Observation of plasma inflows in laser-produced Sn plasma and their contribution to extreme-ultraviolet light output enhancement.

Plasma dynamics are governed by electron density (ne), electron temperature (Te), and radiative energy transfer as well as by macroscopic flows. However, plasma flow-velocity fields (vflow) inside laser-produced plasmas (LPPs) have rarely been measured, owing to their small sizes (< 1 mm) and short lifetimes (< 100 ns). Herein, we report, for the first time, two-dimensional (2D) vflow measurements of Sn-LPPs ("double-pulse" scheme with a CO2 laser) for extreme-ultraviolet (EUV) light sources for semiconductor lithography using the collective Thomson scattering technique, which is typically used to measure ne, Te, and averaged ionic charge (Z) of plasmas. Inside the EUV source, we observed plasma inflow speed exceeding 104 m/s magnitudes toward a plasma central axis from its peripheral regions. The time-resolved 2D profiles of ne, Te, Z, and vflow indicate that the plasma inflows maintain the EUV source at a temperature suitable (25 eV < Te < 40 eV) for EUV light emission at a high density (ne > 3 × 1024 m-3) and for a relatively long time (> 10 ns), resulting increment of total EUV light emission. These results indicate that controlling the plasma flow can improve EUV light output and that there is potential to increase the EUV output further.

Time-resolved two-dimensional profiles of electron density and temperature of laser-produced tin plasmas for extreme-ultraviolet lithography light sources.

Time-resolved two-dimensional (2D) profiles of electron density (n e) and electron temperature (T e) of extreme ultraviolet (EUV) lithography light source plasmas were obtained from the ion components of collective Thomson scattering (CTS) spectra. The highest EUV conversion efficiency (CE) of 4% from double pulse lasers irradiating a Sn droplet was obtained by changing their delay time. The 2D-CTS results clarified that for the highest CE condition, a hollow-like density profile was formed, i.e., the high density region existed not on the central axis but in a part with a certain radius. The 2D profile of the in-band EUV emissivity (ηEUV) was theoretically calculated using the CTS results and atomic model (Hullac code), which reproduced a directly measured EUV image reasonably well. The CTS results strongly indicated the necessity of optimizing 2D plasma profiles to improve the CE in the future.

Synthesis of arbitrary pulse waveforms in QCL-seeded ns-pulse CO2 laser for optimization of an LPP EUV source.

One of the unique features of the quantum-cascade-laser-seeded, nanosecond-pulse CO2 laser, invented for the purpose of generation of extreme UV by laser-produced-plasma, is a robust synthesis of arbitrary pulse waveforms. In the present Letter we report on experimental results that are, to our best knowledge, the first demonstration of such functionality obtainable from nanosecond-pulse CO2 laser technology. An online pulse duration adjustment within 10-40 ns was demonstrated, and a few exemplary pulse waveforms were synthesized, such as "tophat," "tailspike," and "leadspike" shapes. Such output characteristics may be useful to optimize the performance of LPP EUV source.

Efficient multiline nanosecond pulse amplification in planar waveguide CO2 amplifier for extreme UV laser-produced plasma source.

In this Letter, we report on recent experimental results of a short pulse amplification at 10.6 µm wavelength required to drive a tin laser-produced plasma (LPP) extreme ultraviolet (UV) source. We report for the first time, to our best knowledge, a highly efficient pulsed amplification in a multipass amplifier built on RF-discharge-excited, diffusion-cooled CO2, planar waveguide industrial CO2 laser. About 2 kW of output average power was obtained from about 100 W input average power in ∼15 ns pulses at 100 kHz pulse repetition frequency. As much as 60% relative extraction efficiency, as compared to continuous-wave amplification in similar conditions, and 5.8% wall-plug efficiency was recorded and believed to be the highest reported so far. An improvement of extraction efficiency by ∼10% is reported when driving the amplifier with two lines of CO2 regular band in good agreement with expectations.

Multiline short-pulse solid-state seeded carbon dioxide laser for extreme ultraviolet employing multipass radio frequency excited slab amplifier.

In this Letter we describe in more detail a solid-state seeded, nanosecond pulse, multiline CO(2) oscillator designed and built for the extreme ultraviolet (EUV) laser-produced-plasma (LPP) source. Our oscillator featured quantum cascade laser seeders, a diffraction-type seed beam combiner, and a radio-frequency-discharge-excited, diffusion-cooled, slab-waveguide CO(2) gain cell in a compact multipass regenerative amplifier configuration. The oscillator generated pulses of exceptional stability in terms of envelope, energy, and spectrum. Excellent stability of output was achieved without any additional techniques. The output spectrum consisted of two laser lines of a 00(0)1-10(0)0 band of a CO(2) molecule, P20 and P22, with a target of four lines P18-P24. The pulse duration was electronically adjustable between 11 and 35 ns at a repetition frequency from a few hertz to hundreds of kilohertz. Electronic adjustment of the pulse duration was achieved by relative timing offsets of individual seeders, opening an avenue to a range of on-line adjustments of pulse shape and spectral content timing. The jitter-tolerant operation allows for easy synchronization with an external event, such as a droplet target in an EUV LPP source. A resistance to parasitic seeding of more than 40 dB was recorded. The oscillator produced up to 20 W of average output power at a repetition rate of 100 kHz in a near-diffraction-limited beam of M(2)<1.3 and a pointing stability below 50 µrad.

Wavefront measurement of single-mode quantum cascade laser beam for seed application in laser-produced plasma extreme ultraviolet system.

Quantum cascade laser (QCL) is a very attractive seed source for a multikilowatt pulsed CO2 lasers applied for driving extreme ultraviolet emitting plasmas. In this Letter, we investigate output beam properties of a QCL designed to address P18 and P20 lines of 10.6 micron band of CO2 molecule. In particular, output beam quality and stability are investigated for the first time. A well-defined linear polarization and a single-mode operation enabled a use of phase retrieval method for full description of QCL output beam. A direct, multi-image numerical phase retrieval technique was developed and successfully applied to the measured intensity patterns of a QCL beam. Very good agreement between the measured and reconstructed beam profiles was observed at distances ranging from QCL aperture to infinity, proving a good understanding of the beam propagation. The results also confirm a high spatial coherence and high stability of the beam parameters, the features expected from an excellent seed source.

Spectral characteristics of quantum-cascade laser operating at 10.6 µm wavelength for a seed application in laser-produced-plasma extreme UV source.

In this Letter, we investigate, for the first time to our knowledge, the spectral properties of a quantum-cascade laser (QCL) from a point of view of a new application as a laser seeder for a nanosecond-pulse high-repetition frequency CO(2) laser operating at 10.6 µm wavelength. The motivation for this work is a renewed interest in such a pulse format and wavelength driven by a development of extreme UV (EUV) laser-produced-plasma (LPP) sources. These sources use pulsed multikilowatt CO(2) lasers to drive the EUV-emitting plasmas. Basic spectral performance characteristics of a custom-made QCL chip are measured, such as tuning range and chirp rate. The QCL is shown to have all essential qualities of a robust seed source for a high-repetition nanosecond-pulsed CO(2) laser required by EUV LPP sources.