The primary requirement for the development of tools for extreme ultraviolet lithography (EUVL) has been the identification and optimization of suitable sources. These sources must be capable of producing hundreds of watts of extreme ultraviolet (EUV) radiation within a wavelength bandwidth of 2% centred on 13.5 nm, based on the availability of Mo/Si multilayer mirrors (MLMs) with a reflectivity of ~70% at this wavelength. Since, with the exception of large scale facilities, such as free electron lasers, such radiation is only emitted from plasmas containing moderately to highly charged ions, the source development prompted a large volume of studies of laser produced and discharge plasmas in order to identify which ions were the strongest emitters at this wavelength and the plasma conditions under which their emission was optimized. It quickly emerged that transitions of the type 4p64dn − 4p54dn+1 + 4dn−14f in the spectra of Sn IX to SnXIV were the best candidates and work is still ongoing to establish the plasma conditions under which their emission at 13.5 nm is maximized. In addition, development of other sources at 6.X nm, where X ~ 0.7, has been identified as the wavelength of choice for so-called Beyond EUVL (BEUVL), based on the availability of La/B based MLMs, with theoretical reflectance approaching 80% at this wavelength. Laser produced plasmas of Gd and Tb have been identified as potential source elements, as n = 4 − n = 4 transitions in their ions emit strongly near this wavelength. However to date, the highest conversion efficiency (CE) obtained, for laser to BEUV energy emitted within the 0.6% wavelength bandwidth of the available mirrors is only 0.8%, compared with values of 5% for the 2% bandwidth relevant for the Mo/Si mirrors at 13.5 nm. This suggests a need to identify other potential sources or the selection of other wavelengths for BEUVL. This review deals with the atomic physics of the highly-charged ions relevant to EUV emission at these wavelengths. It considers the developments that have contributed to the realization of the 5% CE at 13.5 nm which underpins the production of high-volume lithography tools, and those that will be required to realize BEUV lithography.

https://iopscience.iop.org/article/10.1088/0953-4075/48/14/144025/pdf

Spectroscopy of highly charged ions and its relevance to EUV and soft x-ray source development

We achieved a continuous, stable, ultrashort pulse hard x-ray point source by focusing 1.8-W, 1-kHz, 50-fs laser pulses onto a novel, 30‐µm-diameter, high-velocity, liquid-metal gallium jet. This target geometry avoids most of the debris problems of solid targets and provides nearly 4π illumination. Photon fluxes of 5×108 photons/s are generated in a two-component spectrum consisting of a broad continuum from 4 to 14 keV and strong Kα and Kβ emission lines at 9.25 and 10.26 keV. This source will find wide use in time-resolved x-ray diffraction studies and other applications.

Ultrashort 1-kHz laser plasma hard x-ray source.

Extreme ultraviolet lithography requires a high-efficiency light source at 13 nm that is free from debris. Our mass-limited Sn-based laser plasma source shows 1.2% conversion efficiency. Emission spectra from the source were obtained to observe the effects of Sn concentration and effects of laser intensity. Debris measurements were analyzed, and an enhanced repeller field configuration shows marked improvement in mitigating debris.

/pdf/high-conversion-efficiency-mass-limited-sn-based-laser-59qurhym4t.pdf

High conversion efficiency mass-limited Sn-based laser plasma source for extreme ultraviolet lithography

A high repetition-rate laser plasma target source system and lithography system is disclosed. The target source system comprises in a preferred embodiment a liquid tank source and freezer which freezes microscopic particles into crystal shapes which are projected by a nozzle jet from a high repetition rate liquid-droplet injector into the path of a flashing laser beam, which results in producing soft x-rays of approximately 13 nm. Uncollected and unshot target crystals are collected and reliquified by a heater source in order to be recycled back to the liquid tank source. Optionally an auxiliary source and detector system can be used to allow for instantaneous triggering of the laser beam. The target source system can be incorporated into well known EUV lithography systems for the production of wafer chips.

/pdf/water-laser-plasma-x-ray-point-source-and-apparatus-43k6t36p2j.pdf

Water laser plasma x-ray point source and apparatus

Light sources based on laser plasmas using tin as target material are known to provide high conversion efficiency of laser power to emission in the 13.5 nm spectral region. In addition, laser plasmas produced from microscopic droplet targets enable the utilization of the mass-limited concept which minimizes the effect of target debris produced from the laser plasma interaction. By combining the mass-limited target concept and tin as the choice of target material, we are developing an extreme-ultraviolet (EUV) light source that can supply high power while remaining essentially debris-free. This source uses tin-doped microscopic droplet liquid targets that are generated at high-repetition rates (>30 kHz), which allows convenient upward power scaling when coupled with a high averaged-power laser. 

Detailed studies of the radiation from this source have been made using a precision Nd:YAG laser. Broad parametric studies of the conversion efficiency along with in-band spectroscopy of this EUV source have been performed. The parametric dependence of conversion efficiency is established based on measurements made by the Flying Circus diagnostic tool and a calibrated high-resolution flat-field spectrometer. These measurements have been independently confirmed by the Flying Circus 2 team.

High conversion efficiency microscopic tin-doped droplet target laser-plasma source for EUVL

We propose the use of mass-limited, line emitting cryogenic targets for SXPL, which permit a continuous supply of targets without the problem of particulate debris and excessive heating of multilayer optics by an intense x-ray flux in wavelength regions outside the multilayer bandwidth. In preliminary experiments we measured the oxygen line emission in the vicinity of 13 nm. The x-ray emitting plasma was produced by using a laser intensity of 2 X 1012 W/cm2 on the surface of an ice target. From the observed crater on target we can deduce that clusters are also ejected from cryogenic targets.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Mass-limited laser-plasma cryogenic target for 13 nm point x ray sources for lithography

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Mass-limited laser-plasma cryogenic target for 13 nm point x ray sources for lithography