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SUMMARY We have studied self-absorption effects of a laser-produced plasma (LPP) source at 6.x nm as a function of laser pulse duration and viewing angle. The spectral profiles are shown to have a strong dependence on the viewing angle. Absorption effects are less pronounced when a 150-ps pulse duration is used due to reduced opacity resulting from plasma expansion. Conversion efficiencies and ion energies are also measured as a function of laser pulse duration and laser power density. A maximum conversion efficiency of 0.4% within a 0.6% bandwidth was measured, and lower ion kinetic energy was observed. It is concluded that in order to reduce self-absorption effects and ion yields, and increase conversion efficiencies, a short pulse duration should be used.

Soft X-ray (SXR) spectra from hafnium and tantalum laser produced plasmas were recorded in the 1–7 nm region using two Nd:YAG lasers with pulse lengths of 170 ps and 10 ns, respectively, operating at a range of power densities. The maximum focused peak power density was 2. 3 × 1014 W cm-2 for 170 ps pulses and 1. 8 × 1012 W cm-2 for 10 ns pulses, respectively. Two intense quasicontinuous intensity bands resulting from n = 4 - n = 4 and n = 4 - n = 5 unresolved transition arrays (UTAs) dominate both sets of experimental spectra. Comparison with calculations performed with the Cowan suite of atomic structure codes as well as consideration of previous experimental and theoretical results aided identification of the most prominent features in the spectra. For the 10 ns spectrum, the highest ion stage that could be identified from the n = 4 - n = 5 arrays were lower than silver-like Hf25+ and Ta26+ (which has a 4d 104f ground configuration) indicating that the plasma temperature attained was too low to produce ions with an outermost 4d subshell, while for the 170 ps plasmas the presence of significantly higher stages was deduced and lines due to 4d–5p transitions were clearly evident. Furthermore, we show an enhancement of emission from tantalum using dual laser irradiation, and the effect of pre-pulse durations and delay times between two pulses are demonstrated.

Summary form only given. When plasmas are instantaneously created around an electromagnetic wave, frequency of the wave up-converted to the frequency, which depends on the plasma frequency. This phenomenon is called as the flash-ionization predicted by S. C. Wilks et. al. [1]. The theory requires not only the plasma creation in time much shorter than an oscillation period of the electromagnetic wave but also plasma length much longer than a wavelength of it. We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultra short laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave and plasma length longer than a wavelength of the wave. We observed frequency up-conversion from 0.35 THz to 3.3 THz by the irradiance of the Ti:S laser in ZnSe crystal.

Over the past 10 years Extreme Ultraviolet Lithography (EUVL) sources based on 13.5nm emission from Sn plasmas have been investigated. Sn plasmas produce strong resonant emission due to 4d-4f, 4p-4d and 4d-5p transitions around 13.5 nm, which overlap in adjacent ion stages to yield an intense unresolved transition array (UTA). The UTA consists of hundreds of thousands of near-degenerate resonance lines, therefore high intense emission can be obtained from Sn plasmas at around 13.5 nm. A high power EUV source is an important requirement for EUVL. Shorter wavelength sources can not only be used for EUVL, but also material science, and biological imaging near the water window. According to a previous work, it was shown that the UTA peak wavelength depends on atomic number and will therefore extend down towards the water window region1. This may be of great interest if a high power source can be achieved using UTA emission.

We demonstrate the extreme ultraviolet (EUV) and soft x-ray sources in the 2 to 7 nm spectral region related to the beyond EUV (BEUV) question at 6.x nm and the water window source based on laser-produced high-Z plasmas. Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays (UTAs), extending below the carbon K edge (4.37 nm). An outline of a microscope design for single-shot live cell imaging is proposed based on high-Z plasma UTA source, coupled to multilayer mirror optics.

In this work we present results on the influence of laser pulse duration and irradiating power density on the conversion efficiency (CE) and the ion energy of gadolinium (Gd) laser produced plasmas. Three lasers were used with 10 ns, 150 ps and 140 fs pulse durations. By varying the lasers output energies, experiments could be carried out for a power density range of 10 11 - 10 15 W/cm 2 . A maximum CE of 0.4% was achieved within a 0.6% bandwidth in 2π steradians using the picosecond laser. A faraday cup was used to calculate ion yield and time of flight measurements of each laser. The picosecond laser also showed a reduction in the ion time of flight measurements compared with the nanosecond pulse.

We demonstrate a table-top strong band emission water window source based on laser-produced high-Z plasmas. Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays in the 2 to 4 nm region, extending below the carbon K edge (4.37 nm). Arrays resulting from n = 4-n = 4 transitions are overlaid with n = 4-n = 5 emission and shift to shorter wavelength with increasing atomic number. Under spectral analysis a guideline for microscope construction design for single-shot live cell imaging is proposed based on the use of a bismuth plasma source, coupled with multilayer mirror optics.

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.

Plasmas containing gadolinium have been proposed as sources for next generation lithography at 6.x nm. To determine the optimum plasma conditions, atomic structure calculations have been performed for Gd11+ to Gd27+ ions which showed that n=4 - n=4 resonance transitions overlap in the 6.5 – 7.0 nm region. Plasma modeling calculations, assuming collisional-radiative equilibrium, predict that the optimum temperature for an optically thin plasma is close to 110 eV and that maximum intensity occurs at 6.76 nm under these conditions. The close agreement observed between simulated and experimental spectra from laser and discharge produced plasmas indicates the validity of our approach. Science Foundation Ireland ab, li - TS 27.03.12

We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultra short laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave and plasma length longer than a wavelength of the wave. We observed frequency up-conversion from 0.35 THz to 3.3 THz by the irradiance of the Ti:S laser in ZnSe crystal.

We characterize the emission spectra of a potassium plasma and its temporal behavior at 39 nm. To understanding the potassium spectral behavior without contamination effect, we use a laser-produced plasma to control the plasma parameters by changing the laser intensity and wavelength. Potassium ions produced strong broadband emission around 40 nm ranging from K 3+ to K 5+ ions at a time-averaged electron temperature of about 12 eV. Emission at 39 nm is caused during the recombining phase and it was reproduced by hydrodynamic simulation, which accounted for atomic processes. As the emission spectral behavior of the laser-produced potassium plasma XUV source is similar to that of the hollow cathode-mode discharge-produced plasma spectrum, it indicates that the emission from the discharge-produced plasma occurs in a region of high electron density close to 10 20 cm -3 .

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 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. We also 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 × 10 12 W/cm 2 at an operating wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target.

Presented at a poster session at Advances in X-Ray/EUV Optics and Components VI, Monday 22 August 2011, San Diego, California, USA

When plasmas are instantaneously created around an electromagnetic wave, frequency of the wave up-converted to the frequency, which depends on the plasma frequency. This phenomenon is called as the flash-ionization. The theory requires the plasma creation in time much shorter than an oscillation period of the electromagnetic wave. We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultrashort laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave. We observed frequency up-conversion from 0.2 THz to 0.7–1.5 THz by the irradiance of the laser in ZnSe crystal. The up-conversion to the broad frequencies suggests that the flash ionization occurs in non-uniform plasmas produced in the deeper region of the crystal.

We have demonstrated that the terahertz radiation can be generate by the burst current produced by a laser created ionization front, which is induced an optical-field-induced ionization in air with a pulsed electric field. The peak frequency of the radiation spectrum depends on the rise time of the OFI. The central frequencies of the radiation are observed to be 0.2 and 0.04 THz at the pulse durations of 50 and 300 fs (FWHM), respectively. The power of the linearly polarized THz radiation is linearly increased with the square of the electrostatic field strength applied to the capacitors.

Emission spectra from multiply charged potassium ions ranging from K(3+) to K(5+) have been obtained in the extreme ultraviolet (EUV) spectral region. A strong emission feature peaking around 38 nm, corresponding to a photon energy of 32.6 eV, is the dominant spectral feature at time-averaged electron temperatures in the range of 8-12 eV. The variation of this emission with laser intensity and the effects of pre-pulses on the relative conversion efficiency (CE) have been explored experimentally and indicate that an enhancement of about 30% in EUV CE is readily attainable.

Extreme ultraviolet (EUV) spectra from gold laser produced plasmas were recorded in the 1–7 nm region using two Nd:YAG lasers with pulse lengths of 150 ps and 10 ns, respectively, operating at a range of power densities. The maximum focused peak power density was 9.5 × 1013 W cm−2 for the former and 5.3 × 1012 W cm−2 for the latter. Two intense quasicontinuous intensity bands resulting from n = 4–n = 4 and n = 4–n = 5 unresolved transition arrays dominate the 4–5.5 and 1.5–3.6 nm regions of both spectra. Comparison with atomic structure calculations performed with the Cowan suite of atomic structure codes as well as consideration of previous experimental and theoretical results aided identification of the most prominent features in the spectra. For the ns spectrum, the highest ion stage that could be identified from the n = 4–n = 5 arrays was Au28+ while for the ps plasma the presence of significantly higher stages was deduced and lines due to 4d104f–4d94f2 transitions in Ag-like Au32+ give rise to the strongest observed features within the n = 4–n = 4 array while in the n = 4–n = 5 array it was possible to identify a number of previously unidentified spectral features as resulting from 4f–5g transitions in the spectra of Au XX to Au XXXIII.

Unresolved transition array (UTA) is scalable to shorter wavelengths, and we demonstrate a table-top broadband emission "water window" soft x-ray source based on laser-produced plasmas. Resonance emission from multiply charged ions merges to produce intense UTAs in the 2 to 4 nm region, extending below the carbon K edge (4.37 nm). An outline of a microscope design for single-shot live cell imaging is proposed based on a bismuth (Bi) plasma UTA source, coupled to multilayer mirror optics.

Development of laser-produced plasma “water window” sources poses a major challenge in x-ray research and most effort has focused on line sources for use with zone plate optics. Here, a comparison of carbon and nitrogen line emission with that from both 3d – 4f and 4d – 4f unresolved transition arrays shows that, at power densities available from “table-top” solid-state lasers, 3d – 4f emission from zirconium plasmas is most intense, and calculations show that in an imaging system based on multilayer mirrors, for reflectance bandwidths >1% has superior performance than either line or broader-band sources. For bandwidths

XUV spectra of laser-produced zirconium plasmas in the 20 to 65 ? wavelength range, where 3d?4p and 3d?4f transitions dominate the observed emission, were studied experimentally and theoretically. Atomic structure calculations were performed with the Cowan and flexible atomic codes. Spectator satellite transitions, especially 3dn-14s?3dn-24s4f, where the 4s electron acts as a ?real? spectator, in Mn-, Fe-, Co-, Ni- and Cu-like ions have been found to play an important role. The resulting emission causes arrays resulting from 3dn?3dn4f transitions to broaden and must be included in spectral simulations in order to reproduce the overall spectral profile.

Recent increases in reflectance at 6.x nm of La/B4C multilayer mirrors has led to the adoption of this wavelength for the development of future sources for lithography. The emission from laser produced plasmas of Gd and Tb emit strongly in this wavelength region. The transitions responsible and the optimum plasma conditions for optimum emission have been identified both theoretically and experimentally.

We have demonstrated frequency up-conversion in the terahertz (THz) region by flash ionization, which occurs due a terahertz wave interacting with a plasma created by an ultrashort laser pulse. The upshift frequency is controlled by the plasma density. We performed a proof-of-principle experiment with a plasma creation time scale much shorter than the period of the electromagnetic wave and a plasma length longer than its wavelength. We upshifted the frequency from 0.35 to 3.5 THz by irradiating a ZnSe crystal with a Ti:sapphire laser pulse.

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)

Recent work on multilayer mirror development for beyond extreme ultraviolet lithography indicates that their optimum reflectivity occurs at either 6.63 nm or 6.66 nm which may be too short a wavelength for Gd-based plasma sources. Calculations performed for Tb12+ to Tb28+ ions show that if the mirror reflectivity is fixed at one of these values, Tb may be a better source, though Gd is capable of providing greater intensity if the full reflection curve of the mirrors is exploited. Theoretical simulation shows that the Tb emission peaks close to 6.51 nm at an optimum electron temperature close to 120 eV.

We have demonstrated the effect of viewing angle on the extreme ultraviolet (EUV) emission spectra of gadolinium (Gd) near 6.7 nm. The spectra are shown to have a strong dependence on viewing angle when produced with a laser pulse duration of 10 ns, which may be attributed to absorption by low ion stages of Gd and an angular variation in the ion distribution. Absorption effects are less pronounced at a 150-ps pulse duration due to reduced opacity resulting from plasma expansion. Thus for evaluating source intensity it is necessary to allow for variation with both viewing angle and target orientation. Science Foundation Ireland

We have demonstrated an efficient extreme ultraviolet (EUV) source at 6.7 nm by irradiating Gd targets with 0.8 and 1.06 μm laser pulses of 140 fs to 10 ns duration. Maximum conversion efficiency of 0.4% was observed within a 0.6% bandwidth. A Faraday cup observed ion yield and time of flight signals for ions from plasmas generated by each laser. Ion kinetic energy was lower for shorter pulse durations, which yielded higher electron temperatures required for efficient EUV emission, due to higher laser intensity. Picosecond laser pulses were found to be the best suited to 6.7 nm EUV source generation. Science Foundation Ireland

We demonstrate a table-top broadband emission water window source based on laser-produced high-Z plasmas. Resonance emission from multiply charged ions merges to produce intense unresolved transition arrays in the 2 to 4 nm region, extending below the carbon K edge (4.37 nm). Arrays resulting from n = 4−4 transitions are overlaid with n = 4−5 emission and shift to shorter wavelength with increasing atomic number. From spectral analysis, a guideline for microscope construction design for single-shot live cell imaging is proposed based on the use of a bismuth plasma source, coupled with multilayer mirror optics. Higher Education Authority

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.

We have characterized a discharge-produced potassium plasma extreme ultraviolet (XUV) source. Potassium ions produced strong broadband emission around 40 nm with a bandwidth of 8 nm (full width at half-maximum). By comparison with atomic structure calculations, the broadband emission is found to be primarily due to 3d–3p transitions in potassium ions ranging from K2+ to K4+. The current-voltage characteristics of the microdischarge suggest that the source operates in a hollow cathode mode and consequently the emitting ions may be localized on the potassium electrode surface at the hole into the capillary. To understand the spectral behavior from the potassium plasmas we compared the spectra from the discharge-produced plasma with that from a laser-produced plasma. The spectra from the different (electric and laser) plasmas at the same electron temperature (12 eV) were almost the same. This compact capillary XUV source with a photon energy of 30 eV is a useful XUV emission source for surface morphology app...

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.

Self-compression of 20-mJ laser pulses has been demonstrated in a free-space argon-filled cell. A 130-fs pulse was compressed to less than 60 fs (full width at half maximum) with an output energy of 17 mj at an argon gas pressure of 25 kPa, corresponding to an input peak power of 3.65 times the self-focusing critical power through a single filament in a 10-mJ energy region.