Evolution of Ferroelectricity With Annealing Temperature and Thickness in Sputter Deposited Undoped HfO2 on Silicon

Ferroelectricity in sputtered undoped HfO2 is attractive for composition control for low power and nonvolatile memory and logic applications. Unlike doped HfO2, the evolution of ferroelectricity with annealing and film thickness effect in sputter deposited undoped HfO2 on Si is not yet reported. In the present study, we have demonstrated the impact of postmetallization annealing temperature and film thickness on ferroelectric properties in dopant-free sputtered HfO2 on Si substrate. A rich correlation of polarization (
$2{P}_{r}$
) with phase, lattice constant, and crystallite size and interface reaction is observed. First, anneal temperature shows o-phase saturation beyond 600 °C followed by interface reaction beyond 700 °C to show an optimal temperature window on 600 °C–700 °C. 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
$2 {P}_{r}$
. Beyond 20 nm, crystallite scaling with thickness saturates with the correlated appearance of m-phase and reduction in
$2 {P}_{r}$
. The optimal thickness-temperature window range of 15–20-nm films annealed at 600 °C–700 °C shows
$2{P}_{r}$
of
$\sim 35.5~\mu \text{C}$
/cm2 is comparable to state of the art. The robust endurance of ~10
⁸
cycles with negligible wakeup showcased in the promising temperature-thickness window has been identified systematically for nonvolatile memory applications.