High Power Pulsed CO_2 Laser for EUV Lithography

摘要

Laser produced plasma is the candidate for high quality, 115 W EUV light source for the next generation lithography. Cost effective laser driver is the key requirement for the realization of the concept as a viable scheme. A CO_2 laser driven LPP system with a Xenon or Tin droplet target, is therefore a promising light source alternative. We are developing a high power and high repetition rate CO_2 laser system to achieve 10 W intermediate focus EUV power. High conversion efficiency (CE) from the laser energy to EUV in-band energy, is the primarily important issue for the concept to be realized. Experimental and numerical simulation analysis of a Xenon plasma target shows that a short laser pulse less than 15 ns is necessary to obtain high CE by a CO_2 laser. This paper describes on the development of a CO_2 laser system with a short pulse length less than 15 ns, a nominal average power of a few kW, and a repetition rate of 100 kHz based on RF-excited, axial flow CO_2 laser amplifiers. Output power of 1 kW has been achieved with a pulse length 15 ns at 100 kHz repletion rate in a small signal amplification condition. The phase distortion during the amplification is negligible and the beam is focused down to 100μ m diameter onto a fast Xenon jet. The conceptual design of the CO_2 laser system for LPP EUV light source, and amplification performance in short pulse using RF-excited axial flow laser as amplifiers, are reported. Additional approach to increase the amplification efficiency is discussed.