Reliability of AlN/sapphire bilayer structure for high-temperature SAW applications

This paper explores the possibility to use AIN/sapphire bilayer structure as substrate for surface acoustic wave (SAW) devices operating at high temperature in air atmosphere. Aluminium nitride (AlN) thin films deposited by magnetron sputtering on sapphire substrates were annealed for different periods (2 to 20 hours) and at different temperatures going from 700 to 1000°C. Investigated samples were characterized before and after annealing by X-ray diffraction (XRD), ellipsometry and secondary ion mass spectroscopy (SIMS). This experimental study attests that AIN/Sapphire is reliable up to 700°C in air. These results were also confirmed by in situ measurements of frequency responses coming from SAW devices based on Pt/AIN/Sapphire structure.