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.

Keywords:

• Aluminium nitride (AlN)
• surface acoustic wave devices
• residual stress
• sputtering