Abstract
Two methods improving the brilliance of laser-induced plasmas emitting in the extreme UV (EUV) and soft x-ray (SXR) regions were investigated, using three different gases (nitrogen, krypton, and xenon) from a pulsed gas jet. Utilizing a newly designed piezoelectric valve, up to almost ten times higher gas pressures were applied, resulting in increased target densities and thus, higher conversion efficiencies of laser energy into EUV and SXR radiation. Secondly, geometrically reducing the angle between the incoming laser beam and the observed plasma emission minimizes reabsorption of the emitted short wavelength radiation. Combining both methods, the source brilliance is increased by a factor of 5 for nitrogen. Furthermore, a compact EUV focusing system for metrological applications is presented utilizing the optimized plasma source. An energy density of 1 mJ/cm2 at wavelength λ = 13.5 nm in the focal spot of an ellipsoidal mirror is achieved with xenon as the target gas being sufficient for material removal of PMMA samples with an ablation rate of 0.05 nm/pulse.
Highlights
Extreme ultraviolet (EUV) sources based on laserproduced plasmas (LPP) emitting at a wavelength of 13.5 nm are currently developed with tremendous effort for the generation of semiconductor microlithography.1,2 Besides high power sources for high volume manufacturing, compact extreme UV (EUV) sources of lower average power are needed for metrology purposes, e.g., for mask inspection, actinic material testing, or optics and sensor characterization
Two methods improving the brilliance of laser-induced plasmas emitting in the extreme UV
the source brilliance is increased by a factor
Summary
Extreme ultraviolet (EUV) sources based on laserproduced plasmas (LPP) emitting at a wavelength of 13.5 nm are currently developed with tremendous effort for the generation of semiconductor microlithography. Besides high power sources for high volume manufacturing, compact EUV sources of lower average power are needed for metrology purposes, e.g., for mask inspection, actinic material testing, or optics and sensor characterization. Plasmas generated from gas jets are generally less brilliant due to their lower particle density as compared to liquid or solid targets, such sources are of particular interest for metrological applications due to their simplicity and compactness. The laser plasma is ignited very close to the nozzle, almost no debris is generated since the gas flow protects the nozzle against erosion from the plasma, accomplishing a clean and longterm stable operation. For these reasons, several efforts have been undertaken to enhance the target particle density and the brilliance of gas jet based LPPs without waiving its inherent advantages. The optimized EUV laser plasma was focused with the help of an ellipsoidal mirror, generating a focal spot suitable for surface modification of a PMMA sample at a wavelength of 13.5 nm
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