Enhancing ferroelectric properties of Hf0.5Zr0.5O2 thin films using the HfN/TiN and W/TiN bi-layer bottom electrodes

This study clarifies the influence of single-layer (TiN, HfN, W) and bi-layer (HfN/TiN, W/TiN) bottom electrodes (BEs) on the ferroelectric performance and reliability of the 10-nm-thick Hf_0.5Zr_0.5O₂ (HZO) thin films. A smaller thermal expansion coefficient in HfN or W imposes higher in-plane tensile stress on the HZO thin films, facilitating the polar orthorhombic (o-) phase fraction and enhancing remanent polarization (P_r). However, thicker interfacial layers formed when HfN or W single-layer BE and HZO contacted directly, leading to excessive leakage current and degraded ferroelectric performance. These excessive interfacial layers were effectively suppressed by inserting a thin (5 nm–20 nm) TiN layer on the HfN or W BEs. As a result, the HZO thin films on the HfN/TiN and W/TiN bi-layer BEs decrease the HZO grain size, facilitating the o-phase formation (increasing P_r) and lowering the film's coercive field. However, the higher surface roughness of the W/TiN bi-layer BEs induced excessive leakage current and reliability degradation. In contrast, the HfN BEs with a 10- or 20-nm-thick upper TiN layer lower the surface roughness of the BEs, thereby eliminating the adverse effects. As a result, the HfN 40 nm/TiN 10 nm/HZO/TiN stack exhibited enhanced ferroelectric performance up to 10⁹ switching cycles with a lower cycling field of 2.7 MV/cm than the TiN 50 nm/HZO/TiN stack with a cycling field of 3.7 MV/cm.