Combining Superlattices with Co-Doping -- Toward BEoL Compatible Ferroelectric Materials with Improved Reliability at Bias and Temperature Stress

Interest in the concept of ferroelectric memory has been revived by the discovery of ferroelectricity in thin doped hafnium oxide films. Zirconium doped hafnium oxide (HZO) crystallizes at low temperatures (400°C and below), which makes it a compelling material for back-end of line (BEoL) implementation. Metal-ferroelectric-metal (MFM) capacitors are essential building blocks for realizing BEoL-compliant ferroelectric memory concepts. Placed in BEoL, these devices can be connected to the gate or drain contact of a standard logic device to achieve a one-transistor-one-capacitor (1T1C) ferroelectric field-effect transistor (FeMFET) or a 1T1C FeRAM, respectively. [1]<br/><br/>Since the discovery ferroelectricity in hafnium oxide, scientists have been working to improve key properties of these materials, including remanent polarization, endurance, retention, imprint, and wake-up. Common methods of improvement include exploring different dopants, dopant concentrations, film thicknesses, and stacking options such as interface/electrode materials and superlattices [2].<br/><br/>Despite significant advancements, some reliability challenges persist, such as operation under high bias and temperature stress (BTS). Especially the automotive industry's strict demands for resilience and reliability under challenging BTS conditions exacerbate the issue. Currently, the Automotive Electronic Council's quality requirements for Integrated Circuits (AEC-Q100) serve as the minimum standard for car manufacturers. Fulfilling the Grade 0 specifications, which demand stable performance within the temperature range of -40°C to 150°C, presents a major challenge for ferroelectrics with a fluorite structure.<br/><br/>Recently, two methods have been reported that significantly improve the stability under BTS: (1) using ferroelectric [HfO₂/ZrO₂] superlattices with relatively thick sublayer thicknesses [3], and (2) co-doping, which introduces a small amount of an additional dopant [4].<br/>Herein, we utilize both techniques to achieve optimal reliability properties. Analytical and electrical methods were used to characterize co-doped [HfO₂/ZrO₂] superlattices with various stacking options. To gain a deeper understanding of the structural properties, X-ray diffraction (XRD) and time-of-flight secondary-ion mass spectrometry (ToF-SIMS) were conducted. Additionally, polarization versus electric field characteristics were measured to explore the electrical properties under various BTS conditions. Finally, endurance and retention characteristics under BTS conditions will be utilized to evaluate compatibility with the requirements specified by the automotive industry.<br/> <br/>[1] D. Lehninger et al., <i>2021 IEEE IITC</i>, Kyoto, Japan, 2021, pp. 1-4.<br/>[2] F. Ali et al., Adv. Funct. Mater. 2022, 32, 2201737<br/>[3] D. Lehninger et al., <i>2023 IEEE IMW</i>, Monterey, USA, 2023, pp. 1-4. <br/>[4] A. Sünbül et al., Adv. Funct. Mater. 2023, submitted.