Increasing the magnetic field sensitivity is one of the major challenges in the study of magnetic surface acoustic wave (MSAW) sensors. In this paper, Love wave MSAW sensors are investigated in a Transmissive Single Port Delay Line (TSP-DL) configuration, on ZnO (700 nm)/Y-X LiNbO3. CoFeB (100 nm) or CoFeB(100 nm)/SiO2(3 nm)/CoFeB(100 nm) sensitive magnetoelastic layers are placed in the acoustic path between two electrically connected interdigital transducers. The fundamental Love wave on ZnO(700 nm)/ Y-X LiNbO3 presents a high electromechanical coupling coefficient (K2), resulting in a high signal amplitude, which is favorable for delay lines performances. Additionally, shear waves are highly sensitive to the magnetic field due to the large Delta G effect in the magnetoelastic layers. A maximum frequency shift of 0.36% was achieved using a CoFeB(100 nm)/SiO2(3 nm)/CoFeB(100 nm) stack as the sensitive layer, with a high maximum magnetic sensitivity of 3630 ppm mT-1 (1482 kHz mT-1 at 410 MHz). This sensitivity is among the highest reported in the literature on magnetic SAW sensors.

This paper presents wet chemical etching processes of piezoelectric ZnO (wurtzite atomic structure) films. ZnO films can be etched by most of acid and basic solutions. However, in MEMS processes, many etchants also attack sublayers (electrodes or insulating layers) and are therefore not compatible with MEMS processes. HCl, HF and H3PO4 were selected among the possibilities and carried out etching on Ti/Pt/ZnO sputtered silicon wafers at room temperature. Vertical etching rates are found to be in the 0.2 to 5 mu m/min and lateral etching rates are in between 0.5 to 17 mu m/min. The ratio between lateral to vertical etching speed was found to reach a minimum value for low etchant concentrations of approximately 2%. The morphology of the semi-etched borders of the samples were observed by SEM and practical conclusions are extracted on the adequation of the wet etching process on the top electrode subsequent fabrication step.

In this paper we study and fabricate surface acoustic wave devices that combine magnetic field and temperature measurements and that integrate a radio-frequency identification functionality (RFID). The use of a Transmissive-Reflective Delay Line (TR-DL) design enables to control the amplitude of the RFID signal peaks while maximizing the sensitivity of the sensor. The latter is based on a double layer of magnetoelastic CoFeB film used as sensitive layer to the magnetic field and deposited on top of a ZnO-covered LiNbO 3 Y-X substrate. This structure is used to generate a Love wave which enhances the sensitivity to the magnetic field. The achieved device demonstrates a high sensitivity to the magnetic field, a temperature compensation and reflected signal levels of -20 dB in a wired connection. The obtained sensitivity of 3630 ppm/mT (or 1.482 Hz/nT), is among the highest reported in the literature on magnetic SAW sensors.

Magnetic field measurement including a temperature compensation is essential for a magnetic field sensor. This study investigates a magnetic surface acoustic wave (MSAW) sensor in a reflective delay line configuration with two acoustic propagation paths with and without magnetic field sensitive layer. The delay in path with sensitive layer leads to magnetic field detection and the one without enables temperature measurement and thus compensation for the first path. The developed sensor is based on a ZnO/LiNbO3 Y-cut (X-direction) layered structure as Love wave platform. Love wave as a shear wave being more favorable for magnetic detection. Co-Fe-B is considered as sensitive layer to detect magnetic field changes and is deposited on the top of ZnO, but only on one of the two paths. We combined an original configuration of connected IDTs with a high electromechanical coupling coefficient (K2) mode to improve the signal amplitude. The achieved sensor exhibits a high temperature and magnetic field sensitivity of −63 ppm/°C and −781 ppm/mT, respectively. The temperature compensation method for magnetic field measurement is demonstrated using a differential measurement by subtracting the delay times obtained for the two paths with and without the sensitive layer. Finally, the sensor exhibited good repeatability at various temperatures. Moreover, the device developed allows in addition to the multisensor functionality, the radio frequency identification (RFID) which is necessary for the deployment of sensor networks.

This paper shows the feasibility and the performance of a multifunctional device based on a surface acoustic wave delay line. It combines a magnetic field detection with a temperature sensing and has also a radio frequency identification (RFID) capability. This promising device consists of a Co-Fe-B thin film as a magnetic field sensitive material over a ZnO/LiNbO 3 Y-cut (X-direction) structure, used as a platform for guided Love waves. The experimental results show high sensitivities of -774 ppm/mT and -67.7 ppm/°C for magnetic field and temperature detection, respectively. The device thus allows magnetic field measurements with the possibility of temperature compensation which make it particularly attractive for industrial applications.

For harsh industrial environments, surface acoustic wave (SAW) sensors are attractive because they are fully passive, can sense many physical and chemical parameters and can be remotely queried. In a delay line (DL) configuration, an ID code can be added in the time domain signal, hence the name SAW-RFID or SAW-Tag. In this communication, we showcase recent original examples of SAW RFID sensors for harsh environment temperature and strain sensing, based on innovative electrodes and substrates.

Surface acoustic wave (SAW) strain sensors require an adhesive—typically a glue—for the mounting on the part under measurement. This comes with strain-transfer, reproducibility and aging issues. In this paper, we propose a novel glue-less solution where the SAW sensor is directly fabricated onto the surface of interest, here a metallic substrate. Here, we study the layered structure ZnO/Ti numerically and experimentally, with ZnO as the piezoelectric layer and titanium as the substrate. In this structure, both bulk acoustic waves and SAW can propagate, and we used both of them to monitor temperature variations up to 300 °C and strain levels up to 1800 μϵ . Moreover, reflective delay line (R-DL) designs were used, to provide the future users with radio frequency identification functionalities. In order to overcome the limitations due to the relatively low electromechanical coupling coefficient of ZnO/Ti, a specific R-DL layout with connected inter-digital transducers was used. The obtained experimental results confirm that the proposed glue-less R-DL structure is a promising solution for the independent evaluation of temperature and strain, with identification.

A one‐port surface acoustic wave (SAW) resonator based on Co40–Fe40–B20/SiO2/ZnO/quartz multilayer structure and exhibiting a dual mode, Rayleigh and Love wave modes, is investigated to achieve a multifunctional sensor measuring both temperatures and magnetic fields. The Rayleigh wave mode of the resonator is used for temperature measurement with a temperature sensitivity of −37.9 ppm/°C, and the Love wave mode is used for magnetic field measurement. Co40‐Fe40‐B20 is the magnetic sensitive layer, and quartz crystal is the piezoelectric substrate. ZnO film is also a piezoelectric but considered here in combination with SiO2 as insulating layers and serves to control impedance matching and temperature dependence of the sensor. ZnO and SiO2 thicknesses are selected to realize temperature compensation for the Love wave mode and making this mode highly sensitive to magnetic fields and insensitive to temperatures. The magnetic field sensitivities of −170.4 kHz m−1 T−1 and −621.6 kHz m−1 T−1 are obtained respectively for the fundamental and the third harmonic of the Love wave mode. The proposed structure is beneficial to design reliable hybrid SAW magnetic field and temperature sensors.

The properties of a surface acoustic wave magnetic field sensor (MSAW) based on a piezoelectric /magnetoelastic heterostructure are investigated and presented. It consists of a ZnO piezoelectric layer and a magnetoelastic metallic amorphous ribbon of 1K101 Metglas, which is a Fe-Si-B alloy, as a sensitive material. The aim is to exploit the high ΔE effect, i.e. the change of elastic properties under magnetic field, in Metglas to cause a significant change in the wave velocity and thus of the frequency in SAW resonator. We first describe the design and fabrication of the MSAW resonator, which is based on a Rayleigh wave at 422 MHz. Then, we study the sensor’s resonance frequency that was determined under (a) an in-plane magnetic field in the acoustic wave propagation direction and, (b) an out-of-plane field perpendicular to the plane. The experimental results show very high sensitivities of 2793 ppm/mT and 1482 ppm/mT, respectively, in a quite thin, compact and easy to fabricate sensor. To the best of our knowledge, the out-of-plane sensitivity is the largest reported so far, and it is of great importance for practical industrial implementations.

In the above article [1] , by Yang et al. , published in IEEE Sensors Journal, volume 20, issue 19, pp. 11292–11301, in 2020, we would like to correct the coauthor Daniel Lacour’s biography on page 11300.

The surface acoustic waves (SAW) delay sensitivity to different types of deformation was calculated for different orientations of langasite (LGS) substrate. Besides LGS properties recent experimental data from several sources related to high-temperature SAW sensors are discussed that also show useful properties of other materials. Notably catangasite (CTGS) is a promising candidate for replacing LGS. The applicability of devices based on lithium niobate (LN) was obtained up to 600°C paving the way to wide-bandwidth devices. For mounting the deformation sensors onto DUT testing of different high-temperature adhesives was carried out. Another way to high-temperature deformation sensor development makes use of piezoelectric thin film directly deposited on a metallic object whose deformation is measured. Successful testing of ZnO/metal-based devices is appealing, because the direct deposition of piezoelectric layer on metal can allow to directly measure the strain of the monitored object and thus to eliminate the hysteresis and unreliable strain transfer in a wide range of temperatures.

Surface acoustic wave (SAW) sensors are very promising for structural health monitoring (SHM) applications as they have the advantages of being robust, passive (batteryless), remotely interrogated (wireless) and can even be packageless. This paper describes ultralow-profile SAW resonators that can be directly fabricated and integrated on metallic parts in industrial facilities. They are based on piezoelectric thin films (ZnO) which are directly sputtered on polished industrial titanium (Ti) and stainless steel. With this approach, no sensor glue-bonding to the target is needed, and measurement errors related to this step are avoided. Demonstrator devices have been studied numerically and experimentally. The structural properties of the ZnO thin films were characterized through x-ray diffraction and atomic force microscopy. A preferred orientation (002) was achieved with a roughness of 50 nm on the top surface. Resonators were microfabricated and their functional parameters (i.e. resonance frequency, quality factor and electromechanical coupling) were extracted through impedance measurements and fitted with a Butterworth-van Dyke model. By increasing applied temperatures (up to 450 °C) and the strain (up to 1800 μ ϵ), a linear decrease of the resonance frequency has been shown. A temperature coefficient of frequency of −46.4 ppm °C −1 and a good strain sensitivity (1.49 ppm μ ϵ −1 ) were obtained, thus making the structure promising as a high temperature and strain sensing element in industrial SHM applications.

Herein we present new surface acoustic wave (SAW) sensors integrating a radio-frequency identification code and directly fabricated on metal substrates with piezoelectric thin films. Those sensors overcome by construction the main weakness of strain sensors, namely the need of bonding with adhesives on the monitored equipment. The proof of concept was carried out using a SAW device based on a ZnO/Titanium multilayer structure. Sensors were characterized both in temperature (from 25 to 300 °C) and under strain (from 0 to 1800 µε). Results have shown a good linearity of the sensors’ responses, a good stability at high temperature and a good strain sensitivity.

In this communication, we describe the direct integration of thin film SAW resonators on industrial metals. Two different structures were studied numerically and experimentally: Al/ZnO/Duralumin and Al/ZnO/Titanium. Resonators were manufactured on ZnO thin film directly deposited by RF magnetron sputtering on polished Duralumin and Titanium substrates. The presence of Rayleigh waves was demonstrated. The aim of these resonators is an application in the field of structural health monitoring.

Temperature compensation is critical and important for surface acoustic wave (SAW) magnetic field sensors. In this study, a Love wave mode based SAW device is investigated as a magnetic field sensor. The considered structure is composed of a CoFeB magnetostrictive film as sensitive layer, SiO2, and ZnO film as insulating and temperature compensation layers and ST+90°-cut quartz as substrate. A theoretical model is proposed to study the magnetic field sensitivity and temperature coefficient of frequency (TCF) variations. Optimized structures by calculation were fabricated and characterized and obtained results show a good agreement between experiments and our model simulation. We clearly shown that signal performances as well as the flexibility of the resonator design were improved by adding the isolating SiO2 layer. Thus, a sensor showing a near zero TCF (0.1 ppm/°C) and a magnetic field sensitivity of -420 ppm/mT was achieved with the structure CoFeB(100 nm)/SiO2(250 nm)/ZnO(300 nm)/Quartz(ST-X+90°). This multi-layered structure is beneficial to design reliable SAW magnetic field sensors.

This work aims to present a multifunctional surface acoustic wave (SAW) device that is capable of simultaneously measuring magnetic field as well as temperature. This is achieved in three parts where a multilayered approach is used to first achieve a temperature compensated Love wave structure, followed by a micro-structuration of the sensitive layer to eliminate the effects of temperature on magnetic anisotropy and finally multiple resonances observed allow a multifunctionality in our device.

A temperature compensated magnetic field sensor based on the combination of CoFeB ferromagnetic thin films and Quartz/ZnO Love waveguide platform is developed and optimized. The Love wave is a shear horizontal guided wave and therefore provides an optimal interaction with magnetization in the magneto-elastic thin film resulting in higher acoustic wave magneto-elastic coupling compared to the conventional Rayleigh wave based devices. ST-cut Quartz was chosen as substrate, ZnO as insulating layer for Love wave generation and temperature coefficient of frequency (TCF) compensation and CoFeB as the magnetostrictive layer sensitive to magnetic field. Experimental results show a magneto-acoustic sensitivity of 15.53 MHz T-1 with almost zero TCF.

A multifunctional sensor based on a surface-acoustic-wave (SAW) device is fabricated. It allows inde- pendent measurements of temperature and applied magnetic field. Optimization of the multilayered device structure leads to a temperature-coefficient frequency of the Love-wave resonance reduced to zero. The sensitivity to an applied magnetic field is obtained through magnetostriction of a Co-Fe-B layer. By use of shape anisotropy, the variation of intrinsic magnetic anisotropy with temperature is strongly reduced. On one hand, interrogating the device at the Love-wave-resonance frequency allows us to extract the applied magnetic field independently of the temperature in a [130-370 K] range. On the other hand, the Rayleigh or Leaky waves are less or not sensitive to applied fields but have a high temperature coefficient of frequency. So interrogating the device at the Rayleigh resonance frequency allows us to extract the temperature independently of the magnetic field. In addition, the used resonator geometry offers the possibility for future batteryless and wireless interrogation.