Deposition and characterisation of c-axis oriented AlScN thin films via microwave plasma-assisted reactive HiPIMS

In this work, we demonstrate that highly oriented c-axis aluminium scandium nitride (AlScN) piezoelectric thin films can be deposited via microwave plasma-assisted reactive high power impulse magnetron sputtering (MAR-HiPIMS), without the necessity of substrate heating. A combination of in situ plasma diagnostics, i.e. time-of-flight mass spectrometry (ToF-MS), modified quartz crystal microbalance (m-QCM), and magnetic field measurements allowed to optimise the deposition conditions, in turn maximising the nitrogen supply and ionic flux at the substrate region, while maintaining stable discharge conditions. The AlScN thin films synthesised in this study were deposited as chemically gradient coatings with varying levels of scandium doping, and were characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Obtaining highly textured films was made possible with the addition of microwave plasma to the optimised HiPIMS discharge, where the wurtzite AlScN films (with up to 20 at. % Sc) exhibited a stronger texture in the (0002) orientation compared to films prepared without microwave plasma. Additionally, the use of a microwave plasma led to a significant decrease in oxygen content in the films and increase in nitrogen content, ensuring stoichiometric compositions. Based on the results mentioned above, it is expected that the AlScN thin films fabricated via MAR-HiPIMS would exhibit a strong piezoelectric response

Amélioration des performances des résonateurs à ondes acoustiques de volume en niobate de lithium

Les exigences strictes introduites par les prochaines normes 5G mettent l'accent sur le frontal RF intégré aux objets connectés. L’ouverture de bandes de fréquences larges (> 150 MHz) à hautes fréquences (> 5 GHz) pousse les filtres à ondes acoustiques de volume (BAW) actuels vers leurs limites. Ceci tient au coefficient de couplage électromécanique du matériau piézoélectrique employé dans ces composants, le nitrure d’aluminium (AlN). Le CEA-LETI développe depuis quelques années une technologie de filtres BAW basée sur des couches minces, obtenues par Crystal Ion Slicing, de LiNbO3, matériau possédant des propriétés piézoélectriques supérieures à celles de l’AlN. Dans ce cadre, ma thèse a porté sur la caractérisation de ces couches minces en vue d’améliorer lers performances des composants développés. Une première partie de mes travaux a été consacrée à l’analyse des propriétés microstructurales de ces couches minces et à leur amélioration. Ceci a permis d’augmenter le coefficient de couplage électromécanique du matériau jusqu’à atteindre sa valeur théorique pour des monocristaux. Il s’est cependant avéré que ceci ne pouvait se faire en présence d’électrodes en aluminium. Une seconde partie de ma thèse a donc été consacrée à la recherche d’électrodes alternatives et à leur intégration dans des résonateurs. La réalisation de prototypes possédant des électrodes inférieures en tungstène a montré une augmentation significative des coefficients de qualité de 200 à 600. Finalement, je me suis intéressé à l’analyse des défauts microstructuraux électriquement actifs, par la mesure des courants de fuite dans des capacités Al/LiNbO3/Al. L’établissement d’une loi générale reliant ces courants à la tension, la température et le temps a mis en évidence la présence de défauts également rencontrés dans des substrats massifs de ce matériaux. Ceci tend à indiquer que le procédé de réalisation de résonateurs n’apporte pas de défauts suplémentaires au cristal.The strict requirements introduced by the next 5G standards are putting pressure on the RF front-end of connected objects. The opening of new wide (> 150 MHz) and high frequency (> 5 GHz) bands pushes current Bulk Acoustic Wave (BAW) filters to their limits. This originates from the limited electromechanical coupling factor of the material employed in these devices, aluminum nitride (AlN). CEA-LETI has been developing for several years a BAW filter technology based on LiNbO3 thin-films, a material exhibiting larger piezoelectric properties than AlN, obtained by crystal ion slicing. In this context, my Ph.D. has focused on the characterization of these thin films to improve the performances of BAW devices. The first part of my work has focused on the analysis of the microstructure of LiNbO3 thin films and their improvement. This led to an increase in the electromechanical coupling factor up to reaching the theoretical value for single crystals. However, optimizing the piezoelectric properties proved incompatible with the use of aluminum electrodes. Therefore, the second part of my Ph.D. has been devoted to the development and integration of alternative electrode materials. The fabrication of prototypes with bottom W electrodes has revealed a significant improvement of quality factors, from 200 to 600. Eventually, I have investigated electrically active microstructural defects trough leakage current measurements in Al/LiNbO3/Al capacitors. A general law relating leakage currents to voltage, temperature and time has evidenced defects that have been already reported in studies on the conductivity of bulk wafers. This evidences that the fabrication process does not induce additional crystal defects

Crystallization behavior of ion beam sputtered HfO

In this work, we present our results about the thermal crystallization of ion beam sputtered hafnia on 0001 SiO2 substrates and its effect on the laser-induced damage threshold (LIDT). The crystallization process was studied using in-situ X-ray diffractometry. We determined an activation energy for crystallization of 2.6 ± 0.5 eV. It was found that the growth of the crystallites follows a two-dimensional growth mode. This, in combination with the high activation energy, leads to an apparent layer thickness-dependent crystallization temperature. LIDT measurements @355 nm on thermally treated 3 quarter-wave thick hafnia layers show a decrement of the 0% LIDT for 1 h @773 K treatment. Thermal treatment for 5 h leads to a significant increment of the LIDT values

Emulsion electrospinning of sodium alginate/poly(epsilon-caprolactone) core/shell nanofibers for biomedical applications

Electrospun nanofibers have shown great potential as drug vehicles and tissue engineering scaffolds. However, the successful encapsulation of multiple hydrophilic/hydrophobic therapeutic compounds is still challenging. Herein, sodium alginate/poly(ε-caprolactone) core/shell nanofibers were fabricated via water-in-oil emulsion electrospinning. The sodium alginate concentration, water-to-oil ratio, and surfactant concentration were optimized for the maximum stability of the emulsion. The results demonstrated that an increasing water-to-oil ratio results in more deviation from Newtonian fluid and leads to a broader distribution of the fibers' diameters. Moreover, increasing poly(ε-caprolactone) concentration increases loss and storage moduli and increases the diameter of the resulting fibers. The nanofibers' characteristics were investigated by scanning electron microscopy, transmission electron microscopy, confocal laser scanning microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and water contact angle measurements. It was observed that using an emulsion composition of 10% (w/v) PCL and a water-to-oil ratio of 0.1 results in smooth, cylindrical, and uniform core/shell nanofibers with PCL in the shell and ALG in the core. The in vitro cell culture study demonstrated the favorable biocompatibility of nanofibers. Overall, this study provides a promising and trustworthy material for biomedical applications.ISSN:2516-023

Single-mode high frequency LiNbO 3 Film Bulk Acoustic Resonator

International audienceIn this paper, Y+163°-cut LiNbO 3 (LNO) Film Bulk Acoustic Resonators (FBAR) with patterned bottom electrodes (AlSi or W) and a sacrificial layer cavity have been fabricated using a layer transfer process (4-inch). Unlike previous work based on films oriented towards the X axis [1], the Y+163° orientation provides a single resonance at 2.2 GHz, with an effective electromechanical coupling factor (k t 2 ) of 26 %. Thanks to this single mode behavior and to the energy trapping induced by the heavy tungsten electrodes, the quality factor at antiresonance (Q a ) is increased to 600. Moreover a Temperature Coefficient of Frequency (TCF) of -45 ppm/°C is obtained. This demonstrates a significant improvement towards introducing LiNbO 3 as an alternative to AlN in Bulk Acoustic Wave (BAW) filters for the new generation of RF filters