Exploring the electron density in plasmas induced by extreme ultraviolet radiation in argon

TL;DR

This study measures and analyzes electron density in argon plasmas induced by EUV radiation, revealing how pulse energy and gas pressure influence plasma characteristics relevant for lithography applications.

Contribution

It provides experimental and theoretical insights into EUV-induced plasma electron density dependence on pulse energy and pressure, aiding in understanding plasma effects in lithography.

Findings

Electron density scales linearly with EUV pulse energy.

Electron density exhibits quadratic dependence on gas pressure.

Theoretical estimates support experimental observations.

Abstract

The new generation of lithography tools use high energy EUV radiation which ionizes the present background gas due to photoionization. To predict and understand the long term impact on the highly delicate mirrors It is essential to characterize these kinds of EUV-induced plasmas. We measured the electron density evolution in argon gas during and just after irradiation by a short pulse of EUV light at 13.5 nm by applying microwave cavity resonance spectroscopy. Dependencies on EUV pulse energy and gas pressure have been explored over a range relevant for industrial applications. Our experimental results show that the maximum reached electron density depends linearly on pulse energy. A quadratic dependence - caused by photoionization and subsequent electron impact ionization by free electrons - is found from experiments where the gas pressure is varied. This is demonstrated by our theoretical estimates presented in this manuscript as well.