Tuning laser wavelength and pulse duration to improve the conversion efficiency and performance of EUV sources for nanolithography

Abstract

The optimum conditions for efficient extreme ultraviolet (EUV) sources in laser-produced plasma (LPP) depend on the ideal combination of several parameters such as laser wavelength, intensity, spot size, and pulse duration. These parameters need to be optimized in relation to the actual target size and geometry. The laser wavelength can play a major role in this regard. While target related effects are not significant in the case of a Nd:YAG laser, the achievable conversion efficiency (CE) of EUV sources produced by a CO2 laser from small Sn droplets can be several times lower compared to that from a flat Sn target. Improving the efficiency and performance of EUV sources requires detailed investigation of various laser/target combinations using comprehensive integrated models. We simulated and characterized in detail plasmas produced from the Sn droplet irradiated by lasers with various wavelengths. The optimum laser intensity with respect to the laser and target sizes was determined for a range of laser wavelengths from 1 μm to 10 μm. This work was inspired by the recent developments of lasers with a 2 μm wavelength. The laser pulse duration is another important parameter in the optimization of LPP for EUV sources. Our comprehensive HEIGHTS modeling showed that increasing the pulse duration of the 2 μm laser leads to an increase in the CE of EUV sources. Our results also indicate that a significant reduction in the kinetic energies of ions generated from LPP can be achieved by optimizing the laser pulse duration.