ALD-Derived WO3-x Leads to Nearly Wake-Up-Free Ferroelectric Hf0.5Zr0.5O2 at Elevated Temperatures

Breaking the memory wall in advanced computing architectures will require complex 3D integration of emerging memory materials such as ferroelectrics-either within the back-end-of-line (BEOL) of CMOS front-end processes or through advanced 3D packaging technologies. Achieving this integration demands that memory materials exhibit high thermal resilience, with the capability to operate reliably at elevated temperatures such as 125C, due to the substantial heat generated by front-end transistors. However, silicon-compatible HfO2-based ferroelectrics tend to exhibit antiferroelectric-like behavior in this temperature range, accompanied by a more pronounced wake-up effect, posing significant challenges to their thermal reliability. Here, we report that by introducing a thin tungsten oxide (WO3-x) layer-known as an oxygen reservoir-and carefully tuning its oxygen content, ultra-thin Hf0.5Zr0.5O2 (5 nm) films can be made robust against the ferroelectric-to-antiferroelectric transition at elevated temperatures. This approach not only minimizes polarization loss in the pristine state but also effectively suppresses the wake-up effect, reducing the required wake-up cycles from 105 to only 10 at 125C- a qualifying temperature for back-end memory integrated with front-end logic, as defined by the JEDEC standard. First-principles density functional theory calculations reveal that WO3 enhances the stability of the ferroelectric orthorhombic phase at elevated temperatures by increasing the tetragonal-to-orthorhombic phase energy gap, and promoting favorable phonon mode evolution, thereby supporting o-phase formation under both thermodynamic and kinetic constraints.

💡 Research Summary

The paper addresses a critical bottleneck in the integration of ferroelectric (FE) memories with advanced computing architectures: the loss of ferroelectricity at elevated temperatures typical of back‑end‑of‑line (BEOL) environments (up to 125 °C). Conventional HfO₂‑based ferroelectrics, especially Hf₀.₅Zr₀.₅O₂ (HZO), undergo a tetragonal‑to‑orthorhombic phase transition that degrades polarization and triggers a pronounced wake‑up effect, often requiring 10⁵ electric cycles to reach a stable state. The authors propose inserting an ultra‑thin tungsten oxide (WO₃₋ₓ) layer at the bottom electrode/HZO interface, acting as an oxygen reservoir, to stabilize the orthorhombic (o‑phase) ferroelectric phase at high temperature and suppress wake‑up.

Two fabrication routes are explored: (1) partial oxidation of a sputtered W electrode via O₂ plasma inside an ALD chamber, yielding a ~4 nm WO₃₋ₓ layer with a high concentration of oxygen vacancies (≈28 % V_O); (2) direct plasma‑enhanced ALD (PE‑ALD) deposition of 5 nm or 6 nm WO₃₋ₓ, producing a lower vacancy concentration (≈21 % V_O). Cross‑sectional STEM‑EDS confirms the presence of distinct W, O, and Zr layers, while XPS shows W⁶⁺ peaks and deconvoluted O 1s spectra that quantify lattice versus non‑lattice oxygen. The vacancy‑rich plasma‑treated films are more conductive, whereas the ALD‑grown films are less conductive, leading to higher coercive voltages in the latter due to larger voltage drops across the WO₃₋ₓ layer.

Electrical characterization includes polarization‑voltage (P‑V), switching‑current‑voltage (I_SW‑V), and positive‑up‑negative‑down (PUND) measurements. At room temperature all devices exhibit comparable remanent polarization (2P_r) and similar endurance. However, temperature‑dependent tests reveal stark differences. The reference device (no WO₃₋ₓ) shows a clear double‑peak in I_SW‑V already at 85 °C, indicating the onset of antiferroelectric‑like behavior. In contrast, devices with WO₃₋ₓ display reduced peak separation, and the ALD‑based 5 nm/6 nm samples retain a single dominant switching peak up to 125 °C. Correspondingly, the 2P_r of the reference device drops dramatically at 125 °C, while WO₃₋ₓ‑containing devices maintain >80 % of their room‑temperature polarization.

First‑principles density functional theory (DFT) calculations provide mechanistic insight. The presence of WO₃₋ₓ raises the Helmholtz free‑energy gap between the tetragonal and orthorhombic phases, making the o‑phase thermodynamically favored at higher temperatures. Moreover, WO₃₋ₓ enhances coupling of the X₂′ phonon mode, which stabilizes the orthorhombic lattice and raises the kinetic barrier for the t‑to‑o transition. These effects stem from the lower entropy and increased lattice anisotropy introduced by the oxygen‑deficient tungsten oxide.

Overall, the study demonstrates that a carefully engineered WO₃₋ₓ interfacial layer can (i) suppress the wake‑up effect, reducing required cycling from 10⁵ to just 10 cycles at 125 °C, (ii) preserve ferroelectric polarization under thermal stress, (iii) improve endurance and reduce leakage, and (iv) provide a pathway to meet JEDEC JESD22‑A108 specifications for BEOL‑compatible memory‑on‑logic stacks. The work suggests that further optimization of WO₃₋ₓ thickness, vacancy concentration, and integration into full 3D‑stacked architectures could enable robust, high‑performance Fe‑RAM and Fe‑FET devices for future heterogeneous computing systems.