Colliding plasmas as potential EUV sources towards higher conversion efficiency

Abstract

Development and optimization of EUV sources for nanolithography during the last decade lead to the significantly improved understanding of Laser Produced Plasmas (LPPs) evolution and properties. Many efforts were devoted to the fundamental effects of laser parameters on LPP temporal and spatial characteristics and on EUV light emission and absorption. Comprehensive experimental and theoretical studies allowed prediction of optimum conditions for the efficient sources produced by various lasers. However, improving the quality and reducing the cost of chips production require significant increase in EUV source power. Several studies have shown EUV power limits from laser based sources produced using mass-limited Sn droplets. These limitations can be explained by target/plasma hydrodynamics effects under laser irradiation that can result in reduced transient area of laser/plasma interactions and the high reabsorption of emitted EUV photons in evolving plasma. Therefore, further analysis of the effects of target parameters on EUV output is an important task in the optimization of future high power EUV sources. Colliding plasmas experiments can be very useful in this regard. Development of pre-plasma in these experiments can be regulated by several parameters of two lasers in conjunction with target geometry. The developed pre-plasma characteristics can accurately be measured and optimized for the following main laser (CO2 or Nd:YAG) irradiation to develop very efficient and prolonged EUV emitting area. We used our advanced 3D simulation package (HEIGHTS) for modeling of complex colliding plasmas evolution and EUV source characteristics produced by two simultaneous Nd:YAG lasers interaction with wedge target followed by CO2 irradiation. We used several diagnostics technique and comparison of modeling and experimental results in our CMUXE labs regarding plasma characteristics, laser/plasma/target interactions, and EUV photons emission and absorption. Our results showed that significant enhancement in EUV output can be achieved with optimized parameters of laser beams that could allow approaching the predicted theoretical limit of the conversion efficiency of EUV sources.