M.B. Assouar

Surface acoustic wave propagation in aluminum nitride-unpolished freestanding diamond structures

High-quality surface acoustic wave (SAW) filters based on aluminum nitride (AlN)/diamond layered structures were prepared using the nucleation side of polycrystalline chemical vapor deposition (CVD) diamond, removed from a silicon substrate by wet etching. Highly oriented AlN thin films with optimized piezoelectric properties and with various thicknesses were sputtered onto the nucleation side of freestanding diamond. The effect of AlN thickness on the SAW phase velocity, the coupling coefficient, and the device characteristics were investigated. Experimental results show that the Rayleigh wave and the higher modes are generated. These results agree well with calculated dispersion curves and demonstrate that a high electromechanical coupling coefficient together with a high phase velocity can be obtained by using the nucleation side of freestanding CVD diamond layer.

5GHz surface acoustic wave devices based on aluminum nitride/diamond layered structure realized using electron beam lithography

Very high frequency surface acoustic wave (SAW) devices based on AlN/diamond layered structures were fabricated by direct writing using e-beam lithography on the nucleation side of chemical vapor deposition diamond. The interdigital transducers made in aluminum with resolutions down to 500nm were patterned on AlN/diamond layered structure with an adapted technological process. Experimental results show that the Rayleigh wave and the higher modes are generated. The fundamental frequency around 5GHz was obtained for this layered structure SAW device and agrees well with calculated results from dispersion curves of propagation velocity and electromechanical coupling coefficient.

High-frequency surface acoustic wave devices based on AlN/diamond layered structure realized using e-beam lithography

We report in this paper on the study and the realization of surface acoustic wave devices based on an AlN/diamond layered structure intended for the X band (8 GHz). Both x-ray diffraction and transmission electronic microscopy, used for characterization of the structural properties of the AlN/diamond structure, have shown (002) highly oriented sputtered AlN films on free-standing chemical vapor deposition diamond films. Surface roughness of the AlN/diamond structure was measured by atomic force microscopy and showed a very low surface roughness, less than 1 nm. Low surface roughness is very important to reduce the acoustic propagation losses. SAW devices operating in the range of 8 GHz were realized by the combination of the high velocity of the AlN/diamond layered structure and the high lateral resolution obtained using e-beam lithography (EBL). Due to high electrical resistivity of the AlN film, interdigital transducers with submicronic resolution were patterned by an adapted technological EBL process. ...

Reactive DC magnetron sputtering of aluminum nitride films for surface acoustic wave devices

A piezoelectric film combined with high velocity substrate as sapphire or diamond, seems very promising for surface acoustic wave (SAW) devices operating at high frequency. In this work, we concentrated on the optimization of growth parameters to perform AlN films with required properties for SAW devices: high resistivity, low roughness and high piezoelectricity coupling. AlN films are deposited by reactive DC magnetron sputtering on silicon substrate as a function of N concentration in Ar-N gas 22 mixtures. The duration of the process was modulated to obtain a constant film thickness (2 mm) to permit a better comparison. X-Ray diffraction (XRD) shows that the AlN films deposited in the range of 20-100% N , 400 8C, and 6=10 mbar, exhibit y3 2 a columnar structure textured in (002) orientation corresponding to hexagonal wurtzite structure with a c-axis perpendicular to the surface. In regard to XRD and electrical characterizations, the optimum film properties are obtained in the range of 60-80% N concentration. The higher peak intensity of (002) AlN diffraction and the higher resistivity are obtained for the AlN films 2 synthesized with 75% N. AFM analysis of AlN films demonstrate a low roughness at approximately 3 nm. The SAW device 2 (filter) was formed by development of interdigital transducers of 32 mm wavelength on the AlNySi structure by photolithography. The frequency response shows a center frequency of 158 MHz corresponding to a phase velocity of 5055 mys. This value is almost constant for all samples performed at optimum conditions. However, the insertion loss of device seems varies with N2 percentage and the low attenuation is obtained with 70% of N. This study is extrapolated on a sapphire substrate, and a phase 2 velocity of 5536 mys is recorded showing the effect of substrate to increase the center frequency of SAW devices. � 2002 Elsevier Science B.V. All rights reserved.

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...

Opening of simultaneous photonic and phononic band gap in two-dimensional square lattice periodic structure

We discuss two points related to the simultaneous existence of phononic and photonic band gaps in a two-dimensional crystal constituted by a square array of holes drilled in a matrix. In a first part, using the case of a sapphire sample in the microwave range, we show that in addition to the phononic gap, an absolute photonic gap may be obtained making use of the high values as well as the anisotropy of the dielectric matrix elements in the microwave regime. In a second part, using the case of silicon in the telecom frequency range, we demonstrate that absolute photonic and phononic gaps may be obtained by making a combination of two crystals having slightly different filling factors. The calculations of the band structures and transmission coefficients were mainly computed using the finite difference time domain method.

Deposition of aluminium nitride film by magnetron sputtering for diamond-based surface acoustic wave applications

Diamond/piezoelectric material thin film layered structures are expected to be applied to high frequency surface acoustic wave (SAW) devices because of the high acoustic wave velocity of diamond. Aluminium nitride (AlN) has been chosen as piezoelectric material because of its both high phase velocity and high resistivity. AIN thin films have been deposited by DC pulsed magnetron sputtering on Si(100) substrates. Texture and structure of the films have been investigated by X-ray diffraction, cross-section and in-plane view scanning electronic microscopy observation, and atomic force microscopy. One-micron thick, smooth and (002) oriented AlN films have been successfully deposited on freestanding chemical vapour deposition (CVD) diamond layers. The surface acoustic wave characteristics of AlN/diamond structure were investigated.

Modelling of SAW filter based on ZnO/diamond/Si layered structure including velocity dispersion

Abstract 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.

Growth and characterization of c-axis inclined AlN films for shear wave devices

This paper reports on the growth and the characterization of c-axis inclined AlN thin films synthesized at low temperature. These films are of significant interest for shear wave generation in the shear mode resonators that operate as a liquid sensor. AlN films were deposited on 3 inch (1 0 0) silicon wafers using an RF-magnetron sputtering planar system. A SiO2 buffer layer was used to promote the c-axis inclination. This c-axis inclination of AlN thin films was investigated using scanning electronic microscopy and x-ray diffraction in θ/2θ, χ and rocking curve scan modes. These analyses showed up to 10° of c-axis inclination in our planar charging system at low temperature. An AlN film thickness variation of about ±5% was recorded. In this study, we only presented the effect of the pressure on the c-axis inclination of AlN films. A discussion about the effect of this parameter and the role of the SiO2 buffer layer is reported. A shear mode acoustic wave device based on the deposited c-axis inclined AlN film was constructed and showed a phase velocity of 5832 m s−1. This value of shear velocity is discussed.

STUDY OF ACOUSTICAL AND OPTICAL PROPERTIES OF ALN FILMS FOR SAW AND BAW DEVICES: CORRELATION BETWEEN THESE PROPERTIES

ABSTRACT 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. We have shown that the grain size determined from TEM and FESEM characterisations, increases with film thickness. The AlN/Si SAW filter performed with the film presenting the lower residual stress exhibits the fundamental and third harmonic of resonance frequency of 212 MHz and 629 MHz respectively with very practical suppression band, taking into account the low electromechanical coupling coefficient of AlN/Si layered structure predicted by calculation.