Optical EngineeringTunable narrowband soft x-ray source for projection lithography
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EUV emission from laser plasmas produced on low Z target material such as plastics and araldites that contain 1 to 10% concentrations of the elements 50 ≤ Z ≤ 70 emit narrow regions of intense quasicontinua in the 6 to 13-nm wavelength range. The typical bandwidth of the emission is 0.5 to 1.5 nm and the features are brighter than any individual lines emitted from the host substrate materials. Similar spectra can be obtained from hydrated salts of these elements, so suitable targets can be prepared as compressed pellets of crystalline or hydrated salts or alternatively from oxides mixed into an araldite matrix. Spectra of tin salts prepared in this way provide an excellent narrowband source for 13.5-nm radiation for use with Mo/Si multilayers. Since the peak shifts toward shorter wavelengths with increasing Z, plasmas of these elements provide "tunable" narrowband EUV radiation sources.