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Metrology concepts and related results are discussed for characterization of extreme ultraviolet (EUV) light sources based on laser-produced plasmas using metal foil and droplet targets. Specific designs of narrow-band EUV detectors employing multilayer mirrors and broadband detectors for droplet steering are described. Spatially resolved plasma imaging using in-band EUV pinhole cameras is discussed. A grazing-incidence flat-field EUV spectrometer is described that has been employed for spectroscopy in the 6 nm -22 nm range. In addition, spectroscopic data of out-of-band radiation in the ultraviolet and visible spectral regions are presented. Results obtained for different wavelengths of the incident laser radiation and for both tin-and lithium foil-and droplet-targets are discussed.

Efficient conversion of laser light into EUV radiation is one of the most important problems of the laser-produced plasma (LPP) EUV source. Too low a conversion efficiency (CE) increases the amount of power the drive laser will have to deliver, which, besides the obvious laser cost increase, also increases the thermal load on all the components and can lead to increased debris generation. In order to meet the requirements for a high-volume manufacturing (HVM) tool and at the same time keep the laser power requirements within acceptable limits, a CE exceeding 2.5% is likely to be required. We present our results on optimizing conversion efficiency of LPP EUV generation. The optimization parameters include laser wavelength, target material, and laser pulse shape, energy and intensity. The final choice between parameter sets that leads to the required minimum CE is dependent on the debris mitigation solutions and the laser source available for a particular parameter set.

The source output power and lifetime, including the collector optics lifetime, are among the key issues for EUV lithography systems. In order to meet the requirements for the EUV collector mirror, both the reflectivity and the long-term thermal stability of its multilayer coating have been enhanced considerably during recent development efforts. Sub-aperture ellipsoidal mirrors of different substrate materials with outer diameters of about 320 mm were coated with laterally graded high-temperature multilayers. The interface-engineered Mo/Si multilayer mirror (MLM) coatings were optimized in terms of high peak reflectivity at 13.5 nm and working temperatures above 400°C. Thin barrier layers were introduced on both interfaces to block thermally induced interdiffusion processes of molybdenum and silicon and to provide long-term optical stability of the coating at elevated temperatures. A normal-incidence reflectance of R ~ 60 % at 13.5 nm was measured on Si wafer samples after heating up to 600°C. No degradation of the optical properties of these multilayer coatings occurred during both long-term heating tests and multiple annealing cycles. On highly polished collector substrates with improved surface roughness a reflectance for s-polarized light exceeding peak values of R = 57 % was obtained. With optimized layer gradient the degree of wavelength matching was improved, as well, resulting in peak reflectivity values above 56 % throughout the clear aperture for a series of measurement points across the mirror. The corresponding area-weighted 2% in-band average reflectance for this collector mirror coating exceeds 52 % for unpolarized light

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