Wake-Up of Ferroelectric HZO Films -- Towards a BEoL Compatible Wake-Up Free Material

The discovery of ferroelectricity in thin doped hafnium oxide films has revived interest in the concept of ferroelectric (FE) memory. Zirconium-doped hafnium oxide (HZO) crystallizes at low temperatures (400°C and below), making this material interesting for back-end-of-line (BEoL) implementation. Metal-FE-Metal (MFM) capacitors are critical building blocks for the realization of BEoL-compatible FE memory concepts. Located in the BEoL, they can be connected to either the gate or drain of a standard logic device to realize a one transistor-one capacitor (1T1C) FE field effect transistor (FeFET) or a 1T1C FE random access memory (FeRAM), respectively. [1]<br/>Since its discovery, researchers have made great efforts to improve important properties of hafnium oxide-based ferroelectrics, such as remanent polarization, endurance, retention, imprint, and wake-up. Typical approaches include, but are not limited to, the investigation of different dopant elements, dopant concentrations, film thicknesses, and stacking options such as interface/electrode materials and superlattice structures.<br/>While much progress has been made, certain issues, such as wake-up, remain challenging. Wake-up is the opening of the initially pinched polarization versus electric field hysteresis loop, which leads to a gradually increasing remanent polarization during field cycling until saturation. Since memory applications require stable polarization, an initial field cycling ("wake-up") is necessary. However, the need for this wake-up makes the realization of hafnium oxide based FE memories more complex and reduces the maximum number of endurance cycles. Therefore, a wake-up-free material would be highly desirable.<br/>Several theories have been proposed to explain the wake-up effect. These theories can essentially be summarized as follows [2]: (i) defect redistribution, which reduces internal bias fields and promotes the depinning of domain walls, (ii) defect redistribution and subsequent stabilization of the orthorhombic phase ("field-induced phase transformation"), and (iii) 90° domain wall motion ("ferroelastic switching").<br/>The wake-up effect of FE-HZO films could be mitigated by: (i) using mesa-etched test structures [3]; (ii) reducing the number of defects and carbon contamination by applying longer ozone pulses durations during the atomic layer deposition process [4]; (iii) interface engineering by plasma treatment [5] or epitaxial growth [6].<br/>Herein, we investigate the effect of post-deposition annealing temperature, film stoichiometry, film thickness, and co-doping on the wake-up characteristics of FE HZO films embedded in MFM capacitors. Several structural and electrical characterization techniques, such as X-ray diffraction and polarization versus electric field measurements, are applied to obtain a comprehensive overview. The goal is to present a BEoL-compatible material stack with significantly reduced wake-up without compromising other important parameters such as remnant polarization and reliability.<br/><br/>[1] D. Lehninger et al., <i>2021 IEEE IITC</i>, Kyoto, Japan, 2021, pp. 1-4.<br/>[2] M. Lederer et al., 2021, PSS RRL, 15: 2100086.<br/>[3] J. Bouaziz et al., ACS Appl. Electron. Mater. 2019, 1, 9, 1740–1745<br/>[4] A. Kashir et al., Adv. Eng. Mater., 23: 2000791.<br/>[5] K. -Y. Chen et al., 2017 VLSI Circuits, Kyoto, Japan, 2017, pp. T84-T85<br/>[6] J. Lyu et al., ACS Appl. Electron. Mater. 2019, 1, 2, 220–228