Fully Atomic-Layer-Deposited Vertical Complementary FeRAM with Ultra-High 2Pr > 100 uC/cm2 and High Endurance > 1E10 cycles

Abstract: A limited remanent polarization (Pr) in HfO2-based FeRAM remains a key obstacle to density scaling and reliability, while material and process optimizations offer only incremental improvements. This limitation fundamentally originates from the thickness-constrained switchable polarization and the intrinsic polarization ceiling of HfO2-based ferroelectrics. Here, we propose an all-ALD-grown vertical complementary FeRAM (VCF) architecture, in which the top and bottom stacked FeRAM cells maintain complementary polarization. This complementary dipole configuration converts the readout from a single-layer polarization response into a differential polarization summation, thereby amplifying the effective charge window without increasing the switching field of each individual layer or incurring area overhead. Viewed from top to bottom, an "up-down" polarization pair stores logic '1', whereas a "down-up" pair stores logic '0'. Using a complementary polarization write-read scheme, the VCF achieves an effective differential polarization above 100 uC/cm^2 and retains above 90 uC/cm^2 after 1e10 switching cycles without electrical breakdown. Robust retention (longer than 1e4 s at 85 degC) and strong disturb immunity are demonstrated, with an effective differential polarization above 80 uC/cm^2 under a V/3 scheme after 1e6 disturb pulses. Array-level operation is validated in a 5 x 5 selector-free crosspoint array. The performance enhancement of the VCF arises from the co-optimization of the all-ALD-grown process, device architecture, and operation scheme, enabling high density, a wide memory window, and strong reliability for scalable FeRAM integration.

• The paper introduces a novel ALD vertical complementary FeRAM architecture that doubles the polarization by stacking two HZO layers with complementary orientations.
• The approach achieves ultra-high performance with a measured 2Pr > 100 µC/cm² and endurance exceeding 10¹⁰ cycles, addressing key scaling challenges.
• The method reduces switching energy and enhances disturb immunity, paving the way for scalable, selector-free, high-density nonvolatile memory arrays.

Fully Atomic-Layer-Deposited Vertical Complementary FeRAM: Architecture, Performance, and Integration Prospects

Introduction

Ferroelectric random-access memory (FeRAM) utilizing HfO$_2$-based ferroelectricity has evolved as a credible contender for nonvolatile memory in next-generation embedded and high-density logic applications. Despite significant progress in device miniaturization and back-end-of-line (BEOL) compatibility, scaling is constrained by the intrinsic ceiling on remanent polarization ($P_r$) in ultrathin HfO$_2$ ferroelectrics, which directly limits the achievable sensing margin and storage density. The paper "Fully Atomic-Layer-Deposited Vertical Complementary FeRAM with Ultra-High 2Pr > 100 µC/cm² and High Endurance > 1E10 cycles" (2604.15131) proposes a comprehensive solution, introducing a novel all-atomic-layer-deposited (ALD) vertical complementary FeRAM (VCF) architecture engineered for high polarization, robustness, and integration scalability.

Device Architecture and Fabrication Approach

The VCF cell leverages a vertically stacked two-layer FeRAM configuration with complementary dipole orientations between the top and bottom Hf$_{0.5}$Zr$_{0.5}$O$_2$ (HZO) layers, separated and interfaced by ultrathin TiN electrodes. Each layer is precisely deposited by plasma-enhanced ALD, ensuring atomic-scale controllability, high conformality, and minimal interfacial defects. The choice of ultrathin TiN (≈10 nm) for all electrodes and ferroelectric layers optimizes interfacial stress and suppresses morphological roughness, both crucial for stack reliability and ferroelectric phase stability.

A rapid thermal anneal (RTA) is employed post-deposition to crystallize the desired orthorhombic ferroelectric phase, with careful patterning of the bottom, middle, and top TiN electrodes forming the two-tier crosspoint architecture. This fabrication methodology circumvents the usual polarization degradation arising from thickness downscaling and interface roughness, enabling reliable multilayer stacking.

Complementary Polarization Scheme and Operation

The operation of the VCF is predicated on a complementary polarization scheme: writing logic '1' establishes an "up-down" polarization across the top-bottom stack, while logic '0' uses a "down-up" configuration. Readout is performed differentially, summing the polarization of both layers, which in effect doubles the measurable charge window without incurring an area penalty or increasing the switching field required by each individual layer.

This architecture closely parallels the complementary FET (CFET) paradigm known for 3D monolithic integration in logic, where vertical stacking and symmetric operation mitigate the scaling bottlenecks of planar configurations. The precise tailoring of write/read pulse trains ensures stable, symmetric polarization switching across both layers, leading to well-matched P-V hysteresis loops and robust differential sensing margins.

Performance Metrics

Polarization and Memory Window

The VCF achieves a measured $2P_r > 100$ µC/cm²—substantially higher than conventional single-layer or optimally-doped HfO$_2$-based FeRAM devices, where the $2P_r$ ceiling has typically remained below ≈ 70 µC/cm² even with interface engineering, dopant incorporation, and alternative electrode stacks. Notably, this high polarization is maintained post $10^{10}$ switching cycles (with $P_r$0 µC/cm²), evidencing minimal fatigue and high operational robustness.

Switching Dynamics and Power Efficiency

The differential scheme directly impacts switching kinetics. Target polarization states (e.g., $P_r$1 µC/cm²) are accessible at reduced pulse widths and lower voltages versus single-layer FeRAM, indicating a significant reduction in switching energy and improved scalability for ultra-dense arrays. For instance, a $P_r$2 µC/cm² state, requiring ≈6 V in a single-layer device, is achieved at ≈2.5 V in the VCF architecture. Thus, overall energy consumption per bit operation is strongly reduced, which is central for high-density memory where cumulative power becomes a limiting constraint.

Reliability: Endurance, Disturb Immunity, and Retention

The VCF demonstrates endurance in excess of $P_r$3 P/E cycles without electrical breakdown—a superior result compared to many contemporary HfO$P_r$4 FeRAM reports. Disturb tests (via V/3 biasing schemes consistent with crosspoint array operation) show that VCF cells retain $P_r$5 µC/cm² after $P_r$6 disturb pulses, indicative of robust disturb immunity and suitability for selector-free array topologies. Additionally, retention characteristics at 85°C for > $P_r$7 s confirm thermal stability appropriate for commercial-grade memory retention requirements.

Scalability: Selector-Free Array Demonstration

To validate array-level integration and uniformity, a 5x5 selector-free crosspoint VCF array was fabricated and characterized. Uniform switching polarization ($P_r$8 µC/cm²) and consistent operation across all devices affirm the architectural viability for large-scale integration, addressing array disturb and cell-to-cell variability, which are recurrent challenges in dense FeRAM arrays.

Comparative Benchmarking

When benchmarked against state-of-the-art FeRAM implementations—considering both single and dual-layer configurations, material systems, and process enhancements—the VCF device presents a distinct advantage in terms of $P_r$9, operational endurance, and immunity. Competing approaches (e.g., interface/dopant engineering, epitaxial oxide electrodes, process composition modulation) have yielded incremental polarization improvements but remain limited by the intrinsic properties of single-layer HfO$_2$0. The co-design of architecture and operation in VCF overcomes these ceilings while remaining scalable and process-compatible.

Implications and Future Prospects

Practically, the VCF architecture unlocks avenues for aggressive three-dimensional scaling of nonvolatile memory, circumventing typical area versus performance trade-offs inherent to planar and pseudo-3D FeRAM. The complementary-dipole scheme provides a framework for further extension into deep-trench configurations, orthogonally boosting storage density. The demonstrated endurance and disturb immunity indicate readiness for embedded and stand-alone memory deployment, especially in AI accelerators and edge computing ICs where low-power operation and array scalability are paramount.

From a theoretical perspective, the work challenges conventional limits on achievable polarization in HfO$_2$1 ferroelectrics, suggesting that architecture-level co-optimization (stacking, polarization symmetry, and interface design) is a compelling path forward, complementary to ongoing materials and process innovations. Extensions to more complex stack schemes (triple- or quadruple-layer vertical cells), or hybrid FeRAM/CMOS logic integration, could further enhance nonvolatile memory functionality in advanced nodes.

Conclusion

The fully ALD-grown vertical complementary FeRAM introduced in this study demonstrates ultra-high $_2$2 µC/cm², high endurance exceeding $_2$3 cycles, robust thermal retention, and strong disturb immunity, all within a compact selector-free crosspoint array. Through synergy between deposition process, device structure, and complementary operation, the VCF approach transcends prior limits set by single-layer HfO$_2$4 FeRAM, offering both practical and conceptual advances for the next generation of high-density, reliable nonvolatile memory integration (2604.15131).