High average power CO_2 laser MOPA system for Tin target LPP EUV light source

摘要

Extreme ultraviolet 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 CO_2 laser system to achieve 50 W intermediate focus EUV power with a Tin droplet target. We have achieved CE of 2.8% with solid Tin wire target by a transversely excited atmospheric (TEA) CO_2 laser MOPA system with pulse width, pulse energy and pulse repetition rate as 10～15 ns, 30 mJ and 10 Hz, respectively. 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, fast axial flow CO_2 laser amplifiers is under development. Output power of about 3 kW has been achieved with a pulse length of 15 ns at 130 kHz repletion rate in a small signal amplification condition with P(20) single line. The phase distortion of the laser beam after amplification is negligible and the beam can be focused to about 150um diameter in 1/e~2. The CO_2 laser system is reported on short pulse amplification performance using RF-excited fast axial flow lasers as amplifiers. And the CO_2 laser average output power scaling is shown towards 5～10 kW with pulse width of 15 ns from a MOPA system.