Abstract

Most of the proposed Extreme Ultraviolet Lithography (EUVL) sources utilize the interaction of some high energy density source, i.e. a laser, electron beam, or arc discharge, with a solid target. EUV can be efficiently generated by these techniques, but debris from the interaction region impacting on the reflective optics can substantially degrade the performance of the condenser system. Los Alamos and Grumman are cooperatively investigating a debris free source^1,2 for EUV lithography. This source utilizes the predicted anomalous energy loss^3,4 of a short pulse electron beam in a preformed plasma to heat and ionize the ions to a charge state where efficient radiation at 130Å occurs. Accelerators⁵ developed for the free electron laser program at Los Alamos are used as the electron bunch source. These accelerators use a laser driven photocathode to produce 15 psec electron bunches containing 4 nC of charge with an energy of 15.5 MeV. These micropulses are produced at a repetition rate of 108 MHz and continue for the length of the rf macropulse energizing the accelerator cavities, typically 1-10 μsec. The weakly ionized preformed plasma is created by purely classical collisional ionization caused by the initial few electron micropulses within the macropulse. When a critical electron density is reached, n_e ≅ 1.6x10¹⁵/τ²cm^-3, where τ is the FWHM, in psec, of the electron bunch, the plasma responds collectively to the electron micropulse generating a large amplitude plasma wave. The plasma wave efficiently slows the high energy electron beam while heating the background plasma electrons. The initial electron population rapidly heats and then equilibrates with the bulk ion and electron populations in a few 10's of picoseconds. With neon as the dominant ionic species, an efficient filamentary radiator of line radiation near 130Å is created.

© 1994 Optical Society of America