Radiation induced surface modification and contamination for EUV lithography and fusion applications

Al-Montaser Bellah Al-Ajlony, Purdue University

Abstract

Al-Montaser Bellah Al-Ajlony. Ph.D., Purdue University, May 2014. Radiation Induced Surface Modification and Contamination for EUV Lithography and Fusion Applications. Major Professor: Ahmed Hassanein. The effect of ionizing radiation on materials surfaces is of major interest for many engineering applications. The importance of this topic rises from the severity of the implications that a surface at a certain application might suffer due its interaction with some sort of ionizing radiation. The severity of implication is not always related to the severity of the radiation, in many applications the concern comes from the over-sensitivity of the surface to a low doses of radiations. One example of these sensitive applications is the extreme ultraviolet (EUV) induced surface contaminations of the optics in EUV lithography devices. In this application, a small dose of ionizing radiation (EUV at 13.5 nm wavelength) can cause slight change in the chemical composition of the irradiated surface. This change in chemical composition can cause large change in the surface optical properties of the irradiated surface (EUV optics). This degradation in reflectivity is an issue that needs to be avoided. On the other extreme where intense radiation is implemented, the main concern of the radiation-surface interaction comes from the severity of the irradiation process. The plasma-facing component (PFC) in future thermonuclear devices represent the ultimate example where the materials might be exposed to severe irradiation processes. Under such extreme irradiation processes, some candidate PFC materials exhibit the formation of very fine and fragile nanostructure (Fuzz) that can be washed out into the fusion device plasma and stop the fusion reaction. These two extreme examples of the radiation-surface interaction were selected to be my PhD research topic. The change in chemical properties of Ru surface during exposure to a 13.5 nm wavelength of EUV light radiation was investigated. This study shows a real time tracking to the early stages of the EUV induced carbon contamination process. In another similar study, The changes of chemical composition of Ru mirror surface during 100 eV electron bombardments have been studied using XPS. The reason behind coupling these two studies is due to the fact that the mechanism by which the EUV radiation can alter the chemical composition of the mirror surface is between two main mechanisms; photons dissociation and secondary electrons dissociation processes. Although the results obtained by these two studies were useful to understand the mechanisms in which the surface composition can be altered during the EUV exposure, many vital details about the gradual transformation of the adsorbed hydrocarbon molecules to carbon rich solid contaminant layer are not well understood. These important information was hidden beneath the photoelectron signal interference between the Ru 3d lines and the C 1s line. For this reason we also extensively studied this transformation during the EUV irradiation by changing the target materials to Au. Due to this reason, the changes in surface properties of Au surface in a high vacuum atmosphere during EUV exposure have been also studied. One of the most important finding we observed during the last three studies is that, the adsorption process is at the very early stage of the EUV induced contamination process. Therefore, the rate of adsorption on the irradiated surface always govern the rate of the entire contamination process. In attempt to understand the impact of the low energy electron irradiation on the kinetics of the adsorption process, we also investigated the impact of 100 eV electron beam on the physisorption of hydrocarbons and water molecules on Au surfaces. After this study we moved to the second part of the selected research topic which was investigating the surface morphology evolution of the W surface exposed to high flux of He ion irradiation. Our first investigation was an attempt to understand the basic parameters under which this interesting phenomenon of fuzz formation occurs. For this reason, a series of pure W samples were irradiated by high fluxes of low energy He+ ions of high doses at 900°C. The phenomenon of He ions induced fuzz formation was the most prominent observation that has been noticed in most of the irradiation cases. Several attempts have been made to understand this phenomenon by varying many irradiation parameters such as irradiation dose, ions flux, and the energy of incident He ions. We also studied the effect of carbon contaminations on the He induced surface morphology evolution of W target.

Degree

Ph.D.

Advisors

Hassanein, Purdue University.

Subject Area

Nuclear engineering

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