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 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.
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 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.