Transmission grating spectrometer for broadband characterization of EUV sources

Emission from extreme ultraviolet (EUV) light sources for lithography and metrology applications needs to be maximized in a narrow wavelength band. On the other hand, these sources also emit radiation outside this wavelength band, extending into the deep ultraviolet (DUV) and visible/IR range. To gain a comprehensive understanding of the source conditions, characterization of the in-band as well as the ouf-of-band radiation is needed. Here, we present a broadband spectrometer based on a set of free-standing transmission gratings. The spectrometer combines, high (up to 10,000 lines/mm) and low line density gratings that enable recording of the source spectrum with high resolution and also in a wide bandwidth extending beyond the DUV wavelengths. Compared to a reflective grating configuration, the transmission geometry enables the design to be more compact, and the alignment and calibration to be more straightforward. The components in the spectrometer are controlled by motorized stages that enable adjusting spectral resolution and spectral range without breaking vacuum. We present measurements that were carried out at different types of EUV sources and at various spectral ranges. The measurements provide comprehensive spectral information that is directly available for analysis and optimization of the source conditions.