Lithography transfers circuit pattern drawn on mask to photo resist on silicon wafer. To achieve half pitch of 32 nm or less for DRAM, light source for the lithography must have extreme shot wavelength at 13.5 nm.

In this research, as the EUV light source, rotational Sn disk has been proposed. The disk has many tiny holes which are arranged in a concentric pattern. Each hole is irradiated by intense laser pulse, and the resultant EUV light will be integrated along central axis of hole. Finally intense EUV pulse will come out from the opposite side of hole. After the emission of EUV light, Sn atoms or fine grains will attach to inner wall of hole. Therefore this target has potential for debris-free EUV light source.

Left figure shows schematic diagram for experiment. Excitation laser is pulsed Nd:YAG laser at 1064 nm, and pulse energy is 500mJ/pulse. The laser pulse is focused into tiny hole on Sn plate by lens. Through-hole target is made by micro drill 200 µm diameter.

Small detector for EUV light at around 13.5 nm is made with (1) gold-evaporated silicon mirror, (2) Zr filter, and (3) Si photodiode for EUV light.

Left figure shows a series of experimental results to study angular distribution of EUV emission from through-hole target. The angles indicated in this figure are measured from optical axis of laser beam. Each PD output curve or pink data point is recorded by single laser pulse of 500 mJ. This result represents that EUV emission from through-hole target has sharply-directed radiation property.

Variation in (1) duration of laser pulse (4 ns to 8 ns) and(2) pulse energy (270 mJ to 660 mJ) gave us left-hand figure of changes in EUV conversion efficiency.

ITRS road map shows that DRAM having half pitch of 32 nm or less will be made by extreme ultraviolet lithography (EUVL). Plasma for EUV source emits divergent light, and collection optics are needed to make small EUV spot at intermediate focal spot. Commercial EUV source must have (1) optical power of 115 W at the focal spot and (2) high repetition rate of 7 to 10 kHz. Also one year operation period must be guaranteed.