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The tin (Sn) debris contamination is one of the technical challenges for the development of high power
EUV light source with Sn fuel with a long lifetime of EUV collectors. The debris mitigation techniques
(DMTs) can considerably minimize the Sn debris coming out of the source thereby reducing the need or
effort for cleaning. However, for HVM, which requires higher EUV power output than today, it is
questionable if the DMTs alone will completely eliminate the Sn contamination. Besides, at abnormal
instances, we also need to clean thick Sn debris from the mirror surface. For this purpose, the Center for
Plasma-Material Interactions (CPMI) at University of Illinois at Urbana-Champaign has developed a
plasma-based Sn cleaning method using chlorine plasma with densities and temperature around ~9×109/cm3 and ~ 4 eV respectively. From the previous studies at CPMI, it was shown that chlorine plasma
etching can remove Sn debris from Ru mirror surface in a fast (> 400 nm/min) and in situ manner. In this
study, we applied the same method to clean Sn contamination on the mock-up collector in our XTS13-35
DPP EUV source system. The mock-up is made of two shells with different gap widths (4 cm, 7.5 cm and
10 cm) in similar size with the actual collector optic. The cleaning rate at different locations on the mockup
was experimentally investigated, and it was found that the cleaning rates vary largely with the distance
from the chlorine plasma in the range of 20 - 100 nm/min. In addition, a simple analytical model to predict
the cleaning rate was developed based on the plasma-surface reactions and the plasma transport. The
model describes how plasma transport, chlorine radical distribution and pumping flow affect the Sn
cleaning rate with chlorine plasma. Finally, the model is then compared to the experimental results and
validated. Based on the knowledge of chlorine plasma and Sn interactions obtained in this study, a remote
plasma cleaning technique was also investigated and the results obtained therein are presented. The
experimental results along with the model predictions will help design an integrated cleaning system for
collector optic in the high power EUV source system for HVM.
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