Study of structural and microstructural properties of AlN films deposited on silicon and quartz substrates for surface acoustic wave devices

Abstract

Piezoelectric aluminum nitride thin films were deposited by reactive dc magnetron sputtering under various experimental conditions on a silicon (100) substrates. These films structural and microstructural properties were studied using x-ray diffraction (XRD), selected area electron diffraction (SAED), and transmission electron microscopy (TEM) in order to determine the optimum growth conditions that improve the piezoelectric coupling. A highly c-axis oriented AlN film (002) was identified from XRD and SAED. An average grain column size of about 40 nm was determined from TEM. The root-mean-square surface roughness measured using atomic-force microscopy was less than 7 nm. The optimum experimental parameters found for AlN deposition on Si(100) substrates were then extrapolated to its deposition on a ST-cut quartz substrate for the investigation of the influence of substrates on the surface acoustic wave (SAW) propagation. The AlN/Si and AlN/ST-quartz SAW devices were formed using the photolithography process. Their operating characteristics, phase velocity, temperature coefficient of frequency, and electromechanical coefficient (K2) were measured and analyzed. Experimental results show that the generalized SAW (GSAW) as well as the pseudosurface acoustic wave were excited in this structure. Their phase velocities are, respectively, 3245 m/s and 5117 m/s. The SAW device realized from the AlN/ST-cut quartz structure exhibits higher phase velocity and higher K2 compared to those formed on the ST-quartz substrate.