High-temperature Surface Acoustic Wave Sensors Research Articles

High-temperature Pt–Al2O3 composite nano-thick interdigital electrodes for surface acoustic wave sensors

High-temperature stability of ultra-thin film electrodes is vital for surface acoustic wave (SAW) devices in harsh environments. This paper proposed the Pt–Al2O3 composite Nano-thick interdigital electrodes (IDEs) for langasite-based high-temperature SAW sensors. The sandwich structure of the IDEs was composed of Pt layers and Al2O3 armors. The armor consisting of barrier and coating layers was introduced to reduce the agglomeration, oxidation, and interdiffusion of IDEs at high temperatures. Capacitive bonding pads were proposed to transmit radiofrequency signals through the closed armor. The composite IDEs (226 nm thickness) had good structural stability and impurities concentration less than 1% at 1000 °C (3 h, air condition). Improved quality factors of 8805, and matched electrical impedance of 51 Ω and −16° were achieved through the electroacoustic optimized IDEs. Finally, multiple SAW temperature sensors using the composite IDEs presented fitting errors less than 1% and hysteresis errors below 4% in 80–1000 °C exceeded 9 h, which was 150 °C higher than existing ones. The proposed composite IDEs enable SAW devices to be applicated in 80–1000 °C.

Fabrication of grooved LGS resonators based high temperature SAW sensors and analysis with FEM simulation

Langasite (LGS) based surface acoustic wave (SAW) sensors are widely used in high temperature circumstances due to their advantages of being passive and wireless. In this research, the platinum electrodes are deposited into the grooves of the LGS instead of on the surface to further improve the acoustic properties and prolong the lifetime of the resonators at high temperature. Proper MEMS fabrication techniques are proposed to fabricate such structures. The result of every step is evaluated, and the corresponding process parameters are optimized. With the optimal processes, two types of resonators with different Euler angles of (0∘, 22∘, 30∘) and (0∘, 22∘, 90∘) are fabricated on the LGS substrate. Furthermore, the process-influenced structure parameters such as metal ratio and sidewall angle are well investigated via a weak form nonlinear finite element method simulation model. The temperature dependence of the resonance frequency is measured, and the recorded thermal behaviors match well with the prediction of the simulation. In addition, the further endurance test shows that the devices could work at 1000 ∘C for 12 h. This research proves the validity of the grooved resonator structure and the prominent role of the simulation model in further optimizing the LGS-based SAW devices design.

Novel Multilayer SAW Temperature Sensor for Ultra-High Temperature Environments

Performing high-temperature measurements on the rotating parts of aero-engine systems requires wireless passive sensors. Surface acoustic wave (SAW) sensors can measure high temperatures wirelessly, making them ideal for extreme situations where wired sensors are not applicable. This study reports a new SAW temperature sensor based on a langasite (LGS) substrate that can perform measurements in environments with temperatures as high as 1300 °C. The Pt electrode and LGS substrate were protected by an AlN passivation layer deposited via a pulsed laser, thereby improving the crystallization quality of the Pt film, with the function and stability of the SAW device guaranteed at 1100 °C. The linear relationship between the resonant frequency and temperature is verified by various high-temperature radio-frequency (RF) tests. Changes in sample microstructure before and after high-temperature exposure are analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The analysis confirms that the proposed AlN/Pt/Cr thin-film electrode has great application potential in high-temperature SAW sensors.

Stoichiometric Lithium Niobate Crystals: Towards Identifiable Wireless Surface Acoustic Wave Sensors Operable up to 600 °C

Wireless surface acoustic wave (SAW) sensors constitute a promising solution to some unsolved industrial sensing issues taking place at high temperatures. Currently, this technology enables wireless measurements up to 600-700$^\circ$C at best. However, the applicability of such sensors remains incomplete since they do not allow identification above 400$^\circ$C. The latter would require the use of a piezoelectric substrate providing a large electromechanical coupling coefficient K 2 , while being stable at high temperature. In this letter, we investigate the potentiality of stoichiometric lithium niobate (sLN) crystals for such purpose. Raman spectroscopy and X-ray diffraction attest that sLN crystals withstand high temperatures up to 800$^\circ$C, at least for several days. In situ measurements of sLN-based SAW resonators conducted up to 600$^\circ$C show that the K 2 of these crystals remains high and stable throughout the whole experiment, which is very promising for the future achievement of identifiable wireless high-temperature SAW sensors.

Electroless-Deposited Platinum Antennas for Wireless Surface Acoustic Wave Sensors.

In an effort to develop a cost-efficient technology for wireless high-temperature surface acoustic wave sensors, this study presents an evaluation of a combined method that integrates physical vapor deposition with electroless deposition for the fabrication of platinum-based planar antennas. The proposed manufacturing process becomes attractive for narrow, thick, and sparse metallizations for antennas in the MHz to GHz frequency range. In detail, narrow platinum-based lines of a width down to 40 m were electroless-deposited on -AlO substrates using different seed layers. At first, the electrolyte chemistry was optimized to obtain the highest deposition rate. Films with various thickness were prepared and the electrical resistivity, microstructure, and chemical composition in the as-prepared state and after annealing at temperatures up to 1100 C were evaluated. Using these material parameters, the antenna was simulated with an electromagnetic full-wave simulation tool and then fabricated. The electrical parameters, including the S-parameters of the antenna, were measured. The agreement between the simulated and the realized antenna is then discussed.

The improved Pt/ZnO/Al2O3 electrode for high temperature SAW sensors

In this study, Pt/ZnO/Al2O3 multilayer film were fabricated on La3Ga5SiO14 substrates as the electrodes of surface acoustic wave (SAW) sensors working at high temperature. The Al2O3 film was employed as the first buffer layer to promote the crystalline qualities of the Pt and ZnO layers. This Pt/ZnO/Al2O3 sample was found to significantly improve the electrode’s reliability at high temperature. After high temperature electrical measurement, the varied microstructures of the Pt/ZnO/Al2O3 electrodes were analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM) to investigate its mechanism for stable resistance. The conductive stability of the Pt/ZnO/Al2O3 film was found to be improved at 800 °C by using Al2O3 buffer layer. This proposed ZnO/Al2O3 double buffer layers have great potential in improving the conductive stability of electrodes for SAW devices at high temperature.

Novel AlN/Pt/ZnO Electrode for High Temperature SAW Sensors.

In order to develop a film electrode for the surface acoustic wave (SAW) devices working in high temperature, harsh environments, novel AlN/Pt/ZnO multilayers were prepared using pulsed laser deposition (PLD) systems on langasite (LGS) substrates. The AlN film was used as a protective layer and the ZnO buffer layer was introduced to improve the crystal quality of Pt films. The results show that the resistances of Pt and AlN/Pt film electrodes violently increase above 600 °C and 800 °C, respectively, while the resistances of AlN/Pt/ZnO electrodes have more stable electrical resistance from room temperature to 1000 °C. The AlN/Pt/ZnO electrode, where the ZnO film was deposited at 600 °C, has the best temperature stability and can steadily work for 4 h at 1000 °C. The mechanism underlying the stable resistance of the AlN/Pt/ZnO electrode at a high temperature was investigated by analyzing the microstructure of the prepared samples. The proposed AlN/Pt/ZnO film electrode has great potential for applications in high temperature SAW sensors.

Investigation of low-temperature cofired ceramics packages for high-temperature SAW sensors

Abstract. Surface acoustic wave (SAW) temperature sensor devices have been developed for operating temperatures up to and above 1000 °C. A challenging task to make these devices available on the market is to develop an appropriate housing concept. A concept based on low-temperature cofired ceramics (LTCC) has been investigated and tested under elevated temperatures up to 600 °C. The devices showed promising results up to 450 °C. Thorough analysis of the possible failure mechanisms was done to increase the maximum temperature above this limit in further production cycles.

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.

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.