Ouarda Legrani

Highly textured growth of AlN films on sapphire by magnetron sputtering for high temperature surface acoustic wave applications

Piezoelectric aluminum nitride films were deposited onto 3 in. [0001] sapphire substrates by reactive magnetron sputtering to explore the possibility of making highly (002)-textured AlN films to be used in surface acoustic wave (SAW) devices for high temperature applications. The synthesized films, typically 1 μm thick, exhibited a columnar microstructure and a high c-axis texture. The relationship between the microstructures and process conditions was examined by x-ray diffraction (XRD), transmission electron microscopy, and atomic force microscopy analyses. The authors found that highly (002)-textured AlN films with a full width at half maximum of the rocking curve of less than 0.3° can be achieved under high nitrogen concentration and moderate growth temperature, i.e., 250 °C. The phi-scan XRD reveals the high in-plane texture of deposited AlN films. The SAW devices, based on the optimized AlN films on sapphire substrate, were characterized before and after an air annealing process at 800 °C for 90 min...

In situ high-temperature characterization of AlN-based surface acoustic wave devices

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.

Packageless AlN/ZnO/Si Structure for SAW Devices Applications

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.

AlN/Sapphire: Promising Structure for High Temperature and High Frequency SAW Devices

In this paper, the performance of AlN/sapphire structure in high frequencies is investigated. Several SAW devices are fabricated with various designs (free-space delay, wavelength, metallization ratio, etc.) to study simultaneously different parameters (acoustic velocity, electromechanical coupling (K2), acoustic propagation loss (α), and temperature coefficient of frequency) versus frequency and temperature. Experimental results show that, as expected, α increases with temperature while K2 is enhanced at high temperatures. Because of the antagonistic evolution of these two parameters, insertion loss decreases or increases as function of the free-space delay. We also demonstrate that this structure allows fabrication of devices operating up to 1.5 GHz and that the frequency varies linearly with temperature.

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.

Packageless temperature sensor based on AlN/IDT/ZnO/Silicon layered structure

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.

AlN/IDT/AlN/Sapphire as packageless structure for SAW applications in harsh environments

Packageless heterostructure AlN/IDT/AlN/Sapphire was investigated as a solution protection of the interdigital transducer against agglomeration phenomena at high temperature. For this purpose, the performance of conventional IDT/AlN/Sapphire and protected AlN/IDT/AlN/Sapphire heterostructures were compared. The capability of AlN protective thin film to minimize the deterioration of electrodes was shown.

Potential of Al2O3/GaN/Sapphire layered structure for high temperature SAW sensors

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.