Resonance effects in photoemission from TiO2-capped Mo/Si multilayer mirrors for extreme ultraviolet applications

In the unbaked vacuum systems of extreme ultraviolet (EUV) lithography steppers, oxide formation and carbon growth on Mo/Si multilayer mirrors (MLMs) are competing processes leading to reflectivity loss. A major contribution to this mirror degradation is a series of surface reactions that are thought to be driven in large part by photoemitted electrons. In this paper, we focus on the resonance effects in photoemission from Mo/Si MLMs protected by thin TiO2 cap layers. In the vicinity of the resonant energy of the mirror, the energy flux of the EUV radiation forming standing wave oscillates throughout the multilayer stack. As a result, light absorption followed by the emission of photoelectrons becomes a complex process that varies rapidly with depth and photon energy. The electron emission is characterized as a function of the EUV photon energy, the angle of incidence, and the position of the standing wave with respect to the solid/vacuum interface. In our experiments, the position of the standing wave was controlled both by deliberately varying the thickness of the Si terminating layer (of the Mo/Si stack) and by depositing C films of various thicknesses on the TiO2. The experimental data are compared with model simulations to examine the changes in photoemission yield due to the presence of carbon and to the changes in the position of the standing wave. We find that carbon deposition can have a dramatic impact on the yield and, therefore, on the rates of electron mediated reactions at the surface.