Sn Plasma Research Articles

Study of Sn plasma parameters ablated by a 1064 nm Q-switched laser

In the present work, the parameters of laser-induced Sn plasma are studied using the fundamental of a Q-switched Nd:YAG. An improved iterative Boltzmann plot method is developed to calculate the electron temperature of the plasma to be 5116 K with 14 Sn (I) emission lines. The Sn (I) 303.41 nm emission line is Lorentz fitted and the electron number density is inferred to be 7.6 × 1016 cm−3. Moreover, the validity of local thermodynamic equilibrium is elucidated.

CdTe:Sn thin films deposited by the simultaneous laser ablation of CdTe and Sn targets

CdTe:Sn thin films were grown by pulsed laser deposition on glass substrates at room temperature. The study of the changes in the chemical, structural and optical properties of the films as a function of the density of Sn ions calculated from Langmuir planar probe measurments is presented. Four films were grown by the simultaneous ablation of CdTe and Sn targets in vacuum. The Sn ion density was modified by varying the fluence on the Sn target, while the CdTe plasma density was kept constant for all the deposits. One CdTe film was grown as control sample using the same experimental conditions as in the CdTe:Sn films. The chemical composition was analyzed by XPS, where a dependence of the Sn incorporation into the CdTe lattice, on the Sn ion plasma density was demonstrated. The crystalline structure analysis by XRD showed a hexagonal structure for all the films. When the CdTe and Sn plasmas were combined, a preferential orientation in the plane (110) was observed for the resulting films. Furthermore, as Sn plasma density increased, the intensity of the (110) peak increased as well, suggesting that crystalline re-orientation is an effect of Sn incorporated into the CdTe lattice. Vibrational behavior was analyzed by Raman spectroscopy. A vibrational mode appeared at 118 cm−1 and was related to a Sn-Te vibration, suggesting the incorporation of substitutional Sn into the CdTe lattice. The Sn-Te bonding was confirmed by XPS by the appearance of signals centered at 485 and 494 eV, which correspond to Sn2+ in SnTe. Thin films thicknesses values were between 320 and 460 nm increasing with Sn ion density. The band gap calculated using UV–vis spectrophotometry, resulted in values ranging from 1.42 to 1.46 eV. PL measurements showed a slight blue shift of the near-edge emission as Sn plasma density increased.

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.

EUV light generation using laser induced Sn plasma for semiconductor high volume manufacturing

EUV light generation using laser induced Sn plasma for semiconductor high volume manufacturing

Energy-level structure ofSn3+ions

Laser-produced Sn plasma sources are used to generate extreme ultraviolet (EUV) light in state-of-the-art nanolithography. An ultraviolet and optical spectrum is measured from a droplet-based laser-produced Sn plasma, with a spectrograph covering the range 200 - 800 nm. This spectrum contains hundreds of spectral lines from lowly charged tin ions Sn$^{1+}$ - Sn$^{4+}$ of which a major fraction was hitherto unidentified. We present and identify a selected class of lines belonging to the quasi-one-electron, Ag-like ([Kr]$4d^{10} nl$ electronic configuration), Sn$^{3+}$ ion, linking the optical lines to a specific charge state by means of a masking technique. These line identifications are made with iterative guidance from COWAN code calculations. Of the 53 lines attributed to Sn$^{3+}$, some 20 were identified from previously known energy levels, and 33 lines are used to determine previously unknown level energies of 13 electronic configurations, i.e., $ 7p $, $ (7,8)d $, $ (5,6)f $, $ (6-8)g $, $ (6-8)h $, $ (7,8)i $. The consistency of the level energy determination is verified by the quantum-defect scaling procedure. The ionization limit of Sn$^{3+}$ is confirmed and refined to 328908.4 cm$^{-1}$ with an uncertainty of 2.1 cm$^{-1}$. The relativistic Fock space coupled cluster (FSCC) calculation of the measured level energies are generally in good agreement with experiment, but fail to reproduce the anomalous behavior of the $5d$ $^2$D and $nf$ $^2$F terms. By combining the strengths of FSCC, COWAN code calculations, and configuration interaction many-body perturbation theory (CI+MBPT), this behavior is shown to arise from interactions with doubly-excited configurations.

New trends in high-order harmonics generation using the mid-infrared pulses propagating through the laser-produced plasmas

Various recently emerged applications of mid-infrared pulses for high-order harmonic generation in laser plasmas produced on the surfaces of various materials are reviewed. Among them are (a) the highorder harmonic generation in Ag, Sn, fullerene, and graphene nanoparticle-containing plasmas using tunable two-color mid-infrared pulses, (b) efficiency dependence on the generating particle properties in the case of two-color high-harmonic generation of the longer-wavelength radiation in the plasmas, and (c) high-order sum and difference frequency generation using tunable two- and three-color commensurate and incommensurate mid-infrared pumps of graphite plasma. These studies demonstrate the perspectives of the frequency conversion of mid-infrared radiation in the extended laser-produced plasmas.

Spatial profiles of electron density, electron temperature, average ionic charge, and EUV emission of laser-produced Sn plasmas for EUV lithography

Spatial profiles of the electron density (ne), electron temperature (Te), and average ionic charge (Z) of laser-produced Sn plasmas for EUV lithography, whose conversion efficiency (CE) is sufficiently high for practical use, were measured using a collective Thomson scattering (TS) technique. For plasma production, Sn droplets of 26 µm diameter were used as a fuel. First, a picosecond-pulsed laser was used to expand a Sn target. Next, a CO2 laser was used to generate plasmas. By changing the injection timing of the picosecond and CO2 lasers, three different types of plasmas were generated. The CEs of the three types of plasmas differed, and ranged from 2.8 to 4.0%. Regarding the different plasma conditions, the spatial profiles of ne, Te, and Z clearly differed. However, under all plasma conditions, intense EUV was only observed at a sufficiently high Te (> 25 eV) and in an adequate ne range [1024–(2 × 1025) m−3]. These plasma parameters lie in the efficient-EUV light source range, as predicted by simulations.

Dynamics of low- and high-Z metal ions emitted during nanosecond laser-produced plasmas

Dynamics of metal ions during laser-produced plasmas was studied. A 1064 nm, Nd: YAG laser pulse was used to ablate pure Al, Fe, Co, Mo, and Sn samples. Ion flux and velocity were measured using Faraday cup ion collector. Time-of-flight measurements showed decreasing ion flux and ion velocity with increasing atomic weight, and heavy metal ion flux profile exhibited multiple peaks that was not observed in lighter metals. Slow peak was found to follow shifted Maxwell Boltzmann distribution, while the fast peak was found to follow Gaussian distribution. Ion flux angular distribution that was carried out on Mo and Al using fixed laser intensity 2.5 × 1010 W/cm2 revealed that the slow ion flux peaks at small angles, that is, close to normal to the target ∼0° independent of target's atomic weight, and fast ion flux for Mo peaks at large angles ∼40° measured from the target normal, while it completely absents for Al. This difference in spatial and temporal distribution reveals that the emission mechanism of the fast and slow ions is different. From the slow ion flux angular distribution, the measured plume expansion ratio (plume forward peaking) was 1.90 and 2.10 for Al and Mo, respectively. Moreover, the effect of incident laser intensity on the ion flux emission as well as the emitted ion velocity were investigated using laser intensities varying from 2.5 × 1010 W/cm2 to 1.0 × 1011 W/cm2. Linear increase of fast ion flux and velocity, and quadratic increase of slow ion flux and velocity were observed. For further understanding of plume dynamics, laser optical emission spectroscopy was used to characterize Sn plasma by measuring the temporal and spatial evolution of plasma electron density Ne and electron temperature Te. At 3.5 mm away from the target, plasma density showed slow decrease with time, however electron temperature was observed to decrease dramatically. The maximum plasma density and temperature occurred at 0.5 mm away from target and were measured to be 8.0 × 1017 cm−3 and 1.3 eV, respectively.

Ion and extreme ultraviolet emission features of CO2 laser produced Sn and SnO2 plasmas

The plasma plume expansion, ion debris and extreme ultraviolet (EUV) emission features of CO2 laser-produced Sn and SnO2 plasmas are investigated in this work. The expansion parameters of laser-produced plasma (LPP) are estimated using a fast-gated intensified charge-coupled device (ICCD) imaging system, while the ion debris is detected using a Faraday cup and the time-of-flight (TOF) method. Additionally, the EUV emissions from CO2 laser-produced Sn and SnO2 plasmas are measured using a grazing incidence flat-field spectrograph and EUV photodiodes. The ICCD images show that the SnO2 plasma plume has a broader edge when compared with that of the Sn plasma. Because of the differences between the thermodynamic parameters of the samples, the total ion number and the kinetic energy were both higher for the SnO2 plasma than for the Sn plasma. The maximum estimated conversion efficiencies (CEs) of the Sn and SnO2 plasmas were 1.1% and 0.9%, respectively; the difference is due to the lower Sn fraction in SnO2. The results of the angularly-resolved EUV emission measurements showed that the detected EUV energy decreases as the angle with respect to the laser axis increases.

Investigation of ion characteristics in CO2laser irradiating preformed tin-droplet plasma

AbstractComparative study of CO2laser-produced tin-droplet plasma with and without pre-pulse laser has been presented. A pre-pulse laser and the CO2laser was combined and focused to tin-droplet with a diameter of 180 µm. The emitted Sn ions were detected by several Faraday cups to obtain angular distribution of ions in the laser-produced tin-droplet plasma. The influence of pre-pulse laser energy and delay time between pumping laser and pre-pulse laser on the ion characteristics was investigated. It is illustrated that ion average kinetic energy from CO2laser-produced plasma (LPP) can be reduced when the tin-droplet target has been replaced by the preformed Sn plasma. The obtained optimal delay time with the lowest ion average kinetic energy is about hundreds of nanoseconds. The ion time-of-flight spectra show a twin peak structure in laser-irradiating preformed Sn plasma. And a superimposed Maxwell–Boltzmann (MB) distribution is proposed to describe this twin peak ion time-of-flight spectra. The fitting results quite agree with the raw ion time-of-flight spectra in current experiment. Then, the fitted plasma temperatures and mass-center velocities with various delay times in laser-irradiating preformed plasma are obtained, and the fitted plasma temperatures can be comparable with ion average kinetic energy in double-pulse LPP, which justified the rationality using this superimposed MB distribution.

On the maximum conversion efficiency into the 13.5-nm extreme ultraviolet emission under a steady-state laser ablation of tin microspheres

Theoretical investigation has been performed on the conversion efficiency (CE) into the 13.5-nm extreme ultraviolet (EUV) radiation in a scheme where spherical microspheres of tin (Sn) are simultaneously irradiated by two laser pulses with substantially different wavelengths. The low-intensity short-wavelength pulse is used to control the rate of mass ablation and the size of the EUV source, while the high-intensity long-wavelength pulse provides efficient generation of the EUV light at λ=13.5 nm. The problem of full optimization for maximizing the CE is formulated and solved numerically by performing two-dimensional radiation-hydrodynamics simulations with the RALEF-2D code under the conditions of steady-state laser illumination. It is shown that, within the implemented theoretical model, steady-state CE values approaching 9% are feasible; in a transient peak, the maximum instantaneous CE of 11.5% was calculated for the optimized laser-target configuration. The physical factors, bringing down the fully optimized steady-state CE to about one half of the absolute theoretical maximum of CE≈20% for the uniform static Sn plasma, are analyzed in detail.

In situ collector cleaning and extreme ultraviolet reflectivity restoration by hydrogen plasma for extreme ultraviolet sources

Laser-produced Sn plasmas used to generate extreme ultraviolet (EUV) light for lithography cause the release of Sn ions and neutrals in the EUV source chamber. These Sn atoms condense and deposit on the multilayer collector optic, which reduces its ability to reflect EUV light. This lowers the source throughput and eventually necessitates downtime for collector cleaning. In this paper, an in situ plasma-based collector cleaning technique is presented and experimentally demonstrated. First, the technique is shown to completely clean a 300 mm diameter stainless steel dummy collector. Second, simulations and secondary ion mass spectroscopy depth profiles show that the technique does not erode the real multilayer mirrors. Finally, EUV reflectivity measurements demonstrate the ability of the technique to restore EUV reflectivity to Sn-coated multilayer mirrors. This technique has the potential to be used in conjunction with source operation, eliminating cleaning-related source downtime.

Spectral and ion emission features of laser-produced Sn and SnO2 plasmas**Project supported by the National Natural Science Foundation of China (Grant No. 11304235) and the Director Fund of WNLO, China.

We have made a detailed comparison of the atomic and ionic debris, as well as the emission features of Sn and SnO2 plasmas under identical experimental conditions. Planar slabs of pure metal Sn and ceramic SnO2 are irradiated with 1.06 μm, 8 ns Nd:YAG laser pulses. Fast photography employing an intensified charge coupled device (ICCD), optical emission spectroscopy (OES), and optical time of flight emission spectroscopy are used as diagnostic tools. Our results show that the Sn plasma provides a higher extreme ultraviolet (EUV) conversion efficiency (CE) than the SnO2 plasma. However, the kinetic energies of Sn ions are relatively low compared with those of SnO2. OES studies show that the Sn plasma parameters (electron temperature and density) are lower compared to those of the SnO2 plasma. Furthermore, we also give the effects of the vacuum degree and the laser pulse energy on the plasma parameters.

High-order harmonic generation in Ag, Sn, fullerene, and graphene nanoparticle-contained plasmas using two-color mid-infrared pulses

We demonstrate the application of tunable mid-infrared pulses (70 fs, 1300–1600 nm) and their second harmonics for high-order harmonic generation in the laser-produced plasmas containing Ag, Sn, C60, and graphene nanoparticles. The application of two-color pump allowed generation of harmonics up to the thirties orders, as well as demonstrated the resonance enhancement of tunable single harmonic in the Sn nanoparticle-contained plasma. The demonstration of equal odd and even harmonic generation in nanoparticle-contained plasmas using tunable mid-infrared source is useful for producing the efficient source of short-wavelength ultrashort pulses for various applications and studies of the properties of harmonic emitters.

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.

Influence of a magnetic field on laser-produced Sn plasma

The effects of an external magnetic field on Nd:YAG laser-produced Sn plasma have been investigated. The characteristics of ion debris from Sn plasma, emission spectra, and EUV radiation have been studied by the time-of-flight method and the optical emission spectroscopy. Our results show that the ion kinetic energies of the plume species can be effectively reduced with a modest magnetic field of 0.6 T. With the presence of a magnetic field, the spectral intensities of Sn I and Sn II show significant enhancement and the electron density of plasma is about 2 times higher. We have not found any influence of magnetic field on the characteristics of EUV emissions.

Temporal evolution of Nd:YAG laser-produced Sn plasma

The temporal evolution of Nd:YAG laser-produced Sn plasma in atmospheric pressures from 5 to 104 Pa is investigated. The results show that the extreme ultraviolet radiation exists only at the beginning of the expansion process for 20 ns. The maximum temperature of 18.7 eV and density of 9.6×1017 cm 3 are measured at 73 ns. The effects of air pressure and laser energy on the process of plasma expansion are investigated. The results indicate that the air pressure has an inhibitory effect on kinetic energy, while the electron density and temperature increase with air pressure.

Diagnostics of Tin Plasma Produced by Visible and IR Nanosecond Laser Ablation

AbstractWe present the optical emission spectroscopic studies of the Tin (Sn) plasma, produced by the fundamental (1064 nm) and second (532 nm) harmonics of a Q switched Nd: YAG pulsed laser having pulse duration of 5 ns and 10 Hz repetition rate which is capable of delivering 400 mJ at 1064 nm, and 200 mJ at 532 nm using Laser Induced Breakdown Spectroscopy (LIBS). The laser beam was focused on target material by placing it in air at atmospheric pressure. The experimentally observed line profiles of four neutral tin (Sn I) lines at 231.72, 248.34, 257.15 and 266.12 nm were used to extract the electron temperature (Te) using the Boltzmann plot method and determined its value 6360 and 5970 K respectively for fundamental and second harmonics of the laser. Whereas, the electron number density (Ne) has been determined from the Stark broadening profile of neutral tin (Sn I) line at 286.33 nm and determined its value 5.85 x 1016 and 6.80 x 1016cm–3 for fundamental and second harmonics of the laser respectively. Both plasma parameters (Te and Ne) have also been calculated by varying distance from the target surface along the line of propagation of plasma plume and also by varying the laser irradiance. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Resonant and non-resonant high-order harmonic generation in the plasmas produced by 1 kHz picosecond and femtosecond pulses

We report the application of femtosecond and picosecond laser pulses for plasma formation on metal targets (Al, Cu, In, Sn) for non-resonant and resonant high-order harmonic generation using laser pulses with 1 kHz repetition rate. The resonance enhancement of single harmonics in the picosecond and femtosecond pulse induced In and Sn plasmas has been studied. These studies reveal comparable harmonic yields from both types of plasmas, which indicate that the pump fluence is of central importance for the optimal plasma formation for efficient harmonic generation.

Extreme ultraviolet emission and confinement of tin plasmas in the presence of a magnetic field

We investigated the role of a guiding magnetic field on extreme ultraviolet (EUV) and ion emission from a laser produced Sn plasma for various laser pulse duration and intensity. For producing plasmas, planar slabs of pure Sn were irradiated with 1064 nm, Nd:YAG laser pulses with varying pulse duration (5–15 ns) and intensity. A magnetic trap was fabricated with the use of two neodymium permanent magnets which provided a magnetic field strength ∼0.5 T along the plume expansion direction. Our results indicate that the EUV conversion efficiency do not depend significantly on applied axial magnetic field. Faraday Cup ion analysis of Sn plasma show that the ion flux reduces by a factor of ∼5 with the application of an axial magnetic field. It was found that the plasma plume expand in the lateral direction with peak velocity measured to be ∼1.2 cm/μs and reduced to ∼0.75 cm/μs with the application of an axial magnetic field. The plume expansion features recorded using fast photography in the presence and absence of 0.5 T axial magnetic field are simulated using particle-in-cell code. Our simulation results qualitatively predict the plasma behavior.