Papers by Gerry O'Sullivan
Journal of Physics B: Atomic, Molecular and Optical Physics, 2017
Journal of Physics D: Applied Physics, 2016
Extreme ultraviolet (EUV) light generation by hybrid laser-assisted vacuum arc discharge plasmas,... more Extreme ultraviolet (EUV) light generation by hybrid laser-assisted vacuum arc discharge plasmas, utilizing Sn-coated rotating-disc-electrodes, was investigated. The discharge was initiated by localized ablation of the liquid tin coating of the cathode disc by a laser pulse. The laser pulse, at 1064 nm, was generated by Nd:YAG lasers with variable energy from 1 to 100 mJ per pulse. The impact of shortening the laser pulse from 7 ns to 170 ps on the EUV generation has been investigated in detail. The use of ps pulses resulted in an increase in emission of EUV radiation. With a fixed discharge energy of ~4 J, the EUV conversion efficiency tends to plateau at ~2.4 ± 0.25% for the ps laser pulses, while for the ns pulses, it saturates at ~1.7 ± 0.3%. Under similar discharge and laser energy conditions, operating the EUV source with the ps-triggering resulted also in narrower spectral profiles of the emission in comparison to ns-triggering. The results indicate an advantage in using ps-triggering in laser-assisted discharges to produce brighter plasmas required for applications such as metrology.
Physical Review A, 1994
Spectra of laser-produced plasmas of the elements from tin to iodine contain weak bands of quasic... more Spectra of laser-produced plasmas of the elements from tin to iodine contain weak bands of quasicon-0 tinuum overlaid by weak emission lines in the 70-120-A region. Multiconfiguration-Dirac-Fock calculations show that these features are consistent with theoretical spectra for 4d 4d-'(Sf +6p) transitions in a number of adjacent ion stages which are predicted to produce unresolved transition arrays (UTA) in this spectral region. Moreover, the calculations predict a gradual decrease of 4d 5f osc-illator strength with increasing ionization up to the eleventh spectrum where ggf(4d 5f) =0.-However, inclusion of configuration interaction effects between the 4d'nf levels showed that 4d'4f and 4d~5f mixing shifts this minimum to the thirteenth spectrum. The theoretical data were then parametrized within the UTA formalism and the different order moments corresponding to weighted mean energy, variance, and skewness coefficient evaluated. PACS number(s): 32.70.n, 31.20.d, 32.30.Jc gress [16]. In this work we show that the weak emission bands observed in the vuv at A. &100 A are consistent with unresolved transition arrays of this type.
Journal of Physics: Conference Series, 2020
Synopsis EUV spectra have been recorded from optically-thin germanium laser produced plasmas crea... more Synopsis EUV spectra have been recorded from optically-thin germanium laser produced plasmas created using a high-powered TEA CO2 laser. The Cowan suite of atomic codes has been used to identify previously unidentified lines in Ge VI-XI. The absence in the spectra of Δn=0 transitions for ion stages lower than Ge9+ has been interpreted by calculating line strength ratios for of Δn=0 to Δn=1 transitions for the relevant ion stages.
Applied Physics Letters, 2007
ABSTRACT Extreme ultraviolet lithography requires a light source at 13.5 nm to match the proposed... more ABSTRACT Extreme ultraviolet lithography requires a light source at 13.5 nm to match the proposed multilayer optics reflectivity. The impact of wavelength and power density on the ion distribution and electron temperature in a laser-produced plasma is calculated for Nd:YAG and C O 2 lasers. A steady-state figure of merit, calculated to optimize emission as a function of laser wavelength, shows an increase with a C O 2 laser. The influence of reduced electron density in the C O 2 laser-produced plasma is considered in a one-dimensional radiation transport model, where a more than twofold increase in conversion efficiency over that attainable with the Nd:YAG is predicted.
AIP Advances, 2019
The extension of the roadmap of shorter wavelength extreme ultraviolet and soft x-ray sources is ... more The extension of the roadmap of shorter wavelength extreme ultraviolet and soft x-ray sources is a topic of considerable interest. We have studied the optimized emission from high power and/or high brightness sources based on unresolved transition array (UTA) emission. The peak UTA wavelengths follow a quasi-Moseley's law as λ = 33.82 × R −1 ∞ (Z − 20.86) −1.61 nm for the laser wavelength of 1064 nm (the critical density of 1 × 10 21 cm −3) and λ = 165.8 × R −1 ∞ (Z − 12.44) −1.94 nm for the laser wavelength of 532 nm (the critical density of 4 × 10 21 cm −3), respectively. The photon flux decreased with increasing atomic number. We also mapped the optimum electron temperatures and corresponding charge states required to produce strong soft x-ray UTA emission with a photon flux of the order of 10 14 photons nm −1 sr −1. The present quasi-Moseley's law is sufficient for identifying the optimum element for numerous applications, such as material ablation and ionization, nanolithography, and in vivo biological imaging.
Optics Express, 2019
Our measurement of the soft X-ray emission of Mo plasmas produced by picosecond Nd:YAG lasers emi... more Our measurement of the soft X-ray emission of Mo plasmas produced by picosecond Nd:YAG lasers emitting on the fundamental (1064 nm, 150 ps) and second (532 nm, 130 ps) harmonics is presented. The contrast in intensity between spectral peaks and the intensity outside them is lower for the second harmonic produced plasmas probably due to the presence more intense satellite emission and higher optical thickness. The measured spectra are absolutely calibrated and the observed output photon flux was (7 - 9) × 1013 photons/sr in the water-window (2.3 - 4.4 nm) spectral range for a laser energy of 160 mJ independent of laser wavelength. However, in the short wavelength range 1.5 - 2 nm, the emission using the second harmonic is strongly enhanced and is even higher than for the maximum energy of 220 mJ of the fundamental wavelength, so despite inevitable energy losses, laser wavelength conversion may lead to emission enhancement in certain spectral ranges. This enhancement is attributed to higher absorption of short wavelength laser light and higher charge state generation in denser plasmas.
Atoms, 2019
Soft X-ray spectra from high Z rare-earth (lanthanide) elements have been systematically observed... more Soft X-ray spectra from high Z rare-earth (lanthanide) elements have been systematically observed in optically thin, high-temperature plasmas produced in the Large Helical Device (LHD), a facility for magnetically confined fusion research. It has been demonstrated that the discrete and quasicontinuum (UTA) spectral features from highly charged lanthanide ions are observed depending on the plasma temperature. The analyses of the measured spectra are ongoing by comparisons with theoretical calculations and/or previous experimental data available. The discrete spectra recorded in high-temperature conditions are dominated by individual lines of Ge- to Ni-like ions, while prominent peaks in the narrowed UTA spectra observed in low-temperature conditions are well explained by the transitions of Ag- to Rh-like ions.
Optics letters, 2018
We demonstrated the upper limitation to the number of shots, i.e., target lifetime, together with... more We demonstrated the upper limitation to the number of shots, i.e., target lifetime, together with the number of photons emitted in the water-window soft x-ray spectral region from a number of targets used as sources in this spectral region, for multi-shot irradiation at the same position on the target surface. The spectra involved result from unresolved transition arrays originating from n=3-n=4 transitions in medium-Z element plasmas and from n=4-n=4 transitions originating in high-Z plasmas. The output flux was maintained for the highest number of shots in the case of the high melting point element molybdenum, and the total output in the water window was 7.7×10 photons/sr at a laser power density of 1.2×10 W/cm.
Optics letters, 2018
The effect of optical thickness in a bismuth water-window soft x-ray source is considered by comp... more The effect of optical thickness in a bismuth water-window soft x-ray source is considered by comparing the emission from laser-produced plasmas of a 7.5% atomic density foam target and a solid-density target. The number of photons recorded in the 4 nm region was comparable for both targets at a plasma-initiating laser pulse duration of 6 ns. From experiments at different pulse durations of 150 ps and 6 ns, self-absorption (opacity) effects were found to be relatively small for bismuth plasmas as compared to those of tin, based on the same emission mechanism and which are used in 13.5 nm sources for extreme ultraviolet lithography.
Optics express, Jan 15, 2018
We demonstrate intense emission in the water-window soft x-ray spectral region by controlling the... more We demonstrate intense emission in the water-window soft x-ray spectral region by controlling the spectral behavior through changing the balance between emissivity and self-absorption in an expanding plasma. The number of photons obtained from a dual laser irradiated target with a 150-ps pre-pulse was maximized at 3.8 × 10 photons/sr in λ = 2.34 - 4.38 nm at a pulse separation time of 7 - 10 ns. Enhancement of the number of photons is attributed to efficient coupling with the main laser pulse while maintaining a tiny source size.
Journal of Physics B: Atomic, Molecular and Optical Physics, 2017
The soft x-ray emission spectra of femtosecond-laser-produced plasmas from 2nd row transition ele... more The soft x-ray emission spectra of femtosecond-laser-produced plasmas from 2nd row transition elements from yttrium (Z = 39) to palladium (Z = 46), with the exception of technetium (Z = 43), were measured in the 1–5 nm region. Plasmas were produced by shining pulses from a titanium–sapphire laser with 65 fs pulse duration and an energy per pulse of 4.5 mJ focused to an intensity of 3 × 1015 W cm−2 onto bulk targets. While the emission spectra from yttrium to molybdenum (Z = 42) contain only the unresolved transition arrays (UTA) already observed in nanosecond and picosecond laser-target interactions described in our previous paper (Lokasani et al 2015 J. Phys. B: At. Mol. Opt. Phys. 48 245009), transitions from higher ionization states are clearly demonstrated in the spectra emitted from ruthenium (Z = 44), rhodium (Z = 45) and palladium targets heated by the femtosecond laser. These UTAs are interpreted by comparing the experimental spectra to calculated results, obtained using the Cowan suite of codes, the flexible atomic code as well as previous predictions of isoelectronic trends. The 3d–4f transitions arrays emitted from Ru XXI-XXIII, Rh XXI-XXIII and Pd XXI-XXIII ions are clearly seen in the observed spectra. To our knowledge, such high ionization states are demonstrated for the first time at such moderate laser energies.
X-ray Lasers and Coherent X-ray Sources: Development and Applications, 2017
Laser-produced plasmas are intense sources of XUV radiation that can be suitable for different ap... more Laser-produced plasmas are intense sources of XUV radiation that can be suitable for different applications such as extreme ultraviolet lithography, beyond extreme ultraviolet lithography and water window imaging. In particular, much work has focused on the use of tin plasmas for extreme ultraviolet lithography at 13.5 nm. We have investigated the spectral behavior of the laser produced plasmas formed on closely packed polystyrene microspheres and porous alumina targets covered by a thin tin layer in the spectral region from 2.5 to 16 nm. Nd:YAG lasers delivering pulses of 170 ps (Ekspla SL312P )and 7 ns (Continuum Surelite) duration were focused onto the nanostructured targets coated with tin. The intensity dependence of the recorded spectra was studied; the conversion efficiency (CE) of laser energy into the emission in the 13.5 nm spectral region was estimated. We have observed an increase in CE using high intensity 170 ps Nd:YAG laser pulses as compared with a 7 ns pulse.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017
We have performed ion beam collision experiments using multiply charged tantalum ions and observe... more We have performed ion beam collision experiments using multiply charged tantalum ions and observed EUV (extreme ultraviolet) emission spectra in collisions of ions with molecular targets, N 2 and O 2. Broad UTAs (un-resolved transition arrays) from multiply charged Ta ions were observed, and the mean wavelengths of the UTAs shifted and became shorter at higher charge statea of Ta ions. These UTAs may be attributed to the 4f-5d and 4f-5g transitions. Not only the UTA emission from incident ions, but also the sharp emission lines from multiply charged fragment atomic ions were observed. Production of temporary highly charged molecular ions, their kinetic energy and fragmentation processes have been investigated with coincident detection technique. However, the observation of emission from the fragments might be for the first time. The formation mechanisms of the multiply charged fragment atomic ions from target molecules are discussed.
Optics Communications, 2017
Soft x-ray and extreme ultraviolet (XUV) spectra from lead (Pb, Z=82) laser-produced plasmas (LPP... more Soft x-ray and extreme ultraviolet (XUV) spectra from lead (Pb, Z=82) laser-produced plasmas (LPPs) were measured in the 1.0-7.0 nm wavelength region employing a 150-ps, 1064-nm Nd:YAG laser with focused power densities in the range from 3.1×10 13 W/cm 2 to 1.4×10 14 W/cm 2. The flexible atomic code (FAC) and the Cowan's suite of atomic structure codes were applied to compute and explain the radiation properties of the lead spectra observed. The most prominent structure in the spectra is a broad double peak, which is produced by Δn=0, n=4-4 and Δn=1, n=4-5 transition arrays emitted from highly charged lead ions. The emission characteristics of Δn=1, n=4-5 transitions were investigated by the use of the unresolved transition arrays (UTAs) model. Numerous new spectral features generated by Δn=1, n=4-5 transitions in ions from Pb 21+ to Pb 45+ are discerned with the aid of the results from present computations as well as consideration of previous theoretical predictions and experimental data.
Applied Physics Letters, 2015
Journal of Physics: Conference Series, 2015
ABSTRACT We have observed extreme ultraviolet (EUV) spectra from terbium (Tb) ions in optically t... more ABSTRACT We have observed extreme ultraviolet (EUV) spectra from terbium (Tb) ions in optically thin and thick plasmas for a comparative study. The experimental spectra are recorded in optically thin, magnetically confined torus plasmas and dense laser-produced plasmas (LPPs). The main feature of the spectra is quasicontinuum emission with a peak around 6.5-6.6 nm, the bandwidth of which is narrower in the torus plasmas than in the LPPs. A comparison between the two types of spectra also suggests strong opacity effects in the LPPs. A comparison with the calculated line strength distributions gives a qualitative interpretation of the observed spectra.
Currently the emission of both Xe and Sn are being investigated as sources for EUV lithography. I... more Currently the emission of both Xe and Sn are being investigated as sources for EUV lithography. In Xe the bulk of the emission in the region of interest, 13.5 nm, originates from one ion stage, Xe XI, while in Sn, the emission at this wavelength arises from resonance transitions in a range of stages and thus is potentially more intense.
Uploads
Papers by Gerry O'Sullivan