Fabricated Surface Acoustic Wave Devices Research Articles

Integrated Sensing and Acoustofluidic Functions for Flexible Thin Film Acoustic Wave Devices Based on Metallic and Polymer Multilayers

Surface acoustic wave (SAW) devices are generally fabricated on rigid substrates that support the propagation of waves efficiently. Although very challenging, the realisation of SAW devices on bendable and flexible substrates can lead to new generation SAW devices for wearable technologies. In this paper, we report flexible acoustic wave devices based on ZnO thin films coated on various substrates consisting of thin layers of metal (e.g., Ni/Cu/Ni) and/or polymer (e.g., polyethylene terephthalate, PET). We comparatively characterise the fabricated SAW devices and demonstrate their sensing applications for temperature and ultraviolet (UV) light. We also investigate their acoustofluidic capabilities on different substrates. Our results show that the SAW devices fabricated on a polymer layer (e.g. ZnO/PET, ZnO/Ni/Cu/Ni/PET) show enhanced temperature responsivity, and the devices with larger wavelengths are more sensitive to UV exposure. For actuation purposes, the devices fabricated on ZnO/Ni/Cu/Ni layer have the best performance for acoustofluidics, whereas insignificant acoustofluidic effects are observed with the devices fabricated on ZnO/PET layers. We propose that the addition of a metallic layer of Ni/Cu/Ni between ZnO and polymer layers facilitates the actuation capability for the acoustofluidic applications while keeping temperature and UV sensing capabilities, thus enhancing the integration of sensing and acoustofluidic functions.

Gallium Nitride: A Versatile Compound Semiconductor as Novel Piezoelectric Film for Acoustic Tweezer in Manipulation of Cancer Cells

Gallium nitride (GaN) is a compound semiconductor which has advantages to generate new functionalities and applications due to its piezoelectric, pyroelectric, and piezo-resistive properties. Recently, surface acoustic wave (SAW)-based acoustic tweezers were developed as an efficient and versatile tool to manipulate nano- and microparticles aiming for patterning, separating, and mixing biological and chemical components. Conventional piezoelectric materials to fabricate SAW devices such as lithium niobate suffer from its low thermal conductivity and incapability of fabricating multiphysical and integrated devices. This article piloted the development of a GaN-based acoustic tweezer (GaNAT) and its application in manipulating microparticles and biological cells. For the first time, the GaN SAW device was integrated with a microfluidic channel to form an acoustofluidic chip for biological applications. The GaNAT demonstrated its ability to work on high power (up to 10 W) with minimal cooling requirement while maintaining the device temperature below 32°C. Acoustofluidic modeling was successfully applied to numerically study and predict acoustic pressure field and particle trajectories within the GaNAT, which agree well with the experimental results on patterning polystyrene microspheres and two types of biological cells including fibroblast and renal tumor cells. The GaNAT allowed both cell types to maintain high viabilities of 84.5% and 92.1%, respectively.

Comprehensive characterization of ZnO thin films for surface acoustic wave applications

This paper presents a comprehensive characterization of a very smooth, c-axis oriented, highly piezoelectric and electrically resistive 2.5 µm-thick ZnO thin film deposited by a radio frequency (RF) magnetron sputtering system on SiO2/Si substrate. Thin film properties such as surface roughness, crystallography, stoichiometry, and electrical resistivity are measured. Two-port surface acoustic wave (SAW) devices with bidirectional interdigital transducer (IDT) periods of 16 µm, 20 µm and 24 µm are fabricated on top of the ZnO thin film. A detailed finite element analysis (FEA) of the thin film is elaborated by varying ZnO thickness and IDT configuration. FEA results shows that acoustic wave velocities and resonance frequencies of the SAW devices are decreasing together with increasing ZnO thickness. Frequency response of the fabricated SAW devices are measured with a vector network analyzer (VNA) and compared to FEA results. First two wave modes, namely the Rayleigh and Sezawa waves, of the fabricated SAW devices on the ZnO thin films are interrogated. Resonance frequencies of the SAW devices with wavelengths of 16 µm, 20 µm, and 24 µm are measured as 204.8 MHz, 176.3 MHz, and 155.3 MHz for the Rayleigh mode, 335.9 MHz, 275.3 MHz, and 235.1 MHz for the Sezawa mode, respectively. In addition, omnidirectional wave propagation of ZnO thin film is shown with 45° rotated IDTs. Overall, experimental results are in good agreement with simulation results, demonstrating ZnO thin film fabrication is successfully carried out, and FEA is an appropriate method for modeling SAW devices on thin films.

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.

A stable and highly sensitive strain sensor based on a surface acoustic wave oscillator

A stable and highly sensitive strain sensor based on a surface acoustic wave (SAW) oscillator is presented in this article. A 151MHz SAW delay line was fabricated on a 128° YX LiNbO3 substrate as the oscillator element. A cantilever beam with a tapered cross section was designed to provide a quasi-uniform strain testing platform and analyzed with a finite element method (FEM) model. Single-mode selection capability was achieved by using a comb configuration, and the triple transition interference (TTI) was suppressed by using a single phase unidirectional transducer (SPUDT). Prior to fabrication, a FEM/boundary element method (BEM) simulation was used to simulate and optimize the parameters of the SAW delay line structure. The RF test results of the fabricated SAW devices agree well with the simulation results. Further, the output frequency fluctuation of the developed SAW oscillator was shown to be only approximately 0.7ppm in a temperature-controlled laboratory environment for stability tests. The measured sensitivity was evaluated to be 126Hz/μɛ, and good linearity was observed at strain values ranging from 0 to 400μɛ.

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.

Deposition of Piezoelectric ZnO Films by RF Magnetron Sputtering and its Application to SAW Devices

AbstractHighly c-axis oriented, dense, and fine-grained polycrystalline ZnO films with smooth surface and high resistivity were deposited on 4 inch silicon wafers by employing ZnO targets in a radio-frequency (RF) magnetron sputtering system. By changing applied RF power, substrate temperature and O2/Ar gas ratio, the optimum process parameters were found to be 150 W, 200 °C and 30/70, respectively. Applying the ZnO films deposited under these optimum conditions, surface acoustic wave (SAW) devices of ZnO/IDT/SiO2/Si structure were fabricated by conventional photolithography and etching processes. The interdigital transducers (IDT), made of the aluminum deposited by DC magnetron sputter, were patterned as 2.5/2.5 μm of finger width/spacing. Another type of SAW filter of IDT/ZnO/diamond/Si structure was fabricated. In this structure, high-quality nanocrystalline diamond (NCD) films were deposited on 4 inch silicon wafers by direct current (DC) plasma assisted chemical vapor deposition method using H2-CH4 mixture as precursor gas. On the top of the diamond films, ZnO films were deposited under the optimum conditions. The aluminum IDT pattern was fabricated on the ZnO/diamond layered films. The characteristics of the fabricated SAW devices were evaluated in terms of center frequency, insertion loss, and wave propagation velocity.

Effects of lattice mismatches in ZnO/substrate structures on the orientations of ZnO films and characteristics of SAW devices

Polycrystalline ZnO films are deposited using RF magnetron sputtering on various substrate materials including AlN/Si-(111), sapphire, DLC/Si-(100), SiO 2/Si-(100) and Si-(111). The structural parameters of deposited ZnO films, such as texture coefficient (TC) value for (002)-orientation, crystallite size and full-width at half-maximum (FWHM) at ZnO (002)-peak, are compared. Surface acoustic wave (SAW) devices are also fabricated by using a lift-off method, with the configuration of IDT/ZnO/substrate. The frequency response characteristics (including S 21) of the fabricated SAW devices are measured and the device parameters, such as insertion losses and side-lobe rejection level, are estimated in terms of the substrate materials. Experimental results indicate that the lattice matching as well as the structural similarity between ZnO and substrate may be essential for determining the SAW device characteristics.

Fabrication and characterization of high frequency SAW device with IDT/ZnO/AlN/Si configuration: role of AlN buffer

Surface acoustic wave (SAW) devices with a ZnO/buffer/Si configuration have been fabricated, employing poly-AlN (deposited by RF magnetron sputtering) and epi-AlN (deposited by plasma-assisted MBE) as the buffer layer. In particular, the effects of the AlN buffers on the orientation of ZnO films as well as the performance of SAW devices are studied. Piezoelectric ZnO films are deposited using RF sputtering at a nominal condition of RF power of 300 W, O 2/(Ar+O 2) ratio of 10%, and substrate temperature of 300 °C. For all the deposited AlN and ZnO films, the X-ray diffraction (XRD) spectra and full width at half maximum ( ω FWHM) of rocking curves are measured in terms of the deposition conditions, to characterize the c-axis preferred orientation and crystal quality. The frequency response characteristics (including S 21) of the fabricated SAW devices are also measured. The experimental results indicate that the c-axis orientation and crystal quality of ZnO films are determined mainly by the properties of AlN buffer. Furthermore, the insertion loss of ZnO-SAW devices is found to be dependent on the crystal quality and surface morphology of AlN buffer. The correlations between the properties of AlN buffers and the characteristics of ZnO-SAW devices are also discussed in detail.