Electrode\nElastic\nModulus as the Dominant Factor in\nthe Capping Effect in Ferroelectric Hafnium Zirconium Oxide Thin Films

The discovery of\nferroelectricity in hafnia based thin\nfilms has\ncatalyzed significant research focused on understanding the ferroelectric\nproperty origins and means to increase stability of the ferroelectric\nphase. Prior studies have revealed that biaxial tensile stress via\nan electrode “capping effect” is a suspected ferroelectric\nphase stabilization mechanism. This effect is commonly reported to\nstem from a coefficient of thermal expansion (CTE) incongruency between\nthe hafnia and top electrode. Despite reported correlations between\nferroelectric phase fraction and electrode CTE, the thick silicon\nsubstrate dominates the mechanics and CTE-related stresses, negating\nany dominant contribution from an electrode CTE mismatch toward the\ncapping effect. In this work, these discrepancies are reconciled,\nand the origin of these differences deriving from electrode elastic\nmodulus, not CTE, is demonstrated. Pt/<i>M</i>/TaN/Hf<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub>/TaN/Si devices, where <i>M</i> is platinum, TaN, iridium, tungsten, and ruthenium, were\nfabricated. Sin<sup>2</sup>(ψ)-based X-ray diffraction measurements\nof biaxial stress in the HZO layer reveal a strong correlation between\nbiaxial stress, remanent polarization, and electrode elastic modulus.\nConversely, a low correlation exists between the electrode CTE, HZO\nbiaxial stress, and remanent polarization. A higher elastic modulus\nenhances the resistance to electrode elastic deformation, which intensifies\nthe capping effect during crystallization, and culminates in the tandem\nrestriction of out-of-plane hafnia volume expansion and preferential\norientation of the polar <i>c</i>-axis normal to the plane.\nThese behaviors concomitantly increase the ferroelectric phase stability\nand polarization magnitude. This work provides electrode material\nselection guidelines toward the development of high-performing ferroelectric\nhafnia into microelectronic devices, such as nonvolatile memories.