23 March 2006 Model of Ru surface oxidation for the lifetime scaling of EUVL projection optics mirror
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A chemical model of the surface oxidization of a ruthenium capping layer was constructed for the lifetime scaling of the mirrors of EUVL projection optics. It has two steps: the formation of active oxygen from adsorbed water by EUV irradiation, and the diffusion of active oxygen and its reaction with Ru at the interface between Ru oxide and Ru metal. The rate equations for these steps can be solved independently because the time scale is very different. This paper reports results for Step 1. They revealed the following: (1) The predicted dependence of the amount of oxidation on water pressure is linear over a very wide range. That means that water pressure is a good measure of the degree of acceleration. The reported non-linear dependence on water pressure cannot be explained without the influence of other factors, such as the amount of background hydrocarbons. (2) The rate equation showed the dependence on light intensity also to be linear. That is, a theoretical model based on a simple rate equation cannot predict the reported non-linear intensity dependence. So, we calculated the rise in surface temperature caused by irradiation over a long period from the rate equation for one-dimensional thermal conduction and the Lambert-Beer Law for surface photoabsorption. These calculations revealed that irradiation for a long period of time causes the surface temperature to rise, thus reducing the rate of formation of active oxygen. So, light intensity is a limiting factor in acceleration tests. (3) The rate of oxidation is significantly lower for pulse excitation than for quasi-continuous (synchrotron radiation) light when the EUV light source produces narrow pulses at a low repetition rate.
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Iwao Nishiyama, Iwao Nishiyama, } "Model of Ru surface oxidation for the lifetime scaling of EUVL projection optics mirror", Proc. SPIE 6151, Emerging Lithographic Technologies X, 61510G (23 March 2006); doi: 10.1117/12.655499; https://doi.org/10.1117/12.655499

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