Fabrication and Characterization of Thermally Oxidized Tungsten-Based Thin Films for Application in Cold Field Emission Sources

article in a collection out of WoS and Scopus

English

Thermal oxidation is a methodology employed to generate thin oxide coatings on surfaces. Oxide coatings have demonstrated utility across diverse applications, including cold field electron emission sources. In the realm of commercial microscopy applications, tungsten remains a choice for cold field emission purposes [1] [2]. Its desirable attributes can be further enhanced through the deposition of a thin layer. This not only helps emission characteristics but also extends the operational lifespan of the emitter. Tungsten oxide, recognized for its exceptional resistance to corrosive agents and chemical assaults, owes its durability to its stable crystal lattice and the robust chemical bonds formed between tungsten and oxygen atoms. Several techniques have been employed to achieve this objective, each with distinct advantages and drawbacks [1] [2]. For this work, termooxidation emerged as the chosen method due to its capacity for achieving uniform material coverage and its accessibility without the necessity for sophisticated and costly equipment. Temperature initiates the formation of a thin layer of tungsten oxide on the metal's surface. The thickness and properties of the resultant oxide layer can be controlled by manipulating parameters, such as the temperature, pressure, and duration of the process. Initially, polycrystalline tungsten planar squares were subjected to thermal oxidation under relatively high pressures of 80 Torr, and by a lower pressure of 10 Torr. Their chemical composition and resulting roughness were compared. It became evident that the lower pressure significantly reduced roughness and thus this pressure was retained for further experiments. For these samples, three temperatures were chosen: 550, 650, and 750 °C. These choices were inspired by thermal oxidation analyses from paper [3] to get reasonable thickness. The thermal oxidation procedure encompassed the following steps: chamber pre-cleaning with oxygen for approximately 10 minutes at a higher pressure than the intended one (for 10 Torr working pressure, pre-cleaning occurred at 30 Torr). Then, a 200-minute preheating phase to the desired temperature under the low chamber pressure (around 2 Torr), followed by a 30 min treatment at the chosen pressure (10 Torr). Followed by cooling, reaching around 220 °C after 200 minutes and gradually decreasing to room temperature (completely approximately 350 minutes depending on the selected temperature). To preliminarily determine the suitability of these tungsten oxide layers on electron emitters for use in electron guns and other high-quality electron-emitting devices, we conducted a series of tests to assess morphology and chemical composition. When applying layers to cold field emission cathode tips, it is important for the coating to be relatively uniform and possess lower roughness. As our goal is to achieve a non-conductive thin layer, we would prefer a higher occurrence of WO3 over WO2. Thermal oxidation generally leads to reduced roughness when the initial sample has a rougher surface [4]. However, given that our cold field emitters tend to have lower roughness, it is reasonable to expect an increase in roughness compared to reference after thermal oxidation, as was observed. For a pressure of 80 Torr, a roughness was Ra = 324 nm and for 10 Torr only Ra = 227 nm was achieved, which is significantly lower.

Thermal Oxidation, Tungsten Tips, XPS, AFM, Cold Field Emmision

KOŠELOVÁ, Z.; KNÁPEK, A.; FOHLEROVÁ, Z.

20. 11. 2023

Comenius University

Bratislava, Slovakia

978-80-223-5713-5

Extended Abstract Book of 8th conference SURFINT-SREN VIII

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