Time-resolved two-dimensional profiles of electron density and temperature of laser-produced tin plasmas for extreme-ultraviolet lithography light sources

Kentaro Tomita,Takeshi Kodama,Toshiaki Eguchi,Georg Soumagne,Syouichi Tsukiyama,Katsunobu Nishihara,Yuta Sato,Tatsuya Yanagida,Masahito Kunishima,Yasunori Wada,Hakaru Mizoguchi,Hiroaki Tomuro,Kiichiro Uchino,Kouichiro Kouge,Atsushi Sunahara

doi:10.1038/s41598-017-11685-0

Kentaro Tomita, Takeshi Kodama

Open Access

https://doi.org/10.1038/s41598-017-11685-0

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Abstract

Time-resolved two-dimensional (2D) profiles of electron density (ne) and electron temperature (Te) of extreme ultraviolet (EUV) lithography light source plasmas were obtained from the ion components of collective Thomson scattering (CTS) spectra. The highest EUV conversion efficiency (CE) of 4% from double pulse lasers irradiating a Sn droplet was obtained by changing their delay time. The 2D-CTS results clarified that for the highest CE condition, a hollow-like density profile was formed, i.e., the high density region existed not on the central axis but in a part with a certain radius. The 2D profile of the in-band EUV emissivity (ηEUV) was theoretically calculated using the CTS results and atomic model (Hullac code), which reproduced a directly measured EUV image reasonably well. The CTS results strongly indicated the necessity of optimizing 2D plasma profiles to improve the CE in the future.

Highlights

Time-resolved two-dimensional (2D) profiles of electron density and electron temperature (Te) of extreme ultraviolet (EUV) lithography light source plasmas were obtained from the ion components of collective Thomson scattering (CTS) spectra
This paper determined for the first time that it is important to generate a plasma with an optimal two-dimensional (2D) structure to achieve a large EUV conversion efficiency by adding theoretical analysis to
The CTS signals were collected by lenses at an angle of 120° from the incident laser direction, focusing on the entrance slit of the spectrometer (20 μm width and 5 mm height) and detected by an intensified charge-coupled device (ICCD) camera (Princeton Instruments, PI-MAX4, 45% quantum efficiency at λ0)

Summary

Introduction

Time-resolved two-dimensional (2D) profiles of electron density (ne) and electron temperature (Te) of extreme ultraviolet (EUV) lithography light source plasmas were obtained from the ion components of collective Thomson scattering (CTS) spectra. The highest EUV conversion efficiency (CE) of 4% from double pulse lasers irradiating a Sn droplet was obtained by changing their delay time. A carbon dioxide (CO2) drive laser and a tin droplet target are used as an efficient extreme-ultraviolet (EUV) light source[4,5]. It was confirmed that adequate ne and Te were achieved in the EUV light source plasmas, but this did not provide optimum plasma conditions for high CE17. This paper determined for the first time that it is important to generate a plasma with an optimal two-dimensional (2D) structure to achieve a large EUV conversion efficiency by adding theoretical analysis to www.nature.com/scientificreports/. It is very important to increase the plasma volume, satisfying the optimum conditions for EUV light emission within the range of the permitted etendue condition

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