The spectrum of a 1- μ m-wavelength-driven tin microdroplet laser-produced plasma source in the 5.5-265.5 nm wavelength range

We present a calibrated spectrum in the 5.5-265.5 nm range from a microdroplet-tin Nd:YAG-laser-produced plasma under conditions relevant for the production of extreme ultraviolet (EUV) light at 13.5 nm for nanolithography. The plasma emission spectrum obtained using a custom-built transmission grating spectrometer results from a careful calibration of a series of filters enabling measurements free of any higher diffraction orders. Specifically, Zr, Si, and Al thin-foil filters and bulk LiF, MgF2, and UV fused silica filters are employed. A further filter using four SiC mirrors is used to record the otherwise inaccessible 40-100 nm range. The resulting corrected and concatenated spectra are shown to accurately match in their respective overlap regions. The possibility to measure spectra over this broad range enables the optimization of current and future sources of EUV light for nanolithography by providing the diagnostics required for minimizing the emission of unwanted wavelength bands.