricated and characterized. A quality factor exceeding 3000 has been demonstrated at frequencies of about 885 MHz. The IDT based film plate acoustic resonators (FPAR) technology proved to be preferable when lower costs and higher Qs are pursued. The LW-LFE-based FPAR technology offers higher excitation efficiency at costs comparable to that of the thin film bulk acoustic wave resonator (FBAR) technology     

Ultra-low gas sensing utilizing metal oxide thin films

The structure, functionality and sensing response of metal oxide films is discussed with emphasis on ZnO and InO_x prepared by Aerosol Spray Pyrolysis in ambient atmosphere and DC Magnetron Sputtering techniques under vacuum. Optical, structural and electrical characterization techniques applied for the in-depth analysis of the film properties are described. Sensing response towards ozone is presented utilizing a conventional conductivity technique as well as surface acoustic wave (SAW) structures and devices. It is shown that sensing responses of extremely low ozone concentrations in the range of a few parts per billion (ppb), at room temperature (RT), may be obtained by appropriate control of the film nanostructure. It is also shown that InO_x employed as sensitive layer on top of surface acoustic wave structures can lead to strong frequency shifts for low concentrations of NO₂, H₂ and O₃ gases.