Particle-cluster tin target is presented as the solution of a 100W EUV source for EUVL. Theory for maximizing conversion efficiency of a laser-produced plasma is derived and the theory is experimentally confirmed by using a dispersed SnO<sub>2</sub> particles. The EUV intensity 4 times higher than that from a plasma on a solid Sn plate target is observed at the optimized density. The achieved conversion efficiency for dispersed particles is estimated to be as high as 3%/(2π str 2%BW) or higher from the value for a Sn plate of 0.8% measured by using two multilayer mirrors and a calibrated photodiode. Theoretical consideration reveals that larger diameter plasma enables higher EUV power. The particle-cluster can be delivered at multi kHz rep-rate by using water droplet. Experimental confirmation of delivering particles by droplets is also reported.
In this paper, we described a laser plasma source for Extreme Ultraviolet Lithography (EUVL) based on a water droplet target. We successfully generated stable multi-kHz water droplets with several hundred μm diameter using our experimentaql setup. We realized a good synchronization of laser with droplet by employing droplet-probing photo diode (PD) signal to trig YAG laser timely. We got EUV emission with pulse to pulse stability of 3.4% (1σ) from this droplet region without being destroyed due to hot laser plasma formation from the previous droplet.
Debris-free generation of a tin plasma was demonstrated in the cavity-confined configuration. Narrow band emission at 13.7-nm was observed in an emission spectrum of a cavity confined tin plasma. The spectral efficiency was as high as 12% and we found the conversion efficiency could reach 6%/2π str ultimately while lots of works are required to achieve this value. We also confirmed a magnetic field has some effect of stopping a plasma.