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.
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.
Recent work in UCD has centred on the development of a liquid metal coating process for EUV and soft X-ray collector
optics. The work involves using a room temperature liquid metal coated on a solid metal substrate of the appropriate
form. The advances made demonstrate that a stable thin coating film on the interior surface of a rotating optic substrate is
possible, and this offers promise as a solution to the problem of producing an atomically flat reflector that remains
unspoiled in front of a multi-kilowatt EUV plasma. We report on the results of preliminary EUV tests carried out on a
simple focusing liquid metal mirror.
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.