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.
Buffer gases, used for protection and cleaning of the optical system in the EUV source chamber, as well as the amount of residual vapor and fine mist due to the required high frequency operation for the high-volume manufacture (HVM) may affect the source performance and mitigating system efficiency. Interplay of injected gas flow and evolving Sn plasma/vapor should be studied in conditions closely related to laser produced plasma (LPP) chamber environment.
We expanded and enhanced our models implemented in the HEIGHTS package to simulate LPPs in mixture environment of vapor/plasma created from Sn droplet and the background buffer gas (e.g., Ar) at various pressures. Our integrated models allowed self-consistent simulation of EUV produced and EUV induced plasma evolution in the entire chamber.
We studied tin plasma evolution in single and dual pulse systems in conditions of Ar residual background gas at 3 and 30 Pa pressure. Details of Ar plasma induced by EUV photons were analyzed to predict conditions near mirror surfaces that could change performance of the surface layers and reflectivity of the collecting optical system. Processes of mixture and two plasmas expansion and cooling from both the pre- and the main pulse were simulated for the first time to predict chamber conditions at the next iteration of target/lasers coupling and interaction.
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