Intense short wavelength sources for EUV lithography and other applications

Summary form only given. The development of efficient normal-incidence multilayer reflective coatings in the 13-14 nm wavelength region has led to many new optical applications. One of the most demanding applications is extreme ultraviolet lithography (EUVL). This lithography would use reduction imaging to print microchip features smaller than 0.1 μm. The use of all-reflective optics (reflectivities up to 65% per surface with Mo:Si multilayer coatings) makes it possible to operate a lithographic stepper in the 13-14 nm wavelength range with the necessary throughput required by a commercial microchip manufacturers. The Lyman alpha transition in doubly ionized lithium appears to be an efficient source at 13.5 nm. The ratio of excitation energy to radiated energy for that transition is 53% and only two electrons per atom need to be removed to produce that radiating state. The optimum lithium plasma radiating at 13.5 nm has been estimated to require an electron density of 10¹⁸-10¹⁹ cm^-3 and an electron temperature of 15-20 eV. Low inductance capillary discharges of compact design have been recently shown to be capable of producing these plasma conditions, while maintaining a small source size. Experiments demonstrating the “proof of principle” of this source are described. Future directions include the construction of a lithium vapor discharge lamp operating at temperatures up to 900 °C