Abstract
Laser produced plasma (LPP) soft x-ray and extreme ultraviolet sources utilize various types of targets. Some of them are based on gaseous targets. The most important disadvantage of such targets is the very limited number of elements that can be used in the gaseous form under normal conditions, including chemical compounds in the gaseous state. In this paper, the authors propose a new type of target, constituting an extension of the gaseous target, namely aerosol target, based on solutions of solid chemical compounds in liquids. Unlike previous attempts to produce aerosol targets from overheated liquids, in our system, the target is created by injection of a mist, preformed in a dedicated cell, into the vacuum chamber. Such targets have many advantages: the emission spectra can be adjusted by choosing proper chemical compounds, the aerosol target is elongated, hence, the laser focus can be positioned at a relatively long distance from the nozzle outlet, its average density can be higher compared to gaseous targets. In this work, such a target and its usefulness for the LPP soft x-ray source was demonstrated. The target was investigated by the soft x-ray backlighting method. Its density distribution at the position of the laser focus was determined. The emission spectra from the soft x-ray range, corresponding to multiple charged ions, originating from the solvent and the diluted salt were measured. Based on numerical simulations of the spectra, an ionic composition of the resulting plasmas was determined.
Highlights
Intense development of plasma based, extreme ultraviolet (EUV) sources started over 15 years ago, and was connected with investigations concerning EUV lithography.[1,2,3,4] Optical systems for lithography tools are based on MoSi multilayer mirrors, having the maximum reflection coefficient at 13.5 nm
Such targets have many advantages: the emission spectra can be adjusted by choosing proper chemical compounds, the aerosol target is elongated, the laser focus can be positioned at a relatively long distance from the nozzle outlet, its average density can be higher compared to gaseous targets
An important advantage of the aerosol target formed by our system, in respect to gaseous targets or aerosol targets produced by expansion of superheated vapors, is a possibility of using solutions of various chemical compounds
Summary
Intense development of plasma based, extreme ultraviolet (EUV) sources started over 15 years ago, and was connected with investigations concerning EUV lithography.[1,2,3,4] Optical systems for lithography tools are based on MoSi multilayer mirrors, having the maximum reflection coefficient at 13.5 nm. It was a reason for choosing Sn based plasmas for EUV sources, due to a strong emission in this wavelength range, with the spectral maximum close to 13.5 nm.[4,5] On the other hand, plasma based sources can operate in other wavelength ranges, including soft x-rays (SXRs). EUV or SXR sources having spectra of such form are suitable for experiments concerning absorption spectroscopy.[7,8,9] Isolated lines emitted from plasmas containing elements of low Z were employed in experiments concerning x-ray microscopy.[10,11,12]
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