Extreme ultraviolet (EUV) lithography is currently entering high-volume manufacturing to enable the continued miniaturization of semiconductor devices. The required EUV light, at 13.5 nm wavelength, is produced in a hot and dense laser-driven tin plasma. The atomic origins of this light are demonstrably poorly understood. Here we calculate detailed tin opacity spectra using the Los Alamos atomic physics suite ATOMIC and validate these calculations with experimental comparisons. Our key finding is that EUV light largely originates from transitions between multiply-excited states, and not from the singly-excited states decaying to the ground state as is the current paradigm. Moreover, we find that transitions between these multiply-excited states also contribute in the same narrow window around 13.5 nm as those originating from singly-excited states, and this striking property holds over a wide range of charge states. We thus reveal the doubly magic behavior of tin and the origins of the EUV light.

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.

Experimental scaling relations of the optical depth are presented for the emission spectra of a tin-droplet-based, 1-lm-laser-produced plasma source of extreme-ultraviolet (EUV) light. The observed changes in the complex spectral emission of the plasma over a wide range of droplet diameters (16-65 lm) and laser pulse durations (5-25 ns) are accurately captured in a scaling relation featuring the optical depth of the plasma as a single, pertinent parameter. The scans were performed at a constant laser intensity of 1.4 Â 10 11 W/cm 2 , which maximizes the emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for industrial EUV lithography. Using a one-dimensional radiation transport model, the relative optical depth of the plasma is found to linearly increase with the droplet size with a slope that increases with the laser pulse duration. For small droplets and short laser pulses, the fraction of light emitted in the 2% bandwidth around 13.5 nm relative to the total spectral energy is shown to reach high values of more than 14%, which may enable conversion efficiencies of Nd:YAG laser light into-industrially-useful EUV radiation rivaling those of current state-of-the-art CO 2 -laser-driven sources.

Extreme-ultraviolet (EUV) spectra of Sn 13+ −Sn 15+ ions have been measured in an electron-beam ion trap (EBIT). A matrix inversion method is employed to unravel convoluted spectra from a mixture of charge states typically present in an EBIT. The method is benchmarked against the spectral features of resonance transitions in Sn 13+ and Sn 14+ ions. Three new EUV lines in Sn 14+ confirm its previously established level structure. This ion is relevant for EUV nanolithography plasma but no detailed experimental data currently exist. We used the Cowan code for first line identifications and assignments in Sn 15+. The collisional-radiative modeling capabilities of the Flexible Atomic Code were used to include line intensities in the identification process. Using the 20 lines identified, we have established 17 level energies of the 4p 4 4d configuration as well as the fine-structure splitting of the 4p 5 ground-state configuration. Moreover, we provide state-of-the-art ab initio level structure calculations of Sn 15+ using the configuration-interaction many-body perturbation code AMBiT. We find that the here-dominant emission features from the Sn 15+ ion lie in the narrow 2% bandwidth around 13.5 nm that is relevant for plasma light sources for state-of-the-art nanolithography.

Experimental spectroscopic studies are presented, in a 5.5-25.5 nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, 2µm-wavelength laser light. Emission spectra are compared to those obtained from plasma driven by 1-µmwavelength Nd:YAG laser light over a range of laser intensities spanning approximately 0.3 − 5 × 10 11 W cm −2 , under otherwise identical conditions. Over this range of drive laser intensities, we find that similar spectra and underlying plasma charge state distributions are obtained when keeping the ratio of 1-µm to 2-µm laser intensities fixed at a value of 2.1(6), which is in good agreement with RALEF-2D radiation-hydrodynamic simulations. Our experimental findings, supported by the simulations, indicate an approximately inversely proportional scaling ∼ λ −1 of the relevant plasma electron density, and of the aforementioned required drive laser intensities, with drive laser wavelength λ . This scaling also extends to the optical depth that is captured in the observed changes in spectra over a range of droplet diameters spanning 16-51 µm at a constant laser intensity that maximizes the emission in a 2% bandwidth around 13.5 nm relative to the total spectral energy, the bandwidth relevant for EUV lithography. The significant improvement of the spectral performance of the 2-µm-vs 1-µm driven plasma provides strong motivation for the development of high-power, high-energy nearinfrared lasers to enable the development of more efficient and powerful sources of EUV light.