Development of a short pulse and high average power CO2 laser for EUV lithography

EUV lithography (EUVL) is the candidate for next generation lithography to be introduced by the semiconductor industry to HVM (high volume manufacturing) in 2013. The power of the EUVL light source has to be at least 115W at a wavelength of 13.5nm. A laser produced plasma (LPP) is the main candidate for this light source but a cost effective laser driver is the key requirement for the realization of this concept. We are currently developing a high power and high repetition rate CO2 laser system to achieve 50 W intermediate focus EUV power with a Tin droplet target. A high conversion efficiency (CE) from laser energy to EUV in-band energy is the most important issue for the concept to be realized. A CE of more than 2% has been obtained with a Tin target and a CO2 laser which is also predicted by numerical simulation analysis. The high CE requires a short laser pulse of less than 15 ns. This paper describes the development of a CO2 laser system that is based on RF-excited, axial flow CO2 laser amplifiers. The system produces a short pulse length about 15 ns and a nominal average power of several kW at a repetition rate of 100 kHz, An output power of 2.6 kW has been achieved with a pulse length between 15-30 ns at 130 kHz repetition rate at small signal amplification. The phase distortion of the laser beam after amplification is negligible and the beam can be focused to about 150 mu m diameter in 1/e(2). The conceptual design of the CO2 laser system for LPP EUVL light source, and the laser performance are reported. The further increase of the amplification efficiency is discussed to obtain 5 kW average output power.