Mitigation of debris from a laser plasma EUV source and from focusing optics for thin film deposition by intense EUV radiation

Abstract Studies on extreme ultraviolet (EUV) light sources for practical applications have attracted much attention. Although a laser-driven EUV source in conjunction with focusing mirrors enables the use of intense EUV radiation, mitigation of debris from the source plasma and focusing mirrors is a challenge to satisfy requirements for advanced material synthesis, because even a tiny amount of contamination in the processed materials such as micro-machined circuit patterns or thin film materials drastically changes the physical properties and consequently deteriorates the functions. In this study, we designed and validated two types of optical layout, namely a combination of a planar mirror and an ellipsoidal mirror (PT) and double ellipsoidal mirrors (TT), for the mitigation of debris, including neutral particles. The properties of these layouts were compared with those of a reference layout consisted with a single ellipsoidal mirror. Ray-trace calculations for EUV light showed that the PT and TT layouts can both transfer enough energy to process or ablate materials while preserving the EUV profile at the light source. Furthermore, simulations of the debris trajectory showed that the amount of debris arriving at the EUV ablation target position with the TT layout is reduced to 0.23% of that with the reference layout. Si thin film deposition experiments were conducted and the fraction of elements in the deposited film was characterized by x-ray fluorescence analysis. The proportions of elements other than Si in the thin films were 1.3% with the PT layout and 0.0% with the TT layout, which are significant reductions from that with the reference layout and thus show the feasibility of the proposed optical layouts.