Abstract
Ferroelectric hafnium zirconium oxide (HZO) has emerged as an attractive candidate for next-generation non-volatile memory technologies, owing to its scalability, large remanent polarization, and compatibility with complementary metal-oxide-semiconductor (CMOS) materials and back-end-of-line (BEOL) processes. However, realizing its industrial integration potential requires a comprehensive understanding of the interwoven mechanisms that govern stabilization of the non-equilibrium polar orthorhombic phase of HZO responsible for ferroelectricity. This thesis addresses these challenges through five complementary studies that elucidate the interdependencies within the process-property-structure paradigm of HZO and, through careful analysis, position HZO as a leading material for next-generation microelectronic applications.
First, two deposition techniques are discussed for fabricating HZO thin films. Atomic layer deposition is used extensively throughout this work as a robust, conformal growth process with precise stoichiometric control. Complementarily, RF magnetron sputtering is investigated as a viable and industrially attractive alternative, given the widespread use of sputter tools in semiconductor foundries and the breadth of accessible deposition parameters. Despite these practical advantages, a knowledge gap remains regarding how process conditions govern phase stability and electrical performance when using a stoichiometric ceramic target. Through in-situ investigation of ion energies during deposition, this work demonstrates that processing pressure is a powerful knob for tuning the sputter environment and, in turn, the macroscopic film microstructure, phase assemblage, and ferroelectric response. These results provide a comprehensive characterization for further research and actionable guidelines for integrating RF sputtered HZO into fabrication facilities.
Ferroelectric HZO films are conventionally investigated in metal-ferroelectric-metal geometries, where the HZO is sandwiched between two electrode layers, enabling electrical characterization. Electrode material selection is a well-established driver of ferroelectric phase stabilization, measured remanent polarization, and endurance, due in part to its presence during crystallization annealing, which upholds biaxial stress within the HZO layer, known as the “capping effect.” This work examines the influence of top electrode material selection on ferroelectric phase stabilization. It identifies the material properties governing this stress state and provides clear electrode selection criteria for applications requiring large remanent polarization.
With processing and electrode design established, electrical characterization provides a crucial link in the structure-process-property relationship of HZO thin films. However, device endurance’s dependence on the field-cycling waveform complicates this understanding. In this study, endurance and polarization stability as a function of field-cycling waveform pulse duration are investigated, revealing a time-under-field-dependent degradation mechanism with significant implications for endurance testing design and interpretation. These results demonstrate that pulse duration is a critical yet underappreciated variable, with shorter pulses yielding dramatically improved endurance, informing device integration where nanosecond pulses are anticipated.
Although waveform-dependent degradation is observed, the underlying mechanisms remain unknown. To elucidate the conduction mechanisms, two complementary studies are conducted, isolating the contributions of total field cycles and total time under field. Quasi-static current density measurements reveal a distinct evolution of the dominant conduction mechanism, providing mechanistic insight into how and why degradation progresses with increasing pulse duration. From this, the waveform features that drive degradation are revealed. Furthermore, a comprehensive modeling procedure for potential dielectric conduction mechanisms highlights the evolution of defect-driven conduction under field cycling conditions. These results reveal the charge state and spatial distribution of oxygen vacancies as critical engineering targets for extending device endurance.
Finally, HZO is incorporated into ferroelectric tunnel junction devices, utilizing its large switchable polarization for non-volatile memory. The stability of this polarization over time is evaluated through a series of electrical tests that differentiate between true ferroelectric properties and external interface effects. These findings shed light on the retention mechanisms specific to this device structure and offer valuable insights for circuit design aimed at addressing resistance state drift in practical memory applications.
Collectively, this work provides a cohesive set of guidelines for materials selection and processing, waveform design criteria, and device-level understanding towards the realization of HZO-based ferroelectric memory technologies that meet the performance and reliability demands of next-generation microelectronics.
