Surface Of Surface Acoustic Wave Device Research Articles

Piezoelectric microsensor for selective detection of low concentrations of ammonia

This study developed an ammonia (NH3) sensor based on an electrode-width-controlled/single-phase unidirectional transducer surface acoustic wave (SAW) resonator coated with poly(diketopyrrolopyrrolethiophene-thieno[3,2-b]thiophene-thiophene) (DPP2T-TT) film. This sensor can be operated at room temperature and can detect NH3 at concentrations of the order of parts per billion (ppb). DPP2T-TT films were deposited on the surface of SAW devices using the drop-casting method and were characterized using X-ray diffraction, field-emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Our developed sensor could linearly detect NH3 at concentrations of 50–1000 ppb in a dry air environment. This sensor exhibited a sensitivity of 48 Hz for 400 ppb NH3 in dry air, and the average response and recovery times were typically <45 s. The sensor also exhibited repeatability, reversibility, and long-term stability. Its sensitivity toward NH3 was 13 times higher than that toward other interfering gases, indicating its excellent selectivity.

Novel SAW CWA Detector Using Temperature Programmed Desorption

In this article a temperature programmed desorption (TPD) on an uncoated surface acoustic wave (SAW) device has been explored for selectivity studies of chemical warfare agents (CWA). High sensitivity to mass change on SAW device surface was exploited to study desorption patterns in a novel way. CWA Simulants 2-chloroethyl ethyl sulfide (CEES), Methyl Salicylate (MS) and Dimethyl Methyl phosphonate (DMMP) were investigated at different concentrations. SAW device was cooled and exposed to simulant vapours resulting in physical adsorption of gas molecules on device surface and subsequently by ramp heating the device, vapours were desorbed. The change in SAW oscillation frequency, due to desorption, was monitored during the temperature ramping and desorption spectrum was obtained. Desorption spectra were statistically analyzed using Principal Component Analysis (PCA) and our results indicate that all the three compounds investigated produced distinct desorption patterns independent of concentrations. Through this work we aim to develop a reliable methodology for subsequent development of a hand held TDP-SAW detector for field applications.

Development of a Pd/Cu nanowires coated SAW hydrogen gas sensor with fast response and recovery

Employing palladium and copper nanowires (Pd/Cu NWs) as the sensitive interface, a new surface acoustic wave (SAW) sensor with fast response and recovery was developed successfully for hydrogen gas sensing. The adsorption in Pd/Cu NWs towards hydrogen molecules modulates the SAW propagation by the induced acoustoelectric coupling effect from the changes in the resistivity of Pd, and the corresponding shifts in oscillation frequency proportional to gas concentration was collected as the sensor signal. Larger surface-to-volume of the Pd/Cu NWs contributes to improve the response speed and sensitivity. A 150 MHz SAW device with delay line pattern was fabricated photolithographically on 128° YX LiNbO3 substrate, and the Pd/Cu NWs that synthesized by optimized electrochemical deposition were dissolved in ethanol and deposited onto the SAW device surface by dropping method to build the SAW hydrogen sensors. The prepared SAW sensors were characterized by using the differential oscillation loop experimentally, excellent repeatability, fast response and recovery within 4s, and a sensitivity of 1.5 kHz/%were achieved at room temperature (25°C). Additionally, the response mechanism in hydrogen sensing was investigated experimentally.

Enhancement Method of Surface Acoustic Wave-Atomizer Efficiency for Olfactory Display

Since olfaction is an important sense in human interfaces, we have developed an olfactory display using a surface acoustic wave (SAW) atomizer and micro-dispensers. In this olfactory display, the efficiency of atomization is important in order to avoid smell persistence problems often encountered in human olfactory interfaces. Thus, the SAW device is coated with amorphous Teflon film to change the substrate nature from hydrophilic to hydrophobic. It is also necessary to silanize the piezoelectric substrate surface prior to Teflon coating to enhance the adhesion of the film. A dip coating method was adopted to obtain uniform coating on the substrate. The high-speed solenoid valve was used as micro-dispenser to spout a liquid droplet to the SAW device surface since its accuracy and reproducibility were high. Then, the atomization became easier on the hydrophobic substrate. In this study, the amorphous Teflon coating for minimizing the remaining liquid on the substrate after atomization was studied. The goal of the protocol described here is to show the methods for coating a SAW device surface with amorphous Teflon film and generating the smell using the SAW atomizer and a micro-dispenser, followed by a sensory test.

Enhancement Method of Surface Acoustic Wave-Atomizer Efficiency for Olfactory Display.

Since olfaction is an important sense in human interfaces, we have developed an olfactory display using a surface acoustic wave (SAW) atomizer and micro-dispensers. In this olfactory display, the efficiency of atomization is important in order to avoid smell persistence problems often encountered in human olfactory interfaces. Thus, the SAW device is coated with amorphous Teflon film to change the substrate nature from hydrophilic to hydrophobic. It is also necessary to silanize the piezoelectric substrate surface prior to Teflon coating to enhance the adhesion of the film. A dip coating method was adopted to obtain uniform coating on the substrate. The high-speed solenoid valve was used as micro-dispenser to spout a liquid droplet to the SAW device surface since its accuracy and reproducibility were high. Then, the atomization became easier on the hydrophobic substrate. In this study, the amorphous Teflon coating for minimizing the remaining liquid on the substrate after atomization was studied. The goal of the protocol described here is to show the methods for coating a SAW device surface with amorphous Teflon film and generating the smell using the SAW atomizer and a micro-dispenser, followed by a sensory test.

Monitored vacuum deposition of dielectric coatings over surface acoustic wave devices

We report on our experience in the control of magnetron sputtering process by in-situ monitoring of a surface acoustic wave (SAW) device (resonator or delay line) electrical response during the deposition of dielectric layers on the SAW device surface. While the electrical response changes with the growth of different layers, the response monitoring provides a useful feedback for layer thickness control in a multiple layer system. The monitoring approach is reproducible and gives physical insight into the SAW propagation changes occurring during the fabrication. It serves as a good tool for obtaining acoustic wave dispersion curves and helps in verifying theoretical and design principles of building multiple layer microwave acoustics devices.

Surface acoustic wave ammonia sensor based on ZnO/SiO2 composite film

A surface acoustic wave (SAW) resonator with ZnO/SiO2 (ZS) composite film was used as an ammonia sensor in this study. ZS composite films were deposited on the surface of SAW devices using the sol–gel method, and were characterized using SEM, AFM, and XRD. The performance of the sensors under ammonia gas was optimized by adjusting the molar ratio of ZnO:SiO2 to 1:1, 1:2 and 1:3, and the sensor with the ratio of ZnO to SiO2 equaling to 1:2 was found to have the best performance. The response of sensor was 1.132kHz under 10ppm NH3, which was much higher than that of the sensor based on a pristine ZnO film. Moreover, the sensor has good selectivity, reversibility and stability at room temperature. These can be attributed to the enhanced absorption of ammonia and unique surface reaction on composite films due to the existence of silica.

Oxide thin films (ZnO, TeO2, SnO2, and TiO2) based surface acoustic wave (SAW) E-nose for the detection of chemical warfare agents

Chemical warfare agents (CWA) are deadly chemicals even at low levels of concentration. In the present work, an electronic-nose (E-nose) comprising of an array of four surface acoustic wave (SAW) sensors having different chemical sensitive coatings of ZnO, TeO2, SnO2 and TiO2 is prepared for the detection of CWA. The oxide thin films are coated on the surface of SAW devices by rf sputtering technique. ZnO and SnO2 thin films are poly-crystalline with fine surface morphology whereas TeO2 and TiO2 thin films are amorphous. The E-nose shows good sensitivity toward the four simulants (dimethyl methylphosphonate (DMMP), dibutyl sulfide (DBS), chloroethyl phenyl sulfide (CEPS) and diethyl chlorophosphate (DECP)) of CWA allowing sub ppm level detection possible. Different simulants give distinct characteristic pattern when passed through the surface of E-nose. Using principal component analysis (PCA), the four different simulants of CWA are well separated from each other. Even with the incorporation of interferants (petrol, diesel, kerosene, VOCs and water vapors) in the PCA study, all four simulants are well separated showing the capability of the prepared SAW E-nose to classify simulants of chemical warfare agents efficiently.

Method for nondestructive testing of the film coating behavior of surface acoustic wave (SAW) sensors

This paper presents a practical non-destructive method for studying the film coating behavior of SAW devices by using a water soluble dielectric film (manitol) deposited on the SAW device surface by resistive evaporation. After measuring the electrical parameters of the film coated SAW device, the film can easily be removed from its surface by water rinsing without causing any damage to it. The SAW device can then be used over and over again in a large number of film depositions. The method was tested on a 1 GHz surface transverse wave (STW) resonator coated with manitol of varying thickness. After each coating and evaluation, the STW device was successfully recovered without significant performance degradation. Data is presented on the electrical changes of the STW device as a result of depositing manitol coatings of various thicknesses.

Love파를 이용한 저점성 유체 점도 측정용 표면 탄성파 센서 개발

Love wave is one of the shear horizontal waves and it can propagate between two layers in liquid without energy loss. The SAW (surface acoustic wave) sensor using Love wave is very useful for real time measurement of the viscosity of liquid with high sensitivity. In this study, the 77 MHz and 155 MHz Love wave SAW sensors were fabricated and use to measure the viscosity of low viscous liquid. To generate the surface acoustic wave, the inter-digital transducers were fabricated on the quartz crystal wafer. In order to obtain the optimal thickness of the coating film (novolac photoresist) generating the Love wave on the surface of SAW device, theoretical calculation was performed. The performances of fabricated Love wave SAW sensors were tested. As test liquid, pure water and glycerol solutions having different concentrations were used. Since the determination coefficients of the regression equations for measuring the viscosity of liquid are greater than 0.98, the developed Love wave SAW sensors in this study will be very useful for precise measurement of viscosity of liquid.

Performance optimization of surface acoustic wave chemical sensors

Acoustic wave devices coated with a thin layer of chemoselective material provide highly sensitive chemical sensors for the detection and monitoring of vapors and gases. In this work, a variety of coating materials and coating deposition techniques have been evaluated on surface acoustic wave (SAW) devices. A novel thin film deposition technique, matrix assisted pulsed laser evaporation (MAPLE), is utilized to coat high quality polymer films on SAW devices, and conventional pulsed laser deposition is used to deposit a passivation layer of diamond-like-carbon on a SAW device surface to prevent water adsorption. In addition, chemoselective coatings are formed by covalent attachment of functionalized species to the silica surface of SAW devices. The self-assembled monolayer or near monolayer structures are designed to populate the SAW device surface with the desirable hexafluoroisopropanol moeity. The rapid kinetic signals achievable with the various coated SAW sensors during vapor tests are examined as a function of the coating material and the quality of the thin films. In parallel to the thin film deposition, growth, and vapor testing, the electrical characteristics of the SAW sensor have been characterized. The quality factor and residual phase noise of polymer coated SAW devices are examined, and a prediction of the theoretical limit of the phase noise performance of the loop oscillator is made.

Chemical sensor based on surface acoustic wave resonator using Langmuir-Blodgett film

A one-port surface acoustic wave (SAW) resonators incorporating Langmuir-Blodgett (LB) films has been investigated. SAW sensors are one potential applications of SAW devices. Most of the work reported on SAW sensor concerns delay lines. In this paper we characterize the mass loading effects of one-port resonators by depositing successive monolayers of LB films onto the surface. A 90 MHz SAW gas-phase sensor has been fabricated on an ST cut quartz substrate, and one-port resonator configurations have been used as the sensing element. Ultra thin monolayers of arachidic acid and arachidic acid ethyl ester have been deposited using the LB method. The resonant frequencies and the Q values have been measured as sensor response. Experimental results show that the Q values and the resonant frequencies of the one-port SAW resonator vary with film mass loading on the SAW device surface.

Surface acoustic wave sensors incorporating Langmuir-Blodgett films

Acoustoelectric devices offer many attractive features for applications as physical and chemical sensors. Surface acoustic wave (SAW) oscillators are of particular importance owing to their high sensitivity. This paper describes the use of Langmuir-Blodgett (LB) films as gas absorbent layers on the surface of SAW devices. Areal densities of standard LB film forming materials were measured and found to agree with those obtained from pressure-area isotherms. Sensors incorporating ω-tricosenoic acid and docosylamine overlayers were examined and their responses to alkanoic acids reported. The room temperature chemiresponse of a SAW device coated with monolayers of tetra-4-tert butyl silicon phthalocyanine dichloride showed response and recovery times comparable with those reported for other phthalocyanine-based sensors operated at much higher temperatures. The detection limit of the LB film device was found to be 40 ppb NO 2 in dry air at an operating frequency of 98.6 MHz and an ambient temperature of 22°C. The frequency change was shown to be entirely due to the mass of gas absorbed by the film.