High-radiance LDP source for mask-inspection application

Actinic mask inspection manufactures are currently searching for high-radiance EUV sources for their tools. LDP source, which was previously used for lithography purposes, was found to be a good candidate as it can provide sufficient power and radiance. Introduction of new techniques, modified modules and fine tuning of operational conditions (discharge pulse energy, discharge frequency, laser) has brought radiance level to 180 W/mm2/sr at plasma or 145 W/mm2/sr as clean-photon. The source has been modified in such a way to improve modules reliability, lifetime and radiance stability even though there is still a room for further improvement. Size of the source system is much smaller than that of the lithography source. A debris mitigation system has been tested. Optical transmission was improved to 77 % and several 8-nm-thick Ru samples were exposed to evaluate contamination and erosion of optics. Preliminary results show low sputter and deposition rates, which supports sufficiently long lifetime of the optics.