Spectral characterization of solid-state laser-driven plasma sources of EUV light

In this thesis, the fundamental limits of converting laser radiation via tin plasmas into EUV light in a 2% bandwidth around 13.5nm, relevant to nanolithographic applications, are experimentally investigated. In particular, plasma generation using near- to mid-infrared lasers is studied. This laser wavelength region, in combination with available energy-efficient solid-state laser technology, shows promising opportunities to achieve high plasma-brightness in combination with high overall efficiencies of converting electrical power to useful EUV radiation. In terms of light source optimization, we investigate the effects of varying target morphologies and laser irradiation parameters, e.g., laser wavelength, laser intensity, and beam spot size on key experimental observables such as EUV spectra, the conversion efficiency of laser energy into industrially useful 13.5nm light (CE), the purity of the spectral emission (SP) and the plasma's optical depth. In Chapter 1, the radiative properties of plasmas formed by irradiating spherical tin droplets with high-energy, 1-um-wavelength laser pulses are investigated. Using pulses of constant intensity in space and time the plasma is homogeneously heated. A significant growth of CE and of the overall efficiency in radiating EUV light is found with increasing laser intensity, droplet size and laser pulse duration. Through optimization of these parameters, CE’s of up to 3.2% are obtained. In Chapter 2, the EUV spectra from Chapter 1 are analyzed in the context of the plasma’s optical depth using a one-dimensional, analytic radiation transport model. Using this model, we find an increase in optical depth with increasing droplet diameter and laser pulse duration. This increase in optical depth is explained in terms of a larger plasma scale length. In Chapter 3, experiments using laser pulses of 1 and 2um wavelength are performed on planar-solid targets in order to explore the drive laser wavelength regime between the well-known cases of 1 and 10um. These experiments, performed on spatially extended targets reveal high CE values of approximately 3% in the 2-um-drive laser case, approximately a factor of two higher than that obtained for 1-um-wavelength laser pulses under otherwise similar experimental conditions. The lower CE values in the 1-um-driven plasmas are largely understood via strong self-absorption of 13.5nm radiation within the plasma, a mechanism which is qualitatively explained using an analytic 1D-radiation transport model comprising two plasma zones of different temperatures and densities. In Chapter 4, the emission from tin-droplet-based plasmas driven by pulses of 2um wavelength are investigated and compared to 1-um-driven plasmas under otherwise similar conditions. Experimental results for the scaling of the plasma’s temperature and its average charge state are compared to theoretical predictions based on a simplified model for the plasma's equation-of-state as well as radiation hydrodynamic simulations. These simulations reveal an inverse scaling of electron density with laser wavelength. This scaling implies a near twofold reduction in the optical depth of a 2-um-driven plasma compared to the 1-um case. In Chapters 5, experiments with laser pulses of 1 and 2um wavelength are performed on preformed tin-droplet targets, the targets of choice in industrial EUV light sources. With increasing tin-target diameter, the plasma’s efficiency of radiating 13.5nm light increases and CE values of up to 3% are obtained using the 2-um drive laser. These CE values are almost independent of pulse duration and laser intensity within the studied parameter range and are significantly higher than CE values of approximately 1.8% measured in the 1-um case under otherwise similar conditions. In summary, high CE values are shown for 1 and 2um solid-state laser-driven tin plasmas. Under identical conditions we find higher CE values using the 2um drive laser wavelength and further increases in 2um CE may be obtained through additional laser and target shaping.