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.
This paper describes the extreme ultraviolet and soft x-ray emission recorded in the 2-12 nm region from Mo, Ru, Rh and Pd ions present in the laser produced plasmas. The spectra were found to be dominated by 3p-3d transitions in the 5-8 nm region, which shift slowly to shorter wavelengths with the increasing atomic number, and by 3d-4p and 3d-4f transitions at shorter wavelengths. These transitions, in a number of neighbouring ion stages, were distinguished by comparison with Cowan code calculations and previously reported data. The experimental results show that strong emission can be observed at the 6.X nm region for Ru, Rh and Pd plasmas.
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 1011 - 1015 W/cm2. 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.
The current status of laser-produced plasma source development for extreme ultraviolet lithography is reviewed. The advantages of using Sn as a fuel for 13.5 nm are discussed as is the rationale for using Nd:YAG prepulses followed by CO2 irradiation of mass-limited Sn droplet targets. To date CEs of around 6% have been obtained in the laboratory while much lower values closer to 2% have been achieved in high repetition rate industrial sources suitable for use in high-volume manufacturing (HVM). The discrepancy appears largely to arise from the mismatch between the effective target lifetime and pulse duration as well as incomplete vaporization or fragmentation of the droplets. Improvements in laser technology and droplet delivery systems should see a marked improvement in the near future, so that CEs of 5% to 6% should eventually be realized in industrial sources. To keep pace with Moore's law, development work has begun on sources operating at 6.x nm, the wavelength selected for lithography beyond 13.5 nm. It is shown that Gd plasmas with an electron temperature close to 110 eV will provide the solution at this wavelength and the strongest lines ocurring in a Gd plasma are identified.
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.
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 K3+ to K5+ 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 1020 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 × 1012 W/cm2 at an operating
wavelength of 1064 nm, when self-absorption was reduced by use of a low initial density target.
An engineering prototype high average power 13.5-nm source has been shipped to semiconductor facilities to permit
the commencement of high volume production at a 100 W power level in 2011. In this source, UTA (unresolved
transition array) emission of highly ionized Sn is optimized for high conversion efficiency and full recovery of the
injected fuel is realized through ion deflection in a magnetic field. By use of a low-density target, satellite emission
is suppressed and full ionization attained with short pulse CO2 laser irradiation. The UTA is scalable to shorter
wavelengths, and Gd is shown to have similar conversion efficiency to Sn (13.5 nm) at a higher plasma temperature,
with a narrow spectrum centered at 6.7 nm, where a 70% reflectivity mirror is anticipated. Optimization of short
pulse CO2 laser irradiation is studied, and further extension of the same method is discussed, to realize 100 W
average power down to a wavelength of 3 nm.
Extreme ultraviolet (EUV) spectra from laser-produced tin plasmas have been recorded as a function of time
using an ISAN grazing incidence spectrograph to study the temporal evolution of the tin unresolved transition
array (UTA) responsible for the peak EUV emission. This paper reports the experimental as well as simulated
results for a 10 ns gate width with 2 ns time steps which confirm that the development and collapse of the
UTA follow the temporal behavior of the laser pulse. The self-absorption features at longer wavelengths are
observed particularly during plasma cooling and arise from lower ion stages ranging from Sn VI to Sn XI.
We have reported a discharge-produced plasma extreme ultraviolet source based on a pure potassium vapor.
Potassium ions produced strong broadband emission around 40 nm with a bandwidth of 8 nm [full width at halfmaximum
(FWHM)]. The current-voltage characteristics of discharge suggest that the source operates in a hollow
cathode mode. By comparison with atomic structure calculations, the broadband emission is found to be primarily
due to 3d-3p transitions in potassium ions ranging from K 2+ to K4+.
Two and three dot laser produced plasma extreme ultraviolet sources have been generated using a Fourier diffractive
optical element (DOE). The DOE featured a >90% diffraction efficiency and a power handling capability of
>100 MW. The plasmas were formed on a planar bulk tin target by pulses from a Nd:YAG laser delivering up to
360 mJ per pulse in a time of 15 ns (full-width half-maximum intensity) at the fundamental wavelength of 1064
nm. After passing through the DOE, the laser beam was focused onto the target by a pair of lens. The resulting
spot radius was estimated to be 8.2±0.2 μm 1/e2 on the target. The extreme ultraviolet radiation emitted by
the plasma was imaged using a 122 μm imaging slit in conjunction with the 38 μm slit of the spectrometer. The
one dimensional image of the laser produced plasma extreme ultraviolet source, together with its spectrum, was
recorded by an absolutely calibrated Jenoptic 0.25 m EUV spectrograph. The spectrograph was located at an
observation angle of 45 degrees with respect to the target. The vacuum chamber and spectrograph were both
maintained at a base pressure of 10-6 Torr. The recorded 1D spatial distribution and EUV spectra demonstrate
the feasibility of EUV patterning by the novel optical method. The characteristics and potential applications of
this method are investigated in this paper.
The photoabsorption spectra of Te I-Te IV have been recorded and analysed in the XUV spectral region using the dual laser produced plasma technique. Photoexcitation from the 4d subshell is the dominant process in the 35-150 eV energy region. For photon energies between 35-45 eV discrete structure corresponding to 4d-np (n>4) transitions were obtained. Above the 4d ionisation threshold the spectra of Te I-Te III were found to be dominated by a 4d-ef shape resonance, which peaks at ~88 eV in each case. A transfer of oscillator strength from the resonance to discrete 4d-nf (n>3) transitions with increasing ionisation is clearly evident, and the 4d-4f transitions are the strongest features in the Te IV spectrum. Hartee-Fock with configuration interaction and time dependent local density approximation calculations successfully account for this behaviour and permit identification of the discrete features. The use of a prepulse to maximise the brightness of a tungsten continuum emitting plasma was also investigated.
The photabsorption spectrum of I IV-VI, Ba IV-VI and La V-VIII have
been recorded using the dual laser produced plasma technique. At lower energies, the barium and lanthanum spectra are dominated by 4d→5p transitions in the regions 76-79 eV and 84-94 eV respectively, whereas for iodine the 4d→nf transitions are dominant between 75-105 eV. For each spectrum, the transitions were identified with the aid of multiconfiguration Hartree-Fock calculations.
Laser-produced plasma source development for Extreme Ultra Violet (EUV) lithography has concentrated on xenon, since XeXI emits at 13.5 nm, the wavelength at which the reflectivity of MoSi mirrors is centred. However it is not obvious that the required conversion efficiencies can be achieved using xenon, and tin has been identified as a strong emitter at this wavelength. The transitions responsible in tin are 4p6 4dn-4p5 4dn+1 + 4p6 4dn-14f occurring in a number of adjacent ion stages that merge to form an unresolved transition array (UTA). This UTA is similar to a feature that appears between 10 nm to 11 nm in xenon, which thus provides information directly relevant to tin. The present experimental studies on xenon were performed at the NIST Electron Beam Ion Trap (EBIT). EBITs were developed to perform spectroscopic studies of highly charged ions. The experiments involved changing EBIT parameters, such as the electron beam energy, so that the distribution of ion stages within the plasma changed systematically. Analysis of the corresponding EUV spectra yields information about the contribution of various ion stages to the evolution of the UTA between 10 nm - 11 nm. Previously reported data for the ion stages XeVII through to XeXI are used to identify features occurring in the EBIT spectrum. When the EBIT relative intensities are compared to those from vacuum spark sources they are found to give better agreement with the calculated gA values (statistically weighted Einstein A-coefficients).
The search for a source of EUV radiation for photolithography in the 13.5 nm region has been narrowed down to laser produced or pulsed discharge plasmas containing either xenon or tin. Higher conversion efficiencies can however be obtained with tin based plasmas within this wavelength regime. It is known that EUV photoabsorption by the lower ion stages of xenon reduces the photon flux from a xenon source. This is due to shape resonances from 4d-epsilonf transitions within Xe I-IV. The corresponding resonances for Sn I-IV have been obtained by means of the dual laser plasma (DLP) technique. It is also found that above the 4d ionisation threshold the spectra of Sn I-IV are dominated by a 4d-epsilonf shape resonance which peaks at close to 65 eV in each case. A transfer of oscillator strength from the shape resonance to pseudo-discrete 4d→nf transitions with increasing ionisation is clearly evident. Hartree-Fock with configuration interaction and relativistic time dependent local density approximation calculations successfully account for this behaviour and also permit identification of the discrete features.
The ideal source of radiation for extreme ultraviolet lithography will produce intense light in a 2% bandwidth centred at 13.5 nm, while the debris and out-of-band radiation produced will be limited to prevent adverse effects to the multilayer optics in the lithography system. In this study ways to optimise plasma sources containing tin are presented. The optimum power density for a tin slab target, with a fixed spotsize, is determined, while the effects of power density on ceramic targets, where tin is present only as a few percent in a target of mainly low Z elements, is also investigated. It has been found that the in-band radiation is increased when the concentration is 5-6%, while the out-of-band radiation is dramatically reduced, due the the low average Z of the target constituents, with conversion effciencies of over 2.5% recorded. The power density needed to optimise the emission from ceramic targets was found to be greater than that required for the pure tin case. In addition, if the target is first irradiated with a pre-pulse, the conversion effciency is seen to increase.
The aim of this study is to investigate ways to maximise the efficiency of tin based laser produced plasmas as sources of EUV radiation in the 2% band centered on 13.5 nm. It has been found that targets containing below 15% tin atoms by number emit more brightly in the spectral region around 13.5 nm than pure tin targets. Furthermore, if the remaining material in the target is composed on primarily low-Z atoms, then both plasma continuum radiation and Bremsstrahlung radiation are greatly reduced. In addition, if the target is illuminated with a prepulse, the conversion efficiency shows a distinct increase. The third parameter under examination is the laser power density, which controls the ion distribution in the plasma. The influence of low-Z atoms on the tin ion distribution in the plasma has been investigated and found to be of little consequence.
Measurements were made in the region from 9-17 nm on an absolutely calibrated 0.25-m flat field grazing incidence spectrograph, and on two 2-m grazing incidence spectrographs. Spectra and conversion efficiency data from a range of target materials and illumination regimes are presented.
The emission spectra of laser produced plasmas of pure tin targets are dominated by recombination continuum emission throughout the entire EUV spectral region with intense structure due to line emission dominating the spectra in the 13 - 14 nm region. This feature arises from resonant 4p64dn - 4p54dn+1 + 4p64dn-14f emission lines that are generally concentrated in a narrow band, 5 - 10 eV wide, which overlaps considerably in adjacent ion stages to form an intense unresolved transition array (UTA). Such plasmas are optically thick; the strongest lines are attenuated and frequently appear in absorption. However, if tin comprises a few percent of a predominantly low-Z matrix, the recombination is suppressed and the plasmas can become optically thin to resonance radiation. Under these conditions, resonance line emission can dominate the spectra. The application of a collisional radiative (CR) model, combined with ab initio atomic structure calculations, allows one to estimate the laser plasma parameters that will optimize the UTA as efficient narrow bandwidth emitters of EUV radiation. The dependence on laser power density of both in-band emission and debris generation from pure tin targets is presented. The influence of a pre-pulse on the plasma output is also investigated.
We report here some observations and preliminary findings from a study focussed on the vacuum-UV (λ, 40-60 nm) radiation emitted during the interaction of 150 ps laser pulses (100-400 mJ) with copper pre-plasmas formed by an electro-optically synchronised (0.1 - 0.8 J, 8 ns) long pulse laser. We have observed significant gains in VUV flux that scale with inter-laser delay. We also report preliminary observations on total X-ray emission from the interaction of a superintense 80 fs, 200 mJ laser pulse at the UK ASTRA laser facility with a similar pre-plasma at irradiances approaching 1019 W/cm2
The internationally agreed semiconductor roadmap calls for the development of bright, narrow bandwidth sources of extreme UV radiation for lithography at 13.5 nm. Both pulsed discharge sources and laser produced plasmas have been proposed as possible candidates and xenon is currently the most favored constituent element since it possesses strong emission lines at the correct wavelength. In this paper we show from theoretical considerations that tin containing plasmas provide superior intesitiy, albeit at the expense of increased debris emission. We also show that for laser produced plasma sources, the brightness can be considerably enhanced by varying both the target constituents and the laser pulse profile. Various methods by which the debris problem might be reduced are discussed.
The emission from laser produced plasma has been employed as a source of soft X-ray radiation for nanotechnology. By incorporating high Z elements into low Z solid hosts, targets have been fabricated which, when illuminated with high power Nd:YAG laser pulses, emit a significant proportion of the incident energy in a narrow wavelength region, with reduced continuum emission in the adjacent spectrum. Such a source would be of considerable utility in a projection lithography setup, where the presence of multilayer optics prevents the use of broadband soft X-ray sources. Cesium, barium and cerium were incorporated into different targets, resulting in different peak wavelengths for the emission. 4d - 4f transitions in highly ionized species have been identified as being responsible for the narrow emission feature. In addition, efforts to reduce particulate debris from the target are reported.
The application of extreme ultraviolet (XUV) radiation to studies of core excited atoms, ions, molecules, and solids has led to the demand for suitable XUV continuum light sources. An excellent, compact, reproducible source is the laser produced plasma, which for a suitable choice of target material can be used to generate a line-free continuum throughout the vacuum ultraviolet (VUV) and XUV from 4 to 200 nm. The characteristics and underlying physics of such sources are discussed and their application to inner shell photoexcitation studies of atoms and ions are briefly described.
EUV emission from laser plasmas produced on low Z target material such as plastics and araldites that contain 1 to 10% concentrations of the elements 50 ≤ Z ≤ 70 emit narrow regions of intense quasicontinua in the 6 to 13-nm wavelength range. The typical bandwidth of the emission is 0.5 to 1.5 nm and the features are brighter than any individual lines emitted from the host substrate materials. Similar spectra can be obtained from hydrated salts of these elements, so suitable targets can be prepared as compressed pellets of crystalline or hydrated salts or alternatively from oxides mixed into an araldite matrix. Spectra of tin salts prepared in this way provide an excellent narrowband source for 13.5-nm radiation for use with Mo/Si multilayers. Since the peak shifts toward shorter wavelengths with increasing Z, plasmas of these elements provide "tunable" narrowband EUV radiation sources.
The spectrum of a laser-produced plasma in the XUV-VUV region consists typically of many strong emission lines together with varying degrees of continuum, the latter being due primarily to recombination. In plasmas produced by a Q-switched ruby laser (approximately 1 J output in approximately 25 ns) on a range of high Z targets, in particular the rare earth metals, it was found that the spectrum, over a wide wavelength region, consisted of smooth continuum with apparently no individual lines. The spectrum of samarium, for example, consisted of continuum from approximately 200 to 4 nm with no evidence of lines (except occasional impurity lines, due usually to oxygen). For ytterbium a similar clean continuum extended from 60 to 4 nm (the limit of the authors observations). The dominant stages of ionization in the plasmas are A10+ to A15+, and the observed absence of lines can be explained by a consideration of the electron configurations in these stages. The continua are very useful for experiments such as absorption spectroscopy and other applications in the XUV-VUV region and offer an alternative to the synchrotron when polarized radiation is not required. The sources are simple to mount and operate, are reproducible, intense, and insensitive to ambient pressure. Because of their pulsed mode of operation they are particularly suitable for time-resolved studies. Several applications of the sources are described and some possible future developments are suggested, including the use of excimer lasers.
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