Filter windows for EUV lithography

Extreme-ultraviolet (EUV) lithography based on reflective optics is expected to require at least one filter window to 1) reduce out-of-band radiation in the ultraviolet, visible and infrared, 2) partially protect the optics from debris from the radiation source and any outgassing from the resist on the wafer, and 3) perhaps to serve as a barrier for EUV absorbing gasses. To maximize wafer throughput, the filter window or windows will need to provide the highest possible transmittance at 13.4 nm. EUV filters must operate in a harsh vacuum environment. They will be irradiated with high energy EUV light and will absorb out-of-band radiation that will cause temperature increases of greater than 100 degree(s)C. Outgassing from the filters must be minimal, and they must survive handling as well as pressure differentials during pump-down operation, and return-to-atmospheric pressure. Prototype filters were fabricated for Sandia's Engineering Test Stand (ETS) and are being utilized in on-going EUV lithography demonstrations. Their in and out-of-band transmittance has been measured and found to meet Sandia's performance specifications, and they have been exposed to various environments with good results.