Laser Breakthrough Boosts Next-Generation Chip Manufacturing with Tin Plasmas

Scientists are striving to optimise laser-produced tin plasmas as a viable light source for extreme ultraviolet (EUV) lithography, a crucial technology for manufacturing next-generation microchips. Stanislav Musikhin, Anatoli Morozov, and Alec Griffith, from Princeton Plasma Physics Laboratory and Princeton University, alongside Shurik Yatom and Ahmed Diallo, detail a multi-diagnostic characterisation of these plasmas generated using a novel experimental platform called “SparkLight”. Their research presents a practical, integrated approach to plasma characterisation, combining Thomson scattering, laser interferometry, and emission spectroscopy to accurately measure electron temperatures and densities. By demonstrating that useful EUV emission originates within a limited region under specific plasma conditions, this work provides vital insights for improving source development and ultimately, advancing semiconductor manufacturing.

Characterisation of laser-produced tin plasmas for extreme ultraviolet lithography is crucial for optimizing source performance

Scientists have developed a new integrated diagnostic approach for characterizing laser-produced tin plasmas, a crucial component in extreme ultraviolet (EUV) lithography. This work, conducted using the newly established “SparkLight” experimental platform, addresses the critical need for efficient and well-characterized EUV photon sources for next-generation semiconductor manufacturing.

The research focuses on optimizing plasma parameters, electron density, electron temperature, and tin ion charge state distribution, to maximize spectral purity and conversion efficiency, key challenges in EUV source development. Tin plasmas were generated by irradiating a continuously moving tin-coated wire with laser pulses delivering up to 5.7×1010W/cm2, and subsequently probed using coherent Thomson scattering, laser interferometry, and EUV emission spectroscopy.

Thomson scattering measurements revealed the decay of electron temperatures and densities with increasing distance from the target, providing crucial data on plasma characteristics. Importantly, densities derived from Thomson scattering were rigorously cross-validated against independent measurements obtained through laser interferometry, demonstrating excellent agreement between the two diagnostic techniques.

Correlating the results from these laser diagnostics with spatially resolved EUV spectroscopy indicates that the majority of useful EUV emission originates within 150μm of the target. However, this emission is generated under suboptimal plasma conditions, highlighting areas for further optimization. The study details a compact and cost-effective Thomson scattering implementation, utilizing a Wollaston prism and volume Bragg grating notch filters, substantially lowering the barrier to adoption of this diagnostic technique for plasma characterization. This innovative diagnostic suite enables time- and space-resolved measurements of electron density and temperature, critical for validating radiation hydrodynamic codes used in EUV source optimization.

Plasma characterisation using Thomson scattering and interferometry provides valuable density and temperature profiles

Coherent Thomson scattering underpinned the characterization of laser-produced tin plasmas generated within the SparkLight experimental platform. Laser pulses, with a wavelength of 1064nm, a duration of 10ns, and intensities up to 16W/cm², were focused onto a continuously moving tin-coated wire to create the plasmas relevant to extreme ultraviolet (EUV) radiation.

The scattered light from these plasmas was then collected and analyzed using a high-resolution Thomson scattering system to determine electron temperatures and densities as a function of distance from the target. Laser interferometry served as an independent diagnostic to corroborate the density measurements obtained via Thomson scattering.

Interferograms were recorded and digitally evaluated to provide spatially resolved density profiles of the plasma plume. This technique relies on the phase shift of a probe laser beam as it traverses the plasma, allowing for precise density determination. The Abel transform was employed to reconstruct the two-dimensional density distribution from the line-integrated interferometric data, utilising a Gaussian basis-set expansion method for enhanced accuracy.

Spatially resolved emission spectroscopy was also implemented to investigate the elemental composition and radiative properties of the plasma. The emitted light was dispersed and detected, enabling the identification of spectral lines from various tin ions. Correlating spectroscopic data with the electron density and temperature profiles derived from Thomson scattering and interferometry allowed researchers to pinpoint the region where the majority of useful EUV emission originates, identifying a peak within 150m of the target. This integrated diagnostic suite provides a practical methodology for characterizing plasmas in the development of EUV sources.

Electron density and temperature profiles in laser-produced tin plasmas for EUV source development are presented

Researchers characterized laser-produced tin plasmas generated using the new “SparkLight” experimental platform, achieving densities and temperatures crucial for extreme ultraviolet (EUV) lithography. Thomson scattering measurements revealed electron densities and temperatures decaying with distance from the target, with densities validated by laser interferometry demonstrating excellent agreement between the two diagnostic techniques.

The bulk of useful EUV emission was found to originate within 150μm of the target, although generated under suboptimal plasma conditions. Tin plasmas were created by irradiating a continuously moving tin-coated wire with 1064nm laser pulses lasting 10ns, with intensities reaching up to 5.7×1010W/cm2. Measurements of electron densities and temperatures were obtained using coherent Thomson scattering, a diagnostic technique implemented with a compact design utilizing a Wollaston prism and volume Bragg grating notch filters.

This approach provides comparable performance to traditional multi-grating spectrometers while reducing implementation costs. The experimental setup maintained a base chamber pressure of 10−4 mTorr, increasing to approximately 1 mTorr during operation due to wire ablation. A 250μm diameter tin-coated copper wire was continuously pulled at a speed of 10mm/s to ensure a fresh target surface for each 12, 38 mJ laser pulse.

Calculated peak laser intensities, based on a Gaussian profile, reached values determined by laser energy and focal spot size. Spatially resolved EUV spectroscopy, coupled with the Thomson scattering and interferometry data, indicated that the most significant EUV emission originates from a narrow region near the target.

The high-resolution X-ray spectrometer detected EUV emission with a spectral resolution of 0.05nm, limited by the CCD pixel size and slit width, accumulating data over 10s from 200 laser shots. This integrated diagnostic suite provides a practical approach for characterizing plasmas in EUV source development.

Laser-driven tin plasma characterisation and extreme ultraviolet emission localisation are presented

Tin plasmas generated by a laser irradiation source exhibit characteristics relevant to extreme ultraviolet ( ) light production. Detailed diagnostics, including coherent Thomson scattering, laser interferometry, and emission spectroscopy, were integrated to comprehensively characterise these plasmas.

Measurements of electron temperature and density demonstrate a decay with increasing distance from the target, with densities ranging from 1018 to 1017 cubic centimetres as distance increases from the target. Independent measurements using laser interferometry corroborate the density values obtained via Thomson scattering, validating the diagnostic approach.

Analysis correlating spectroscopic data with the laser diagnostics indicates that the most substantial emission originates within 150 micrometres of the target, although under conditions that are not necessarily optimal for efficient radiation. Observed plasma fluid velocities, derived from Thomson scattering spectral shifts, range from 104 to 105 metres per second near the target, decreasing to approximately zero with increasing distance.

While these magnitudes align with previously reported values for tin laser-produced plasmas, a reduction in velocity with distance was noted, potentially due to geometric factors or complex internal plasma dynamics. The authors acknowledge limitations in the current detection setup preventing measurements closer to the target and propose future angle-resolved Thomson scattering measurements to better distinguish between geometric misalignment and intrinsic plasma flows. This integrated diagnostic suite represents a practical method for characterising plasmas during source development, offering valuable insights into optimising extreme ultraviolet light generation.