Rapid cooling process-driven enhancement of an orthorhombic phase in ferroelectric HfZrOx of sub-3 nm ultrathin films by atomic layer deposition

In recent decades, fluorite-structured HfZrO x (HZO) has been spotlighted as a promising ferroelectric material for next-generation non-volatile memory devices. On an ultrathin scale, HZO thin films face challenges in the phase transformation to an orthorhombic (111) structure for ferroelectric properties. The thermal energy governs the crystallinity of the ferroelectric HZO thin films during atomic layer deposition (ALD) process and post-annealing. Together with the post-metallization annealing (PMA) process, the most common method for enhancing and transforming ferroelectric properties, we determined that the cooling process after PMA is also crucial. In this study, two different cooling processes with -1.5 and -13.3 °C/s for TiN/HZO/TiN structure were conducted after the PMA process. At higher cooling rates, the crystallinity of the HZO, especially that of the orthorhombic (111) phase, dramatically improved. To further improve the crystallinity of the HZO films, the choice of precursors (Hf and Zr) and oxidants was studied. These results suggest that the cooling rate is an additional significant factor in controlling the crystallinity of HZO thin films and that rapid cooling could play a key role in ultrathin (< 5 nm) HZO thin films.