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 ${\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ substrates by reactive radio frequency-sputtering technique at $180°\mathrm{C}$ . The $\mathrm{AlN}$ films, $0.022–6.2\mu \mathrm{m}$ 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 $\left(0002\right)$ rocking curve in the range from $1.6°$ to $1.0°$ for AlN film thickness ranging between $0.022$ and $6.2\mu \mathrm{m}$ . The crystalline quality of $\mathrm{AlN}$ films was extremely good even at high thicknesses, as shown by the presence of the $\mathrm{AlN}\left(0004\right)$ reflection and by the narrow $\left(0.12°–0.20°\right)$ diffraction peaks. Optical measurements of the transmission in the visual and infrared region demonstrated that the $\mathrm{AlN}$ films have low absorption and scattering. The extinction and the absorption coefficients, $\alpha$ and $K_e$ , were estimated at $\lambda ⩾600\mathrm{nm}\left(\alpha =850\pm 50{\mathrm{cm}}^{-1},K_e=0.0040\pm 0.0005\right)$ . The piezoelectric strain constant ${\mathrm{d}}_{33}$ was measured for all the sputtered films: the mean ${\mathrm{d}}_{33}$ value was $(4.2\pm 0.7)\times {10}^{-12}$ $\mathrm{C}∕\mathrm{N}$ , which is very close to the value of the $\mathrm{AlN}$ single crystal. Surface acoustic wave (SAW) delay lines were photolithographically defined on the free surface of the $\mathrm{AlN}$ films grown on bare or metallized ${\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ substrates. Harmonic modes operating at frequencies up to about $2.4\mathrm{GHz}$ were obtained just by using a conventional photolithographic technique with $7.5\mu \mathrm{m}$ linewidth. The phase and group velocities of SAWs propagating in