AlN films, 1.6–6.3 μm thick, were sputtered at 200 °C on Si(100) and Si(111) substrates. The films were crack-free, uniform, and c-axis oriented. The experimental phase velocities of surface acoustic waves (SAW) propagating in the AlN/Si structures were estimated and showed only a small discrepancy (20–40 m/s) compared to the calculated theoretical values. A SAW resonator (SAWR)-based chemical sensor, operating at about 700 MHz, was implemented on AlN/Si. The SAWR surface was covered with a polymer film sensitive to relative humidity (RH) changes, already tested for RH sensing in previous works on SAW delay lines implemented on AlN/Si and ZnO/Si and operating at about 130 MHz. The RH mass sensitivity and the detection limit of the SAWR sensor improved by 38% and by one order of magnitude, respectively, compared to the delay line-based sensors previously tested.

The sensitivity to relative humidity (RH) variations in the range 0%–90% of a platinum polyyne, namely poly-[1,4-dihexadecyloxy-2,5-diethynylbenzene-bis(triphenylphosphine) platinum(II)] (Pt-P-HDOB) membrane was investigated by means of a surface acoustic wave (SAW) based sensor. A thin film of polymeric membrane was spin deposited on the free surface of the device and the resulting acoustic velocity and attenuation perturbations allowed the acoustic characterization of the membrane by means of the perturbation theory. The SAW sensor was able to reveal even very low $\text{(<10\%}$ RH) humidity conditions at room temperature, with high reproducibility, repeatability and stability.

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