Laser-produced Tin Plasma Research Articles

The wide spectral range characteristics and dynamic evolution of laser-produced tin plasmas

The energy levels and transitions of highly charged Sn ions are key data for analyzing emission spectra, radiative opacity, and conversion efficiency of laser-produced Sn plasmas. Temporally–resolved spectra in the 6–45-nm range were acquired via high-precision, spatio–temporally resolved techniques. In addition, the Cowan code, based on Hartree–Fock multi-configurational interactions, was used to perform structural calculations for Sn4+–Sn13+ ion spectra. Emission spectra in the 11–18-nm range were primarily attributed to Sn8+–Sn12+ ions, with transitions involving principal quantum numbers Δn=0 and Δn=1. Whereas, emission spectra in the 6–11-nm and 18–45-nm ranges were mainly attributed to Sn9+–Sn13+and Sn4+– Sn9+ions, with transitions involving principal quantum numbers Δn=1,2. Furthermore, Segmented simulations were conducted based on the assumptions of a normalized Boltzmann distribution among the excited states and a steady-state collisional-radiative model to explain the characteristics of the radiation spectra and variations in state parameters variations during laser-produced plasma expansion from the core to the outer periphery. A specific analysis was conducted on the temporal evolution of the electron temperature and density, while the charge state of the plasma was analyzed over the 6–18-nm range. Based on the spectral diagnostics, this work focused on the impact of non-uniformity in the expansion of the Sn plasma.

Experimental Study on the Temporal Evolution Parameters of Laser–Produced Tin Plasma under Different Laser Pulse Energies for LPP–EUV Source

The laser–produced plasma extreme ultraviolet (LPP–EUV) source is the sole light source currently available for commercial EUVL (extreme ultraviolet lithography) machines. The plasma parameters, such as the electron temperature and electron density, affect the conversion efficiency (CE) of extreme ultraviolet radiation and other critical parameters of LPP–EUV source directly. In this paper, the optical emission spectroscopy (OES) was employed to investigate the time–resolved plasma parameters generated by an Nd:YAG laser irradiation on a planar tin target. Assuming that the laser–produced tin plasma satisfies the local thermodynamic equilibrium (LTE) condition, the electron temperature and electron density of the plasma were calculated by the Saha–Boltzmann plot and Stark broadening methods. The experimental results revealed that during the early stage of plasma formation (delay time < 50 ns), there was a significant presence of continuum emission. Subsequently, the intensity of the continuum emission gradually decreased, while line spectra emerged and became predominant at a delay time of 300 ns. In addition, the evolution trend of plasma parameters, with the incident laser pulse energy set at 300 mJ, was characterized by a rapid initial decrease followed by a gradual decline as the delay time increased. Furthermore, with an increase in the incident laser pulse energy from 300 mJ to 750 mJ, the electron temperature and electron density of laser–produced tin plasma exhibiting a monotonically showed increasing trend at the same delay time.

Joint measurement of electron density, temperature, and emission spectrum of Nd:YAG laser-produced tin plasma

We present the results of joint measurements of electron density (ne), temperature (Te), and emission spectra of an Nd:YAG-driven tin plasma. Collective Thomson scattering provides space- and time-resolved ne and Te data during drive laser irradiation, and extreme ultraviolet (EUV) emission spectra, which is space-resolved in the target normal, are measured using a flat-field grazing incidence spectrometer (GIS). As the distance from the target increased, the emission intensity quickly decreases, and the peaks of the spectra gradually shift to longer wavelengths. This can be explained by the rapid decrease in ne, and thus, self-absorption, with an increase in distance. We obtain the EUV spectra by calculating the transfer of photons along the line-of-sight of the GIS, using theoretical emissivity and opacity, and applying the measured spatial distribution of ne and Te. The results quantitatively demonstrate that the self-absorption effect is significant; the emission from the core regions is mostly reabsorbed by the surrounding plasma. The calculated spectra are compared with the measured spectra. While good agreement is achieved in the spectral region of 13.3 and 15.3 nm, considerable differences are found in the 12.5–13 and 15.5–17.5 nm region. The results demonstrate the significance of this joint measurement for further validation of the atomic process model considering self-absorption effect, which is critical for the future high-density, solid laser-driven EUV source.

Dependence of ion charge-energy emission from Nd:YAG-laser-produced plasma on laser intensity in the 0.4–40×1010 W/cm2 range

We experimentally characterize the ionic emission, including the individual charge states Snz+ (z=1, …, 8), from laser-produced tin plasma as a function of the intensity of the employed ns-pulsed laser. The plasma is generated in a vacuum from tin microdroplets (diameter ranging from 17 to 35 μm) using pulsed Nd:YAG laser light (laser wavelength λ=1.064 μm) over a range of intensities (0.4–40×1010 W/cm2). We measure charge-state-resolved and integrated ion energy distributions at seven angular positions around the plasma using seven retarding field analyzers. We highlight peak features in both types of spectra and describe the dependence of their energies on laser intensity with power-law functions. The resulting power laws match those derived from plasma radiation hydrodynamics theory. The analytical scaling laws exhibit strong isotropy, while the ion energy spectra are highly anisotropic.

Material-specific high-order harmonic generation in laser-produced plasmas for varying plasma dynamics

We present a new experimental setup for high-order harmonic generation in laser-produced plasmas, allowing the generation of coherent VUV and EUV light, as well as characterisation of the laser-produced plasmas by studying the emitted harmonics. We have successfully generated high-order harmonics in laser-produced Al, Ni, Ag, In, and Sn plasmas. Large differences in harmonic spectra and signal yields have been observed for these different targets. Harmonics up to order 25, corresponding to a wavelength of 62.4 nm and photon energy of 19.9 eV, have been measured with tin plasmas. Scanning laser parameters and delay between pump and fundamental laser pulses allows us to optimise the harmonic yield and observe the temporal dynamics of the laser-produced tin plasma.

Toward streaked collective Thomson scattering measurements on an extreme ultraviolet plasma light source

We show through forward modeling calculations that streaked collective Thomson scattering measurements are feasible on laser-produced tin plasmas generated under conditions relevant for extreme ultraviolet lithography. Using a 532 nm probe laser beam, the feasibility of simultaneous measurements of electron plasma wave (EPW) and ion acoustic wave (IAW) spectra is investigated. Absolute photon counts for laser scattering off both waves are calculated. Probe laser electron heating and bremsstrahlung background radiation effects are accounted for. While a large spatiotemporal region can be successfully probed based on the IAW feature, only one measurement location can be accessed through the EPW as a result of the low signal to noise ratio. A portable/traveling tabletop system is proposed.

Enhanced extreme ultraviolet lighting using carbon nanotube-based cold cathode electron beam irradiation

Laser-produced tin (Sn) plasma (LPP) is used to produce high-performance semiconductors using extreme ultraviolet (EUV), but LPP generates debris that limits the lifespan of the optical system, so research on debris mitigation is being conducted. When electrons are directly irradiated to a solid target, occurrence of debris can be minimized, and compact and low-cost EUV lighting can be manufactured. Here, we demonstrated enhanced EUV generation by optimizing the irradiation of Sn with electrons emitted from carbon nanotube-based cold cathode electron beam (C-beam). To minimize debris, a usable zone in which Sn does not melt was identified. It was demonstrated that using two C-beams in the usable zone increased the EUV intensity by about two times compared to using one C-beam. The multiple C-beam irradiation technique enables high-output EUV lighting by overlapping EUV light, which should be led by the development of advanced lithography and inspection technology.

COMPARISON STUDY BETWEEN SPECTROSCOPIC ANALYSIS FOR (ZN,SN) PLASMA BY LIBS

this article a spectroscopic research on laser-produced Tin and Zinc plasmas using the optical emission spectroscopy (OES) technique. Plasmas can be produced from a solid tin and zinc targets irradiated with a pulsed laser in room environments. The spectrum is recorded for the Sn, Zn laser plasma Nd: YAG with a wavelength of (1064) nm, a duration of (9) ns, and a frequency of (6) Hz and a focal length of (10) cm within the energy range (300-800)mj. By using the ratio line strength formula, the electron temperature (Te) can be calculated and the result is for Zinc (Zn) plasma (2.11 ev) and tin (Sn) plasma (1,227 ev). The Saha-Boltzmann equation will be used to calculate electron density (ne) in this method and the values for zinc (Zn) (3.3 cm-3)and tin (Sn) (2.1 cm-3). The plasma parameters, such as plasma (fp), Debye duration (λD), and Debye number (ND), were calculated in the proposed document.

Time-resolved spatial profiles of electron density and temperature in hydrogen plasmas induced by radiation from laser-produced tin plasmas for extreme ultraviolet lithography light sources

We observed the spatial and temporal changes of the electron density (n e) and the electron temperature (T e) of hydrogen plasmas around a laser-produced Sn plasma EUV source. The plasma parameters were measured by the laser Thomson scattering (LTS) method. In the experiment, the Sn plasmas are produced in H2 gas at a pressure of 50–200 Pa and the hydrogen plasmas were induced by radiation from the Sn plasmas. The LTS measurements were performed at distances 30–90 mm away from the Sn plasmas. In all cases, the strong bremsstrahlung radiation of the Sn plasmas easily overwhelmed the weak LTS signals. To suppress noise due to the radiation, the solid angle of radiation from the Sn plasmas was restricted. The experimental results show that the n e was in the order of 1017 m−3 and T e was around 0.7 eV.

Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography

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.

激光锡等离子体极紫外光谱的优化研究

激光锡等离子体极紫外光谱的优化研究

Calculation of the extreme-ultraviolet radiation conversion efficiency for a laser-produced tin plasma source

This article presents the calculation results on the conversion efficiency (CE) of 1.064 μm laser-produced plasmas (LPPs) extreme-ultraviolet (EUV) tin (Sn) light sources with the Gaussian and a triangular-flat-topped like laser pulse temporal shapes. The computational model includes a collisional-radiative model and 1D hydrodynamics code that predicts reported experimental and theoretical results on the CE of 1.064 μm and 10.6 μm LPP EUV sources with the planar and mass-limited spherical Sn targets. The calculations for the case of a spherical target reveal that an optimum triangular-flat-topped like laser pulse generates a higher CE compared to the Gaussian pulse, especially, for the longer laser pulse duration than ≈ 30 ns. The study demonstrated that a rising intensity rate of the laser pulse has a vital role to optimize the CE as well as to prolong the in-band (13.5 ± 0.135 nm) spectral emission of a small Sn spherical target. The model predicts a ≈ 30 ns rising time duration for a linearly increasing intensity of triangular-flat-topped 1.064 μm laser pulse is necessary to obtain a maximum CE with a typical ≈ 40 μm diameter liquid Sn droplet.

Temporally and spatially resolved ion dynamics of droplet-based laser-produced tin plasmas in lateral expansion direction

The temporal and spatial plasma ion dynamics in the lateral direction generated by a Nd:YAG laser irradiated droplet target were studied with a hemispherical electrostatic probe array. The ion dynamics produced from 1.6 × 10+11 W/cm2 irradiation with a pulse duration of 23.9 ns FWHM were measured simultaneously from 50° to 130° from the laser axis with radial probe distances d from 1.5 to 7 cm to the plasma ignition point at an ambient argon gas pressure of 2 × 10−2 mbar. The collected ion charge and expansion velocities were derived from the ion profiles. It was found that the collected ion charge Q around the droplet scales with Q ∼ d−2 indicating that the main driving mechanism relates to the three-dimensional plasma expansion and not recombination processes. An anisotropic ion bulk expansion in the laser forward and backward propagation direction was deduced ranging from 2.9 cm/μs to 2.1 cm/μs, respectively. The gradients of the ion bulk expansion velocities along d were found to be constant within the error margin across the measurement range. The leading edge of the ion profiles showed an anisotropic behavior around the droplet, suggesting recombination effects scale differently in the laser forward and backward propagation direction which was linked to the higher expansion velocities in the laser forward direction. The broadening of the ion current waveform with increasing radial distance was studied and it was observed that the ion profile shape did not change for d > 5 cm, suggesting negligible recombination.

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 (ne) and electron temperature (Te) 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.

Detection of laser-produced tin plasma emission lines in atmospheric environment by optical emission spectroscopy technique

A spectroscopic study on laser-produced tin plasma utilizing the optical emission spectroscopy (OES) technique is presented. Plasma is produced from a solid tin target irradiated with pulsed laser in room environment. Electron temperature is determined at different laser peak powers from the ratio of line intensities, while electron density is deduced from Saha-Boltzmann equation. A limited number of suitable tin lines are detected, and the effect of the laser peak power on the intensity of emission lines is discussed. Electron temperatures are measured in the range of 0.36 eV–0.44 eV with electron densities of the order 1017 cm–3 as the laser peak power is varied from 11 MW to 22 MW.

Evolution analysis of EUV radiation from laser-produced tin plasmas

SynopsisWe present a simplified radiation hydrodynamic model based on the fluid dynamic equations and the radiative transfer equation to rapidly investigate the radiation properties and dynamics in laser-produced tin plasmas. The self-absorption features of EUV spectra measured at an angle of 45° to the direction of plasma expansion have been successfully simulated and explained. The evolution of the plasma temperature has also been evaluated.

Detailed analysis of self-absorption profiles of highly-charged tin ions in the EUV region

Plasma dynamics simulation is a very effective method for the spectral diagnosis and analysis of plasmas, which has been widely used in the interpretation of spectral structure and evolution analysis of the laser-produced plasmas. In this paper, a simplified radiation hydrodynamic model based on the fluid dynamic equations and the radiative transfer equation, and combined with a steady-state collisional-radiative model was presented and used to analyze the origin of extreme ultraviolet (EUV) spectrum of middle- and high-Z highly charged ions from laser-produced plasma, and has been successfully applied to analyze and simulate the spectrum of laser-produced tin plasma in the EUV spectral region. The results show that there is a good agreement between theoretical simulation and experimental spectrum at a laser power density of 1.90×1011 W/cm2. By comparison of the emission profiles and self-absorption profiles of 4d-4f, 4p-4d and 4d-5p, 5f transition arrays from Sn7+ to Sn11+ ions, the influence of opacity effect to the emission profile was identified. We are succeeded in explaining the origin of self-absorbed band and self-absorbed dips around 13.5 nm in tin spectrum. Our results are expected to provide a reference for the applications of extreme ultraviolet radiation in the fields of lithography, metrology and biological imaging.

Evolution analysis of EUV radiation from laser-produced tin plasmas based on a radiation hydrodynamics model

One of fundamental aims of extreme ultraviolet (EUV) lithography is to maximize brightness or conversion efficiency of laser energy to radiation at specific wavelengths from laser produced plasmas (LPPs) of specific elements for matching to available multilayer optical systems. Tin LPPs have been chosen for operation at a wavelength of 13.5 nm. For an investigation of EUV radiation of laser-produced tin plasmas, it is crucial to study the related atomic processes and their evolution so as to reliably predict the optimum plasma and experimental conditions. Here, we present a simplified radiation hydrodynamic model based on the fluid dynamic equations and the radiative transfer equation to rapidly investigate the evolution of radiation properties and dynamics in laser-produced tin plasmas. The self-absorption features of EUV spectra measured at an angle of 45° to the direction of plasma expansion have been successfully simulated and explained, and the evolution of some parameters, such as the plasma temperature, ion distribution and density, expansion size and velocity, have also been evaluated. Our results should be useful for further understanding of current research on extreme ultraviolet and soft X-ray source development for applications such as lithography, metrology and biological imaging.

Spatial diagnostics of the laser-produced tin plasma in air

We present here new experimental studies on the laser-produced tin plasma generated by focusing the beam of a Q-switched Nd:YAG laser (532 nm) on the sample in air at atmospheric pressure. The optical emission spectra were recorded with a set of five spectrometers covering the spectral range from 200–720 nm. The electron temperature has been calculated to be about (10 600 ± 600) K using three methods; the two-line ratio, Boltzmann plot and the Saha–Boltzmann plot method, whereas the electron number density of about (9.0 ± 0.8) × 1016 cm−3 has been calculated using the Stark broadened line profiles of tin lines and the hydrogen Hα-line. Furthermore, the branching fractions have been deduced for 15 spectral lines of the 5p5d → 5p2 transition array in tin, whereas the absolute values of the transition probabilities have been calculated by combining the experimental branching fractions with the lifetimes of the excited levels. Our measured values are compared with those reported in the literature and NIST data base, showing good agreement.

Development of a collective Thomson scattering system for laser-produced tin plasmas for extreme-ultraviolet light sources

Spatial profiles of electron density (ne) and electron temperature (Te) of laser-produced Sn plasmas for extreme-ultraviolet (EUV) light sources have been obtained using a new collective Thomson scattering system, which has been optimized for the measurement of the ion feature spectrum. The system has an 18 pm spectral resolution, a 5 ns temporal resolution, a 50 µm spatial resolution, and sufficient stray-light rejection near the probing laser wavelength. With this system, measurements of the laser-produced Sn plasmas in the parameter ranges of 3 × 1023 < ne < 1025 m−3 and 10 < Te < 20 eV have been performed.