Film Based Surface Acoustic Wave Research Articles

Influence of diamond matrix morphology on ZnO surface morphology and preferred orientation

The signal propagation loss and conversion efficiency are two critical performances of diamond/ZnO film-based surface acoustic wave (SAW) device, and depend on roughness and preferred orientation degree (POD) of ZnO surface layer at plane (002). In-situ improvement of operational performance of ZnO layer is achieved by adjusting morphology of diamond matrix layer via diamond particle size. Diamond layer is deposited on silicon substrate using hot-filament chemical vapor deposition (HFCVD) technique by varying Ar/(Ar+H2) ratio. The ZnO layer is then deposited on the diamond film using radio-frequency magnetron sputtering (RFMS). The results show that, by regulating the diamond particle size, it is possible to adjust morphology of the diamond layer and facilitate variation in morphology and preferred orientation of ZnO layer for a better operational performance of SAW device. When the diamond particle size is decreased from 1.38 to 0.12 µm, the root-mean-square (RMS) roughness of ZnO layer initially drops and then rises, and the lowest roughness is 6.4 nm. Also, POD of ZnO (002) increases from 2.61 to 2.89, and then drops to 2.83. For a diamond particle size of 0.35 µm, the RMS roughness of ZnO layer has minimum value of 6.4 nm, and the POD of ZnO (002) approaches the maximum of 2.89 with a half-peak width of 0.36˚. The influence of diamond layer morphology on ZnO morphology and POD is thus proposed.

Integrated transparent surface acoustic wave technology for active de-fogging and icing protection on glass

There have been great concerns on poor visibility and hazardous issues due to fogging and ice/frost formation on glass surfaces of windshields, windows of vehicles/airplanes, and solar panels. Existing methods for their monitoring and removal include those active ones (such as using resistance heating) or passive ones (such as using surface icephobic treatments), which are not always applicable, effective or reliable. In this study, we proposed a novel strategy by implementing transparent thin film surface acoustic wave (SAW) devices by directly coating ZnO films onto glass substrate and studied their de-fogging, active anti-icing and de-icing mechanisms using the SAW technology. Effects of powers and wavelengths of SAW devices were investigated and influences of acousto-heating and surface hydrophobic treatments were evaluated. Results showed that de-fogging time was dramatically decreased with the increase of SAW powers when the thin film-based SAW devices were exposed to humid air flow for different durations. The icing accretion was significantly delayed under the applied SAW agitation, and SAW application has also effectively promoted de-icing on glass substrate, due to the interfacial nanoscale vibration and localized heating effect.

Machine learning empowered thin film acoustic wave sensing

Thin film-based surface acoustic wave (SAW) technology has been extensively explored for physical, chemical, and biological sensors. However, these sensors often show inferior performance for a specific sensing in complex environments, as they are affected by multiple influencing parameters and their coupling interferences. To solve these critical issues, we propose a methodology to extract critical information from the scattering parameter and combine the machine learning method to achieve multi-parameter decoupling. We used the AlScN film-based SAW device as an example in which the highly c-axis orientated and low stress AlScN film was deposited on silicon substrate. The AlScN/Si SAW device showed a Bode quality factor value of 228 and an electromechanical coupling coefficient of ∼2.3%. Two sensing parameters (i.e., ultraviolet or UV and temperature) were chosen for demonstration, and the proposed machine learning method was used to distinguish their influences. Highly precision UV sensing and temperature sensing were independently achieved without their mutual interferences. This work provides an effective solution for decoupling of multi-parameter influences and achieving anti-interference effects in thin film-based SAW sensing.

Specific Sensing Mechanism Investigation of Surface Acoustic Wave Humidity Sensors Coated With Uniform Graphene Oxide Membrane

Surface acoustic wave (SAW) humidity sensors promise broad prospects for Internet of Things (IoT) applications due to the advantages of low cost, portability, and high sensitivity. However, previous researches mainly focused on developing SAW devices with higher resonant frequencies, or using new sensing materials to improve the performance of SAW humidity sensors. Few researches studied the specific response mechanism of SAW humidity sensors, which restricted further development of the sensors. In this paper, the sensing mechanism of a graphene oxide (GO) film-based SAW humidity sensor is thoroughly studied by investigating the mechanical and electrical parameters of the uniform GO membrane at different relative humidity (from 10% RH to 90% RH). After taking these parameters into consideration for the mechanism analysis, we quantitatively indicate that the mass loading (~800 ppm) and viscoelastic effects (~60 ppm) are the primary and secondary influence factors of the sensor’s humidity response respectively, while the acoustoelectric interaction has little effect on the sensitivity. [2020-0024]

AlScN thin film based surface acoustic wave devices with enhanced microfluidic performance

This paper reports the characterization of scandium aluminum nitride (Al1−xScxN, x = 27%) films and discusses surface acoustic wave (SAW) devices based on them. Both AlScN and AlN films were deposited on silicon by sputtering and possessed columnar microstructures with (0 0 0 2) crystal orientation. The AlScN/Si SAW devices showed improved electromechanical coupling coefficients (K2, ~2%) compared with pure AlN films (<0.5%). The performance of the two types of devices was also investigated and compared, using acoustofluidics as an example. The AlScN/Si SAW devices achieved much lower threshold powers for the acoustic streaming and pumping of liquid droplets, and the acoustic streaming and pumping velocities were 2 × and 3 × those of the AlN/Si SAW devices, respectively. Mechanical characterization showed that the Young’s modulus and hardness of the AlN film decreased significantly when Sc was doped, and this was responsible for the decreased acoustic velocity and resonant frequency, and the increased temperature coefficient of frequency, of the AlScN SAW devices.

Wave Propagation Direction and c-Axis Tilt Angle Influence on the Performance of ScAlN/Sapphire-Based SAW Devices.

Some previously reported surface acoustic wave (SAW) devices using bulk piezoelectric substrates showed higher acoustic power radiated in either forward or backward wave propagation direction depending on their crystal orientations and are called natural single-phase unidirectional transducers (NSPUDT). While these reports were based on bulk piezoelectric substrates, we report directionality in the c-axis tilted 44% scandium doped aluminum nitride thin piezoelectric film-based SAW devices on sapphire. It is worth noting that our observance of directionality is specifically in Sezawa mode. We produced a c-axis tilt up to 5.5° over the single wafer and examined the directionality by comparing the forward and backward insertion loss utilizing split finger electrodes as a receiver. The wave propagation direction and c-axis tilt angle influence on the performance of SAW devices is evaluated. Furthermore, return loss and insertion loss data are presented for various SAW propagation directions and c-axis tilt angles. Finally, the comparison for both acoustic modes, i.e., Rayleigh and Sezawa, is reported.

High performance AlScN thin film based surface acoustic wave devices with large electromechanical coupling coefficient

AlN and AlScN thin films with 27% scandium (Sc) were synthesized by DC magnetron sputtering deposition and used to fabricate surface acoustic wave (SAW) devices. Compared with AlN-based devices, the AlScN SAW devices exhibit much better transmission properties. Scandium doping results in electromechanical coupling coefficient, K2, in the range of 2.0% ∼ 2.2% for a wide normalized thickness range, more than a 300% increase compared to that of AlN-based SAW devices, thus demonstrating the potential applications of AlScN in high frequency resonators, sensors, and high efficiency energy harvesting devices. The coupling coefficients of the present AlScN based SAW devices are much higher than that of the theoretical calculation based on some assumptions for AlScN piezoelectric material properties, implying there is a need for in-depth investigations on the material properties of AlScN.

Characterisation of aluminium nitride films and surface acoustic wave devices for microfluidic applications

Aluminium nitride (AlN) films with different thicknesses (from 2.3 to 4.7μm) were deposited onto high resistivity silicon substrates using magnetron sputtering. Crystalline and bonding structures of the deposited AlN films were characterised. The AlN films showed a highly c-axis texture. AlN film based surface acoustic wave (SAW) devices were fabricated and characterised. The SAW devices showed Rayleigh wave transmission band with a large side-lobe suppression of ∼15dB. With the increase in film thickness, both the central band frequency and electromechanical coupling coefficient were increased, and values of temperature coefficient of frequency was increased linearly from −21.3 to −27.4ppm/K. Microfluidic manipulations including streaming, pumping and jetting have been realised using AlN SAW devices. The applied RF power boundary between streaming and pumping and that between the pumping and jetting decreased with the increase of film thickness. The measured streaming and pumping velocities as well as device surface temperatures increased with the film thickness.

Nano-Crystalline SnO2 Thin Film Based Surface Acoustic Wave Sensor for Selective and Fast Detection of NO2 Gas

Nano-Crystalline SnO₂ Thin Film Based Surface Acoustic Wave Sensor for Selective and Fast Detection of NO₂ Gas

BCB film based SAW humidity sensor

Novel benzocyclobutene (BCB) film based surface acoustic wave (SAW) humidity sensors were successfully designed and fabricated. A sensitivity of 0.26 dB/RH% was successfully demonstrated between 8.6 and 90.6% relative humidity (RH), with a linear relationship between RH level and resonance amplitude. The temperature stability of resonance frequency and amplitude has also been studied.

Dynamic chemical vapor sensing with nanofibrous film based surface acoustic wave sensors

We report on the use of electrospun nanofibrous film for surface acoustic wave (SAW) sensors to enhance its chemical detection capability. Ultrafine (100–300 nm) polyethylene oxide (PEO) fibers with controlled thickness and porosity were electrospun-coated on the surface of a ST-cut quartz SAW sensor. The nanofibrous film provides a high surface area to volume ratio, which can not only offer more adsorption sites for vapor molecules, but also shortens the diffusion length of vapor molecules into polymer material. Compared to conventional thin film coating techniques, the nanofiber-coated SAW sensor shows a higher sensitivity and faster response (shorter adsorption/desorption times). In addition, a theoretical analysis was performed to understand SAW sensor response with nanofibrous film coating. It is concluded that the nanofiber film holds a great potential in enhancing SAW sensor performance for trace level detection of chemical analytes.

Microfluidic pumps employing surface acoustic waves generated in ZnO thin films

ZnO thin film based surface acoustic wave (SAW) devices have been utilized to fabricate microfluidic pumps. The SAW devices were fabricated on nanocrystalline ZnO piezoelectric thin films deposited on Si substrates using rf magnetron sputtering and use a Sezawa wave mode for effective droplet motion. The as-deposited ZnO surface is hydrophilic, with a water contact angle of ∼75°, which prevents droplet pumping. Therefore, the ZnO surface was coated using a self-assembled monolayer of octadecyltrichlorosilane which forms a hydrophobic surface with a water contact angle of ∼110°. Liquid droplets between 0.5 and 1 μl in volume were successfully pumped on the hydrophobic ZnO surface at velocities up to 1 cm s−1. Under acoustic pressure, the water droplet on an hydrophilic surface becomes deformed, and the asymmetry in the contact angle at the trailing and leading edges allow the force acting upon the droplet to be calculated. These forces, which increase with input voltage above a threshold level, are found to be in the range of ∼100 μN. A pulsed rf signal has also been used to demonstrate precision manipulation of the liquid droplets. Furthermore, a SAW device structure is demonstrated in which the ZnO piezoelectric only exists under the input and output transducers. This structure still permits pumping, while avoiding direct contact between the piezoelectric material and the fluid. This is of particular importance for biological laboratory-on-a-chip applications.

ZnO film based surface acoustic wave micro-pump

In this study, a micro-pump unit based on surface acoustic wave (SAW) on piezoelectric ZnO film is designed and fabricated as a micro-fluidic device. It employs a mechanical wave, which is generated electrically using an aluminum interdigital transducer (IDT), and propagates on the surface of the ZnO film. The ZnO film was used in this study because it has a high electromechanical coefficient and an excellent bonding with various substrate materials, in particular silicon. The sputtering parameters for ZnO film deposition have been optimized, and the ZnO films with different thickness from 1 micron to 5.5 microns were prepared. The film properties have been characterized using different methods, such as scanning electron microscopy, X-ray diffraction and atomic force microscopy. Aluminum IDT with a finger width and spacing of 8 microns was patterned on the ZnO film using a lift-off process. The frequency generated was measured using a network analyzer, and it varies from 130 MHz to 180 MHz as a function of film thickness. A signal generator was used to generate the frequency with a power amplifier to amplify the signal, which was then applied to aluminum IDT to generate the surface acoustic wave. If a liquid droplet exists on the surface carrying the acoustic wave, the energy and the momentum of the SAW will be coupled into the fluid, causing the liquid to vibrate and move on film surface. The strength of this movement is determined by the applied voltage and frequency. The volume of the liquid drop loaded on the SAW device in this study is of several hundreds of nanoliters. The movement of the liquid inside the droplet and also on the ZnO film surface can be demonstrated. The performance of ZnO SAW device was characterized as a function of film thickness.

Selective etching of Al/AlN structures for metallization of surface acoustic wave devices

Metallization is a critical step in the fabrication of high frequency AlN thin film based surface acoustic wave devices. Both state-of-the-art lithography as well as high selectivity and anisotropy during etching of Al with respect to AlN are required for low loss and high performance devices. In this work, the etch rates of reactively sputtered AlN, sputtered Al, thermal SiO2, and Shipley 1813 photoresist as well as the selectivity among Al/AlN, Al/SiO2, and resist/Al have been systematically studied during inductively coupled plasma (ICP) etching. Emphasis is focused on obtaining high Al etch rates, while at the same time keeping the etch rate of AlN and that of the resist sufficiently low. High anisotropy is obtained by passivating the sidewalls by the addition of oxygen. The recipe developed is based on a modified Al etch using a mixture of BCl3, Cl2, O2. The parameters varied were gas composition, and substrate bias. Generally it is found that the Al etch rate exhibits a maximum with the O2 flow, while the AlN etch rate decreases monotonically. Substrate bias is found also to be an important parameter with respect to both etch rate and selectivity. At optimized conditions with respect to process selectivity (500 W ICP power, 83 V bias, 50 sccm BCl3, 25 sccm Cl2, 10 sccm O2, pressure 10 mTorr) the Al etch rate is 1000 nm/min with a selectivity of 1000 toward AlN and 9 toward the resist. The same recipe, slightly modified, has also shown similar Al etch rates when etching Al over SiO2 with a selectivity of up to 200. High anisotropy of the Al etch rate is observed with increasing O2 flow. The former has been determined from cross-sectional scanning electron microscope observations.

Strain effect in the YBa 2Cu 3O 7− x thin film based surface acoustic wave structures

Results of the strain effect theoretical investigation for YBa 2Cu 3O 7− x thin film based surface acoustic wave (SAW) structures are presented. Third order elastic moduli of YBa 2Cu 3O 7− x polycrystals at room temperature were evaluated and used to calculate the coefficients of SAW phase velocity sensitivity with respect to deformations and rotations both for the YBa 2Cu 3O 7− x polycrystalline substrate and YBa 2Cu 3O 7− x \\SrTiO 3 laminated structures with various crystallographic orientations of SrTiO 3 single crystal substrate and YBa 2Cu 3O 7− x polycrystalline film thicknesses. It has been shown that strain effect values in such structures are determined, above all, by the YBa 2Cu 3O 7− x layer thickness rather than by substrate characteristics and substantially exceed all the literature data on other SAW materials and structures. © 1997 Elsevier Science S.A. All rights reserved.

Speciation of linear and branched hydrocarbons by a fluorinated polyimide film based surface acoustic wave sensor

We report a surface acoustic wave (SAW) device-based sensor that uses a fluorinated polyimide film as the chemically sensitive interface to preferentially detect linear over branched alkane isomers. Linear hydrocarbons penetrate the film, producing a change in mass that is over 100 times larger than for comparable concentrations of the branched isomer of the same alkane, consistent with very limited penetration by the latter. Results were confirmed using real-time FTIR external-reflection spectroscopy (ERS), which also shows that the penetration of normal alkanes results in conformational changes to the polymer; no similar effects are observed as a result of exposure to branched alkanes. We believe that the speciation results because the chains of this polymer pack together unusually closely and uniformly, with access to the interconnected `free volume` of the polymer film being through openings large enough to pass methyl or methylene groups, but too small for a CH-CH{sub 3} unit to go through `sideways`. The range of hydrocarbons to which this effect extends, the dependence of separation efficiency on film preparation techniques, and models of polymer chain packing to explain these results are under investigation. 12 refs., 2 figs.