Langasite as Piezoelectric Substrate for Sensors in Harsh Environments: Investigation of Surface Degradation under High-Temperature Air Atmosphere.

Langasite crystals (LGS) are known for their exceptional piezoelectric properties at high temperatures up to 1000 °C and more. In this respect, many studies have been conducted in order to achieve surface acoustic wave (SAW) sensors based on LGS crystals dedicated to high-temperature operations. Operating temperatures of more than 1000 °C and 600 °C for wired and wireless sensors, respectively, have been reached. These outstanding performances have been obtained under an air atmosphere since LGS crystals are not stable in high-temperature conditions under a low-oxygen atmosphere due to their oxide nature. However, if the stability of bulk LGS crystals under a high-temperature air atmosphere is well established, the surface deterioration under such conditions has been hardly investigated, as most of the papers dedicated to LGS-based SAW sensors are essentially focused on the development of thin film electrodes that are able to withstand very elevated temperatures to be combined with LGS crystals. Yet, any surface modification of the substrate can dramatically change the performance of SAW sensors. Consequently, the aim of this paper is to study the stability of the LGS surface under a high-temperature air environment. To do so, LGS substrates have been annealed in an air atmosphere at temperatures between 800 and 1200 °C and for durations between one week and one month. The morphology, microstructure, and chemical composition of the LGS surface was examined before and after annealing treatments by numerous and complementary methods, while the surface acoustic properties have been probed by SAW measurements. These investigations reveal that depending on both the temperature and the annealing duration, many defects with a corolla-like shape appear at the surface of LGS crystals in high-temperature prolonged exposure in an air atmosphere. These defects are related to the formation of a new phase, likely an oxiapatite ternary compound, the chemical formula of which is La14GaxSi9-xO39-x/2. These defects are located on the surface and penetrate into the depth of the sample by no more than 1-2 microns. However, SAW measurements show that the surface acoustic properties are modified by the high-temperature exposure at a larger deepness of at least several tens of microns. These perturbations of the LGS surface acoustic properties could induce, in the case of LGS-based SAW sensors operating in the 434 MHz ISM band, temperature measurement errors around 10 °C.

Epitaxial Growth of Sc0.09Al0.91N and Sc0.18Al0.82N Thin Films on Sapphire Substrates by Magnetron Sputtering for Surface Acoustic Waves Applications.

Scandium aluminum nitride (ScxAl1-xN) films are currently intensively studied for surface acoustic waves (SAW) filters and sensors applications, because of the excellent tradeoff they present between high SAW velocity, large piezoelectric properties and wide bandgap for the intermediate compositions with an Sc content between 10 and 20%. In this paper, the growth of Sc0.09Al0.91N and Sc0.18Al0.82N films on sapphire substrates by sputtering method is investigated. The plasma parameters were optimized, according to the film composition, in order to obtain highly-oriented films. X-ray diffraction rocking-curve measurements show a full width at half maximum below 1.5°. Moreover, high-resolution transmission electron microscopy investigations reveal the epitaxial nature of the growth. Electrical characterizations of the Sc0.09Al0.91N/sapphire-based SAW devices show three identified modes. Numerical investigations demonstrate that the intermediate compositions between 10 and 20% of scandium allow for the achievement of SAW devices with an electromechanical coupling coefficient up to 2%, provided the film is combined with electrodes constituted by a metal with a high density.

AlN/Pt/LN-Y128 Packageless Acoustic Wave Temperature Sensor.

Batteryless, wireless, and packageless acoustic wave sensors are particularly desirable for harsh high-temperature environments. In this letter, an acoustic wave sensor based on a lithium niobate (Y + 128° cut, abbreviated LN-Y128) substrate with a buried platinum interdigital transducer (IDT) in an aluminum nitride (AlN) overlayer is investigated. Previously, it was demonstrated theoretically that due to the specific properties of LN-Y128, Rayleigh-type guided waves can propagate at the AlN/IDT(Pt)/LN-Y128 interface. Here, this structure is, for the first time, studied experimentally, including the growth and properties of the AlN layer onto irregular platinum IDTs. Both Shear Horizontal and Rayleigh-type waves have been identified after the AlN deposition and the velocities are consistent with the fitted SDA-FEM-SDA (a combination of finite element modeling with spectral domain analysis) simulations. Electrical measurements with a surface perturbation and temperature measurements show that the AlN/IDT(Pt)/LN-Y128 bilayer structure is promising as a packageless high-temperature sensor.

AlN/ZnO/LiNbO3 Packageless Structure as a Low-Profile Sensor for Potential On-Body Applications.

Surface acoustic wave sensors find their application in a growing number of fields. This interest stems in particular from their passive nature and the possibility of remote interrogation. Still, the sensor package, due to its size, remains an obstacle for some applications. In this regard, packageless solutions are very promising. This paper describes the potential of the AlN/ZnO/LiNbO3 structure for packageless acoustic wave sensors. This structure, based on the waveguided acoustic wave principle, is studied numerically and experimentally. According to the COMSOL simulations, a wave, whose particle displacement is similar to a Rayleigh wave, is confined within the structure when the AlN film is thick enough. This result is confirmed by comprehensive experimental tests, thus proving the potential of this structure for packageless applications, notably temperature sensing.

AlN/IDT/AlN/Sapphire SAW Heterostructure for High-Temperature Applications.

Recent studies have evidenced that Pt/AlN/Sapphire surface acoustic wave (SAW) devices are promising for high-temperature high-frequency applications. However, they cannot be used above 700°C in air atmosphere as the Pt interdigital transducers (IDTs) agglomerate and the AlN layer oxidizes in such conditions. In this paper, we explore the possibility to use an AlN protective overlayer to concurrently hinder these phenomena. To do so, AlN/IDT/AlN/Sapphire heterostructures undergo successive annealing steps from 800°C to 1000°C in air atmosphere. The impact of each step on the morphology, microstructure, and phase composition of AlN and Pt films is evaluated using optical microscopy, scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS). Finally, acoustical performance at room temperature of both protected and unprotected SAW devices are compared, as well as the effects of annealing on these performance. These investigations show that the use of an overlayer is one possible solution to strongly hinder the Pt IDTs agglomeration up to 1000°C. Moreover, AlN/IDT/AlN/Sapphire SAW heterostructures show promising performances in terms of stability up to 800°C. At higher temperatures, the oxidation of AlN is more intense and makes it inappropriate to be used as a protective layer.

Investigations of AlN thin film crystalline properties in a wide temperature range by in situ X-ray diffraction measurements: Correlation with AlN/sapphire-based SAW structure performance.

Aluminum nitride on sapphire is a promising substrate for SAW sensors operating at high temperatures and high frequencies. To get a measure of the suitability and temperature stability of such devices, an experimental relationship between the SAW performance and the structural properties of the AlN thin films was investigated in the temperature range between the ambient temperature and 1000°C. The crystalline structure of the AlN films was examined in situ versus temperature by X-ray diffraction. The results reveal that the AlN films remain (002) oriented even at high temperatures. A gradual increase of the tensile stress in the film due to the thermal expansion mismatch with the substrate has been observed. This increase accelerates around 600°C as the AlN film crystalline quality improves. This phenomenon could explain the amelioration in the SAW performance of AlN/sapphire devices observed previously between 600°C and 850°C. At higher temperatures, surface oxidation of the AlN films reduces the SAW performance. The potential of ZnO thin films as protective layers was finally examined.

A promising method to derive the temperature coefficients of material constants of SAW and BAW materials. first application to LGS.

Langasite (LGS) is a promising material for SAW applications at high temperature. However, the temperature coefficients of LGS material constants are not accurate enough to perform reliable simulations, and therefore to make good use of available design tools, above 300°C. In the first part of the paper, we describe a new possible way to derive these coefficients in a wider temperature range. The method is based on Simulated Annealing, a well-known optimization algorithm. The algorithm converges toward a set of optimized temperature coefficients of the stiffness constants which are used to perform accurate simulations up to at least 800°C. In the second part, a deeper analysis of the algorithm outputs demonstrates some of its strengths but also some of its main limitations. Possible solutions are described to predict and then improve the accuracy of the optimized coefficient values. In particular, one solution making use of additional BAW target curves is tested. A promising solution to extend the optimization to the temperature coefficients of piezoelectric constants is also discussed.

A numerical method to derive accurate temperature coefficients of material constants from high-temperature SAW measurements: application to langasite.

The design of wireless SAW sensors for high-temperature applications requires accurate knowledge of the constitutive materials' physical properties in the desired temperature range. In particular, it is crucial to use reliable temperature coefficients of the stiffness, piezoelectric, dielectric, and expansion constants of the propagation medium to achieve correct simulations of the considered devices. Currently, the best-suited piezoelectric material for high-temperature SAW applications is langasite (LGS). Unfortunately, the available coefficients do not allow for precise prediction of the temperature dependence of LGS-based SAW devices above 300°C. A novel method, based on a simulated annealing algorithm coupled with a Rayleigh wave simulation program, was developed to find optimal LGS temperature coefficients. This approach has proven to yield accurate results up to at least 800°C.

Experimental and theoretical investigations of some useful langasite cuts for high-temperature SAW applications.

Passive high-temperature sensors are a most promising area of use for SAW devices. Langasite (La3Ga5SiO14; LGS) has been identified as promising piezoelectric material to meet high-temperature SAW challenges. Because it is necessary to know the material behavior for an accurate device design, the frequency¿temperature behavior of Rayleigh SAW (R-SAW) and shear-horizontal SAW (SH-SAW) LGS cuts is investigated on delay line and resonator test structures up to 700°C by RF characterization. In the range of the 434-MHz ISM band, the (0°, 22°, 90°) SH-SAW cut shows thermal behavior similar to the (0°, 138.5°, 26.7°) R-SAW cut. Associated with the (0°, 22°, 31°) cut, in which SAWs present mixed types of polarization, the (0°, 22°, 90°) SH-SAW orientation might allow differential measurements on a single substrate. In the temperature range of 400 to 500°C, delay line test devices using the SH-SAW cut show a considerable drop of signal. Theoretical analysis indicates that this newly described behavior might be a result of anisotropy effects in this cut, occurring in case of any slight misorientation of electrode alignment.

Iridium interdigital transducers for high-temperature surface acoustic wave applications.

Iridium is investigated as a potential metal for interdigital transducers (IDTs) in SAW devices operating at high temperatures. SAW delay lines based on such IDTs and langasite (LGS) substrate are fabricated and electrically characterized. The results show reliable frequency responses up to 1000°C. The strong increase of insertion losses beyond this temperature, leading to the vanishing of the signal between 1140 and 1200°C, is attributed to surface transformation of the LGS crystal, consisting of relevant gallium and oxygen losses, as evidenced by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and secondary ion mass spectroscopy.

Investigations on AlN/sapphire piezoelectric bilayer structure for high-temperature SAW applications.

This paper explores the possibility of using AlN/sapphire piezoelectric bilayer structures for high-temperature SAW applications. To determine the temperature stability of AlN, homemade AlN/sapphire samples are annealed in air atmosphere for 2 to 20 h at temperatures from 700 to 1000°C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000°C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700°C. Beyond this temperature, both methods indicate ongoing surface oxidation of AlN. Additionally, Pt/Ta and Al interdigital transducers are patterned on the surface of the AlN film. The resulting SAW devices are characterized up to 500°C and 300°C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around -75 ppm/°C. Between room temperature and 300°C, both electromechanical coupling coefficient K(2) and propagation losses increase, so the evolution of delay lines' insertion losses with temperature strongly depends on the length of the propagation path.

Behavior of platinum/tantalum as interdigital transducers for SAW devices in high-temperature environments.

In this paper, we report on the use of tantalum as adhesion layer for platinum electrodes used in high-temperature SAW devices based on langasite substrates (LGS). Tantalum exhibits a great adhesive strength and a very low mobility through the Pt film, ensuring a device lifetime at 900°C of about one hour in an air atmosphere and at least 20 h under vacuum. The latter is limited by morphological modifications of platinum, starting with the apparition of crystallites on the surface, followed by important terracing and breaking of the film continuity. Secondary neutral mass spectroscopy (SNMS), Auger electron spectroscopy (AES), X-ray diffraction (XRD) measurements, and comparison with iridium-based electrodes allowed us to show that this deterioration is likely intrinsic to platinum film, consisting of agglomeration phenomena. Finally, based on these results, we present a solution that could significantly enhance the lifetime of Pt-based IDTs placed in high-temperature conditions.