Frequency Surface Acoustic Wave Devices 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).

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.

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-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.

Multiple-layer guided surface acoustic wave (SAW)-based pH sensing in longitudinal FiSS-tumoroid cultures

A constitutively increased intracellular pH that is higher than the extracellular pH is emerging as a hallmark of cancer and determining pH could play a significant role in the measurement of drug responsiveness of tumor cells. However, a non-invasive, touch-free and real-time pH sensing as a research tool is lacking and remains a major unmet need. The purpose of the current study is to investigate a microfluidic surface acoustic wave (SAW) sensor platform capable of monitoring pH in cell and tumoroid cultures. A novel multi-layer guided SAW sensor integrated into a microfluidic channel was investigated theoretically and experimentally in detail for pH bio-sensing. Sensitivity and capability of the layer guided Love wave device was modeled using the finite element simulation. The model was verified experimentally, and a study monitoring pH of cell growth media is presented. This novel pH sensor is based on a 13.91 MHz center frequency SAW device coated with ZnO (500 nm) and IrO2 (30 nm) layers to increase the sensitivity. A change in mechanical and electrical properties of the conductive IrO2 layer was observed resulting from electrical corrosion induced by pH solutions affecting the charge distribution, SAW phase velocity and attenuation. By measuring the frequency shift induced by the change in SAW phase velocity between the test group and control group, the pH value of cell culture media from H460 cancer cell culture plates from day 0 to day 5 can easily be determined. To improve the sensitivity and stability of the sensor, a finite element method was used to optimize the layer thicknesses. Taken together, the results of experiments show the potential application of this device to be integrated with microfluidic channels and used in determining pH changes in longitudinal tumor cell cultures.

High performance 33.7 GHz surface acoustic wave nanotransducers based on AlScN/diamond/Si layered structures

Surface acoustic wave (SAW) devices are essential devices for communication and sensing, but usually have an operation frequency limit well below 20 GHz due to the constraints of material properties and fabrication capability. By using an AlScN/diamond layered structure with a high electromechanical coupling coefficient K2 and our proposed two-step exposure electron beam lithography (EBL) process for ultra-fine patterns, we have fabricated SAW devices with resonant frequency up to 33.7 GHz in the Ka-band, the highest one ever reported for SAW devices electrically excited by interdigital transducers (IDTs). Combined with finite element analysis, we identified that series resonances are fundamental and high order Rayleigh modes, and K2 are in the range of 1.21%–2.32%, 200% higher compared to those of traditional AlN/diamond-based SAW devices. The high order modes become stronger and dominant, particularly suitable for the development of ultrahigh frequency SAW devices and applications. In addition, the proposed EBL process showed its superb capability to make ultra-fine IDTs down to the nano-scale with excellent smooth edges and uniform patterns, suitable for ultrahigh frequency SAW development.

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.

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.

Use of fluorine-doped silicon oxide for temperature compensation of radio frequency surface acoustic wave devices

This paper investigates acoustic properties, including the temperature coefficient of elasticity (TCE), of fluorine-doped silicon oxide (SiOF) films and proposes the application of the films to the temperature compensation of RF SAW devices. From Fourier transform infrared spectroscopy (FT-IR), SiOF films were expected to possess good TCE properties. We fabricated a series of SAW devices using the SiOF-overlay/Cu-grating/LiNbO(3)-substrate structure, and evaluated their performance. The experiments showed that the temperature coefficient of frequency (TCF) increases with the fluorine content r, as we expected from the FT-IR measurement. This means that the Si-O-Si atomic structure measurable by the FT-IR governs the TCE behavior of SiO(2)-based films even when the dopant is added. In comparison with pure SiO(2) with the film thickness h of 0.3 wavelengths (λ), TCF was improved by 7.7 ppm/°C without deterioration of the effective electromechanical coupling factor K2 when r = 3.8 atomic % and h = 0.28λ. Fluorine inclusion did not obviously influence the resonators' Q factors when r < 8.8 atomic %.

Low-loss unidirectional transducer for high frequency surface acoustic wave devices

A multi-track unidirectional transducer for surface acoustic wave devices is presented. This transducer consists of periodic cells containing in each of the multiple tracks, only two electrodes and two gaps with quarter period width. So the structure has maximal possible dimensions of its elements for a cell period equal to one wavelength. In spite of current technological limitations this permits to implement unidirectional transducers in GHz range. In contrast to known structures with active tracks only, the structure contains alternating both active transducer tracks and passive reflector tracks with different apertures comparable to surface acoustic wave (SAW) wavelength. The tracks strongly interact due to diffraction of waves excited by such electrode structure on a piezoelectric substrate. A structure analysis by means of finite element method shows that complete unidirectionality can be reached. First experimental results are given.

Growth of highly c-axis oriented (B, Al)N film on diamond for high frequency surface acoustic wave devices

Surface acoustic wave (SAW) devices based on an aluminum nitride (AlN)/diamond layered structure are attractive due to their high operating frequency. To enhance the operating frequency of a diamond SAW device, we demonstrated one piezoelectric layer on diamond by doping AlN with boron. In this study, highly c-axis-oriented wurtzite boron–aluminum nitride (B, Al)N films were deposited on diamond by a co-sputtering technique. The resulting films exhibit a higher piezoelectric coefficient d 33 and higher Young's modulus than AlN films. Moreover, the greater rigidity of (B, Al)N film further boosts the resonance frequency of a diamond SAW device. Considering the SAW wavelength (λ = 2 μm), the calculated surface acoustic velocities ( V S) of (B, Al)N on diamond is 8860 m/s that is higher than AlN on diamond (8720 m/s). We also find that the electromechanical coupling coefficient ( K 2) of a SAW device based on (B, Al)N on diamond was the same (~ 0.5%) as that of one based on AlN on diamond.

Deposition of ZnO thin films on SiO 2/Si substrate with Al 2O 3 buffer layer by radio frequency magnetron sputtering for high frequency surface acoustic wave devices

ZnO films with c-axis (0002) orientation have been grown on SiO 2/Si substrates with an Al 2O 3 buffer layer by radio frequency magnetron sputtering. Crystalline structures of the films were investigated by X-ray diffraction, atomic force microscopy and scanning electron microscopy. The center frequency of the surface acoustic wave (SAW) device with a 4.8 μm thick Al 2O 3 buffer layer was measured to be about 408 MHz, which was much higher than that (265 MHz) of ZnO/SiO 2/Si structure and approaches that (435 MHz) of ZnO/sapphire. It is a possible way as an alternative for the sapphire substrate for the high frequency SAW device applications, and is also useful to integrate the semiconductor and high frequency SAW devices on the same Si substrate.

High Frequency Surface Acoustic Wave Devices Based on Multilayered LiNbO3/Diamond/AlN Substrates: A Finite Element Study

We have used 3-D coupled field structural finite element models to study the acoustic wave propagation characteristics of diamond/AlN/LiNbO3 multi-layered piezoelectric surface acoustic wave devices under the influence of fluid loading for applications in chemical and biological sensing. These devices were studied as a method to increase device frequency and sensitivity, and maintain standard fabrication procedures. Although recent experimental investigations have realized GHz frequency devices based on such multilayered substrates, very little is known about the acoustic wave propagation characteristics in these devices. Identifying the optimum configuration and thickness of the various layers involved still represents a challenge which is addressed in this work.

High Frequency Surface Acoustic Wave Devices Based on Multilayered LiNbO3/Diamond/AlN Substrates: A Finite Element Study

not Available.

Very high frequency SAW devices based on nanocrystalline diamond and aluminum nitride layered structure achieved using e-beam lithography

Very high frequency surface acoustic wave (SAW) devices based on the AlN/diamond layered structure are fabricated by direct writing using e-beam lithography on the nucleation side of nanocrystalline diamond (NCD) films deposited by microwave plasma assisted chemical vapor deposition process. The NCD nucleation side is characterized from the point of view of microstructure, morphology and surface topography. Surface roughness as low as 6 nm is reached, which enhances the deposition of AlN film on this flat surface. The interdigital transducers IDTs made in aluminum with lateral resolution down to 600 nm are successfully patterned on the AlN/NCD layered structure with an adapted technological process. Experimental results show that the Rayleigh wave and the higher mode are generated. A high frequency around 4 GHz (mode 1) is obtained for the considered layered structure SAW device, exhibiting a phase velocity of 9200 m/s taking into account the wavelength of 2.4 μm. This value agrees well with calculated values determined from dispersion curves of phase velocity.

Interfacial characterization and residual stress analysis in diamond films on LiNbO3

Diamond films were deposited via microwave plasma chemical vapor deposition on lithium niobate (LN) substrates. Characterization of the interfacial regions formed between diamond films and LN substrates was carried out by several techniques. Secondary ion mass spectroscopy (SIMS) was used to determine the depth profiles of carbon, lithium, niobium, and oxygen in the LN substrate covered with diamond nuclei and in the substrate without nucleation. Results indicate that the diamond nuclei promoted growth of diamond, and in addition, a reduced depth of the lithium deficient zone formed in the LN substrate was observed. Grazing incidence x-ray diffraction and transmission electron microscopy observations corroborated the results obtained by SIMS. Residual stresses determined experimentally by an x-ray method or by the shift in the characteristic diamond peak in Raman spectroscopy were much smaller than the calculated thermal residual stresses. The results further emphasize that the interfacial phases are responsible for relieving the thermal stresses generated during cooling of the layered structure. The thickness of the interfacial region was found to be well below for attenuation of the surface acoustic waves in the gigahertz frequency surface acoustic wave devices.

Nanocrystalline diamond films for surface acoustic wave devices

Surface acoustic wave (SAW) devices are critical components in wireless communication systems. In this paper the microstructural characteristics of NCD films deposited in Ar/H 2/CH 4 microwave discharges and the potentialities of AlN/NCD/Si layered structures for high frequency SAW device achievement are investigated by both experimental and theoretical approaches. A model describing the SAW propagation in such a structure is developed and used in order to determine the acoustic phase velocity ( V) and the electromechanical coupling coefficient ( K 2). A comparison between the phase velocity estimated from the high frequency characterisation of SAW devices and numerical calculations demonstrates the relevance of the model and shows that acoustic velocity up to 9500 m/s may be reached. According to theoretical results, the best combination of V and K 2 is obtained for the IDT/AlN/NCD/Si configuration, kh AlN = 3 and kh NCD ≥ 4. The paper points out that nanocrystalline diamond is a promising material for high frequency SAW devices, since high-velocity smooth films with the required thickness may be deposited in a few hours and can be used as-grown.

Simulations of surface acoustic wave devices built on stratified media using a mixed finite element/boundary integral formulation

The demand for high frequency surface acoustic wave devices for modern telecommunication applications imposes the development of devices able to answer the manufacturer requirements. The use of high velocity substrates for which a piezoelectric layer is required to excite and detect surface waves has been widely investigated and requires the implementation of accurate theoretical tools to identify the best combinations of material. The present paper proposes a mixed formulation combining finite element analysis with a boundary integral method to accurately simulate the capability of massive periodic interdigital transducers to excite and detect guided acoustic waves in layered media. The proposed model is exploited for different typical configurations.

A Novel High Frequency Surface Acoustic Wave Device Based on Piezoelectric Interdigital Transducers

We report on the fabrication of a novel surface acoustic wave device based on piezoelectric interdigital transducers designed in high quality films of Pb(Zr0.2Ti0.8)O3 grown by RF magnetron sputtering. The approach followed is to replace the conventional metallic interdigital transducers by piezoelectric ones consisting of a series of ferroelectric domains with alternating polarization states. The artificial modification of the ferroelectric domain structure is realized by using an atomic force microscope as a local electric field source. A 1.2 μ m wavelength device has been realized and its transmission properties simulated.

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.