Synthesis and microstructural characterisation of reactive RF magnetron sputtering AlN films for surface acoustic wave filters

Abstract

Wurtzite aluminium nitride (AlN) thin films were deposited by RF reactive magnetron sputtering technique on (1 0 0) silicon substrates at low temperature (400 °C). The microstructural properties of sputtered AlN films were investigated using X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and atomic force microscopy (AFM), to aim improvement of piezoelectric coupling for surface acoustic wave (SAW) devices. It was found that the AlN films deposited in the optimum experimental conditions, as revealed by XRD and selected area electron diffraction, exhibit a high (0 0 2) preferred orientation, where columnar crystals are grown in a non-epitaxial pattern and aligned almost perpendicular to silicon substrate. The ω rocking curve, shows that the standard deviation of columns of thin AlN films is less than 1°, which exhibit a high quality of these films. Furthermore, the AFM found a very low surface roughness less than 7 Å, which is very crucial to decrease a loss propagation in AlN films. The films synthesised with optimum conditions were used to perform SAW devices by developing inter-digital transducer on AlN/Si structure. Frequency characteristics show that the realised SAW devices exhibit good filtering performances and a good compromise between phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency.