Langasite Surface Acoustic Wave Research Articles

Surface Acoustic Wave Strain Sensor With Ultra-Thin Langasite

Langasite (LGS) surface acoustic wave (SAW) sensor is considered as an ideal wireless passive sensing technology application in complex environments. However, existing LGS-based SAW strain sensor has low strain range and its sensing accuracy is affected by temperature greatly as well as strain, which can not meet the application requirements. By exploiting ultra-thin substrate and a custom calibration algorithm, this paper proposes an ultra-thin 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> LGS SAW with (0°, 138.5°, 72°) Euler angle to extend strain range to 1200 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$500^{\circ }\text{C}$ </tex-math></inline-formula> , which is twice higher than the state-of-the-art. The ultra-thin SAW sensor has lower temperature coefficient and hysteresis loop effect, compared with thick LGS SAW. Aiming to improve sensing accuracy, the mechanism of temperature effects on strain sensitivity is investigated in different temperature stages, including thermal expansion mismatch effect and high-temperature glue strain transfer ratio effect. The mechanism can explain strain sensitivity curve under various temperature nicely. Based on the mechanism, a new calibration method is also developed to eliminate temperature effects. The testing results show that this calibration method can improve the measured strain accuracy effectively.

Langasite Bonding via High Temperature for Fabricating Sealed Microcavity of Pressure Sensors

We proposed a novel Langasite (LGS) bonding method only using high temperature to solve the manufacturing difficulty of the sealed microcavity of pressure sensors. The optimal bonding parameters by comparative experiments were defined as 1350 °C for 3 h. Due to simple experimental conditions, low experimental cost, and be suitable for bonding wafers with various sizes, the method is convenient for popularization and mass-production, thus promoting the development of surface acoustic wave (SAW) devices at high temperatures. Simultaneously, an intact microcavity was observed by scanning electron microscopy, and a tight and void-free bonding interface with a transition layer thickness of 2.2 nm was confirmed via transmission electron microscopy. The results of tensile and leakage experiments indicated that the bonded wafer with the sealed microcavity exhibited a high bonding strength of 4.02 MPa and excellent seal performance. Compared to the original wafer, the piezoelectric constant of the LGS bonded wafer had a reduction of only 4.43%. The above characteristics show that the sealed microcavity prepared by this method satisfies the conditions for fabricating the LGS SAW pressure sensors. Additionally, based on the bonding interface characterizations, the mechanism of LGS bonding has been investigated for the first time.

Surface acoustic device for high response NO2 gas sensor using p-phenylenediamine-reduced graphene oxide nanocomposite coated on langasite

In this study, we fabricated a surface acoustic wave (SAW) device using a langasite (LGS) substrate. The p-phenylenediamine-reduced graphene oxide (PrGO)-coated LGS substrate resulted in high response sensor systems for very short time scale detection of NO2 and CO. With the catalytic reaction of PrGO, the resonant frequency of 100 ppm of NO2 gas changed by ∼3.50 kHz, which is nearly eight times higher than that of the pristine LGS SAW device. Further, we conducted selectivity tests for NO, NH3, H2S, and CO gases; the results showed that the PrGO-coated LGS had higher selectivity for NO2 compared to other gases. Accordingly, the underlying mechanism for the superior NO2 sensing was discussed in relation to the morphological characteristics and configuration of PrGO. In addition, the effect of humidity on the SAW performance was studied under ambient NO2.

A langasite surface acoustic wave wide-range temperature sensor with excellent linearity and high sensitivity

Langasite (LGS) surface acoustic wave (SAW) temperature sensors have a large second order temperature coefficient of frequency, which leads to their non-linear and non-monotonic responses in a wide temperature range and possibly makes the measured temperatures non-unique for a specific sensor response. Here, we propose a high-temperature Pt/LGS SAW sensor with linear frequency–temperature behavior in a wide temperature range, which was fabricated on a LGS (0°, 138.5°, 72°) substrate. The sensor has a single acoustic mode, but dual resonances with similar second- and third-order temperature sensitivities that were utilized for achieving a linear frequency–temperature response by a method of difference. A linear relationship between the measured sensor output (Δfm21) and temperature was obtained in the range of −60 °C to 700 °C, consistent with the theoretical analysis. The sensor has a high sensitivity of −167 ppm/K for the whole temperature range. All the results demonstrated exciting prospects of the sensor for wide temperature monitoring in harsh environments.

Strain Measurements With Langasite SAW Resonators at High Temperature

The strain sensitivities of the langasite surface acoustic wave resonators are strongly dependent on the temperature, which causes measurement strain to shift greatly at high temperature. To cancel the effect of temperature, a surface acoustic wave strain sensor which consisted of three langasite surface acoustic wave resonators was presented in this work. The surface acoustic wave resonators are configured to ensure that the propagation directions of the surface acoustic wave in the three surface acoustic wave resonators are parallel, perpendicular and at an orientation of 30° to the applied strain. The strain sensitivities of the three surface acoustic wave resonators from room temperature to 250 °C were investigated. Functional relationships of the resonance frequency shifts of the three surface acoustic wave resonators due to both the temperature and applied strain were established. It was deduced that the strain was linearly represented by the resonance frequency shifts of the three surface acoustic wave resonators. Experimental results demonstrated that the langasite surface acoustic wave strain sensor was well worked within the temperature range from room temperature to 250 °C. The average values of the calculated strains were in agreement with the practical applied strains. There was a little fluctuation between the calculated and preset strains. The measurement error was about 1%. The maximal standard deviation was about 18.5 με.

High temperature CO2 sensing and its cross-sensitivity towards H2 and CO gas using calcium doped ZnO thin film coated langasite SAW sensor

Development of indigenous CO2 sensing element operating at high temperature (≥ 350 °C) is highly desirable. Use of langasite (LGS) based surface acoustic wave (SAW) gas sensors is extremely beneficial compared with other commercially available sensors as it can operate at a higher temperature (> 800 °C). In the present work, we have shown CO2 sensing and its cross-sensitivity performance using LGS based SAW sensor. Compared with LGS with other sensing electrodes, calcium doped ZnO thin film (Ca_ZnO) coated LGS substrate shows enhanced CO2 sensing performance. The interdigitated electrodes (IDT) made of Pt was plated and Ca_ZnO thin film was synthesized and deposited on the LGS substrate. Additionally, we have compared the chemo-resistive gas sensing characteristics of Ca_ZnO thin film with the Ca_ZnO coated SAW sensors. Furthermore, temperature effect on CO2 sensing was studied where optimum response (change in resonant frequency ˜2.469 KHz) was observed at 400 °C. Also, the gas adsorption effect which leads to the change of the resonant frequency of the sensors has been deliberated. Besides, we have discussed the predominant effect of sensor conductivity than mass loading which enhances the reduction of the resonant frequency for thin film coated SAW sensors has been deliberated. Additionally, we have demonstrated the cross-sensitivity of H2 and CO gas in CO2 sensing and their mixed gas sensing behavior operating at 400 °C.

Temperature characteristics of langasite surface acoustic wave resonators coated with SiO2 films

In this paper, the SiO2 temperature compensation layers were deposited on LGS SAW resonators. The temperature characteristics of SAW resonators with different thicknesses of SiO2 coating layers were investigated from room temperature to 450 °C. The results show that the SAW velocity, the turnover temperature, the first and second order temperature coefficients of frequency (TCF1 and |TCF2|) increase as the SiO2 films thickness increasing. The Q factor of the SAW resonator decreases greatly due to the poor quality of the SiO2 film. Zero TCF1 is achieved when the SiO2 coating layer is about 0.25 μm. The results show that the LGS SAW resonators can be temperature compensated by SiO2 coating layer.

Effects of AlN Coating Layer on High Temperature Characteristics of Langasite SAW Sensors.

High temperature characteristics of langasite surface acoustic wave (SAW) devices coated with an AlN thin film have been investigated in this work. The AlN films were deposited on the prepared SAW devices by mid-frequency magnetron sputtering. The SAW devices coated with AlN films were measured from room temperature to 600 °C. The results show that the SAW devices can work up to 600 °C. The AlN coating layer can protect and improve the performance of the SAW devices at high temperature. The SAW velocity increases with increasing AlN coating layer thickness. The temperature coefficients of frequency (TCF) of the prepared SAW devices decrease with increasing thickness of AlN coating layers, while the electromechanical coupling coefficient (K2) of the SAW devices increases with increasing AlN film thickness. The K2 of the SAW devices increases by about 20% from room temperature to 600 °C. The results suggest that AlN coating layer can not only protect the SAW devices from environmental contamination, but also improve the K2 of the SAW devices.

A method for achieving monotonic frequency–temperature response for langasite surface-acoustic-wave high-temperature sensor

To achieve the monotonic frequency–temperature response for a high-temperature langasite (LGS) surface-acoustic-wave (SAW) sensor in a wide temperature range, a method utilizing two substrate cuts with different propagation angles on the same substrate plane was proposed. In this method, the theory of effective permittivity is adopted to calculate the temperature coefficients of frequency (TCF), electromechanical coupling coefficients (k2), and power flow angle (PFA) for different propagation angles on the same substrate plane, and then the two substrate cuts were chosen to have large k2 and small PFA, as well as the difference in their TCFs (ΔTCF) to always have the same sign of their values. The Z-cut LGS substrate plane was taken as an example, and the two suitable substrate cuts with propagation angles of 74 and 80° were chosen to derive a monotonic frequency–temperature response for LGS SAW sensors at −50 to 540 °C. Experiments on a LGS SAW sensor using the above two substrate cuts were designed, and its measured frequency–temperature response at −50 to 540 °C agreed well with the theory, demonstrating the high accuracy of the proposed method.

Surface acoustic wave devices for harsh environment wireless sensing.

Langasite surface acoustic wave devices can be used to implement harsh-environment wireless sensing of gas concentration and temperature. This paper reviews prior work on the development of langasite surface acoustic wave devices, followed by a report of recent progress toward the implementation of oxygen gas sensors. Resistive metal oxide films can be used as the oxygen sensing film, although development of an adherent barrier layer will be necessary with the sensing layers studied here to prevent interaction with the langasite substrate. Experimental results are presented for the performance of a langasite surface acoustic wave oxygen sensor with tin oxide sensing layer, and these experimental results are correlated with direct measurements of the sensing layer resistivity.

Analysis of contributions of nonlinear material constants to stress-induced velocity shifts of quartz and langasite surface acoustic wave resonators

Stress-induced surface acoustic wave velocity shifts are analyzed for langasite (LGS) SAW resonators. The analytical methodology has been verified by comparing experimental results and analytical results for quartz resonators. LGS SAW resonators with Euler angles which are most sensitive and least sensitive to diametrical forces are determined and their applications in force sensors and resonators with minimum acceleration sensitivity are discussed. Sensitivity of the analytical results to different groups of nonlinear material constants is discussed; it was found that for specific configurations, failure to include the third-order piezoelectric constants, dielectric constants and electrostrictive constants may lead to a significant calculation error. Surface acoustic waves propagating on an LGS square plate subject to bending moment along the propagation direction and normal to the propagation direction are analyzed; it was found that the average momentinduced velocity shift of LGS resonators are comparable to quartz resonators. Analyses of the sensitivity of the results to different groups of nonlinear material constants shows that for some specific wave propagation directions, failure to include the third-order piezoelectric constants, dielectric constants, and electrostrictive constants may lead to large errors.

Langasite surface acoustic wave gas sensors: modeling and verification

We report finite element simulations of the effect of conductive sensing layers on the surface wave velocity of langasite substrates. The simulations include both the mechanical and electrical influences of the conducting sensing layer. We show that three-dimensional simulations are necessary because of the out-of-plane displacements of the commonly used (0, 138.5, 26.7) Euler angle. Measurements of the transducer input admittance in reflective delay-line devices yield a value for the electromechanical coupling coefficient that is in good agreement with the three-dimensional simulations on bare langasite substrate. The input admittance measurements also show evidence of excitation of an additional wave mode and excess loss resulting from the finger resistance. The results of these simulations and measurements will be useful in the design of surface acoustic wave gas sensors.

Langasite surface acoustic wave sensors: Fabrication and testing

We report on the development of harsh-environment surface acoustic wave sensors for wired and wireless operation. Surface acoustic wave devices with an interdigitated transducer emitter and multiple reflectors were fabricated on langasite substrates. Both wired and wireless temperature sensing was demonstrated using radar-mode (pulse) detection. Temperature resolution of better than ±0.5°C was achieved between 200°C and 600°C. Oxygen sensing was achieved by depositing a layer of ZnO on the propagation path. Although the ZnO layer caused additional attenuation of the surface wave, oxygen sensing was accomplished at temperatures up to 700°C. The results indicate that langasite SAW devices are a potential solution for harsh-environment gas and temperature sensing.

Jet-and-Flash Nano Imprint Lithography: Fabrication of Passive Wireless Surface Acoustic Wave Sensors

Jet-and-flash nano imprint lithography (NIL) has successfully been utilized for the fabrication of surface acoustic wave (SAW) sensors. These SAW sensors will be used for passive wireless sensing of physical parameters, e.g. the temperature inside gas turbine engines. This is the first time metal lift-off processes based on jet-and-flash NIL have been established on the piezoelectric substrate material lanthanum gallium silicate (Langasite). NIL produced Langasite SAW temperature sensors have successfully been fabricated on various crystal cuts and SAW electrode metallization using Ti/Pt and Ti/Ni have been realized. Langasite based SAW sensors are to be used for passive wireless temperature monitoring at up to 950 degrees Celsius.

High temperature LGS SAW gas sensor

Novel surface acoustic wave (SAW) devices using the recent langasite (LGS) family of crystals have been designed, fabricated, and tested for high temperature (up to 750 °C) gas sensor applications. The devices were fabricated using platinum (Pt) and palladium (Pd) thin film technology as electrodes and sensing films to withstand temperatures in excess of 1000 °C. Combinations of platinum and platinum/tungsten trioxide (Pt/WO 3) films have been used in the detection of C 2H 4 in N 2 (C 2H 4/N 2). Original all palladium metallic structures have been employed in the detection of H 2 in a N 2 carrier gas (H 2/N 2). SAW resonator structures using these films have been fabricated and continuously cycled from 250 to 750 °C for periods up to 3 weeks, with degradations in the |S 21| response up to 7 dB. Constantly held at 250 °C for 12 weeks, the maximum degradation in the |S 21| observed dropped to 3 dB. In order to perform the required gas testing, a high temperature, air sealed, stainless steel, dual chamber has been designed and fabricated to house these devices. The two-port SAW resonator structures fabricated with Pt, Pt/WO 3, and the original all-Pd films mentioned above have been tested with exposures to H 2/N 2 and C 2H 4/N 2 in the temperature range of 250–450 °C. Frequency variations up to 10 kHz for the 167 MHz high temperature SAW sensors were measured. The high temperature LGS SAW devices and experiments reported in this work show the capability of these devices to withstand prolonged exposure to temperatures ranging from 250 to 750 °C and to perform as high temperature gas sensors.

Platinum and palladium high-temperature transducers on langasite

There is a pressing need for the fabrication of surface acoustic wave (SAW) devices capable of operating in harsh environments, at elevated temperature and pressure, or under high-power conditions. These SAW devices operate as frequency-control elements, signal-processing filters, and pressure, temperature, and gas sensors. Applications include gas and oil wells, high-power duplexers in communication systems, and automobile and aerospace combustion engines. Under these high-temperature and power-operating conditions, which can reach several hundred degrees Centigrade, the typically fabricated aluminum (A1) thin film interdigital transducer (IDT) fails due to electro and stress migration. This work reports on high temperature SAW transducers that have been designed, fabricated, and tested on langasite (LGS) piezoelectric substrates. Platinum (Pt) and palladium (Pd) (melting points at 1769 degrees C and 1554.9 degrees C, respectively) have been used as thin metallic films for the SAW IDTs fabricated. Zirconium (Zr) was originally used as an adhesion layer on the fabricated SAW transducers to avoid migration into the Pt or Pd metallic films. The piezoelectric LGS crystal, used as the substrate upon which the SAW devices were fabricated, does not exhibit any phase transition up to its melting point at 1470 degrees C. A radio frequency (RF) test and characterization system capable of withstanding 1000 degrees C has been designed and constructed. The LGS SAW devices with Pt and Pd electrodes and the test system have been exposed to temperatures in the range of 250 degrees C to 750 degrees C over periods up to 6 weeks, with the SAW devices showing a reduced degradation better than 7 dB in the magnitude of transmission coefficient, /S21/, with respect to room temperature. These results qualify the Pt and Pd LGS SAW IDTs fabricated for the above listed modern applications in harsh environments.

Applicability of LiNbO/sub 3/, langasite and GaPO/sub 4/ in high temperature SAW sensors operating at radio frequencies

The applicability of LiNbO3, langasite and GaPO4 for use as crystal substrates in high temperature surface acoustic wave (SAW) sensors operating at radio frequencies was investigated. Material properties were determined by the use of SAW test devices processed with conventional lithography. On GaPO4, predominantly surface defects limit the accessible frequencies to values of 1 GHz. Langasite SAW devices could be operated up to 3 GHz; however, high acoustic losses of 20 dB/micros were observed. On LiNbO3, the acoustic losses measured up to 3.5 GHz are one order of magnitude less. Hence, SAW sensors capable of wireless interrogation were designed and processed on YZ-cut LiNbO3. The devices could be successfully operated in the industrial-scientific-medical (ISM) band from 2.40 to 2.4835 GHz up to 400 degrees C.