Impact of rapid thermal annealing on the structural and piezoelectric properties of AlN thin films

Aluminum nitride (AlN) is a biocompatible piezoelectric material with energy harvesting properties. This study examines the effects of post-deposition rapid thermal annealing (RTA) on AlN thin films used in microelectromechanical systems (MEMS) resonators. RTA is performed in a N_2-purged chamber at temperatures ranging from 200°C to 1000°C on AlN thin films deposited via DC reactive magnetron sputtering. The effects of RTA temperature on the crystallinity, morphology, composition, and piezoelectric properties of AlN are systematically analysed. A practical temperature threshold is established at 400°C for AlN, above which the material properties change markedly due to rapid oxidation of the material. Up to that temperature, the grain size decreases, and the grains reorient toward the c-axis of the wurtzite phase. The piezoelectric coefficient (d₃₃) increased approximately 160%, and AlN/Si based MEMS showed an amplification of the electromechanical properties to 400 pm/V altogether with an output power density of ∼0.12 µW/cm³. Furthermore, this work identifies the thermal stability within the CMOS-compatible temperature range. Devices employing Ti/Pt electrodes exhibit a degradation threshold between 400°C and 600°C, where the metals undergo diffusion and form undesired bonds and structures. Meanwhile, the AlN/Ta/Nb devices remained stable for temperatures up to 600°C, indicating their suitability for integration into high-temperature MEMS fabrication.