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While the understanding of the properties of gratings is essential for any SAW design, high temperature stable electrodes, which are generally made of heavy materials, are not easily accessible by simulation. We have therefore studied this type of electrodes both experimentally and by simulation. Motivated by the demands made by SAW gas sensors operating at high temperatures we opted for a combination of langasite as substrate material and platinum as metallization of the electrodes. Delay line designs with free propagation paths as well as open and shorted gratings were used to assess the velocity inside the grating and the reflection coefficient. The operation frequency and the height of the electrodes were varied in several production runs to vary the ratio of metallization/wavelength. The experimental results are compared with calculated values using different simulation methods.

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.

International audience; 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 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 calculated coefficients allow for accurate predictions up to at least 800°C. In the second part, a deeper analysis of the algorithm outputs demonstrates some of its main limitations. Possible solutions are described to predict then improve the accuracy of the optimized coefficients values.

SAW Pirani sensors are a promising solution for low pressure monitoring. We present here the first results obtained with a new packaging solution based on CSSP technology. Pressure sensitivity and good response time are demonstrated experimentally. A model is developed to account for the sensor's behavior, including an observed slow drift. The paper concludes with a short discussion of the SAW Pirani sensor's industrial potential.

This paper describes the use of a general purpose modified simulator, first developed by K. Hashimoto to determine a priori the device responses of the specify Pt/LGS Y-X structure. The parameters for the analysis are determined as a function of the electrode type (physical constants, thickness) and substrate. The simulation responses are analyzed and compared to the experimental results and we should conclude how effectively the modified simulator is applicable to assessing Pt/LGS Y-X device performances as a function of temperature.

International audience; The authors have already presented a novel surface acoustic wave device for vacuum pressure measurement by employing a piezoelectric substrate that has a high value of temperature coefficient of frequency. Frequency-shift measurements as a function of vacuum pressure can be used to extract information about the pressure sensitivity of the device. In this paper, we report that the deposition of an aluminum thin-film layer and rise in the sensor's operating temperature significantly improve the sensitivity of the device. The results are crucial for improving the lower limit of the vacuum pressure measurement, which currently stands around 10-3 Pa.

International audience; This work is oriented on the used of effective simulation tools to design high temperature SAW sensor. Physical constants of platinum and LGS and related temperature coefficients recently determined were implemented in FEM/SDA program. Plessky's parameters of the Pt/LGS structure were extracted for various temperatures and injected in COM software to calculate frequency response of SAW devices. A comparison between experimental and theoretical results showed good agreement up to 250°C and the requirement of a new LGS constants set.

International audience; In this paper we present a new experimental set-up leading to characterize SAW sensor properties in high temperature up to 900°C. The characterization method consists in hanging a small piece of self-warming piezoelectric SAW device in a vacuum chamber. The device is made of the piezoelectric material to be tested equipped with its IDT plus a heating resistance, both in Platinum. The whole system is suspended from a PCB by mean of classical bonding wires. It is therefore thermally isolated from the rest of the experimental set-up. This allows using standard low-cost circuitry, to connect the SAW device to the measurement apparatus (standard coaxial feed-lines and SMA connectors). The warming being localised on the piezoelectric substrate, it also becomes possible to reach very high temperature, quickly and at low energy cost. This allows easy making of temperature cycles to test the aging of materials. In a first step, TCF values for Quartz ST and LiNbO3 Y-128° were measured in the range [20-500 °C], then compared to calculated ones in order to validate the method. In a second step, one LGS Y-X SAW delay line with Pt/Ta IDT was characterized using this test method in the range [20-900 °C].

In this paper, we show that the stress and strain fields induced in a layered SAW structure by the thermal expansion of the different layers must be taken into account to compute the global structure temperature coefficient of delay (TCD). Experimental and numerical results are provided. The numerical model is described. It is based at the same time on the well- known Campbell and Jones method, the Bolotin equation and a simple way to approximate the strain field in a double layer structure. The model is then applied to test three sets of temperature coefficients for A1N thin film elastic constants. The comparison between AIN/Sapphire already published experimental data and theoretical results leads to the selection of one of the three sets. The recently released A1N 3ld order elastic constants are used here to compute the thermal strain effect. Once chosen, the set is used to compute the TCD of AIN/Diamond structures. The theoretical results are compared with new experimental data. The observed discrepancies are discussed.

A new concept of SAW powered lab on chip able to actuate and to perform accurate sensing is presented. Using IdT/ZnO/IdT/Quartz structure, it is possible to obtain simultaneously Rayleigh and Love waves. The first wave will be used to mix or heat whereas the second one will be use to sense. In order to determine the optimal characteristics, a theoretical approach has been developed. A comparison between our former classical microfluidic device and the new structure is presented. First results are explained.

The new layered structure diamond/langasite is studied for high temperature, high frequency and relatively high electromechanical coupling (K2) SAW devices. Theoretical study was performed in order to calculate the evolution, as a function of diamond film thickness, of phase velocity and K2 of the Rayleigh mode and its higher modes. The calculation results show that phase velocities up to 12000 m/s and up to 9500 m/s are obtained for the mode 2 and mode 1, respectively, while the mode 0 exhibits high K2 values up to 2.9 %. In order to realize the considered layered structure, first experiments were performed to investigate the nanocrystalline diamond (NCD) films deposition on the non conventional substrate, Langasite. Characterization results show that NCD films with good properties (low grain size, low surface roughness, satisfactory purity) can be grown on Langasite. However, in the considered conditions only an optimal NCD thickness estimated around 1 mum permits to avoid film delamination and cracking when returning to room temperature

Aluminium nitride (AlN) is a promising material for challenging sensor applications such as process monitoring in harsh environments (e.g., turbine exhaust), due to its piezoelectric properties, its high temperature stability and good thermal match to silicon. Basically, the operational temperature of piezoelectric materials is limited by the increase of the leakage current as well as by enhanced diffusion effects in the material at elevated temperatures. This work focuses on the characterization of aluminum nitride thin films after post deposition annealings up to temperatures of 1000 °C in harsh environments. For this purpose, thin film samples were temperature loaded for 2 h in pure nitrogen and oxygen gas atmospheres and characterized with respect to the film stress and the leakage current behaviour. The X-ray diffraction results show that AlN thin films are chemically stable in oxygen atmospheres for 2 h at annealing temperatures of up to 900 °C. At 1000 °C, a 100 nm thick AlN layer oxidizes complete...

An original application for surface acoustic wave (SAW) subatmospheric pressure sensor was developed to measure pressure below 100mTorr with very high precision. The basic operating principles and the most significant experimental results of the sensor are presented here. A simple theoretical model is proposed. This sensor provides an efficient measuring solution in a wide range of subatmospheric pressure, which has been inaccessible in past by conventional diaphragm-based SAW sensors.

In this work, bar- and disk-shaped resonators were fabricated using Al(0.8)4Sc(0.16)N thin film Sensitivity analysis of the resonators' resonance frequencies with respect to the piezoelectric material parameters revealed that resonance analysis can be used for material parameter extraction. An optimization algorithm is employed to determine the piezoelectric material parameters S-11(E), s(12)(E), and d(31) of Al0.84Sc0.16N from the length- and radial-extensional mode. Using the piezoelectric constitutive equations and combining the results obtained in this work with the results obtained in a previous work, the stress-charge form of elastic and piezoelectric parameters could be derived.

This thesis deals with the design and characterization of multilayer acoustic wave micro-devices. The fifth generation of communication (5G) requires more efficient acoustic resonators (frequencies > 3GHz, wider bandwidth). In this context, we have designed and optimized using FEM simulation, the geometry of Lamb wave resonator based on AlScN. The final device, consisting of a layer composed of 30% Sc and deposited on a Bragg W/SiO2 mirror, shows excellent performance (coupling coefficient of 5% and quality factor of 768) as well as a good agreement with the simulation. To characterize the surface of BAW and SAW over the 5G frequency range, we also designed and developed a heterodyne interferometer. The latter has been used successfully to characterize surface vibrations with amplitudes between 1 and 10 pm at 5.95 GHz. Furthermore, thanks to their robustness and ability to be wirelessly interrogated,SAW sensors are used in harsh environments and are of great interest for medical applications and structural health monitoring. Recently, the introduction of multi-material stacks offers new development opportunities. We thus studied a pressure sensor composed of two complementary layers, as well as a so-called package- less sensor using different acoustic impedance layers. To design these new sensors, we have developed a simulation tool based on the extraction of mode coupling parameters and taking into account the effects of temperature, stresses and strains to estimate their sensitivity.; Cette thèse porte sur la conception et la caractérisation de micro-dispositifs à ondes acoustiques multicouches. La cinquième génération de communication (5G) nécessite des résonateurs acoustiques plus performants (fréquences > 3GHz, bande passante plus large). Dans ce contexte, nous avons conçu et optimisé par simulation FEM la géométrie de résonateurs à ondes de Lamb à base d’AlScN. Le dispositif final, constitué d’une couche composée de 30% de Sc et déposée sur un miroir de Bragg W/SiO2, montre d’excellentes performances (coefficient de couplage de 5% et facteur de qualité de 768) et un bon accord avec la simulation. Pour caractériser la surface de BAW et SAW sur la gamme de fréquence 5G, nous avons également conçu et développé un interféromètre hétérodyne. Ce dernier a été utilisé avec succès pour caractériser des vibrations de surface d'une amplitude comprise entre 1 et 10 pm à 5,95 GHz. Par ailleurs, grâce à leur robustesse et leur capacité à être interrogé sans-fil, les capteurs SAW sont utilisés dans des environnements difficiles et suscitent un grand intérêt pour les applications médicales et de contrôle de santé intégré. Récemment, l’introduction d’empilements multi-matériaux offre de nouvelles opportunités de développements. Nous avons ainsi étudié un capteur de pression composé de deux couches complémentaires, ainsi qu'un capteur dit package-less utilisant des couches d’impédances acoustiques différentes. Pour concevoir ces nouveaux capteurs, nous avons développé un outil de simulation reposant sur l'extraction de paramètres de couplage de modes et tenant compte des effets de la température, des contraintes et des déformations pour estimer leur sensibilité.