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International audience; Remote interrogation of surface acoustic wave ID-tags imposes a high signal amplitude which is related to a high coupling coefficient value (K 2) and low propagation losses (α). In this paper, we propose and discuss an alternative configuration to the standard one. Here, we replaced the conventional configuration, i.e. one interdigital transducer (IDT) and several reflectors, by a series of electrically connected IDTs. The goal is to increase the amplitude of the detected signal using direct transmission between IDTs instead of the reflection from passive reflectors. This concept can therefore increase the interrogation scope of ID-tags made on conventional substrate with high K 2 value. Moreover, it can also be extended to suitable substrates for harsh environments such as high temperature environments: the materials used exhibit limited performances (low K 2 value and relatively high propagation losses) and are therefore rarely used for identification applications. The concept was first tested and validated using the lithium niobate 128°Y-X cut substrate, which is commonly used in ID-tags. A good agreement between experimental and numerical results was obtained for the promising concept of connected IDTs. The interesting features of the structure were also validated using a langasite substrate, which is well-known to operate at very high temperatures. Performances of both substrates (lithium niobate and langasite) were tested with an in-situ RF characterization up to 600°C. Unexpected results regarding the resilience of devices based on congruent lithium niobate were obtained. Index Terms-high temperature, lithium niobate, radio frequency identification (RFID), surface acoustic wave (SAW)

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.

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.

A biosensor based on the release of the enzyme substrate from its structure was developed for the inhibitive detection of benzoic acid. A polyurethane support comprising two perforated microcapsules (800 μm in diameter) filled with methylene blue as a model compound and covered with a conductive deposit of multiwalled carbon nanotubes, continuously released this stored dye for 24 h. An increase in methylene blue concentration of 0.5–0.75 μmol L-1 h-1 and 1.5–2 μmol L-1 h-1, in the presence and absence of the multiwalled carbon nanotube coating, respectively, was demonstrated by UV–vis spectroscopy in a 2 mL UV cuvette. The same configuration with microcapsules filled with catechol was modified by a laponite clay coating containing tyrosinase enzyme. The resulting biosensor exhibits a constant cathodic current at −0.155 V vs AgCl/Ag, due to the reduction of the ortho-quinone produced enzymatically from the released catechol. The detection of benzoic acid was recorded from the decrease in cathodic current due to its inhibiting action on the tyrosinase activity. Reagentless biosensors based on different deposited quantity of tyrosinase (100, 200, 400 and 600 μg) were investigated for the detection of catechol and applied to the detection of benzoic acid as inhibitor. The best performance was obtained with the 400 μg-based configuration, namely a detection limit of 0.4 μmol L-1 and a sensitivity of 228 mA L mol−1. After the inhibition process, the biosensors recover 97–100% of their activity towards catechol, confirming a reversible inhibition by benzoic acid.

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.

International audience; Aluminium nitride piezoelectric thin films grown on sapphire are strong candidates for high-temperature surface acoustic wave (SAW) sensors, due to their thermal stability, large bandgap, high acoustic velocity and suitable electromechanical coupling. However, thin-film resonators need more design efforts than those based on bulk crystals, due to the usually limited thickness of the piezoelectric films, and to acoustic properties disparities between the latters and their host substrate. This work presents an optimization of AlN/Sapphire-based SAW resonators with high quality factors for high-temperature applications. It combines specifically grown, 3 µm-thick aluminium nitride films, with the use of aluminium electrodes for their low density and resistivity, as an alternative to heavier electrodes like Pt. These electrodes allow for much lower mechanical losses and higher quality factors, in spite of needing passivation for increased lifetime. A standard resonator design is first presented and used for preliminary tests, in order to monitor the AlN/Sapphire structure with unprotected aluminium electrodes, for temperatures up to 600°C. A quasi-synchronous, optimized design is then proposed for higher quality factors and wireless sensing compliance. The high temperature characterizations confirmed that much larger quality factors can be retrieved from this optimized design. The quasi-synchronous resonators proposed in this study remain well-tuned for temperatures up to 400°C, and show high quality factors, as high as 3400 at 400°C.

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.

Surface acoustic wave devices are increasingly used as sensors, especially because they can be batteryless, wireless and packageless. With these features they become particularly interesting in the field of flexible electronics for the monitoring of the human body parameters. By combining a thinned lithium niobate Z-cut substrate to ultrathin and stretchable antennas, a remote temperature measurement has been demonstrated. The addition of two protective layers to obtain a packageless structure seems promising to achieve wireless temperature measurements on the skin.

Remote interrogation of SAW ID-tags imposes a high coupling coefficient value ($\text{K}^{\mathbf {2}}$), which many of the most promising substrates for high temperature applications do not possess. In this paper, we propose and discuss an alternative configuration to the standard one. Here, we replaced the standard configuration, i.e. one interdigital transducer (IDT) and several disconnected reflectors, by a series of electrically connected IDTs. The goal is to increase the amplitude of the reflected signals, using direct transmission between IDTs instead of the reflection on passive reflectors. Numerical simulations show promising results. A test structure was also characterized experimentally. As a first step, the test structure was based on the well-known lithium niobate $128 ^{\circ}\mathrm {Y} -\mathrm {X}-$cut substrate. This concept will be extended to other substrates with lower $\text{K}^{\mathbf {2}}$ value, which should prove especially interesting for high temperature applications.

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.

International audience; In this paper, we report on the use of tantalum and iridium as adhesion layers for platinum electrodes used in high temperature SAW devices based on langasite substrates (LGS). Unlike iridium, tantalum exhibits a great adhesive strength, and a very low mobility through the Pt film, ensuring a device lifetime of at least half an hour at 1000°C. The latter is limited by morphological modifications of platinum, starting by the apparition of crystallites on the surface, and followed by important terracing and breaking of the film continuity. SNMS and XRD measurements allowed us to show that these phenomena are likely intrinsic to platinum film, whatever be the nature of the adhesion layer. Finally, after having outlined a possible scenario leading to this deterioration, we consider some solutions that could replace platinum in order to increase the lifetime of LGS-based SAW devices in high temperatures conditions.

International audience; Surface acoustic wave (SAW) 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. Index Terms-Surface acoustic wave SAW, temperature sensor, waveguiding layer acoustic wave WLAW, packageless, low-profile.

International audience; Over the last decades, the use of Surface Acoustic Waves (SAW) has emerged as a promising technology in many applications such as filters, signal processing but also sensors. We report the fabrication and the characterization of a SAW delay line magnetic field sensor using uniaxial multi-layered 14x[TbCo2(3.7nm)/FeCo(4nm)] nanostructured thin film deposited on Y36° Lithium Niobate (Figure 1.a). The sensor shows an interesting dependency to a tunable bias magnetic field with different orientations relative to the easy axis. The obtained results are well explained using an equivalent piezo-magnetic model described in a previous work [1,2].

We present an innovative and practical pressure sensor based on Pirani effect enabling high sensitivity, high resolution, and high dynamic range around atmospheric pressure. The structure is composed of 3μm width and 1mm long metallic resistors suspended by periodic SiO2 micro-bridges to improve structure toughness and temperature uniformity. One resistor is acting as a heater and another one is the sensing element. A constant 100nm nano-gap separates the wires from the substrate. The sensor is especially designed to obtain the greatest sensitivity around atmospheric pressure. The pressure experimentally measured ranges from 1kPa to 150kPa without reaching the saturation limit. The sensor design and experimental characteristics are presented.

International audience; Surface acoustic waves (SAW) technology is very promising to achieve high-temperature wireless sensors. However, there is currently a need for piezoelectric substrates with a high electromechanical coupling coefficient (K2 > 1%), able to operate under harsh environments, especially in the intermediate temperature range (300-600°C). None of the conventional SAW substrate can face this challenge. In particular congruent lithium niobate, whose K2 can exceed 5%, shows serious limitations from 300°C, mainly related to Li vacancies. Recent studies have demonstrated the potential of stoichiometric lithium niobate (s-LN) for high-temperature bulk acoustic waves applications. In this paper, we investigate this piezoelectric material for high-temperature SAW applications. In particular, we examine carefully the potential structural and chemical changes that s-LN surface can undergo during a high-temperature exposure. Finally, SAW resonators based on s-LN substrates are in situ characterized up to 600°C.

This paper presents a high sensitive micro-sensor designed for pressure measurements in a wide range around atmospheric pressure, for application in aerodynamics. The sensor is a temperature-resistance transducer operating with the Pirani effect, which states that below a certain pressure limit, the thermal conductivity of a gas is pressure-dependent. The sensor presents a wide measurement range between 10 kPa and about 800 kPa, in both constant current and constant temperature mode. The last mode enables high-sensitive measurements with a maximum of sensitivity around atmospheric pressure, enabling the use of the sensor for applications in aerodynamics and fluid dynamics, such as active flow control.

Waveguiding layer acoustic waves (WLAW) technology is considered as a packageless solution for extreme miniaturization of acoustic wave devices. WLAW solution is also original and suitable for high-temperature applications. Indeed, the achievement of a stable high-temperature packaging still constitutes a technological lock, in particular regarding sealing performance. In this context, we consider AlN/GaN/Sapphire as a promising WLAW structure for high-temperature applications. Indeed, the high-temperature stability of these three materials was previously demonstrated. However, in order to develop fully operational WLAW sensors based on this layered structure, it is necessary to have a better knowledge of the behaviour of each constitutive material with respect to the temperature. Consequently, the aim of this work is to experimentally validate GaN and AlN elastic constants sets in a large temperature range, which is a prerequisite to make the design of WLAW sensors according to the targeted performance.

This study describes the implementation of accurate and fully coupled physics models leading to investigate Surface Acoustic Wave (SAW) devices sensitivity to an extern applied magnetic field. The model was firstly validated using experimental published data of SAW resonator based on Quartz substrate and Nickel Interdigital Transducers (IDT). Original layered structures, Ni/ZnO/IDT/LiNbO3 and Ni/Al2O3/IDT/LiNbO3 were then investigated and optimized to enhance their sensitivity to magnetic field intensity. Results show that Ni/Al2O3/IDT/LiNbO3 exhibits a sensitivity of 10ppm/mT, 14 times higher than the structure with ZnO one.

Functional polymers containing Lewis bases are known as promising materials for the elaboration of highly sensitive micro-sensors for acid gas detection. In this work, poly(urethane-imide)s (PUIs) with Lewis bases were synthesized in two steps only from various functional diols (N-methyldiethanolamine, N-tert-butyldiethanolamine, N-phenyldiethanolamine and 1,4-diethanolpiperazine) containing one or two tertiary amine groups with different chemical structures. The synthesis led to high yields, high molecular weights and a good control of the PUI amine content. These good film-forming polymers were then used as SO2 sensitive coatings on Love surface acoustic wave (L-SAW) micro-sensors. The physical and mechanical properties required for an optimal micro-sensor design were determined by High Resolution Brillouin Spectroscopy of PUI thin layers. Three-layer micro-sensors were developed by respecting the conditions for the L-SAW generation. The three-layer structure included Quartz ST-90 as the piezoelectric substrate, a ZnO guiding layer and a PUI sensitive layer. These micro-sensors were weakly sensitive to temperature and thus compatible with the targeted application. The experimental results for SO 2 detection showed that all the PUI coatings greatly improved the sensitivity compared to a micro-sensor without polymer coating. The basicity of the amine groups was not determining for the micro-sensor sensitivity to the SO2 acid gas and the key factor was their steric hindrance. Finally, the micro-sensor sensitivity increased with the accessibility of the amine groups in the order Piperazine-diol < tBu-DEA ≅ Ph-DEA ≪ MDEA. © 2013 Elsevier B.V. All rights reserved.

Équipe 401 : Nanomatériaux pour la vie et développement durable; International audience; The possibility to perform a packageless structure for acoustic wave sensors applications based on AlN/interdigital transducer/ZnO/Si structure was investigated. The effect of thicknesses of AlN and ZnO thin films on structure performance was simulated by 2-D finite element method. Theoretical predictions were confirmed by in-situ measurements of frequency, insertion loss, and thickness during deposition of AlN layer on ZnO/Si.

International audience; This paper describes the investigation of the potential of packageless structures for body-centric acoustic wave sensor applications based on Al2O3/ZnO/Lithium Niobate structures, which are investigated numerically and experimentally.

International audience; We present a theoretical calculation and experimental results for a waveguiding layer acoustic wave (WLAW). The experimental device is modeled by the finite element method (FEM) for the AlN/ZnO/diamond structure. It was found that the AlN thickness must be at least larger than 3λ/2 to obtain negligible surface displacement. In the same way, the ZnO thickness for a fixed value of AlN thickness at 2λ must be larger than λ/4 to confine the acoustic wave. The electromechanical coupling of the wave presents an optimum around λ/2 for the ZnO layer thickness. A first experimental AlN/ZnO/diamond device has been developed and shows the WLAW at 412 MHz.

This study deals with using Brillouin spectroscopy to determine the temperature variation of potassium niobate elastic constants. Two different samples have been used (Y- and Z-cut), and all the measurements were done in the temperature range between 300 K and 420 K. The calculations concern only the first-order coefficients. The acoustic velocity is then determined for each sample in 3 directions of propagation. Results are given for C22, C55, and C66 elastic constants and accuracy problems are discussed.

Suitable deposition conditions yielding to thin zinc oxide (ZnO) films with inclined c-axis have been determined in this work. These films were deposited on silicon (100) and on the nucleation side of self-standing diamond substrates. We used r.f. magnetron and a zinc-oxyde target. We characterized by X-ray diffraction (XRD) the film texture and by scanning electron microscopy (SEM) the film morphology. XRD has revealed that theses films are polycrystalline in nature having a hexagonal wurtzite crystal structure. We have measured a c-axis angle of inclination of 45° in the case of ZnO/Diamond sample.

The sensitivity of a ZnO/Quartz SAW (Surface Acoustic Waves) sensor has been evaluated under alcohol vapour exposure. Ethanol, isopropanol and butanol have been tested. In order to better understand the SAW frequency shifting effect due to the alcohol molecules, a Langmuir/B.E.T analysis has been carried out. Possible shifting mechanisms have been proposed and tested by numerical simulation.

AlN films with c-axis oriented perpendicular to the surface were deposited on silicon substrates by reactive RF magnetron sputtering method, at various temperatures (without heating −400°C). The structural, morphological and optical properties of AlN films were investigated by X-ray diffraction, scanning electron microscope, atomic force microscopy and Fourier transform infrared absorbance spectroscopy. It was found that the AlN films showed the same highly (002) preferred orientation with low full with of half maximum of rocking curve, which is about 2° for all the deposited films. The surface roughness of AlN films determined by AFM is less than 1nm for the film grown at low temperature. This result is very important and means that films with good crystalline quality, low surface roughness can be processed at low temperature. Elastic properties of deposited AlN films were evaluated by realization and characterization of AlN/silicon SAW device.

Due to its exceptional properties, semiconducting diamond is expected to be used for electrically active devices which can be operated in harsh environments. Such devices need reliable ohmic contacts that can also stand hostile environments. Titanium nitride (TiN) is a chemically stable material with good electrical conductivity. In this work, TiN contacts on boron-doped diamond have been made and characterised. TiN films were deposited by reactive magnetron sputtering. Boron-doped diamond layers were deposited by plasma enhanced chemical vapour deposition. Optimal deposition conditions have been determined to obtain TiN films with low resistivity (∼100 µΩ · cm), high reflectance in the IR region and low stress. TiN contacts show ohmic behaviour after annealing at 750 °C.

International audience; This paper describes the investigation of the potential of packageless structures for acoustic wave sensor applications based on Al2O3/IDT/GaN/Sapphire structure. The finite elements modeling predicts the possibility of acoustically isolated waveguiding layer acoustic wave (WLAW) to propagate inside this structure with very low displacement at the top surface of the topmost Al2O3 layer. This layer serves as a protecting layer against oxidation of internal layers as well as an acoustically isolating layer, providing a mean for packageless packaging of this structure. The modeling results get confirmation in experiments with the precursor IDT/GaN/Sapphire structure thus supporting hopes on the predicted correct operation of the final Al2O3/IDT/GaN/Sapphire structure.

Polycrystalline aluminium nitride films were deposited on Si(100) substrates by RF reactive sputtering method. We have carried out experiments to evaluate the effect of stress in AlN thin films on the surface acoustic wave (SAW) velocity by studying AlN films with various thickness (100 nm to 3 μ m). Experimental results show a clear dependence of the residual stress in AlN films on SAW velocity of AlN/silicon structure. Optical properties of films were investigated by Fourier transform infrared absorbance spectroscopy (FTIR). The obtained spectra show absorption bands attributed to vibrational modes of Al─N bonds, in particular E1(TO) at 678 cm−1 and A1(TO) at 620 cm−1. The microstructural analysis were realised by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) and show a columnar structure of AlN films and very dense and crack free films. The analysis of AlN films orientation by X-ray diffraction (XRD) exhibits a (002) high preferred orientation. ...

It has been reported that a Rayleigh wave propagating on a zinc oxide film has a relatively low temperature coefficient of frequency (TCF) and a large electromechanical coupling factor (K2). In this study, we investigate the performance of a SAW pressure sensor based on a ZnO/quartz structure. We used the quartz Y–X in this structure owing to the zero TCF value that this quartz produces when it is combined with a thin ZnO film of particular thickness. This film thickness was determined here not only to achieve such zero value, but also to increase the electromechanical coupling coefficient (K2 = 1%) to seven times higher than that of quartz Y–X. The theoretical approach to select the optimal structure is presented, which is followed by discussions on the effect of the operating frequency and on the ZnO film contribution to the pressure sensitivity.

This article reports on the growth and characterization of polycrystalline ZnO films having c axis inclined up to 16° with respect to the substrate normal. These films allow the excitation of shear and longitudinal waves with comparable electromechanical coupling constants and are of significant interest for thin film bulk acoustic resonators (FBARs). The films are deposited on silicon substrates covered by Al2O3 and SiO2 buffer layers under low pressure using a modified reactive dc-pulsed magnetron sputtering system. A blind has been positioned between target and substrate, allowing oblique particle incidence without tilting the wafer. The study of structural properties of the deposited ZnO films by x-ray diffraction and scanning electron microscopy has permitted to show the presence of the inclined structure. Electromechanical coupling constants K up to 13% have been extracted for shear-mode excitation using highly overmoded FBARs.

Surface acoustic wave (SAW) devices are critical components in wireless communication systems. In this paper the microstructural characteristics of NCD films deposited in Ar/H2/CH4 microwave discharges and the potentialities of AlN/NCD/Si layered structures for high frequency SAW device achievement are investigated by both experimental and theoretical approaches. A model describing the SAW propagation in such a structure is developed and used in order to determine the acoustic phase velocity (V) and the electromechanical coupling coefficient (K2). A comparison between the phase velocity estimated from the high frequency characterisation of SAW devices and numerical calculations demonstrates the relevance of the model and shows that acoustic velocity up to 9500 m/s may be reached. According to theoretical results, the best combination of V and K2 is obtained for the IDT/AlN/NCD/Si configuration, khAlN = 3 and khNCD ≥ 4. The paper points out that nanocrystalline diamond is a promising material for high frequency SAW devices, since high-velocity smooth films with the required thickness may be deposited in a few hours and can be used as-grown.

Wurtzite aluminium nitride (AlN) thin films were deposited by RF reactive magnetron sputtering technique on (1 0 0) silicon substrates at low temperature (400 °C). The microstructural properties of sputtered AlN films were investigated using X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and atomic force microscopy (AFM), to aim improvement of piezoelectric coupling for surface acoustic wave (SAW) devices. It was found that the AlN films deposited in the optimum experimental conditions, as revealed by XRD and selected area electron diffraction, exhibit a high (0 0 2) preferred orientation, where columnar crystals are grown in a non-epitaxial pattern and aligned almost perpendicular to silicon substrate. The ω rocking curve, shows that the standard deviation of columns of thin AlN films is less than 1°, which exhibit a high quality of these films. Furthermore, the AFM found a very low surface roughness less than 7 A, which is very crucial to decrease a loss propagation in AlN films. The films synthesised with optimum conditions were used to perform SAW devices by developing inter-digital transducer on AlN/Si structure. Frequency characteristics show that the realised SAW devices exhibit good filtering performances and a good compromise between phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency.

Due to its high-acoustic velocity, diamond, when combined with piezoelectric material, has a promising potentiality for high frequency surface acoustic wave (SAW) devices. In the present work, low-surface roughness nanocrystalline diamond (NCD) were deposited on silicon substrate by microwave plasma assisted chemical vapour deposition process and piezoelectric aluminium nitride films were grown by DC reactive magnetron sputtering. For the first time, SAW devices based on AlN/NCD/Silicon layered structure were successfully performed. The influence of inter-digital transducers periodicity and NCD film thickness on the acoustic velocity of the layered structure was studied. The high frequency characterisation showed that NCD presents a high-surface acoustic velocity similar to that of polycrystalline CVD diamond. Thus, taking advantage of the remarkable surface smoothness of as-grown NCD films, the polishing step of diamond substrates may be no more required prior to SAW device achievement.

AlN thin films have been deposited by reactive triode sputtering. The effect of the nitrogen contents in the discharge on films stoichiometry and crystal orientation has been investigated. AlN films have been studied by means of Fourier-transform infrared spectroscopy, X-ray diffraction and Raman spectroscopy. Dense AlN layers with very high electrical resistivity, high index of refraction and large optical band gap were obtained at high deposition rates. Finally, the optimized films were applied for the synthesis of surface acoustic waves filters. Acoustic velocities up to 5130 m s −1 for AlN/Si structure, 5548 m s −1 for AlN/sapphire structure, and 9900 m s −1 for AlN/diamond structure were obtained.

Diamond based surface acoustic wave (SAW) devices are extremely versatile devices that are just beginning to realize their commercial potential for use from sensors till high frequency (HF) filters for wireless telecommunications. One of the most promising piezoelectric materials for diamond based HF-SAW devices is aluminium nitride (AlN) thin film. The ability of AlN and diamond to be used for SAW applications depends both on the piezoelectric AlN layer properties and the diamond substrate properties. In this work, optimised piezoelectric (002) oriented AlN layers have been deposited on polycrystalline diamond substrates aiming at HF-SAW filter applications. Polycrystalline diamond layers were deposited on silicon substrates by microwave plasma enhanced chemical vapour deposition (MW-PECVD). SAW filters with unique characteristics have been obtained due to exceptional diamond's mechanical properties [1,2]. One of the important characteristics of CVD diamond substrate concerns its surface roughness. Smooth diamond surfaces were obtained without polishing by a wet chemical etching of the silicon substrate at the diamond layer nucleation side. Very low surface roughness (R MS ≤1 nm) can be achieved by this technique for bias enhanced nucleated (BEN) samples. In this paper, we report the structural characterization of the AlN films and diamond substrates by X-ray diffraction, atomic force microscopy, and transmission electron microscopy methods.

Piezoelectric AlN films crystallised in a hexagonal wurtzite structure with the c-axis oriented perpendicular to the surface were deposited on silicon substrates by dc reactive magnetron sputtering. The evolution of the crystallographic and electrical properties of these films with a nitrogen concentration between 10 and 100% in the Ar-N 2 gas mixture was studied using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS) and four-point probe methods. It was found that the electrical properties are correlated to the film structure and to the deposition parameters. The AlN films having a high electrical resistivity (> 10 14 Ω cm at room temperature) also exhibit a high crystallographic orientation, as determined from the XRD peak intensity. An optimum for the nitrogen concentration is identified in the range 60-80% of N 2 in the gas mixture. The Arrhenius plots of electrical resistance give an activation energy of 2.2 eV between 200°C and 450°C. This energy is attributed to oxygen impurities. EDXS analysis reveals that oxygen is the only element present in the deposited films, along with Al and N.

Highly oriented aluminium nitride (AlN) has been successfully deposited on silicon substrates and on the back side of unpolished, thick, freestanding, polycrystalline diamond films. Structural and electrical properties of the (0 0 2) oriented AlN films have been investigated. Using optimised AlN and chemical vapour deposited diamond films, high quality surface acoustic wave (SAW) filters were constructed by deposition of aluminium inter-digital transducers. The effects of the AlN thickness on the diamond-based SAW filter properties and the SAW propagation were investigated. A phase velocity of 17.1 km/s was observed, which is, to the best of our knowledge, the highest reported value for diamond. The dependence of the phase velocity (v) on the AlN thickness was compared to theoretical predictions. SAW filters with rather low insertion loss, high suppression and high electromechanical coefficient could be obtained. We also report on piezoelectric d33 measurements of AlN films by atomic force microscopy.

International audience; This letter describes and discusses the design and testing of an effcient nanogap Pirani micro-sensor for pressure measurements in a wide range with a maximum sensitivity around atmospheric pressure. The structure combines a substrate-free heated wire and mechanical support using silicon oxide micro-bridges allowing both a constant nanoscale gap between the wire and the substrate and a 1 mm long and 3 µm wide wire. The high aspect ratio of wire provides a uniform heating profile along the wire and contributes to low pressure detection. At the opposite, both the nanoscale gap and the short wire length between two micro-bridges contributes to shift the high pressure limit. Tested between 10 kPa and 800 kPa, the sensor presents a wide measurement range, not fully reached by the experiments, with a maximum of sensitivity close to the atmospheric pressure and performances with up to 38 %/dec sensitivity when operation in constant temperature mode with an overheat of 20 °C.

Diamond films were elaborated in CH 4 –H 2 –N 2 and CH 4 –CO 2 –N 2 gas mixtures in a pulsed microwave plasma under the same working conditions, varying the percentage of nitrogen from 0.1 to 5.2%. Raman spectroscopy was used to probe the quality of the film and the incorporation of nitrogen within diamond films was appreciated using cathodoluminescence (CL) spectroscopy. The intensities of peaks due to nitrogen-vacancy pairs showed that the quantity of incorporated nitrogen increased with that of the nitrogen introduced in the gas mixture. Arrhenius plots of the electrical conductivity showed that the incorporation of low quantities of nitrogen in the diamond film led to an increase in its resistivity. It can be speculated that small quantities of incorporated nitrogen balance the natural p -doping of microwave plasma chemical vapor deposition (MPCVD) diamond and, thus, enhance its insulating properties. Nevertheless, as higher quantities of nitrogen were incorporated, the resistivity of the diamond film decreased due to a possible n -doping. The doping hypothesis was confirmed by I–V measurements which exhibited a rectifier behavior of the n -diamond/ p -Si heterojunction structure. This behavior was observed only in the case of a CO 2 –CH 4 gas mixture, where a high concentration of nitrogen could be incorporated without affecting the continuity of the film.

A simulator based on the coupling of mode (COM) theory, previously developed for modelling the bulk substrate surface acoustic wave (SAW) devices, was modified to be adapted for layered structures. The frequency response of ZnO/diamond/Si SAW filter was calculated and the results were compared with experimental ones extracted from the literature. A good agreement is obtained for the frequencies within and close to the pass-band of the filter. Outside of this pass-band, the experimental frequency response exhibits an asymmetry, which is not reproduced by the simulation. This asymmetry is attributed to the dispersion, as a function of frequency, of SAW velocity (VP) and electromechanical coupling coefficient (K2), which cannot be neglected in the case of layered structures. In the original program developed for bulk structures, K2 and V were assumed to be constant. To take into account the effect of dispersion, the program was modified by the introduction of a dispersive model. The confrontation between the results obtained by simulation, including the dispersive model, and by experimental measurements shows a good agreement.

High performance Brillouin microscopy has been used in order to characterize the spatial distribution of piezoelectrically induced acoustic fields excited at microwave frequencies in a ZnO film deposited on silicon directly within the area of the electrically excited inter-digital transducer (IDT). For a IDT of 300 micrometers length, the maximum of intensity is close to the edge of the transducer and the minimum lies between 200 and 250 micrometers from this maximum. Moreover, an acoustic second harmonic mode is observed for a fundamental excitation frequency of 1 GHz.

International audience; The possibility to generate simultaneously Surface Acoustic Wave (SAW) and Waveguiding layer acoustic wave (WLAW) in layered structures AlN/ZnO/Silicon was investigated. A delay line operating at 525 MHz was tested versus temperature in air and in contact with liquid. Experimental characterizations were also supported by modeling using the commercial software (COMSOL Multiphysics). The full delay line was simulated and liquid modeled by an additional layer on the top of AlN film.

We report on in situ electrical measurements of surface acoustic wave delay lines based on AlN/sapphire structure and iridium interdigital transducers between 20 °C and 1050 °C under vacuum conditions. The devices show a great potential for temperature sensing applications. Burnout is only observed after 60 h at 1050 °C and is mainly attributed to the agglomeration phenomena undergone by the Ir transducers. However, despite the vacuum conditions, a significant oxidation of the AlN film is observed, pointing out the limitation of the considered structure at least at such extreme temperatures. Original structures overcoming this limitation are then proposed and discussed.

In this paper, we present a new microdroplet device assisted by Rayleigh Surface Acoustic Wave (R-SAW) for discrete microfluidic applications and biological reactions where microdroplet heating is needed. A multilayer structure is reported which consists to bury the Inter-digital Transducers (IdT) under a sputtered Aluminium Nitride (AlN) layer to electrically insulate. Thus, microdroplets can be deposited directly on the IdT, which improves the efficiency of the device. Indeed, effect of AlN layer on 128°Y-X cut lithium niobate (LiNbO3) substrate is studied. Based on simulations, we show that electromechanical coupling coefficient can be enhanced with AlN layer leading to a better heating of microdroplet. Heating is next studied as a function of liquid viscosity and AlN layer thickness.

International audience; 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.

International audience

The possibility to create a packageless structure for acoustic wave sensors applications based on AlN/IDT/ZnO/Si structure is investigated. The effect of thicknesses of AlN and ZnO thin films on structure performance was calculated by two dimensional finite element method. Theoretical predictions were confirmed by in-situ measurements of frequency and insertion loss that were performed during deposition of AlN layer together with AlN film thickness monitoring by interferential reflectometry method.

A Surface Acoustic Wave (SAW) Pirani vacuum sensor has been fabricated on a lithium niobate substrate and mounted on a printed circuit board. The pressure range of the sensor is at the state of the art, it is 10−5 mbar up to atmospheric pressure with a good sensitivity. Two functioning modes of the transducer have been tested and compared in term of sensitivity and response time. By changing the functioning mode of the sensor, it is shown in this paper that the response time can be divided by 60.