Efficient Extreme Ultraviolet Sources Research Articles

Tuning laser wavelength and pulse duration to improve the conversion efficiency and performance of EUV sources for nanolithography

The optimum conditions for efficient extreme ultraviolet (EUV) sources in laser-produced plasma (LPP) depend on the ideal combination of several parameters such as laser wavelength, intensity, spot size, and pulse duration. These parameters need to be optimized in relation to the actual target size and geometry. The laser wavelength can play a major role in this regard. While target related effects are not significant in the case of a Nd:YAG laser, the achievable conversion efficiency (CE) of EUV sources produced by a CO2 laser from small Sn droplets can be several times lower compared to that from a flat Sn target. Improving the efficiency and performance of EUV sources requires detailed investigation of various laser/target combinations using comprehensive integrated models. We simulated and characterized in detail plasmas produced from the Sn droplet irradiated by lasers with various wavelengths. The optimum laser intensity with respect to the laser and target sizes was determined for a range of laser wavelengths from 1 μm to 10 μm. This work was inspired by the recent developments of lasers with a 2 μm wavelength. The laser pulse duration is another important parameter in the optimization of LPP for EUV sources. Our comprehensive HEIGHTS modeling showed that increasing the pulse duration of the 2 μm laser leads to an increase in the CE of EUV sources. Our results also indicate that a significant reduction in the kinetic energies of ions generated from LPP can be achieved by optimizing the laser pulse duration.

Stagnation layer development from two colliding Sn plasmas for an efficient EUV source

The collision of two laser-produced plasmas (LPP), individually generated from the opposing faces of a tin wedge, was studied experimentally and through the HEIGHTS computer simulation package to explore onset and development of the stagnation layer and extreme ultraviolet (EUV) photoemission. The colliding Sn plasma offers promise for a powerful source of 13.5 nm light, the industry-identified wavelength being targeted for commercial EUV lithography light sources. A second laser pulse could efficiently couple with the material accumulated along the collision midplane to produce EUV with high power. Imaging of the LPP configuration is accomplished using an ICCD camera while electron temperature and density of the stagnation layer were determined using spectroscopic methods—both were verified using HEIGHTS which was also used to show the potential of an EUV source with 700 W power at the intermediate focus for efficient high-volume manufacturing.

Optimizing conversion efficiency and reducing ion energy in a laser-produced Gd plasma

We have demonstrated an efficient extreme ultraviolet (EUV) source at 6.7 nm by irradiating Gd targets with 0.8 and 1.06 μm laser pulses of 140 fs to 10 ns duration. Maximum conversion efficiency of 0.4% was observed within a 0.6% bandwidth. A Faraday cup observed ion yield and time of flight signals for ions from plasmas generated by each laser. Ion kinetic energy was lower for shorter pulse durations, which yielded higher electron temperatures required for efficient EUV emission, due to higher laser intensity. Picosecond laser pulses were found to be the best suited to 6.7 nm EUV source generation.

Combined effects of prepulsing and target geometry on efficient extreme ultraviolet production from laser produced plasma experiments and modeling

Laser produced plasmas (LPPs) are currently a promising source of an efficient extreme ultraviolet (EUV) photon source production for advanced lithography. Optimum laser pulse parameters with adjusted wavelength, energy, and duration for a simple planar or spherical tin target can provide 2% to 3% conversion efficiency (CE) in laboratory experiments. Additional effects such as targets with complex geometry and tin-doped targets using prepulsing of laser beams can significantly increase CE. Recent studies showed that such improvements in an LPP system are due to reduction in laser energy losses by decreasing photons transmission (higher harmonic of Nd:yttrium-aluminum-garnet laser) or photons reflection (for CO2 laser). Optimization of target heating using prepulses or ablating low-density and nanoporous tin oxide can further improve LLP sources by creating more efficient plasma plumes and, as a result, increase CE, the most important parameter for EUV sources. We investigated the combined effects of prepulsing with various parameters and different target geometries on EUV conversion efficiency and on energetic ions production. The much higher reflectivity of CO2 laser from a tin target leads to two possible ways for system improvement using prepulses with shorter laser wavelengths or using more complex targets geometries with special grooves as developed previously by the authors.

Development of an Efficient EUV Source by Use of a Liquid Tin Oxide Micro-Target

We report on a low-debris, efficient extreme ultraviolet (EUV) source by use of a liquid micro-target containing low-concentration tin oxide nano-particles. Conversion efficiency from a laser pulse energy to an in-band EUV energy (2% bandwidth (BW) and 2πIsr solid angle) depended on various experimental parameters, such as tin oxide concentration, laser intensity, and pulse separation time between dual laser pulses. By using a low concentration (6%) colloidal target together with dual laser pulses under the optimum separation time, we observed the EUV conversion efficiency of 1.2% with undetectable debris.

Potential of discharge-based lithium plasma as an extreme ultraviolet source

Extreme ultraviolet (EUV) discharge-based lamps for EUV lithography need to generate extremely high power in the spectrum band of 13.5±0.135nm. A model was developed to investigate the wavelength-integrated Lyman-α lines light outputs in hydrogen-like lithium ion. The analysis reveals that the commonly observed low conversion efficiency is largely due to a transient nature of Z discharge-based plasma and that a magnetically confined lithium plasma is an efficient EUV source even at low electron temperature. Calculation shows necessary confinement time that raises the conversion efficiency up to half the spectral efficiency.

Debris characteristics of a laser-produced tin plasma for extreme ultraviolet source

We measured debris characteristics of a tin (Sn) plasma produced by a 10-ns infrared Nd:YAG laser. A maximum kinetic energy of 7keV of tin ions was observed. Such suprathermal tin ions emitted from a solid planar target consisted of singly and doubly ionized tin ions. Both suprathermal ions and neutral fragments emitted from a target showed the angular distributions of cos4θ which were narrower than the 13.5-nm extreme ultraviolet (EUV) emission distribution of cos0.5θ. These measurements would give important information on debris mitigation for efficient EUV sources in the next generation lithography.