Correlation between structural properties of AlN/Sapphire and performances of SAW devices in wide temperature range

Aluminum nitride (AlN) on sapphire is a promising substrate for surface acoustic wave (SAW) sensors operating at high temperatures and high frequencies. To get an experimental measure of the suitability and temperature stability of such devices an experimental relationship between surface acoustic wave (SAW) performances of AlN thin films and their structural properties was investigated in the same range of temperature between 50°C and 1000°C. The crystalline structure of AlN films was examined in-situ versus temperature (50-1000°C) by X-ray diffraction. The results reveal that the AlN film remains well oriented (002) even at the high temperature. A significant change in the values of the c-lattice parameter has been observed due to the thermal expansion mismatch and strain relaxation between the AlN crystal and substrate. The correlation between the crystalline quality and the SAW performances of AlN/Sapphire reveal that the evolution of the crystalline quality of the AlN film with temperature (in a range of 600-850 °C) follows similar trends as the evolution of relative insertion loss. The surface oxidation of the film has been observed at high annealing temperature (>850°C) which reduce the crystalline quality and piezoelectric response of the AlN film.