Effect of a 3C-SiC Buffer Layer on the SAW Properties of AlN Films Grown on Si Substrates

This paper aims to describe the influence of a polycrystalline (poly) 3C-SiC buffer layer on the surface acoustic wave (SAW) properties of poly aluminum nitride (AlN) thin films by comparing the center frequency, the insertion loss, the electromechanical coupling coefficient (k(2)), and the temperature coefficient of frequency (TCF) of an inter-digital transducer (IDT)/AlN/3C-SiC structure with those of an IDT/AlN/Si structure. The poly-AlN thin films with a (0002) preferred orientation were deposited on silicon (Si) substrates by using a pulsed reactive magnetron sputtering system. Experimental results show that the insertion loss (21.92 dB) and the TCF (-18 ppm/degrees C) of the IDT/AlN/3C-SiC structure were improved because of the closely-matched coefficients of thermal expansion (CTE) and the small lattice mismatch (1%) between AlN and 3C-SiC. However, the drawback is that the k(2)(0.79%) and the SAW velocity (5020 m/s) of the AlN/3C-SiC SAW device are reduced by the appearance of some non-(0002) AlN planes, such as (10 (1) over bar2) and (10 (1) over bar3) AlN planes, in the AlN/SiC film. Although disadvantages still exist, the use of the AlN/3C-SiC structure for SAW applications at high temperatures is suitable. The characteristics of the AlN thin films were also evaluated using Fourie transform infrared spectra and both X-ray diffraction and atomic force microscopy images.