Low-Temperature (450 ∘C) Crystallization of Al:HfO2 Ferroelectric Thin Films Enabled by Microwave Annealing Process

The crystallization of HAO ferroelectric (FE) material requires high annealing temperatures ranging from $700~^{\circ }$ C to $1000~^{\circ }$ C, because of the smaller ionic radius of Al dopants. This extreme temperature requirement poses substantial challenges for the broad application of the HAO FE layer. Herein, we address this problem by introducing the microwave annealing (MWA) system, capable of inducing the phase transformation essential for FE material crystallization efficiently through a synergistic application of heat and microwaves. The MWA system supplies the activation energy required for the crystallization of hafnia FE materials via a combination of thermal and microwave energy. This allows for a significant reduction in the annealing temperature. Ultimately, we successfully achieved high-quality orthorhombic (o-) phase ( $2{P}_{\text {r}}$ of $24.63~\mu $ C/cm ${}^{{2}}\text {)}$ crystallization of HAO at a notably lower temperature of $450~^{\circ }$ C. The MWA system-based low-temperature crystallization process significantly improves the interface quality, yielding better results than typical rapid thermal annealing (RTA) at $700~^{\circ }$ C. These advancements not only enhance reliability but also diminish the leakage current of HAO. This research is noteworthy as it demonstrates successful polarization switching in HAO FE material under low thermal budget conditions ( $\le 450~^{\circ }$ C) enabled by the MWA system.