Electrode Elastic Modulus as the Dominant Factor in the Capping Effect in Ferroelectric Hafnium Zirconium Oxide Thin Films

Abstract

The discovery of ferroelectricity in hafnia based thin films has catalyzed significant research focused on understanding the ferroelectric property origins and means to increase stability of the ferroelectric phase. Prior studies have revealed that biaxial tensile stress via an electrode "capping effect" is a suspected ferroelectric phase stabilization mechanism. This effect is commonly reported to stem from a coefficient of thermal expansion (CTE) incongruency between the hafnia and top electrode. Despite reported correlations between ferroelectric phase fraction and electrode CTE, the thick silicon substrate dominates the mechanics and CTE-related stresses, negating any dominant contribution from an electrode CTE mismatch toward the capping effect. In this work, these discrepancies are reconciled, and the origin of these differences deriving from electrode elastic modulus, not CTE, is demonstrated. Pt/M/TaN/Hf_0.5Zr_0.5O₂/TaN/Si devices, where M is platinum, TaN, iridium, tungsten, and ruthenium, were fabricated. Sin²(ψ)-based X-ray diffraction measurements of biaxial stress in the HZO layer reveal a strong correlation between biaxial stress, remanent polarization, and electrode elastic modulus. Conversely, a low correlation exists between the electrode CTE, HZO biaxial stress, and remanent polarization. A higher elastic modulus enhances the resistance to electrode elastic deformation, which intensifies the capping effect during crystallization, and culminates in the tandem restriction of out-of-plane hafnia volume expansion and preferential orientation of the polar c-axis normal to the plane. These behaviors concomitantly increase the ferroelectric phase stability and polarization magnitude. This work provides electrode material selection guidelines toward the development of high-performing ferroelectric hafnia into microelectronic devices, such as nonvolatile memories.

Keywords: HZO; capping effect; elastic modulus; ferroelectric; thin film.

Figure 1

Schematic of the metal-ferroelectric-metal geometries of the devices fabricated with the investigated electrode materials. The multicolored layer represents the five different electrode materials investigated.

Figure 2

Grazing incidence X-ray diffraction patterns taken from 26 to 33° 2θ angle, with indexing of reflections characteristic of the equilibrium monoclinic phase at 28.5 and 31.8°, and nonequilibrium o-III, o-I, and tetragonal phases at nominally 30.4° for samples prepared with each of the investigated top electrode materials.

Figure 3

Measured P(E) loops taken at 2.5 MV cm^–1 field at decade intervals from the pristine state to 10³ cycles using a 2.0 MV cm^–1 field 1 kHz square cycling wave for (a) platinum, (b) iridium, (c) TaN, (d) ruthenium, and (e) tungsten, respectively. (f) Remanent polarization measured from PUND measurements taken at each cycling interval and plotted against the measured elastic modulus of each material. The color bar on the right indicates the number of field cycles.

Figure 4

(a) Sin²(ψ)-derived HZO biaxial stress plotted versus the measured top electrode elastic modulus for each electrode material. (b) Awoken P_r extracted from PUND versus the top electrode elastic modulus.

Figure 5

Schematic depiction of devices fabricated with a tungsten (larger E) top electrode exhibiting a greater capping effect, resulting in more significant biaxial stress within the HZO layer and concomitant preferential c-axis out-of-plane o-III phase ferroelastic texture, and a platinum top electrode (lesser E) resulting in an opposing trend. The length of the arrows depicts the magnitude of stresses.

Figure 6

(a) Sin²(ψ) HZO biaxial stress plotted versus reported⁻ and measured linear CTE values for each electrode material. (b) Awoken P_r versus top electrode linear CTE.

Figure 7

Compilation of reported remanent polarization values of devices fabricated with differing top electrode materials, plotted with respect to the electrode elastic moduli and linear CTEs.^,,−,−