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.
Two-color multiphoton ionization of atomic helium was investigated by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. In the photoelectron spectrum, lines associated with direct ionization and above-threshold ionization show strong variations of their amplitudes as a function of both the intensity of the optical dressing field and the relative orientation of the linear polarization vectors of the two fields. The polarization dependence provides direct insight into the symmetry of the outgoing electrons in above-threshold ionization. In the high field regime, the monochromaticity of the XUV radiation enables the unperturbed observation of nonlinear processes in the optical field. DOI: 10.1103/PhysRevLett.101.193002 PACS numbers: 32.80.Fb, 32.80.Rm Multiphoton single-color ionization in intense optical or infrared laser fields has been the subject of multiple experimental and theoretical studies for more than two decades and is by now a very well understood process (e.g., [1]). The extension of these studies to multiphoton absorption in the photoionization continuum was followed by the discovery that high order harmonics of the fundamental laser frequency are emitted in the extreme ultraviolet (XUV) when a strong femtosecond optical laser pulse interacts with a gas jet (e.g., [2,3]). The combination of different wavelengths, one in the XUV and the other in the visible or near infrared, opens new opportunities. It has recently permitted the investigation of above-threshold ionization (ATI) as the result of the combined interaction of both fields [4][5][6]. In this case the dominant contribution comes from processes in the course of which the emitted electron exchanges photons with the dressing laser field via stimulated emission (or absorption) resulting in a comb of sidebands disposed on both sides of the main photoelectron line.Theoretical studies have established that the sideband intensity depends on the electron kinetic energy as well as on the strength and polarization state of the optical laser field [7]. Fitting theoretical profiles to the measured sideband signals should yield the main parameters which govern the photon-atom interaction in this regime. For example, changing the polarization of either of the radiation beams gives rise to ''dichroic effects'' in the photoelectron spectrum. It therefore opens the possibility to control the relative contributions of photoionization channels with different angular momenta.This approach has been extensively used in studies of atomic ionization by weak monochromatic radiation from synchrotrons and continuous lasers, where at least one resonant intermediate state is involved, and the basic photon-electron interaction is completely dominated by this resonant excitation [8]. The use of high harmonic XUV sources to generate similar processes in the nonresonant continuum is complicated by very difficult analysis, since contributions from several harmonics and their mutual interferenc...
An examination of the influence of target composition and viewing angle on the extreme ultraviolet spectra of laser produced plasmas formed from tin and tin doped planar targets is reported. Spectra have been recorded in the 9–17nm region from plasmas created by a 700mJ, 15ns full width at half maximum intensity, 1064nm Nd:YAG laser pulse using an absolutely calibrated 0.25m grazing incidence vacuum spectrograph. The influence of absorption by tin ions (SnI–SnX) in the plasma is clearly seen in the shape of the peak feature at 13.5nm, while the density of tin ions in the target is also seen to influence the level of radiation in the 9–17nm region.
One key aspect in the drive to optimize the radiative output of a laser-produced plasma for extreme ultraviolet lithography is the radiation transport through the plasma. In tin-based plasmas, the radiation in the 2% bandwidth at 13.5 nm is predominantly due to 4d-4f and 4p-4d transitions from a range of tin ions (Sn7+ to Sn12+). The complexity of the configurations involved in these transitions is such that a line-by-line analysis is, computationally, extremely intensive. This work seeks to model the emission profiles of each ion by treating the transition arrays statistically, thus greatly simplifying radiation transport modeling. The results of the model are compared with experimental spectra from tin-based laser-produced plasmas.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.