The thesis addresses several methods to characterize as well as enhance the Extreme UV emission of laser produced plasmas generated from gaseous targets. Xenon was selected as the target material due to the presence of a spectral peak near 13.5 nm in Xe plasma radiation and the absence of debris particles and contamination by the plasma EUV source. In principle some of the methods developed are also valid for plasmas generated from other gaseous systems including discharge produced plasmas. Generally, the EUV yield of plasmas generated from gaseous targets dramatically depends on the target density profile, i.e. the geometry of the target system including the expansion of target material. Several methods have been applied to increase the EUV yield. These included the increase of the target gas density, the use of (super)sonic nozzles with collimated jets, the application of a concentric flow of transparent buffer gas around the target, and the generation of shock-waves in the plasma. Increased target densities did increase the EUV production but also led to stronger EUV absorption in the outer parts of the gaseous target. Applying an EUV transparent buffer gas from an annular, concentric nozzle did reduce that EUV absorption significantly. Thirdly, shockwaves were shown to lead to a local increase of the gas density resulting in a considerable (up to a factor of five) increase of the EUV yield. The different mechanisms causing these effects, are discussed. Finally, the relation between EUV emission and the duration of the laser pulse was investigated, indicating that critical processes to create the relevant ionization stages take place on a time scale of the order of relatively short laser pulses (few ns). A number of XUV diagnostics were constructed and applied to characterize the plasma and it’s emission properties. X-ray backlighting was used to record X-ray shadowgraphs of the gas jet. By using Abel inversion, the density profiles were reconstructed. Raleigh scattering was used to determine the change in average particle size, which was used to determine the degree of condensation. Characterization of the XUV emission properties of the plasmas was performed using an XUV spectrograph based on a transmission grating, a narrow-band XUV diagnostic based on the use of a multilayer mirror, and an XUV pinhole camera. In addition to the selection of different target geometries, several debris mitigation techniques have been applied in order to mitigate any residual contamination from the plasmas. The foil trap technique, which is based on adsorption of thermalized atoms from the source at the surface of radially positioned foils, was characterized, using a fast rotating target as a source of debris of different composition. The diagnostics included collection of debris on witness plates studied by optical microscopy and SEM, oscillating quartz crystals to measure the deposited mass, and monitoring of the reflectivity of exposed multilayer mirrors. The mechanisms active in the mitigation of the debris were analyzed, and relations were found between debris composition and suppression techniques.
|Qualification||Doctor of Philosophy|
|Award date||7 Jun 2004|
|Place of Publication||Eindhoven|
|Publication status||Published - 2004|