Frequency Of Surface Acoustic Wave Device Research Articles

Ultrahigh frequency shear-horizontal acoustic wave humidity sensor with ternary nanocomposite sensing layer

Surface acoustic wave (SAW) technology is promising for humidity monitoring due to its digital output, small size, large-scale production and wireless passive capability, but there are major challenges to achieve ultra-high sensitivity and fast responses using the conventional SAW devices. Herein, ultrahigh frequency (4.7 GHz and 5.9 GHz) shear-horizontal (SH) SAW devices were developed and a ternary nanocomposite strategy of graphene quantum dots/polyethyleneimine/silicon dioxide nanoparticles (GQDs-PEI-SiO2 NPs) was proposed as a sensitive layer to achieve ultrahigh sensitivity and fast response. This ternary material system was constructed by modifying the surface of SiO2 NPs with the PEI through an electrostatic force, and then adsorbing the GQDs onto the PEI through hydrogen bonds. Compared with the conventional low frequency SAW devices, the ultrahigh frequency SH-SAW devices showed exceptionally ultra-high sensitivity (2.4 MHz/%RH, 1000 times as high as a 202 MHz SAW device), fast response (20 s/5 s), excellent linearity, and good repeatability in the range of 20–80% RH. These superior performances are attributed to ultrahigh frequency of SAW devices, large specific surface areas of the nanocomposite (which exposed multiple hydrophilic groups in PEI and GQDs), and high vapor pressure of convex spherical curved liquid surface (which accelerated the adsorption and desorption of water molecules).

Mm-band surface acoustic wave devices utilizing two-dimensional boron nitride

The simple structure, low power consumption, and small form factor have made surface acoustic wave (SAW) devices essential to mobile communication as RF filters. For instance, the latest 5G smartphones are equipped with almost 100 acoustic wave filters to select a specific frequency band and increase communication capacity. On the arrival of the newest communication standard, 5G, mm-band up to 39 GHz is supposed to be utilized, whereas the conventional SAW filters are limited to below 3 GHz, leaving a critical component missing. Here, we show an emerging 2D material—hexagonal boron nitride—can become a key enabler of mm-band SAW filter. Our study, based on first principles analysis and acousto-electric simulation, shows the operating frequency of SAW devices can reach over 20 GHz in its fundamental mode and 40 GHz in its interface mode with high electromechanical coupling coefficient (K2) and low insertion loss. In addition to the orders of magnitude improvement compared to the conventional SAW devices, our study provides a systematic approach to utilizing van der Waals crystals with highly anisotropic acoustic properties for practical applications.

3D Layout of Interdigital Transducers for High Frequency Surface Acoustic Wave 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.

Analysis of propagation characteristics of AlN/diamond/Si layered SAW resonator

Surface acoustic wave (SAW) devices have attracted much attention in the field of mobile communication, such as filters and duplexers. With the upgrading of mobile networks, the requirements for the working frequency of SAW devices are gradually increasing. Traditional single-layer piezoelectric SAW devices can not meet the requirements of today’s performance. However, a new type of layered structure not only meets various performance requirements, but also provides a new direction for the development of SAW devices. In this paper, a layered structure of AlN/diamond/Si SAW resonator is designed. A two-dimensional unit structure model is established. Modal analysis and admittance characteristic analysis are carried out. The optimal design structure is obtained by choosing form the thickness of different electrodes and piezoelectric layers. Following this, the transient analysis of the two-dimensional complete resonator structure is carried out. Next, the propagation characteristics are obtained, and finally the input and output signals are compared. Compared with the single piezoelectric layer structure, the phase velocity and electromechanical coupling coefficient are increased by 32.2% and 49.6% respectively. The work in this paper lays a good foundation for the fabrication of high frequency layered SAW resonators above GHz.

Carbon Dioxide Gas Sensor Using SAW Device Based on ZnO Film

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.

The effects of ZnO films on surface acoustic wave properties of modified lead titanate ceramic substrates

Poly-crystal zinc oxide (ZnO) films with c-axis (002) orientation have been successfully grown on the strontium (Sr) modified lead titanate ceramic substrates with different Sr dopants by r.f. magnetron sputtering technique. Highly oriented ZnO films with c-axis normal to the substrates can be obtained under a total pressure of 10 mTorr containing 50% argon and 50% oxygen and r.f. power of 70 W for 3 hours. Crystalline structures of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency of surface acoustic wave (SAW) devices with ZnO/IDT/PT (IDT, inter-digital transducer; PT, PbTiO3 ceramics) structure were investigated. The devices with ZnO/IDT/PT structure shows that the ZnO film effectively raise the electromechanical coupling coefficient (kappa2) from 3.8% to 9.9% of the device with the concentrations of Sr dopants of 0.15. It also improves the temperature coefficient of frequency of SAW devices.

Wireless passive SAW sensors using single-electrode-type IDT structures as programmable reflectors

Sensor information can be read out wirelessly by an antenna if the surface acoustic wave (SAW) delay line is connected to an external classical sensor. This is a passive device also called impedance-loaded SAW sensor. In the last decade, many researchers have made great efforts researching such devices, in which the double-electrode-type interdigital transducer (IDT) has been used as programmable reflector because its short-circuit reflection coefficient is very small. In this paper, the single-electrode-type IDT structure is proposed to be used as programmable reflector. The metallic periodic structure deposited on 128°YX LiNbO 3 shows an extraordinary characteristic: for a certain ratio of the film thickness to the periodicity, the reflection of the short-circuited single-electrode-type IDT almost disappears. This characteristic enables great amplitude modulation when the impedance-loaded SAW sensor employs the single-electrode-type IDT as programmable reflector. The fundamental operating frequency of SAW devices using the single-electrode-type IDT structure is twice that of SAW devices using the double-electrode-type IDT structure with the same critical dimension. This means higher operating frequencies for such devices can be reached with the achievable linewidth in current manufacturing technologies.

Reactive DC magnetron sputtering of aluminum nitride films for surface acoustic wave devices

A piezoelectric film combined with high velocity substrate as sapphire or diamond, seems very promising for surface acoustic wave (SAW) devices operating at high frequency. In this work, we concentrated on the optimization of growth parameters to perform AlN films with required properties for SAW devices: high resistivity, low roughness and high piezoelectricity coupling. AlN films are deposited by reactive DC magnetron sputtering on silicon substrate as a function of N2 concentration in Ar–N2 gas mixtures. The duration of the process was modulated to obtain a constant film thickness (2 μm) to permit a better comparison. X-Ray diffraction (XRD) shows that the AlN films deposited in the range of 20–100% N2, 400 °C, and 6×10−3 mbar, exhibit a columnar structure textured in (002) orientation corresponding to hexagonal wurtzite structure with a c-axis perpendicular to the surface. In regard to XRD and electrical characterizations, the optimum film properties are obtained in the range of 60–80% N2 concentration. The higher peak intensity of (002) AlN diffraction and the higher resistivity are obtained for the AlN films synthesized with 75% N2. AFM analysis of AlN films demonstrate a low roughness at approximately 3 nm. The SAW device (filter) was formed by development of interdigital transducers of 32 μm wavelength on the AlN/Si structure by photolithography. The frequency response shows a center frequency of 158 MHz corresponding to a phase velocity of 5055 m/s. This value is almost constant for all samples performed at optimum conditions. However, the insertion loss of device seems varies with N2 percentage and the low attenuation is obtained with 70% of N2. This study is extrapolated on a sapphire substrate, and a phase velocity of 5536 m/s is recorded showing the effect of substrate to increase the center frequency of SAW devices.