Evolution of ferroelectricity with annealing temperature and thickness in sputter deposited undoped HfO$_2$ on silicon

Ferroelectricity in sputtered undoped-HfO$_2$ is attractive for composition control for low power and non-volatile memory and logic applications. Unlike doped HfO$_2$, evolution of ferroelectricity with annealing and film thickness effect in sputter deposited undoped HfO$_2$ on Si is not yet reported. In present study, we have demonstrated the impact of post metallization annealing temperature and film thickness on ferroelectric properties in dopant-free sputtered HfO$_2$ on Si-substrate. A rich correlation of polarization with phase, lattice constant, and crystallite size and interface reaction is observed. First, anneal temperature shows o-phase saturation beyond 600 oC followed by interface reaction beyond 700 oC to show an optimal temperature window on 600-700 oC. Second, thickness study at the optimal temperature window shows an alluring o-phase crystallite scaling with thickness till a critical thickness of 20 nm indicating that the films are completely o-phase. However, the lattice constants (volume) are high in the 15-20 nm thickness range which correlates with the enhanced value of 2Pr. Beyond 20 nm, crystallite scaling with thickness saturates with the correlated appearance of m-phase and reduction in 2Pr. The optimal thickness-temperature window range of 15-20 nm films annealed at 600-700 oC show 2Pr of ~35.5 micro-C/cm$^2$ is comparable to state-of-the-art. The robust wakeup-free endurance of ~$10^$8 cycles showcased in the promising temperature-thickness window has been identified systematically for non-volatile memory applications.