Computational modeling and analysis of SAW devices

The development of mobile communication sets higher demands on high frequency surface acoustic wave (SAW) devices. The layout of interdigital transducers (IDT) in SAW devices is in plane and the traditional methods of improving frequency for SAW devices are the increase of acoustic velocity and the reduction of wavelength. However, these methods have a limit due to the limited acoustic velocity and the photolithography limit. This work proposes a new way for the development of high frequency SAW devices by redesigning the layout of IDT in three dimensions (3D). The set of IDTs is split into two layers, i.e. the ground electrodes on one layer and the signal electrodes on the other layer. This 3D layout of IDTs dramatically narrows the horizontal gap between two adjacent electrodes, which can significantly increase the frequency. The frequency and the electromechanical coupling factor (K 2 ) of the four structures of SAW devices with the 3D layout of IDTs were studied by the finite element method (FEM). The results show that the frequency can be doubled under the same critical resolution of lithography due to the shortening of wavelength, and the 3D layout of IDTs is available for SAW devices based on piezoelectric thin film or piezoelectric single crystal. This work develops a new alternative to increase the working frequency of SAW devices.

In the past few years, high-velocity surface acoustic wave (SAW) devices have been demonstrated using multilayers containing diamond. These devices have exhibited SAW phase velocities as large as 10 km/s. The high velocities attainable in diamond-based multilayer systems enables the fabrication of high frequency (>2 GHz) SAW devices without requiring submicron device fabrication technologies. Furthermore, higher velocity pseudo-SAWs (PSAWs) with theoretically calculated velocities approaching 18 km/s have also been predicted. In this work, high velocity pseudo-SAWs have been experimentally observed in a ZnO/diamond/Si multilayer. The fabricated SAW devices were delay line structures with uniform transducers on the ZnO surface, and metal shorting planes at the ZnO/diamond interface. Electrical characterization has been made via RF probing, and SAW and PSAW responses have been observed. Of particular interest is a high velocity PSAW with a phase velocity in excess of 16 km/s. The results are in good agreement with simulated dispersive relationships for this multilayer system.

Synthesis of new microsensor coatings and their response to test vapors 2,4,6-trisubstituted-1,3,5-triazine derivatives

ABSTRACTIn this paper, we utilized a Stroh based formulation for solving problems of surface waves in layered piezoelectric media, and then, applied it to analyze surface acoustic wave (SAW) devices. The determination of the optimal cut of a piezoelectric crystal and the choice of the best propagation of SAW devices were given. The dispersion induced by a thin metal layer on SAW propagation in a SAW device was analyzed and discussed. Finally, we applied the formulation to calculate the effective permittivity and phase velocity dispersion of a LiNbO3/Diamond layered SAW device. Both of the null frequency bandwidth and the insertion loss of the dispersive SAW device were obtained.

A novel NO 2 gas sensor using dual track SAW device

We investigate the excitation and propagation of acoustic waves in polycrystalline aluminum nitride films along the directions parallel and normal to the c-axis. Longitudinal and transverse propagations are assessed through the frequency response of surface acoustic wave and bulk acoustic wave devices fabricated on films of different crystal qualities. The crystalline properties significantly affect the electromechanical coupling factors and acoustic properties of the piezoelectric layers. The presence of misoriented grains produces an overall decrease of the piezoelectric activity, degrading more severely the excitation and propagation of waves traveling transversally to the c-axis. It is suggested that the presence of such crystalline defects in c-axis-oriented films reduces the mechanical coherence between grains and hinders the transverse deformation of the film when the electric field is applied parallel to the surface.

High-performance wideband surface acoustic wave (SAW) devices are essential components for wireless communication systems in the new age, and spurious modes and temperature stability are two important properties for wideband devices. Nevertheless, preparing wideband spurious-free SAW devices with low temperature drift is still a big challenge. To achieve four strengths including large bandwidth, low temperature drift, suppressed Rayleigh mode and suppressed transverse modes in the meantime, this work proposes and theoretically investigates SAW devices based on three structures including SiO2/Cu/Y-X LiNbO3/69°Y-90°X quartz, Cu/Y-X LiNbO3/69°Y-90°X quartz, and Al/41°Y-X LiNbO3/69°Y-90°X quartz. The characteristics of these devices are revealed and some optimized designs based on these structures are provided. Wideband SAW devices with nearly-zero temperature coefficients of frequency and suppressed Rayleigh mode can be achieved when SiO2/Cu/Y-X LiNbO3/69°Y-90°X quartz structure is applied. Transverse modes are weakened by quartz and can be more effectively suppressed if transverse mode suppression designs of interdigital transducers are applied, so the four strengths can be achieved meanwhile. The devices based on the other two structures can have at least two strengths as well as some unique properties such as ultra-large electromechanical coupling factor of 34.20% and suppressed transverse mode generation, which are suitable for different applications. This work provides more possible structures and designs for the engineers in filter design, and promotes the development of high-performance wideband SAW devices.

Nanocrystalline diamond films for surface acoustic wave devices

We present a new method of evaluating the information of interest in the output response of surface acoustic wave (SAW) sensors. A well-known spread-spectrum technique is used to get the sensor information from an individually addressed SAW sensor. On-off keying-coded SAW sensors are picked out of a number of sensors by correlating the sensor response signal with a replica of the known response signal of a particular sensor. The influence of a measurement quantity (e.g., temperature, pressure, current, voltage,...) on a SAW sensor can be observed as a scaling of time and shape of the sensor response signal. This scaling factor is evaluated by use of correlative signal processing techniques. A main advantage of this method is the capability of multiple access, i.e., to distinguish different sensors in the range of a single interrogation system. Since this technique makes it possible to deal with sensor response signals overlapping in the time domain, sensors can remain short and, therefore, cheap. The principle of operation, limits of the method, and experimental results for temperature measurements are also presented.

Surface acoustic wave (SAW) devices are widely used for physical, chemical, and biological sensing applications, and their sensing mechanisms are generally based on frequency changes due to mass-loading effects at the acoustic wave propagation area between two interdigitated transducers (IDTs). In this paper, a new sensing mechanism has been proposed based on a significantly enhanced mass-loading effect generated directly on Au IDT electrodes, which enables significantly enhanced sensitivity, compared with that of conventional SAW devices. The fabricated ultrahigh-frequency SAW devices show a significant mass-loading effect on the electrodes. When the Au-electrode thickness increased from 12 to 25 nm, the Rayleigh mode resonant frequency decreased from 7.77 to 5.93 GHz, while that of the higher longitudinal leaky SAW decreased from 11.87 to 9.83 GHz. The corresponding mass sensitivity of 7309 MHz·mm2·μg-1 (Rayleigh mode) is ∼8.9 × 1011 times larger than that of a conventional quartz crystal balance (with a frequency of 5 MHz) and ∼1000 times higher than that of conventional SAW devices (with a frequency of 978 MHz). Trinitrotoluene concentration as low as 4.4 × 10-9 M (mol·L-1) can be detected using the fabricated SAW sensor, proving its giant mass-loading effect and ultrahigh sensitivity.

Nanotechnology for SAW devices on AlN epilayers

Carbon dioxide (CO2) gas sensor using surface acoustic wave (SAW) device based on Zinc oxide (ZnO) was developed and fabricated in this paper. The center frequency of SAW device is 203.528 MHz. The input and two output interdigital transducers (IDT) apodized by Morlet wavelet function can improve the side lobe rejection compared with uniform IDT. The ZnO film sensitive to CO2 gas was fabricated in measurement acoustic track of SAW device. Experiments results confirm that the CO2 gas sensor using SAW device based on ZnO film has good response characteristics to different concentrations CO2 gas. Furthermore, the CO2 gas sensor using SAW device based on ZnO film has good stability and linearity.

Sezawa SAW devices: Review of numerical-experimental studies and recent applications

This work demonstrates super-high frequency wave-guided surface acoustic wave (SAW) devices based on LiNbO3/SiO2/Si layered substrates. SAW resonators with fundamental operating frequency to the 7-GHz range were developed, and wave modes like the Rayleigh, Shear-horizontal (SH), Pseudo-Bulk, etc., were observed. Furthermore, effective coupling coefficients (k2eff) and quality factor (Q) of the devices have been investigated systematically. Specifically, spurious-free SAW resonators with the SH wave frequency over 7.40 GHz and the high k2eff value of ∼7.8% were obtained at a nanoscale wavelength of 480 nm. Finally, high-performance filters with a bandwidth over 300 MHz were achieved. The demonstrated filters show sharp roll-off and spurious-free responses within the passband with insertion loss <−5 dB and a small temperature coefficient of frequency of ∼−45.8 ppm/K at a super-high frequency of 7.17 GHz. Upon further optimizations, high-performance SAW devices built on a single crystalline LiNbO3 film can potentially enable the low-loss and wideband signal processing functions, promising for the next-generation radio frequency front-end system applications.

Surface acoustic wave (SAW) devices are a technology of choice for passive, radio-interrogable sensor applications operating under extreme conditions. Suitably designed SAW devices can withstand e.g. temperatures exceeding 400°C. At high temperatures (HT), thermal energies reach values corresponding to the activation energies of reactions between gas components and the crystal's substrate elements and/or the metallisation elements, respectively. Thus, the atmosphere in the hermetic packaging becomes a crucial factor for the SAW device's stability. This work investigates the influence of various potential packaging atmospheres on SAW devices at temperatures up to 650°C. The SAW test structures consist of two delay lines with different lengths, which have been processed with Pt - based thin films. Substrate materials were either langasite (LGS), lithium niobate (LN) or stochiometric lithium niobate (sLN). The devices were annealed in a tube oven equipped with a HT-stable radio frequency (RF) measurement system in different atmospheres at several temperature levels up to 650°C. Afterwards, the SAW surfaces were characterised microscopically.