M. El Hakiki

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.

High velocity SAW using aluminum nitride film on unpolished nucleation side of freestanding CVD diamond

High performances surface acoustic wave filters based on AlN/diamond layered structure were performed. C-axis oriented aluminum nitride films with various thicknesses were sputtered on unpolished nucleation side (NS) of freestanding polycrystalline CVD diamond obtained by silicon substrate etching. Experimental results show that high order modes as well as Rayleigh waves are excited. Experimental results are in a good agreement with the calculated dispersion curves which were determined by software simulation with Greens function formalism. We demonstrate clearly that SAW devices combining high phase velocity, high electromechanical coupling coefficient and low temperature coefficient can be performed with AlN/diamond structure.

Theoretical and Experimental Identification of Love Wave Frequency Peaks in Layered Structure ZnO/Quartz SAW Device

In this paper, the layered structure ZnO/Quartz (90deg rotated ST-cut) is investigated theoretically and experimentally. Both waves, Rayleigh and Love, are analyzed. Dispersion curves of phase velocities, electromechanical coupling coefficient (K 2) and temperature coefficient of frequency (TCF) were calculated as a function of normalized thickness ZnO film (kh ZnO = 2pih ZnO /lambda) and the optimum value of h ZnO was determined for experimental study. Experimental results combined with simulation lead to clearly identify the generated waves and their higher modes in this structure except the mode 0 that shows comparable velocity for both Rayleigh and Love waves. The identification of the wave type was performed by studying the frequency response of the device with or without a droplet of water in the wave path. We also demonstrate that the highest elastic velocity is obtained for the mode 1 of the Love wave. This Love wave mode exhibits very interesting electrical characteristics, good K 2, high-frequency rejection, low TCF, and very low attenuation in liquid making it very attractive for gas and liquid sensor applications.

Elastic and piezoelectric constants of potassium niobate studied by brillouin spectroscopy

In this paper we present potassium niobate elastic and piezoelectric constants measurements. These experiments have been performed by Brillouin spectroscopy which gives a direct access to the elastic coefficient by measuring the acoustic velocity. To avoid problems related to the angle precision on Brillouin spectroscopy, three different samples corresponding to X-cut, Y-cut and Z-cut have been used. The complete set of elastic constants is given and results are compared with those obtained previously by other authors. A discussion around the accuracy of the results is developed to explain discrepancy between previous results given in the literature.

High SAW velocity and high electromechanical coupling coefficient with the new three layered structure: ZnO/AlN/diamond

A new diamond based SAW device layered structure, combining high acoustic velocity (V/spl phi/) and high electromechanical coupling coefficient (K/sup -/), is investigated for GHz-band applications. In fact, to overcome the trade-off relationship between V/spl phi/ and K/sup 2/, we propose the use of a three-layer structure ZnO/AlN/diamond that combine the advantages of both piezoelectric materials: high K/sup 2/ of ZnO and high velocity of AlN. A theoretical study based on the Campbell and Jones model was performed to calculate the phase velocity and K/sup 2/ dispersion curves of the Rayleigh mode; its higher modes as well as the leaky waves generated in this structure. Three configurations depending on IDT position in the structure: ZnO surface, ZnO/AlN or AlN/diamond interfaces were calculated considering various thicknesses of ZnO and AlN layers. Both high values of K/sup 2/ larger than 4%, and of V/spl phi/ more than 15 km/s are expected on this new structure according to the theoretical results.

Very large bandwidth IF SAW filters using leaky waves on LiNbO 3 and LiTaO 3

It was previously reported that a leaky wave propagating on 41°Y-X LiNbO₃ and 36°Y-X LiTaO₃ substrates, has a relatively low temperature coefficient of frequency (TCF) and a large electromechanical coupling factor (K²). In this study, we investigated the performance of wide band IF SAW filters using a FAN - shaped structure and leaky wave mode. The optimization of Hanma Hunsinger cells on 41°Y-X LiNbO₃ and 36°Y-X LiTaO₃ substrates enables to have a good reflection coefficient (r) and a large electromechanical coupling factor (K²) for a Leaky wave. The experimental results show the IF SAW filter on 36°Y-X LiTaO₃ gives a good steepness of 1.5% with a reasonable relative bandwidth (9%). Concerning the filter on 41°Y-X LiNbO₃, it presents a large relative bandwidth (55%), low losses (-13dB) and a good Return Loss (-9 dB). These results are very promising compared to those of conventional wide band IF filters using Rayleigh waves.

P3O-5 High Frequency and High Temperature SAW Devices Based on Nanocrystalline Diamond Film on Langasite Substrate

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

6K-1 Temperature Coefficients of Potassium Niobate (KNbO3) Elastic Constants Studied by Brillouin Spectroscopy

In this paper we present the study of potassium niobate KNbO3 single crystals with different cuts in order to observe the temperature dependence of the acoustic velocity of bulk waves in chosen crystallographic directions. The crystal was temperature controlled to determine the variation of the elastic constant versus the temperature. The novelty of this work concerns the determination for the first time for KNbO3, in the best of our knowledge, of the temperature coefficients of elastic constants C11 and C44. All measurements have been performed by Brillouin spectroscopy, in the range of (240-420 K)