Hydrogen atom based tin cleaning

  • V.M.C. Crijns

Student thesis: Master


In next-generation lithography machines, tin resulting from the EUV source is regarded as a seriously contaminating element of the EUV optics. Atomic hydrogen appears to be an appropriate candidate for selective in-situ tin cleaning. However, understanding of the hydrogen atom based tin cleaning process is lacking. Therefore, the goal of this thesis is to identify and understand the mechanisms involved in the tin cleaning process. Initially, tin cleaning is compared with hydrogen atom based carbon cleaning - carbon being another contaminating element of EUV optics. The cleaning rates for both carbon and tin are determined as a function of atomic hydrogen flux at various pressures. Atomic hydrogen is generated through hydrogen dissociation at the surface of a hot tungsten filament. In order to quantify the atomic hydrogen flux, an isothermal catalytic probe is successfully employed. Atomic hydrogen was generated within a pressure range of 5 - 100 Pa and with filament temperatures between 1300 °C and 1950 °C. The resulting flux values were determined within a range of 1015 - 5?1017 at cm-2 s-1 with an absolute error of 4.4?1016 at cm-2 s-1. Both carbon samples and various types of tin containing samples were exposed to quantified atomic hydrogen fluxes. Results of the carbon cleaning rate demonstrate a linear relation with flux, as is expected from literature. When thick layers of tin (~160 nm) on Si substrates are cleaned, the tin cleaning rate appears not to be proportional to the atomic hydrogen flux. The results indicate a cleaning threshold for very low fluxes as well as a decreasing cleaning efficiency at the high end of the studied flux range. Next, the thick layered tin samples are exposed to a fixed atomic hydrogen flux for various exposure times. Studying the tin cleaning rate as a function of exposure time displays a threshold period of approximately 50 minutes, during which the cleaning rate is relatively low. This period corresponds to the time after which the substrate underneath the tin layer becomes partially uncovered as a result of atomic hydrogen exposure. When cleaning thin layers of tin (~30 nm) on ZrN and Mo substrates, the cleaning rate shows a reverse dependency on time. Within the initial minutes of the cleaning experiments, the cleaning rates for both types of samples are relatively high. During this time, the tin structure changes rapidly. After less than 20 minutes, the cleaning rate has declined to a steady value and distinct tin islands have been formed. The islands become more spherical as the cleaning experiments continue, which appears to significantly impede the cleaning process.
Date of Award28 Feb 2014
Original languageEnglish
SupervisorM. van Kampen (Supervisor 1) & Gerrit M.W. Kroesen (Supervisor 2)

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