Deirdre Kilbane

Rare-earth plasma extreme ultraviolet sources at 6.5–6.7 nm

We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5–6.7 nm region based on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these resonant lines around 6.7 nm (in-band: 6.7 nm ±1%) suggest that the in-band emission increases with increased plasma volume by suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in maximized emission.

4f collapse, level density inflation and the emergence of 'compound-like' atomic states in rare earth ions

4f collapse in rare earth ions produces an enormous level density due to the occurrence of near-degeneracies of 4f/5p and 4f/5s binding energies. As a result, there is a complete breakdown of the single-particle approximation and the level and line spectra satisfy many of the distributions associated with random matrix theory. Similar behaviour has been previously reported in spectra associated with wavefunction contraction effects or atoms containing open 4f subshells; here the effects of combining both phenomena are explored. In particular it is shown that the adjacent level separations satisfy a Wigner distribution, while transitions between them obey the Porter-Thomas distribution. Detailed Hartree-Fock calculations show that there is a strong correlation between line strength and transition energy. This correlation is pursued and parametrized for the particular case of 4f→5d, 5p→5d and 5s→5p transitions in Sm IX within the unresolved transition array model.

Extreme ultraviolet source at 6.7 nm based on a low-density plasma

We demonstrate an efficient extreme ultraviolet (EUV) source for operation at λ = 6.7 nm by optimizing the optical thickness of gadolinium (Gd) plasmas. Using low initial density Gd targets and dual laser pulse irradiation, we observed a maximum EUV conversion efficiency (CE) of 0.54% for 0.6% bandwidth (BW) (1.8% for 2% BW), which is 1.6 times larger than the 0.33% (0.6% BW) CE produced from a solid density target. Enhancement of the EUV CE by use of a low-density plasma is attributed to the reduction of self-absorption effects.

Systematic investigation of self-absorption and conversion efficiency of 6.7 nm extreme ultraviolet sources

We have investigated the dependence of the spectral behavior and conversion efficiencies of rare-earth plasma extreme ultraviolet sources with peak emission at 6.7 nm on laser wavelength and the initial target density. The maximum conversion efficiency was 1.3% at a laser intensity of 1.6×1012 W/cm2 at an operating wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target. Moreover, the lower-density results in a narrower spectrum and therefore improved spectral purity. It is shown to be important to use a low initial density target and/or to produce low electron density plasmas for efficient extreme ultraviolet sources when using high-Z targets.

Extreme ultraviolet emission spectra of Gd and Tb ions

Theoretical extreme ultraviolet emission spectra of gadolinium and terbium ions calculated with the Cowan suite of codes and the flexible atomic code (FAC) relativistic code are presented. 4d–4f and 4p–4d transitions give rise to unresolved transition arrays in a range of ions. The effects of configuration interaction are investigated for transitions between singly excited configurations. Optimization of emission at 6.775 nm and 6.515 nm is achieved for Gd and Tb ions, respectively, by consideration of plasma effects. The resulting synthetic spectra are compared with experimental spectra recorded using the laser produced plasma technique.

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

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.

Analysis of tungsten laser produced plasmas in the extreme ultraviolet (EUV) spectral region

Tungsten will be used as a wall material in ITER and therefore will be present as an intrinsic plasma impurity with the resulting emission having the potential to be used as a plasma diagnostic. We have recorded spectra of tungsten laser produced plasmas in the 1‐7 nm region using Nd:YAG lasers operating at a range of power densities. We have analysed these spectra, giving special attention to the unresolved transition arrays in the 3 nm region that appear at the highest laser power densities. We compare our results to those from previous work and also use new atomic structure calculations to identify a number of new features. (Some figures may appear in colour only in the online journal)

Interpretation of spectral emission in the 20 nm region from tungsten ions observed in fusion device plasmas

We have measured extreme ultraviolet spectra from tungsten ions in the 20 nm region in plasmas produced in the Large Helical Device at the National Institute for Fusion Science. The spectra following injection of a tungsten pellet into a hydrogen plasma were monitored by a grazing incidence spectrometer. A quasicontinuum spectral feature arising from an unresolved transition array (UTA) was observed around 20 nm in plasmas with temperatures below 1.0 keV. This structure is reasonably considered to be the same as observed in another tokamak device or laser-produced plasmas under low-temperature conditions. Atomic structure calculations have been performed for tungsten ions with open 5p, 5s and 4f subshells (W7 +–W27 +) to interpret the observed feature around 20 nm. Wavelengths and strengths for these transitions were calculated, and mean wavelengths and extent of the UTAs were compared with the observations, which suggests that the emission largely arises from n = 5–5 transitions in stages lower than W27 +.

Variable composition laser-produced Sn plasmas—a study of their time-independent ion distributions

The time-independent ion distributions of variable composition laser-produced Sn plasmas are studied for a wide range of electron temperatures and atomic number densities, the purpose of which is to elucidate the effect that varying the number density of Sn within a mixed species plasma has upon the steady state populations of Sn and its ions. Particular emphasis will be placed on binary mixtures of Sn with Li, C, O or Sm and more specifically the charge states Sn8+ to Sn13+ within these mixed plasmas, where it will be assumed that the plasma is optically thin. It is found that using these composites has relatively little effect upon the Sn ion population distributions for plasma atomic number densities of less than approximately 1019.5 cm−3. However, for greater values of number densities the Sn ion populations can be shifted by as much as 10–15 eV for Li mixtures. These results are of particular relevance to current research being carried out on extreme ultraviolet lithographic technologies for the optimization of XUV sources in the 13.5 nm wavelength region, which include composite target investigations.

Sources for beyond extreme ultraviolet lithography and water window imaging

Lithography tools are being built and shipped to semiconductor manufacturers for high volume manufacturing using extreme ultraviolet lithography (EUVL) at a wavelength of 13.5 nm. This wavelength is based on the availability of Mo/Si multilayer mirrors (MLMs) with a reflectivity of ~70% at this wavelength. Moreover, the primary lithography tool manufacturer, ASML, has identified 6.x nm, where x~7, as the wavelength of choice for so-called Beyond EUVL, based on the availability of La/B4C MLMs, with theoretical reflectance approaching 80% at this wavelength. The optimum sources have been identified as laser produced plasmas of Gd and Tb, as n = 4–n = 4 transitions in their ions emit strongly near this wavelength. However, to date, the highest conversion efficiency obtained, for laser to EUV energy emitted within the 0.6% wavelength bandwidth of the mirror is only 0.8%, pointing to the need to identify other potential sources or consider the selection of other wavelengths. At the same time, sources for other applications are being developed. Conventional sources for soft x-ray microscopy use H-like line emission from liquid nitrogen or carbon containing liquid jets which can be focused using zone plates. Recently the possibility of using MLMs with n = 4−n = 4 emission from a highly charged Bi plasma was proposed and subsequently the possibility of using Δn = 1 transitions in 3rd row transition elements was identified. All of these studies seek to identify spectral features that coincide with the reflectance characteristics of available MLMs, determine the conditions under which they are optimized and establish the maximum conversion efficiencies obtainable. Thus, there is a need for systematic studies of laser produced plasmas of a wide range of elements as some of the challenges are similar for all of these sources and some recent results will be presented.