Synthesis and Characterization of Mo Precursors for ALD process and Li2S for Solid Electrolytes

Abstract

Part 1. In recent times, molybdenum has emerged as a promising alternative to copper and tungsten in wiring applications, owing to its favorable combination of low resistance and cost-effectiveness when compared to precious metals like gold, silver, ruthenium, and rhodium. Notably, copper and tungsten wiring face challenges related to increased resistance values due to electro-migration in microstructures measuring just a few nanometers. As a result, there is an urgent need for the development of metals and corresponding technologies that can serve as effective substitutes. The industrial production of such metal wiring predominantly relies on various vapor deposition methods. Particularly, after vapor depositing a volatile, complex compound containing a metal onto a thin film, a specific chemical reaction ensues involving various reactive gases, the thin film, and the metal complex compound. Subsequently, a vapor deposition process known as ALD (Atomic Layer Deposition), capable of producing atomic-scale compounds ranging from a few Å to several nanometers, is commonly employed. The ALD process involves vaporizing the organometallic compound by heating the container, in which it is stored, at a temperature of approximately 100 to 110°C for an extended period before deposition onto the substrate, and transferring it to a gas phase. Representative molybdenum complexes utilized in this ALD process include inorganic complexes coordinated with halogen elements like MoCl5, MoO2Cl2, or Mo(CO)6, along with carbon monoxide, and organic ligands such as amides, imides, and beta-ketonates. Examples of organic molybdenum complexes are Mo(NR)2(NR2)2, MoO2thd2, Mo(NtBu)2(dpamd)2, etc. During the process of vaporizing the solid-state precursor mentioned above and transitioning it to the gas phase, spontaneous intermolecular reactions occur, leading to the generation of multi-component compounds. This not only complicates the control of thin film thickness but also hinders the attainment of thin films with excellent physical properties. Consequently, this study delves into molybdenum liquid precursors to produce high-purity thin films and maintain a constant vapor pressure under the aforementioned conditions. In Chapter 2, a liquid precursor MoTHD(2-x)Cl(4-x), where x=1 or 2) was synthesized by reacting THD with MoCl5. The precursor's chemical properties were confirmed through 1H-NMR and FT-IR, while purity and thermal stability were assessed using ICP-OES and TGA analysis, respectively. The Cl group in precursors 1 and 2 was replaced with NR2 (R=Et, iPr, SiMe3) to address halide drawbacks, resulting in three liquid precursors. TGA analysis revealed that the precursor substituted with NiPr2 exhibited favorable properties. In Chapter 3, Mo(CpNMe)(NR2)2 was synthesized to overcome oxygen content challenges in ALD processes. Chemical properties were confirmed, purity assessed, and thermal stability verified for three synthesized precursors. Only the NiPr2-containing compound exhibited clean evaporation with stable vapor pressure increase. Chapter 4 describes the deposition of a MoNx thin film by ALD using the synthesized MoTHDCl3(THF) precursor. Two deposition methods were employed, and the resulting thin film was characterized using various techniques, revealing successful deposition at 500°C in the supercycle process. The study suggests that the developed precursor could lead to superior Mo2N film characteristics for potential applications in memory and logic devices. In summary, Chapters 2 and 3 detail the synthesis and characterization of Molybdenum liquid precursors for ALD processes, addressing challenges related to halides and oxygen content. Chapter 4 demonstrates the successful deposition of a MoNx thin film using the synthesized precursor, with a focus on process optimization and characterization techniques. The findings suggest the potential application of a superior Mo2N film in future electronic devices.

Part 2. Recently, the demand for high-energy/high-capacity batteries has surged with the advancement and widespread adoption of electric vehicles (EVs), high-performance portable devices, and energy storage systems (ESS). Consequently, extensive global research on next-generation batteries is actively underway. Lithium-sulfur batteries are prominently featured among these studies due to their high theoretical specific energy and the abundance of raw materials. However, practical applications face challenges. The sulfur anode, while having high theoretical potential, exhibits low electrical conductivity and inefficiency in practical usage. An alternative material to address these issues is lithium sulfide, although its high production cost has prompted ongoing industry-wide process developments. Existing methodologies primarily employ mechanical alloying (mechanical ball milling) in several stages, hindering the attainment of optimized morphology and productivity. In response to these challenges, this study focuses on the liquid process manufacturing of lithium sulfide to control particle size, aiming for efficient production by enhancing morphology and distribution within the electrolyte. In Chapter 2, lithium sulfide (Li2S) was synthesized through the reaction of (NH4)2S with lithium hydroxide (LiOH) in methanol solvent, followed by pyrolysis. The chemical properties of the resulting Li2S were confirmed using 7Li-NMR, and XRD was employed to assess crystallinity and chemical properties. The use of high-boiling-point solvents in the heat treatment process improved purity compared to conventional methods, although the reaction of residual ethanol and LiSH led to the formation of LiOEt, causing a decline in purity. To overcome this issue, LiH was introduced to suppress LiOEt production, and efficient heat treatment using a high-boiling-point solvent was achieved, as confirmed by TGA results indicating the absence of LiOEt generation. In Chapter 3, various lithium sources with reducing power were utilized to synthesize Li2S at lower temperatures using different thiourea sources, presenting a method to reduce the reaction temperature for Li + S reactions. Particle size analysis and purity assessments revealed an oxygen content of around 2%, suggesting potential applications in diverse fields, including batteries. Chapter 4 focused on synthesizing the sulfide-based solid electrolyte, argyrodite (Li6PS5Br), using Li2S produced in Chapter 2, 3. Crystallinity and chemical properties were evaluated using XRD and Raman spectroscopy, and TEM analysis indicated variations in the solid electrolyte size with the particle size of the synthesized Li2S. Electrochemical current spectroscopy measured a conductivity of 1.1 mS/cm, suggesting potential applications in various fields for the synthesized lithium sulfide. In summary, Chapters 2 and 3 detail the synthesis and characterization of Lithium sulfide for solid Sulfide electrolyte, addressing challenges related to price and new methods. Chapter 4 demonstrates the successful preparation of argyrodite using the synthesized Li2S, with a focus on process optimization and characterization techniques. The findings suggest the potential application of solid sulfide electrolyte in future battery materials.