Abstract
An experimental study of laser-produced plasmas is performed by irradiating a planar tin target by laser pulses, of 4.8 ns duration, produced from a KTP-based 2-µm-wavelength master oscillator power amplifier. Comparative spectroscopic investigations are performed for plasmas driven by 1-µm- and 2-µm-wavelength pulsed lasers, over a wide range of laser intensities spanning 0.5 - 5 × 1011 W/cm 2. Similar extreme ultraviolet (EUV) spectra in the 5.5-25.5 nm wavelength range and underlying plasma ionicities are obtained when the intensity ratio is kept fixed at I1µm/I2µm = 2.4(7). Crucially, the conversion efficiency (CE) of 2-µm-laser energy into radiation within a 2% bandwidth centered at 13.5 nm relevant for industrial applications is found to be a factor of two larger, at a 60 degree observation angle, than in the case of the denser 1-µm-laser-driven plasma. Our findings regarding the scaling of the optimum laser intensity for efficient EUV generation and CE with drive laser wavelength are extended to other laser wavelengths using available literature data.
Highlights
Extreme-ultraviolet (EUV) lithography has successfully entered high-volume manufacturing, enabling the continued miniaturization of semiconductor devices
CO2-gas lasers operating at λ = 10.6 μm wavelength are used to drive the EUV-emitting plasma at a high conversion efficiency (CE) of laser energy into ‘in-band’ radiation, i.e. in a 2% wavelength bandwidth centered at 13.5 nm emitted into the half-sphere back towards the laser that is relevant for state-of-the-art EUV lithography
Efficiencies of converting laser energy into in-band EUV photons into a 2π-sr half-sphere (CEs) in excess of 3% have been achieved from a Sn plasma driven by a 2-μmwavelength laser system assuming isotropic emission
Summary
Extreme-ultraviolet (EUV) lithography has successfully entered high-volume manufacturing, enabling the continued miniaturization of semiconductor devices. Solid-state lasers, operating at near- or mid-infrared wavelengths, may become a viable alternative in the future Such laser systems would potentially provide a smaller footprint, a significantly higher efficiency in converting electrical power to laser light, and may be scaled to much higher pulse energies and output powers which will enable even more powerful EUV light sources. Simulation efforts indicate that a global optimum of the efficiency of converting drive laser light into useful EUV radiation lies in between the well-known 1- and 10-μm cases [21]. In this range, thulium lasers, operating at 1.9-μm wavelength, appear promising [22]. No experimental studies of plasmas driven by lasers in the 1- and 10-μm range, under conditions relevant for EUV emission, are yet available
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