Appl. Phys. B 108, 743–747 (2012)

Sho Amano and Tomoaki Inoue from the University of Hyogo in Japan claim to have created the first extreme-ultraviolet source that can generate radiation continuously at a wavelength of 6.7 nm and is potentially scalable to industrially relevant powers. This wavelength is of particular interest for performing next-generation extreme-ultraviolet lithography. Their device is a laser plasma X-ray source that uses radiation from a high-energy-density plasma produced by laser irradiation of a target. The target is a drum-shaped copper surface, cryogenically cooled with liquid nitrogen, onto which xenon gas is blown to form a solid xenon layer. The researchers rotated the position of the drum to ensure that a fresh area of xenon on the target surface is available for each shot from their Q-switched Nd:YAG laser. Maximum emission at 6.7 nm was obtained with irradiation at a wavelength of 1,064 nm. Emission with a narrow spectral bandwidth of 0.6% — which is particularly desirable for lithography applications — was enabled by using La/B4C mirrors instead of the usual Mo/Si structures, although this came at the cost of reduced power. They achieved an average emitted power of 80 mW (at a repetition rate of 320 Hz and an average pump power of 100 W), which is well below the ∼100 W needed for industrial mass-production applications. However, the researchers report that in principle up to 120 W can be obtained if the average pumping laser power is increased to 80 kW. A repetition rate of 100 kHz may be possible by enlarging the drum diameter from 10 cm to 50 cm, thereby providing a greater amount of xenon target.