Origin of Ferroelectric Phase Stabilization via the Clamping Effect in Ferroelectric Hafnium Zirconium Oxide Thin Films

Abstract

The presence of the top electrode on hafnium oxide‐based thin films during processing has been shown to drive an increase in the amount of metastable ferroelectric orthorhombic phase and polarization performance. This “Clamping Effect,” also referred to as the Capping or Confinement Effect, is attributed to the mechanical stress and confinement from the top electrode layer. However, other contributions to orthorhombic phase stabilization have been experimentally reported, which may also be affected by the presence of a top electrode. In this study, it is shown that the presence of the top electrode during thermal processing results in larger tensile biaxial stress magnitudes and concomitant increases in ferroelectric phase fraction and polarization response, whereas film chemistry, microstructure, and crystallization temperature are not affected. Through etching experiments and measurement of stress evolution for each processing step, it is shown that the top electrode locally inhibits out‐of‐plane expansion in the HZO during crystallization, which prevents equilibrium monoclinic phase formation and stabilizes the orthorhombic phase. This study provides a mechanistic understanding of the clamping effect and orthorhombic phase formation in ferroelectric hafnium oxide‐based thin films, which informs the future design of these materials to maximize ferroelectric phase purity and corresponding polarization behavior.

Understanding the Effect of Top Electrode on Ferroelectricity in Atomic Layer Deposited Hf0.5Zr0.5O2 Thin Films

“…It is reported that the stress of the HZO thin films is related to the coefficients of thermal expansion mismatch with the employed electrodes. , The Figure c summarizes the 2Pr (with 4 MV/cm electric field) of different electrode devices and the corresponding thermal expansion coefficient. It can be seen that the thermal expansion coefficient is 4.5 × 10 –6 /°C, 5.2 × 10 –6 /°C, and 8.5 × 10 –6 /°C for W/Mo/ITO electrodes, respectively.…”

Asymmetric Electrode Work Function Customization via Top Electrode Replacement in Ferroelectric and Field‐Induced Ferroelectric Hafnium Zirconium Oxide Thin Films

“…The relative volume fraction of t + o phases (f t+o ) can be calculated through comparison between the integrated intensity of the Pearson VII shape fit to the t + o peak and the integrated intensities of shapes fit to all t + o and monoclinic peaks, as conducted in prior work. [46] These fractions were determined to be 0.97 ± 0.05, 0.94 ± 0.09, and 0.95 ± 0.04, for the films processed with TaN, Pt, and W, respectively. The PMA sample processed with an Hf top electrode has comparatively little orthorhombic phase, as evidenced by the large total intensity from the (111) and (111) monoclinic (m 1 and m 2 , respectively) diffraction peaks and low f t+o of 0.33 ± 0.05.…”

“…Pt [64] 5.65 0.90 with observations in previous work. [46] Specifically, the decrease in diffraction angle is resultant from a reduction in the biaxial tensile stress state of the HZO and the transformation of 5% of the volume of the film from the t + o phases to the monoclinic phase. This phase transformation occurs because the removal of the top electrode allows for local volumetric expansion and formation of the high molar volume monoclinic phase.…”

“…This dependence is related to mechanical boundary conditions and chemical interactions that occur between the HfO 2 -based film and electrode layers during thermal processing. [41,[44][45][46][47] As such, the majority of investigations that have examined the ferroelectric properties of HfO 2 -based MIM devices have employed binary nitride electrodes. [1,4,43,45,[48][49][50][51] These materials provide a combination of high cycles-tobreakdown during field cycling [52] and high film polarization responses [41,42,48] due to the presence of large volume fractions of the ferroelectric orthorhombic phase following thermal processing.…”

Ferroelectric Orthorhombic ZrO2 Thin Films Achieved Through Nanosecond Laser Annealing

“…[10,11,12] One of the most promising strategies used to stabilize the orthorhombic (o-) phase and enhance the ferroelectric properties of HZO films was the use of tensile stress. [13,14,15] Different tensile stress levels can be obtained by the appropriate choice of the electrodes, since the difference in the thermal expansion coefficient (TEC) of HZO and the electrode is one of the important origins of tensile stress when the films are cooled down to room temperature.…”