High-power, high-repetition-rate pulsed CO2 lasers and their application in EUV lithography sources

The development of high power CO₂ lasers used for EUV sources with power of 250 W and beyond is determined by the requirements for CO₂ laser parameters, generation and amplification of short duration CO₂ laser pulses, target formation for CO₂ laser absorption and EUV emission, isolation technology to mitigate self-lasing and reverse laser power, and other related technologies. The CO₂ laser consists of a seed laser, preamplifiers, power amplifiers, beam transport system, focusing unit, control and metrology systems. Seed lasers generates laser pulses with high repetition rate (>50 kHz), short pulse duration, and high beam quality. Preamplifiers and power amplifiers boost its power and intensity enough to heat plasma to high temperature to emit 13.5 nm EUV radiation. Mode matching is required to efficiently extract gain from the amplifiers. Gain distribution through amplifiers need to be balanced to obtain efficient amplification and avoid self lasing. Efficient amplification of short pulse duration CO₂ laser is enabled by employing high power seed systems. Pre-pulse is employed to form the desired target for more efficient laser absorption and EUV emission. Reverse laser power from plasma is a critical challenge in the development of high power CO₂ lasers, i.e., the reverse power could damage system or reduce system performance. EUV power scaling requires higher CO₂ laser power, isolation technology is needed to match the requirement of higher CO₂ laser power.