Effect of target mass on CO₂-driven EUV emitting tin plasma for nanolithography

State-of-the art nanolithography for the semiconductor industry employs Extreme Ultraviolet (EUV) light produced by illuminating tin targets with a CO₂ laser [1]. The resulting high charge state plasma (Z > 10) has a significant emission in the 13.5 nm ± 1% band [2,3] in which the Mo/Si mirrors used to guide the light towards the wafer have a peak reflectivity. These targets start as droplets of ~30 μm diameter, which are deformed by a laser pulse into thin disks of ~200 μm of radius and a thickness of ~50 nm. During this deformation process, between half and three-quarters of the initial droplet mass is lost due to fragmentation [4]. This results in less mass available to form the EUV emitting plasma with the CO₂ laser.

In this work, we quantify the effect of the available mass on the production of in-band light in EUV sources for nanolithography. [5] Two-dimensional rad-hydrodynamic simulations performed with the RALEF-2D code [6] are presented. A distinct trend of increasing conversion efficiency (CE) of laser energy into in-band energy with a higher target mass is identified. We find that the maximum CE is obtained when the target feeds mass to the plasma during the whole pulse. A laser energy sweep indicates that overheating the plasma in cases that conserve more than 50% of the droplet mass leads to an increase in in-band radiation without a significant decrease in CE.

[1] J. Fujimoto et al., Journal of Micro/Nanolithography, MEMS, and MOEMS, 11(2), 021111 (2012)

[2] O. O. Versolato, Plasma Sources Science and Technology, 28(8), 83001 (2019)

[3] I. Fomenkov et al., Advanced Optical Technologies, 6(3), 173 (2017)

[4] B. Liu et al., Journal of Applied Physics, 129(5), 053302 (2021)

[5] J. Gonzalez, J. Sheil, Phys. Plasmas 31(5): 050701, (2024).

[6] M. M. Basko, "RALEF-2D: A 2D hydrodynamic code with heat conduction and radiation transport. II. Solution of the radiation transfer equation." Darmstad: GSI (2009)