Ferroelectric Properties of HfO2-Based Thin Films and Their Application for Low Power and Scalable Ferroelectric Tunnel Junctions

• Main Author: Shekhawat, Aniruddh Singh
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2021
• Subjects: Materials science; Mechanical engineering; Nanotechnology
• ISBN: 9798380309721

Ferroelectric random-access memories (FRAM) based on conventional perovskite materials are non-volatile with fast read/write (ns access times) operations but suffer from lack of CMOS compatibility, scalability limitation, and a destructive reading scheme. Therefore, the FRAM R&D projects were discontinued after the 130 nm and 90 nm technology nodes in the mid-2000s. Recently, ferroelectricity is discovered in Si-doped HfO2 thin films for low doping concentration of Si (2.5-4.0 %) after annealing the films in the presence of the TiN capping layer. Several other dopants such as Zr, Al, Y, La, Sr, and Gd have also been identified to induce ferroelectricity in the HfO2 thin films. Ferroelectric HfO2-based thin films are CMOS compatible and an attractive candidate for future non-volatile memory (NVM) technology such as Ferroelectric tunnel junctions (FTJ) and Ferroelectric field-effect transistors due to their simple structure, scalability, low power consumption, high operation speed, and non-destructive readout operations.In the following study, ferroelectricity of solid solution of hafnium zirconium oxide (HfxZr1-xO2), is examined as a function of film thickness, annealing temperature, and ambient with careful consideration given to Back-end-of-line (BEOL) device integration and device applications. Among all the dopants previously mentioned, HfxZr1-xO2 has been explored in this study due to its relatively lower annealing temperature, ferroelectricity over a broad composition range (most ferroelectric when x=0.5) and for DRAM mass production, both HfO2 and ZrO2 have already been utilized. Therefore, it emerges as one of the most attractive candidates for integration in non-volatile memory device applications such as FTJ.It is shown that Hf0.5Zr0.5O2 thin films down to 5 nm in thickness can be crystallized in a desired ferroelectric orthorhombic phase by rapid thermal annealing (RTA) at 500˚C for 20 s under N2 ambient. Additionally, phase transition kinetics study is performed on RTA crystallized films after furnace annealing at different temperatures and under different ambient conditions to mimic the temperature profile during the formation of the interconnect structure, where temperatures up to 500˚C are subjected several times that last from minutes to hours. The prolonged furnace anneals (500˚C, 1 hour, N2 ambient) of the RTA crystallized films of varying thicknesses (4 nm to 11 nm) exhibit an increase in grain growth particularly for the thinner films. While furnace annealing did not have much impact on the ferroelectricity of the 11 nm film, it enhanced the ferroelectricity of the 4.5 nm film by more than 200 % and induced ferroelectricity in the 4 nm RTA annealed film that was paraelectric (i.e. nonferroelectric) before extended furnace annealing. These unique effects of furnace annealing depending on the film thickness are explained using material and electrical characterization techniques. One critical process integration issue with ferroelectric materials based on perovskite structure (SBT, PZT, etc) is the use of low temperature (∼400°C–550°C) forming gas (FG) anneal. FG anneal is used to passivate dangling bonds at the Si/Si02 interface and to reduce interface-trapped charges. Unfortunately, hydrogen damages both the PZT and SBT capacitors, and they lose their ferroelectric characteristics. By switching the furnace annealing ambient from N2 to FG (i.e. N2 (96%) + H2 (4 %) ), we first time demonstrate full compatibility of the ferroelectric Hf0.5Zr0.5O2 thin films against FG annealing (i.e. an increment in the remnant polarization (Pr) with lower leakage current and improved wake-up effect without degradation of Pr). Two different strategies have been addressed to integrate ferroelectric HfO2-based thin films in FTJ. Interface-engineering approach based on the H2-plasma surface treatment of Ge has been shown to improve upon the tunneling electroresistance effect and data retention of the bilayer oxide heterostructure (Al2O3/Hf0.5Zr0.5O2) FTJ. Integration of a thick ferroelectric layer (Hf0.5Zr0.5O2) with a thin interface layer (Al2O3) resulted in the highest ON/OFF ratio of the FTJ device. Furthermore, the effect of Hf0.5Zr0.5O2 and Al2O3 films, and Ge-Al2O3 interfacial property on the ON/OFF ratio is explained using time-of-flight secondary ion mass spectrometry in-depth profiling mode. The study also provides evidence of strong coupling between Hf0.5Zr0.5O2 and Al2O3 films in controlling the ON/OFF ratio of the FTJ. The present results are expected to provide new insights into integrating ferroelectric hafnium oxide-based films for future low power and scalable NVM applications.