Broadband transmission grating spectrometer for measuring the emission spectrum of EUV sources

Extreme ultraviolet (EUV) light sources and their optimization for emission within a narrow wavelength band are essential in applications such as photolithography. Most light sources however also emit radiation outside this wavelength band and have a spectrum extending up to deep ultraviolet (DUV) wavelengths. This out-of-band radiation can be hazardous in the rest of the lithography process, hence monitoring of it is necessary. In this article we present a broadband spectrometer based on a transmission grating for spectral monitoring of EUV sources from EUV to DUV wavelengths. The transmission geometry enables a compact design and a straightforward alignment. Measurements that were carried out with the spectrometer at two different EUV sources provide detailed spectral information that is immediately available for analysis and optimization of the source conditions.