High-frequency Surface Acoustic Wave Devices Research Articles

Effect of interdigital transducers structure on insertion loss of high-frequency surface acoustic wave devices

Effect of interdigital transducers structure on insertion loss of high-frequency surface acoustic wave devices

High-Frequency Surface Acoustic Wave Resonator with Diamond/AlN/IDT/AlN/Diamond Multilayer Structure.

A high-frequency surface acoustic wave (SAW) resonator, based on sandwiched interdigital transducer (IDT), is presented. The resonator has the structure of diamond/AlN/IDT/AlN/diamond, with Si as the substrate. The results show that its phase velocity and electromechanical coupling coefficient are both significantly improved, compared with that of the traditional interdigital transduce-free surface structure. The M2 mode of the sandwiched structure can excite an operation frequency up to 6.15 GHz, with an electromechanical coupling coefficient of 5.53%, phase velocity of 12,470 m/s, and temperature coefficient of frequency of −6.3 ppm/°C. This structure provides a new ideal for the design of high-performance and high-frequency SAW devices.

Fabrication of high frequency SAW devices using tri-layer lift-off photolithography

A tri-layer lift-off photolithography technique is presented to enable the fabrication of surface acoustic wave (SAW) devices with near GHz fundamental operating frequencies. SAW devices require high-quality micron-scale features for high frequency operation, a requirement that challenges traditional UV photolithography techniques. The presented process is a modified version of a previously reported tri-layer photolithography process intended for Si and SiO2 substrates which allows for compatibility with materials that are piezoelectric and pyroelectric, often used as the substrate in SAW devices. The process uses a lithographic tri-layer consisting of layers of lift-off resist (LOR) on the bottom, back anti-reflection coating (BARC) in the middle, and photoresist (PR) on top. The addition of the BARC layer prevents back reflection of exposure light, improves the structural integrity of the lithographic stack, and decouples the PR and LOR development, improving resolution by a factor of two over traditional lift-off photolithography techniques. We demonstrate the fabrication of a SAW device with an interdigital transducer (IDT) pitch of 4 μm (minimum feature size of 1 μm) on 128° Y-X cut lithium niobate. The device produces a dip in the measured S11 spectrum at 994.5 MHz, corresponding to the fundamental operating frequency of the device. The fundamental operating frequency of the SAW device is also determined theoretically via numerical modelling and found to be 995.5 MHz. The process thus enables the reliable fabrication of high frequency SAW devices that exhibit excellent agreement with numerical results.

Microstructure and bending piezoelectric characteristics of AlN film for high-frequency flexible SAW devices

Flexible surface acoustic wave (SAW) device is one of hot point areas for wearable communication technology because of their ductility and bendability, but the deterioration problem of device performance after bending experiments is limiting its wide application for future wearable communication devices. In this work, the effect of sputtering power on microstructure properties and bending piezoelectric characteristics of aluminum nitride (AlN) film on polyimide (PI) with molybdenum (Mo) buffer layer for high-frequency flexible SAW device was discussed in detail from 200 W to 280 W. Microstructure results show that the high oriented (002) AlN films with narrow full width at half maximum (FWHM) of 0.29° and low roughness of 3.2 nm are obtained under 240 W sputtering power. Piezoelectric and frequency results of SAW devices after cyclic bending indicate that the relative piezoelectric coefficient d*33 is about 8.01 pm/V, and the center frequency of the flexible SAW devices is as high as 4.95 GHz, especially the piezoelectric and frequency characteristics can maintain high satiability after 10,000 bending cycles. This work may be of useful for the flexible high-frequency SAW devices in the wearable communication area.

Surface acoustic wave modes characteristics of CMOS compatible SiO2/AlN/SiO2/Si multilayer structure with embedded electrodes

In this work, a CMOS compatible surface acoustic wave (SAW) device has been simulated and analyzed on SiO2/AlN/SiO2/Si. The simulation results have been used to characterize a typical fabricated device as well. The phase velocity, coupling coefficient, and temperature coefficient of frequency (TCF) of surface acoustic waves in the proposed structure have been investigated using the finite-element (FE) simulation. The simulation results show that a high velocity and a large effective coupling factor can be simultaneously obtained. Besides, the excellent nearly zero TCF is also achieved. The analysis further shows potential for designing high-frequency SAW devices that are CMOS compatible and temperature stable utilizing the Sezawa wave mode.

Optimization of LiNbO3/Quartz Substrate for High-Frequency Wideband SAW Devices Using Longitudinal Leaky Waves.

Multilayered structures can successfully replace single crystal substrates in high-performance surface acoustic wave (SAW) devices. A combination of high propagation velocity and strong piezoelectric coupling is required for acoustic modes used in wideband high-frequency SAW devices. These characteristics can be achieved simultaneously in SAW devices arranged on a lithium niobate (LN) thin plate bonded to quartz and using longitudinal leaky SAW (LLSAW) if the orientations of both the crystals and the plate thickness are optimized. This article describes an optimization procedure developed to find low-attenuated LLSAWs in layered structures that can be used in LN/quartz applications. The symmetry consideration of two crystals was followed by the optimization of the LN orientation to achieve the largest electromechanical coupling and by an analysis of quartz anisotropy as a crucial factor of the existence of nonattenuated LLSAWs. Finally, the quartz orientation and LN thickness were optimized by a rigorous numerical simulation of resonator admittances and by the extraction of LLSAW attenuation at resonant and antiresonant frequencies. The discovered structures enable the propagation of LLSAWs with velocities in the range of 5400-6000 m/s, electromechanical coupling up to 18%, and negligible attenuation. High Q -factors can be achieved at resonance and antiresonance by the variation of the duty factor in the same layered structure. The specific behavior of the LLSAW attenuation with a variation of the LN thickness and quartz cut angle is illustrated by contour plots for each of the optimal structures discovered.

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.

High-frequency surface acoustic wave devices based on epitaxial Z-LiNbO3 layers on sapphire

Filter market demands push the development of new piezoelectric materials to address the modern telecommunication challenges. Combining composite wafers with an epitaxial piezoelectric layer and a said high velocity and acoustic quality substrate is a promising way to answer those demands. However, the fabrication of high-quality LiNbO3 films with reproducible physical properties is complicated by the difficulty in controlling volatile Li2O incorporation into the film and of measuring its composition. So far, large-scale production of films with physical properties suitable for the targeted applications is not available. In this paper, lithium niobate films with controlled nonstoichiometry were deposited by means of pulsed injection metalorganic vapor phase deposition. We have demonstrated a high acoustical performance for surface acoustic wave devices operating in the frequency range of 3.7 GHz up to 5.3 GHz and based on grown epitaxial Z-axis oriented LiNbO3 films on sapphire. An electromechanical coupling of 8% for the Rayleigh wave at 5.3 GHz was demonstrated experimentally.

High-velocity non-attenuated acoustic waves in LiTaO3/quartz layered substrates for high frequency resonators

Multilayered substrates for Surface Acoustic Wave (SAW) devices are able to combine SAW characteristics that cannot coexist in a single crystal substrate and, thus, meet the strong requirements of the new class of SAW devices developed for the next generations of communication systems. Recently, high performance resonators arranged on LiTaO3/quartz bonded wafers and utilizing shear horizontally polarized acoustic waves were reported. Leaky SAWs with quasi-longitudinal polarization propagate faster and can facilitate fabrication of high frequency SAW devices but generally leak strongly into the substrate. This paper describes how the LiTaO3/quartz structure can be optimized to allow longitudinal SAWs to propagate without attenuation. Due to the symmetry consideration, which is supplemented by a rigorous numerical simulation of the admittance functions of SAW resonators and an accurate extraction of the propagation losses, the found optimal LiTaO3 and quartz orientations with the optimized LiTaO3 thickness ensure the propagation of acoustic waves with a velocity exceeding 5400 m/s and an electromechanical coupling of 6.8% in resonators with Q factors up to 10,000. The optimal LT/quartz structures with plate thicknesses varying between 0.32 and 0.68 wavelengths can be employed in SAW resonators operating at high frequencies, up to 5 GHz. The existence of numerous orientations in quartz supporting the propagation of non-attenuated longitudinal SAWs is explained based on the concept of exceptional bulk waves, which is a part of SAW theory.

High-Frequency Surface Acoustic Wave Devices Based on ZnO/SiC Layered Structure

In this letter, highly (0002) oriented ZnO piezoelectric films were deposited on high acoustic velocity SiC substrates. Surface acoustic wave (SAW) devices on ZnO/SiC layered structure operating in the fundamental mode with high frequency to the 7-GHz range were successfully fabricated for the first time. The comprehensive experimental and theoretical investigations about phase velocities ${V}_{p}$ , the electromechanical coupling coefficients ${K}^{{2}}$ , and quality factors Q of one-port SAW resonators have been studied considering various ZnO normalized thicknesses ${h}/\lambda $ . The resonators with large ${K}^{{2}}$ and high Q are implemented over 5–7 GHz demonstrating ${K}^{{2}}$ of 0.47%–2.83% and Q of 146–549. Specifically, a high ${K}^{{2}}$ of 2.83% and a large Q of 549 are simultaneously achievable for the resonator at 5.19 GHz. Finally, with the high ${V}_{p}$ of ZnO/SiC structure being up to 6800 m/s at ${h}/\lambda = {0.14}$ , a 6.8-GHz SAW filter was achieved. Our work shows that the ZnO/SiC structure is of great potential for high-frequency SAW devices application.

Enhanced coupling factor of surface acoustic wave devices employing ScAlN/diamond layered structure with embedded electrodes

A layered structure ScAlN/diamond configured with embedded electrodes is studied for high frequency and wide band SAW devices. Theoretical studies are performed to calculate the phase velocity and K2 dispersion curves of the Rayleigh mode and its higher modes. The influences of various ScAlN film thickness and electrode thickness as well as different electrode materials on K2 are also investigated. The maximum electromechanical coupling factor (K2) of surface acoustic wave (SAW) resonator based on such structure has been theoretically shown up to be 14.5%, which is about 2.5 times larger than the previously reported. Such an enhancement of K2 makes this layered structure great potential application to ultrawide band SAW filters operating in 2–3GHz range.

AlN/SiO2/Si3N4/Si(100)-Based CMOS Compatible Surface Acoustic Wave Filter With −12.8-dB Minimum Insertion Loss

A CMOS compatible AlN/SiO2/Si3N4/Si(100) surface acoustic wave (SAW) device has been fabricated and will be compared with standard AlN/SiO2-based devices. The presented filter demonstrates high potential for CMOS integrated high-frequency SAW devices. The filter insertion loss could be improved to −12.8 dB. The device exhibits high crosstalk suppression of −50 dB on a standard Si-substrate (10 $\Omega $ cm). X-ray diffraction, (scanning) transmission electron microscopy, and energy dispersive X-ray spectroscopy studies correlate the signal quality with $c$ -axis orientation of aluminum nitride films on interdigitated transducer finger electrodes. Finite-element method simulations are in good agreement with the electric measurements and show typical Rayleigh particle displacement.

Using the Effective Surface Permittivity Method to Analyse Multilayer Piezoelectric Substrates for Surface Acoustic Wave Filters

AbstractIn this study, the properties of surface acoustic wave (SAW) filters, including phase velocity and electromechanical coupling coefficient (K2) are investigated. The effective surface permittivity (ESP) method was employed to estimate the K2 of bulk materials (single layer) and multi-layer (double-layer and trilayer) structures. In the cases of bulk materials, the calculation results agree with the experimental data, and the errors are less than 7% for quartz. In the cases of double-layer materials, the phase velocity and K2 of various materials, such as ZnO/Diamond and LiNbO3/Diamond, were acquired, and the results demonstrate that LiNbO3/diamond is the optimal choice for high-frequency SAW devices. For the cases of trilayer, the structure of ZnO/PZT/diamond has relatively high K2 and phase velocity. Therefore, this structure is the optimal trilayer structure for high-frequency SAW devices. The study demonstrates that ESP method can be successfully used for estimating SAW properties in piezoelectric multi-layer structures even though the structures contain nonpiezoelectric film (diamond). The proposed numerical computation has the potential to shorten the developing time of SAW device.

Design and Fabrication of Nanoscale IDTs Using Electron Beam Technology for High-Frequency SAW Devices

High-frequency Rayleigh-mode surface acoustic wave (SAW) devices were fabricated for 4G mobile telecommunications. The RF magnetron sputtering method was adopted to grow piezoelectric aluminum nitride (AlN) thin films on the Si3N4/Si substrates. The influence of sputtering parameters on the crystalline characteristics of AlN thin films was investigated. The interdigital transducer electrodes (IDTs) of aluminum (Al) were then fabricated onto the AlN surfaces by using the electron beam (e-beam) direct write lithography method to form the Al/AlN/Si3N4/Si structured SAW devices. The Al electrodes were adopted owing to its low resistivity, low cost, and low density of the material. For 4G applications in mobile telecommunications, the line widths of 937 nm, 750 nm, 562 nm, and 375 nm of IDTs were designed. Preferred orientation and crystalline properties of AlN thin films were determined by X-ray diffraction using a Siemens XRD-8 with CuKαradiation. Additionally, the cross-sectional images of AlN thin films were obtained by scanning electron microscope. Finally, the frequency responses of high-frequency SAW devices were measured using the E5071C network analyzer. The center frequencies of the high-frequency Rayleigh-mode SAW devices of 1.36 GHz, 1.81 GHz, 2.37 GHz, and 3.74 GHz are obtained. This study demonstrates that the proposed processing method significantly contributes to high-frequency SAW devices for wireless communications.

Temperature-Compensated High-Frequency Surface Acoustic Wave Device

We report high-frequency surface acoustic wave (SAW) devices with excellent temperature stability using a layered structure consisting of single-crystal LiNbO3 thin film on SiO2/LiNbO3 substrate. SAW devices with a wavelength of 2 μm have been fabricated and several wave modes ranging from ~ 1.5 to 2.1 GHz have been obtained. With the SiO2 interlayer providing the temperature compensation and the top single-crystal Z-cut LiNbO3 piezoelectric thin film for acoustic wave excitation, the fabricated SAW devices exhibit excellent temperature coefficients of frequency. Theoretical calculations are presented to elucidate temperature compensation of the proposed layered structure.

Surface acoustic wave resonators based on (002)AlN/Pt/diamond/silicon layered structure

Performance of surface acoustic wave (SAW) resonators based on (002)AlN/Pt/diamond/silicon layered structure is demonstrated. Three resonators with different wavelengths and center frequencies in the GHz range are designed and constructed without using submicron device fabrication processes, due to the high acoustic velocity of the layered structure. The enhanced performance of the SAW resonator by introducing a thin metal layer between the diamond and AlN layers is demonstrated theoretically and experimentally. Dependence of the SAW propagation characteristics on the normalized AlN film thickness is determined and the experimental results are in good agreement with theoretical predictions. In particular, for the 1st wave mode, a very high SAW velocity of 13,656m/s and an electromechanical coupling coefficient of 0.37% are obtained. These results demonstrate that the (002)AlN/Pt/diamond/silicon layered structure is very promising for high-frequency SAW device applications.

Significant enhancement in electromigration resistance and texture of aluminum films using an ultrathin titanium underlayer

Obtaining electrodes with high electromigration resistance, which are used in high-frequency surface acoustic wave devices, is imperative for the rapidly developing mobile communications industry. The effect of a Ti underlayer on the electromigration resistance and texture of Al films was studied comprehensively. It was found that crystalline orientation, crystalline size and electromigration resistance varied significantly with the thickness of the Ti underlayer. A highly (111) oriented Al film with a full width at half maximum value of the rocking curve of 1.7° and a crystalline grain with a bamboo-like structure was successively deposited on an ultrathin titanium underlayer (∼1nm). The median time to failure of Al interconnects with the ultrathin Ti underlayer was enhanced by one order of magnitude higher than those without the Ti underlayer. The influence of the Ti underlayer on the texture of Al films was investigated using a combination of X-ray diffraction and transmission electron microscopy techniques. The results revealed that the ultrathin discontinuous Ti underlayer facilitated the Al (111) texture growth. The mechanism of texturing was discussed in terms of surface-interface energy balance along with high resolution transmission electron microscopy analysis.

Ultrananocrystalline Diamond-Based High-Velocity SAW Device Fabricated by Electron Beam Lithography

Surface acoustic wave (SAW) devices have been used extensively for a variety of applications such as telecommunications, electronic devices, and sensors. The emerging need for high-bit data processing at gigahertz frequencies and the requirement of high-sensitivity sensors demand the development of high-efficiency SAW devices. With the objective of exploiting the high acoustic velocity of diamond, we report on an optimally developed nanodiamond thin film with crystal size of 3–5 nm, embedded in an amorphous carbon matrix with grain boundaries of 1–1.5 nm, that is integrated with aluminum nitride (AlN) to extend the operating frequency of SAW transducers. We utilize this attractive property of diamond through facile synthesis of a bilayer structure consisting of sputtered AlN deposited on an ultrananocrystalline diamond (UNCD) film. We report the realization of a high-frequency SAW resonator, using a device architecture based on an UNCD layer. The UNCD films were synthesized using a microwave plasma-enhanced chemical vapor deposition (MWPECVD) technique and were used to enhance the SAW velocity in the AlN thin film, thus opening the way for the application of CMOS compatible high-frequency SAW devices. The deposition and characterization of UNCD thin films are presented and highlighted for the realization of the SAW resonators. The high velocity associated with the UNCD/AlN bilayered approach together with the high lateral resolution of the interdigital transducers obtained with electron beam lithography is essential for the realization of high-frequency SAW devices. The fabricated devices demonstrate resonance frequencies of 11.3 and 6.2 GHz corresponding to spatial periods of 800 and 1600 nm, respectively, yielding a SAW velocity of 9040 and 10 064 m/s, respectively.

ZnO Surface Acoustic Wave Sensor for the Enhanced Detection of DMMP

This article reports the development of ZnO coated one port Surface Acoustic Wave (SAW) resonators for the detection of DMMP (dimethyl methyl phosphonate), a simulant of chemical warfare agent Sarin at room temperature. For enhanced sensitivity high frequency SAW devices (433Mhz) were used. Frequency shift with DMMP exposure was found to increase with increase in thickness of ZnO film from 20 to 80 nm with maximum shift of 52 kHz. The cross sensitivity of the sensor with commonly available volatile organic compounds (VOCs), gases and water vapors has been performed and was found that the sensitivity towards other interferants is very less as compared to that of DMMP vapors. The sensing mechanism has been explained in detail. It has been shown that there was no significant change in the resistance of the film with exposure to DMMP vapors and hence the contribution of the acoustoelectric interaction is negligible. Further mass loading was not dominant. The negative differential frequency shifts proved that change in elasticity of the film with exposure to DMMP vapors is the dominant sensing mechanism.

Effect of Alumina Film on Surface Acoustic Wave Properties of ZnO Thin Film Surface Acoustic Wave Devices

ZnO films with c-axis (0002) orientation have been successfully grown by RF magnetron sputtering on Al2O3/SiO2/Si substrates. The alumina films were deposited on SiO2/Si substrates by electron beam evaporation. Crystalline structures of the films were investigated by X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The phase velocity of the surface acoustic wave (SAW) device with a 8-µm-thick alumina film was measured to be about 4733 m/s, which is much higher than that (4069 m/s) of the ZnO/SiO2/Si structure and approaches (89.6%) that (5283 m/s) of ZnO/sapphire. The coupling coefficient and temperature coefficient of the frequency of the ZnO/Al2O3/SiO2/Si structure with 8-µm-thick alumina film were close to those of the ZnO/sapphire structure. The experimental result is beneficial to the replacement of the expensive single-crystalline sapphire substrate with alumina film at lower cost for high-frequency SAW devices, and is also useful for integrating the semiconductor and high-frequency SAW devices on the same Si substrate.