Plasma cleaning of lithium off of collector optics material for use in extreme ultraviolet lithography applications

One of the critical issues within extreme ultraviolet lithography is mirror lifetime and the degradation due to debris from the pinch. This research investigated and showed the efficacy of using a helium secondary plasma and heat for removal of Li debris from collecting on the surface of collector optics. A He helicon plasma, which minimizes selfbiasing and sputtering, has good extreme ultraviolet (EUV) photon wavelength transmission and preferential sputtering of lithium compared to other collector optics material. Through the combined use of heating and a He secondary plasma, EUV collector sample surface roughness and surface composition was able to be maintained near as-received status. The use of the He secondary plasma while the collector optics sample is exposed to Li debris shows promise as an in situ cleaning process for collector optics and can extend the lifetime of collector optics.