Structural, optical, and acoustic characterization of high-quality AlN thick films sputtered on Al2O3(0001) at low temperature for GHz-band electroacoustic devices applications

Aluminum nitride thin and thick films were grown on Al2O3(0001) substrates by reactive radio frequency-sputtering technique at 180 deg. C. The AlN films, 0.022-6.2 μm thick, were stress-free, uniform, transparent, and extremely adhesive to the substrate. Their structural properties were investigated by x-ray diffraction measurements: the resulting films were highly c-axis oriented, with a full width at half maximum of the (0002) rocking curve in the range from 1.6 deg. to 1.0 deg. for AlN film thickness ranging between 0.022 and 6.2 μm. The crystalline quality of AlN films was extremely good even at high thicknesses, as shown by the presence of the AlN(0004) reflection and by the narrow (0.12 deg. -0.20 deg.) diffraction peaks. Optical measurements of the transmission in the visual and infrared region demonstrated that the AlN films have low absorption and scattering. The extinction and the absorption coefficients, α and Ke, were estimated at λ≥600 nm (α=850±50 cm-1,Ke=0.0040±0.0005). The piezoelectric strain constant d33 was measured for all the sputtered films: the mean d33 value was (4.2±0.7)x10-12 C/N, which is very close to the value of the AlN single crystal. Surface acoustic wave (SAW) delay lines were photolithographically defined on the free surface of the AlN films grown on bare or metallized Al2O3(0001) substrates. Harmonic modes operating at frequencies up to about 2.4 GHz were obtained just by using a conventional photolithographic technique with 7.5 μm linewidth. The phase and group velocities of SAWs propagating in AlN/Al/Al2O3 and in AlN/Al2O3 structures, along and normal to the Al2O3 a axis, were estimated for different AlN thicknesses. The experimental measurements were compared with the theoretical data and found to be in good agreement