Extreme Ultraviolet Lithography Source Research Articles

Elastic scattering cross sections and transport of tin ions in extreme ultraviolet lithography sources

Energetic tin ions are created by laser produced plasmas in extreme-ultraviolet lithography sources where hydrogen must be used as a buffer gas to protect critical optical components. In this study, the quantum chemistry code NWChem is used to calculate the interatomic potential between singly ionized tin and molecular hydrogen. The interatomic potential was fit by an inverse-power potential, a modified universal ZBL potential, and a two-piece-Lennard-Jones potential which were in turn used to calculate the classical distance of closest approach, scattering angle, total elastic scattering cross sections. Furthermore, the universal Ziegler-Biersack-Littmark (ZBL) potential was used in the open-source binary collision approximation codes RustBCA as well as Stopping Range in Matter and Transport of Ion in Matter (SRIM/TRIM) to calculate ion ranges, straggling, and stopping cross sections in a hydrogen gas target.

RHDLPP: A multigroup radiation hydrodynamics code for laser-produced plasmas

In this paper, we introduce the RHDLPP, a flux-limited multigroup radiation hydrodynamics numerical code designed for simulating laser-produced plasmas in diverse environments. The code bifurcates into two packages: RHDLPP-LTP for low-temperature plasmas generated by moderate-intensity nanosecond lasers, and RHDLPP-HTP for high-temperature, high-density plasmas formed by high-intensity laser pulses. The core radiation hydrodynamic equations are resolved in the Eulerian frame, employing an operator-split method. This method decomposes the solution into two substeps: first, the explicit resolution of the hyperbolic subsystems integrating radiation and fluid dynamics; second, the implicit treatment of the parabolic part comprising stiff radiation diffusion, heat conduction, and energy exchange. Laser propagation and energy deposition are modeled through a hybrid approach, combining geometrical-optics ray-tracing in sub-critical plasma regions with a one-dimensional solution of the Helmholtz wave equation in super-critical areas. The thermodynamic states are ascertained using an equation of state, based on either the real gas approximation or the quotidian equation of state (QEOS). For ionization calculations, the code employs a steady-state collisional-radiation (CR) model using the screened-hydrogenic approximation. Additionally, RHDLPP includes RHDLPP-SpeIma3D, a three-dimensional spectral simulation post-processing module, for generating both temporally-spatially resolved and time-integrated spectra and imaging, facilitating direct comparisons with experimental data. The paper showcases a series of verification tests to establish the code's accuracy and efficiency, followed by application cases, including simulations of laser-produced aluminium (Al) plasmas, pre-pulse-induced target deformation of tin (Sn) microdroplets relevant to extreme ultraviolet lithography light sources, and varied imaging and spectroscopic simulations. These simulations highlight RHDLPP's effectiveness and applicability in fields such as laser-induced breakdown spectroscopy, extreme ultraviolet lithography sources, and high-energy-density physics.

Absolute density measurement of hydrogen radicals in XUV induced plasma for tin contamination cleaning via laser-induced fluorescence

Effective cleaning of tin contamination on the collecting mirrors in extreme ultraviolet source is one of the key techniques to improve throughput and cost performance of extreme ultraviolet lithography. Hydrogen radicals produced in hydrogen plasma that is induced by wideband extreme ultraviolet radiation are expected to be utilized for in situ tin contamination cleaning in extreme ultraviolet sources. In this Letter, we clarified absolute density and cleaning ability of the hydrogen radicals produced by intense extreme ultraviolet pulse through ground state population density measurement by laser-induced fluorescence technique. The experimentally obtained radical parameters coincided well with simulation results and collisional radiative model. It was found that the extreme ultraviolet induced plasma was in quasi-steady state with abundant amount of hydrogen radicals in ground state. Further, it was found that the in situ tin contamination cleaning in extreme ultraviolet lithography source would become more practical with increase in operational parameters, such as extreme ultraviolet emission intensity, gas pressure, and radical production cross section.

Reconstruction and analysis of transient evolution images of laser-produced plasma plumes.

We introduce a method for the analysis and simulation of transient images of laser-produced plasma (LPP) plumes. This method comprises three steps: (i) calculating the two-dimensional distribution of plasma parameters using a radiation hydrodynamics model, (ii) constructing radiation paths through ray tracing, and (iii) solving the radiation transport equation along these paths. In our simulations, we have meticulously considered factors that could influence the imaging results, including the quantum efficiency to different radiation wavelengths, the imaging lens' transmittance, the target surface's reflectivity, and the absorption, emission, and scattering quantum effect of the detector processes occurring in the plasma. We applied this method to analyze and simulate the transient images of aluminum plasma plumes in a background air environment at a pressure of 2000 Pa. The results demonstrate that our method not only produces simulated images that align with experimental results but also provides a reliable distribution of plasma state parameters and clearly identifies the ion species radiating in different bands. Given its capability in transient image reconstruction and its adaptability as a tool for spectral simulation and analysis of LPPs, we believe this method holds significant potential for spectral diagnostics in fields such as laser-induced breakdown spectroscopy, extreme ultraviolet lithography sources, and high-energy-density physics, among others.

Tin removal by an annular surface wave plasma antenna in an extreme ultraviolet lithography source

Tin contamination of the collector mirror surface remains one of the crucial issues of EUV (Extreme Ultraviolet) sources, directly impacting the availability of the tool. Hydrogen plasma-based tin removal processes employ hydrogen radicals and ions to interact with tin deposits to form gaseous tin hydride (SnH4), which can be removed through pumping. An annular surface wave plasma (SWP) source developed at the University of Illinois—Urbana Champaign is integrated into the cone and perimeter of the collection mirror for in situ tin removal. The SWP is characterized by high ion and radical densities, low electron temperature, and local generation where etching is needed. This method has the potential to significantly reduce downtime and increase mirror lifetime. Radical probe measurements show hydrogen radical densities in the order of 1019 m−3, while Langmuir probe measurements show electron temperatures of up to 6 eV and plasma densities on the order of 1017–18 m−3. The generated ions are essential to the tin cleaning and have sufficiently low energy to cause no damage to the collector capping layer. Tin etch rates of up to 270 nm/min were observed in a variety of experimental conditions, including various powers, pressures, flowrates, and temperatures. The high etch rates demonstrated in this study exceed the expected contamination rate of the EUV source.

High power light source for future extreme ultraviolet lithography based on energy-recovery linac free-electron laser

The development of ways to increase the intensity of extreme ultraviolet (EUV) light sources for future EUV lithography is important to realize high throughput fine patterning. The energy-recovery linac (ERL) free-electron laser (FEL), which is an accelerator based light source, is a candidate for this. We clarify the design concept of the ERL-FEL for EUV light sources for future lithography, delivery systems of the FEL light to multiscanners, and future development items of the accelerator technologies and a possibility of the beyond EUV.

Research Progress and Development Trend of Extreme Ultraviolet Lithography Source

Research Progress and Development Trend of Extreme Ultraviolet Lithography Source

Fast EUV Lithography Source Optimization Using Cascade Compressed Sensing

Source optimization (SO) is intensively used in extreme ultraviolet (EUV) lithography to improve the image fidelity of multiple mask clips on the integrated circuit chip. However, if new layout data is inserted to the mask clip library, the optimal source pattern needs to be recalculated. This paper proposes a fast SO method based on cascade compressed sensing (CCS-SO) to circumvent the redundant computation problem. First, the optimized source patterns for different mask clips are individually calculated using compressed sensing algorithm, where the robustness to defocus is considered. Then, the global optimal source is formulated as the superposition of these pre-optimized source patterns, where the weighting coefficients are solved by the gradient-based algorithm. When new data is included in the mask clip library, we can rapidly update the weighting coefficients instead of re-optimizing the source pattern over again. The proposed CCS-SO approach is shown to achieve superior imaging performance and computational efficiency over the traditional SO approach.

Programmable pulse synthesizer for the generation of Joule-level picosecond laser pulses of arbitrary shape.

We report the demonstration of a pulse synthesizer based on spatial beam splitting and pulse stacking for the generation of picosecond laser pulses of Joule-level energy with arbitrary shape. An array of liquid crystals is used to control the amplitude of ten individual sub-pulses, and sliding retroreflectors are used to adjust their temporal separations. The synthesizer was used in combination with a λ=1.03 µm diode-pumped cryogenically-cooled Yb: YAG chirped pulse amplification laser to synthesize 1.3 J pulses or pulse trains of arbitrary shapes up to 9 ns duration with a temporal resolution as short as 8 ps. This pulse synthesizer offers the opportunity to incorporate a self-learning system to search for the optimal laser pulse shapes for various applications including optimized plasma conditions in laser-plasma based soft x-ray lasers and plasma sources for extreme ultraviolet lithography.

Modeling multilayer coating profiles with defects on EUV collector with grating

Extreme ultraviolet lithography (EUVL) is recognized as a leading technology in next-generation lithography. Achieving spectral purification while ensuring extreme ultraviolet (EUV) reflectance is one of the key technologies for industrializing EUVL. An EUV collector mirror with phase grating can be used in the spectral purification of an EUVL source. However, it also induces a considerable loss of EUV. We propose a deposition model for calculating the multilayer coating profile on the surface of an EUV collector with grating based on a geometric line tracing method. In addition, it also analyzes the coating defects that influence the EUV reflectance, and the evolution of the coating defects with the grating positions. Experimental results reveal that the model accurately predicts the multilayer coating profile deposited on the surface of the collector, which helps improve how EUV collector is deposited.

Emission of water-window soft x-rays under optically thin conditions using low-density foam targets.

The effect of optical thickness in a bismuth water-window soft x-ray source is considered by comparing the emission from laser-produced plasmas of a 7.5% atomic density foam target and a solid-density target. The number of photons recorded in the 4nm region was comparable for both targets at a plasma-initiating laser pulse duration of 6ns. From experiments at different pulse durations of 150ps and 6ns, self-absorption (opacity) effects were found to be relatively small for bismuth plasmas as compared to those of tin, based on the same emission mechanism and which are used in 13.5nm sources for extreme ultraviolet lithography.

A double-stream Xe:He jet plasma emission in the vicinity of 6.7 nm

We present the results of investigations of extreme ultraviolet (EUV) light emission in the range from 5 to 10 nm. The light source was a pulsed “double-stream” Xe:He gas jet target irradiated by a laser beam with a power density of ∼1011 W/cm2. The radiation spectra were measured with a Czerny-Turner monochromator with a plane diffraction grating. The conversion efficiency of the laser energy into EUV radiation caused by Xe+14…+16 ion emission in the range of 6–8 nm was measured using a calibrated power meter. The conversion efficiency of the laser radiation into EUV in the vicinity of 6.7 nm was (2.17 ± 0.13)% in a 1 nm spectral band. In the spectral band of the real optical system (0.7% for La/B multilayer mirrors) emitted into the half-space, it was (0.1 ± 0.006)%. The results of this study provide an impetus for further research on laser plasma sources for maskless EUV lithography at a wavelength of 6.7 nm.

Ultraviolet out-of-band radiation studies in laser tin plasma sources

Out-of-band long wavelength emission measurements from high power, high-repetition-rate extreme-ultra-violet lithography (EUVL) laser plasma sources are imperative to estimating heat deposition in EUV mirrors, and the impact of short wavelength light transported through the imaging system to the wafer surface. This paper reports a series of experiments conducted to measure the absolute spectral irradiances of laser-plasmas produced from planar tin targets over the wavelength region of 124 to 164 nm by 1.06 μm wavelength, 10 ns full-width-at-half-maximum Gaussian laser pulses. The use of spherical targets is relevant to the EUVL source scenario. Although plasmas produced from planar surfaces evolve differently, there is a close similarity to the evolution of current from 10.6 μm CO2 laser EUVL sources, which use a pre-pulse from a lower energy solid-state laser to melt and reform an initial spherical droplet into a thin planar disc target. The maximum of radiation conversion efficiency in the 124–164 nm wavelength band (1%/2πsr) occurs at the laser intensity of 1010 W cm−2. A developed collisional-radiative model reveals the strong experimental spectra that originate mainly from the 4d105p2-4d105s5p, 4d105p-4d105s resonance lines, and 4d95p-4d95s unresolved transition arrays from Sn III, Sn IV, and Sn V ions, respectively. The calculated conversion efficiencies using a 2D radiation-hydrodynamics model are in agreement with the measurements. The model predicts the out-of-band (100–400 nm) radiation conversion efficiencies generated by both 1.06 and 10.6 μm pulses. The 10.6 μm laser pulse produces a higher conversion efficiency (12%/2πsr) at the lower laser intensity of 109 W cm−2.

Spectroscopy of highly charged ions for extreme ultraviolet lithography

After almost three decades of research, commercial sources for extreme ultraviolet lithography (EUVL) at 13.5nm are now being manufactured and EUVL is expected to be introduced finally in 2018 or 2019. The source is based on laser produced plasmas (LPPs) from tin micro-droplets using double pulse irradiation in a pre-pulse/main pulse configuration. The currently accepted configuration uses a CO2 laser main pulse to minimize self-absorption in the transitions responsible for the emission at 13.5nm. The conversion efficiency (CE) of laser to in-band EUV energy in commercial systems is currently around 4% and values up to 6% have recently been observed. However the possibility of attaining significantly higher figures has been predicted by plasma modelling and research is still ongoing to increase the CE. The present paper describes the physical processes that occur in Sn LPPs and possible routes to attaining higher CE values.

Demonstrative setup of EUVL source from three plasmas to be coupled in capillary discharge

Extreme ultraviolet (EUV) source with short wavelength of 13.5nm sets a solid foundation for extreme ultraviolet lithography (EUVL) that will provide distinguishable limited line width of 30nm below and enough depth of focus in lithographic process. On the basis of a new scheme of EUV source from three plasmas to be coupled in capillary discharge, the demonstrative setup of three plasmas EUVL source in capillary discharge has been built and tested. By experiment, it is found that the single plasma source just emits very little power of radiation, while three independent plasmas source outputs intense radiation when three anode electrodes are concurrently discharged to the corresponding cathode electrodes. However, it is impossible for three independent plasmas to be coupled in capillary tube without the effect of the additional laser-produced plasma (LPP). As for the next work such as the weak positional coupling, the input of the laser beam and the delay time between the concurrent discharge and the ignition of the laser, it is worthy of doing.

Spectral purification and infrared light recycling in extreme ultraviolet lithography sources

We present the design of a novel collector mirror for laser produced plasma (LPP) light sources to be used in extreme ultraviolet (EUV) lithography. The design prevents undesired infrared (IR) drive laser light, reflected from the plasma, from reaching the exit of the light source. This results in a strong purification of the EUV light, while the reflected IR light becomes refocused into the plasma for enhancing the IR-to-EUV conversion. The dual advantage of EUV purification and conversion enhancement is achieved by incorporating an IR Fresnel zone plate pattern into the EUV reflective multilayer coating of the collector mirror. Calculations using Fresnel-Kirchhoff's diffraction theory for a typical collector design show that the IR light at the EUV exit is suppressed by four orders of magnitude. Simultaneously, 37% of the reflected IR light is refocused back the plasma.

Laser produced plasma sources for nanolithography—Recent integrated simulation and benchmarking

Photon sources for extreme ultraviolet lithography (EUVL) are still facing challenging problems to achieve high volume manufacturing in the semiconductor industry. The requirements for high EUV power, longer optical system and components lifetime, and efficient mechanisms for target delivery have narrowed investigators towards the development and optimization of dual-pulse laser sources with high repetition rate of small liquid tin droplets and the use of multi-layer mirror optical system for collecting EUV photons. We comprehensively simulated laser-produced plasma sources in full 3D configuration using 10–50 μm tin droplet targets as single droplets as well as, for the first time, distributed fragmented microdroplets with equivalent mass. The latter is to examine the effects of droplet fragmentation resulting from the first pulse and prior to the incident second main laser pulse. We studied the dependence of target mass and size, laser parameters, and dual pulse system configuration on EUV radiation output and on atomic and ionic debris generation. Our modeling and simulation included all phases of laser target evolution: from laser/droplet interaction, energy deposition, target vaporization, ionization, plasma hydrodynamic expansion, thermal and radiation energy redistribution, and EUV photons collection as well as detail mapping of photons source size and location. We also simulated and predicted the potential damage to the optical mirror collection system from plasma thermal and energetic debris and the requirements for mitigating systems to reduce debris fluence. The debris effect on mirror collection system is analyzed using our three-dimensional ITMC-DYN Monte Carlo package. Modeling results were benchmarked against our CMUXE laboratory experimental studies for the EUV photons production and for debris and ions generation.

Recent results on the development of extreme ultraviolet sources for lithography and metrology at 6.x nm

Recent increases in reflectance at 6.x nm of La/B4C multilayer mirrors has led to the adoption of this wavelength for the development of future sources for lithography. The emission from laser produced plasmas of Gd and Tb emit strongly in this wavelength region. The transitions responsible and the optimum plasma conditions for optimum emission have been identified both theoretically and experimentally.

Laser-plasma extreme ultraviolet source at 6.7 nm using a rotating cryogenic Xe target

A laser-plasma source comprising a rotating cryogenic solid-state Xe target has been studied for use in extreme ultraviolet lithography (EUVL) systems equipped with La/B4C mirrors. The laser-to-EUV power conversion efficiency (CE) of the cryogenic Xe target was improved to achieve a maximum CE of 0.15 % at 6.7 nm with 0.6 % bandwidth. We successfully demonstrated the continuous generation of EUV light with an average power of 80 mW at 6.7 nm with 0.6 % bandwidth using a Nd:YAG slab laser at a repetition rate of 320 Hz and an average power of 100 W. Scaling-up of the laser-plasma source for use as a future EUVL source is also discussed.

Development of laser-produced plasma sources for extreme ultraviolet lithography

The current status of laser-produced plasma source develop- ment for extreme ultraviolet lithography is reviewed. The advantages of using Sn as a fuel for 13.5 nm are discussed as is the rationale for using Nd:YAG prepulses followed by CO2 irradiation of mass-limited Sn droplet targets. To date CEs of around 6% have been obtained in the laboratory while much lower values closer to 2% have been achieved in high repetition rate industrial sources suitable for use in high-volume manufacturing (HVM). The discrepancy appears largely to arise from the mismatch between the effective target lifetime and pulse duration as well as incomplete vaporization or fragmentation of the droplets. Improvements in laser technology and droplet delivery systems should see a marked improvement in the near future, so that CEs of 5% to 6% should eventually be realized in industrial sources. To keep pace with Moore's law, development work has begun on sources operating at 6.x nm, the wavelength selected for lithography beyond 13.5 nm. It is shown that Gd plasmas with an electron temperature close to 110 eV will provide the solution at this wavelength and the strongest lines ocurring in a Gd plasma are identified. © 2012 Society of Photo-Optical Instru-