Laser-produced Plasma Extreme Ultraviolet Source Research Articles

Spectral investigations of low-temperature plasma induced in CO2 gas by nanosecond pulses of extreme ultraviolet (EUV)

In this work, low-temperature plasmas, induced in a gaseous CO2 by intense extreme ultraviolet (EUV) pulses were investigated with a purpose to determine their ionic/molecular composition and the resulting, potentially reactive species. Two laser-produced plasma EUV sources based on a xenon gas puff target were used to irradiate and ionize of the CO2 gas. The sources, driven by Nd:YAG lasers of different parameters, delivered EUV beams created using reflective, focusing collectors. The CO2-based, low-temperature plasmas induced using both systems, emitted radiation in a wide wavelength range, from vacuum ultraviolet (VUV) to visible light (VIS). The radiation was measured using spectrometers and a streak camera operating in these spectral ranges. In the VUV range, multiple emission lines corresponding to ionic and atomic species together with the CO molecular bands were acquired. Spectra from the UV–VIS range were mainly composed of the CO2 + molecular bands. Numerical simulations of the molecular spectra allowed us to estimate rotational and vibrational temperatures of the EUV induced plasmas. As could be expected, plasmas created in both experimental systems were characterized by different temperatures and intensity ratios of the ionic–atomic spectral lines. The spatio-temporal measurements performed using the streak camera indicated a few times longer lifetime of the EUV induced plasmas, compared to the driving, EUV pulses.

A Magnetohydrodynamic System for Pumping Liquid Metal Media in a Plasma Source of Extreme Ultraviolet Radiation

A sealed compact system based on two series-connected magnetohydrodynamic pumps for pumping liquid metal media with an operating temperature as high as 450°C is described. The system is designed to produce a renewable target in the form of a liquid lithium jet in a pulsed-repetition rate source of extreme ultraviolet (EUV) radiation based on laser plasma. The system provides a head pressure of 2 bar and a flow rate of up to 80 cm3/s and is characterized by reliability, simplicity of design, the absence of moving parts, convenient handling, and the ability to pump aggressive conductive liquids. The system is easily scalable in both the head pressure and the flow rate. The long-term operation of the system has been demonstrated in a laser-produced plasma EUV source, where it provided a jet velocity of up to 22 m/s.

Temporal measurements of extreme ultraviolet (EUV) emission, from low temperature, EUV-induced plasmas

AbstractIn this work, extreme ultraviolet (EUV) emission, from EUV induced, low-temperature microplasmas, were investigated. To perform temporal measurements of the EUV pulses of low intensity, in a medium vacuum, under the pressure of the order of 0.1–0.01 mbar a special detection system was prepared. The system was based on an EUV collector and a semiconductor detector, sensitive for the EUV photons. The collector consisted of two identical grazing incidence, paraboloidal mirrors, and allowed to focus a part of the radiation emitted from the microplasma onto the detector surface. An absorption filter, mounted between the collector and the detector, allowed for selection of an interesting wavelength range. Plasmas were created by irradiation of small portions of gases, injected into the vacuum chamber, using a laser produced plasma EUV source. Three gases were used for the EUV induced plasma formation: neon, krypton, and xenon. Low-temperature plasmas, created in these gases, contained multiply charged ions, emitting radiation in similar wavelength ranges. Two detectors, AXUV20HS1 and AXUVHS5, were used for the measurements. It was shown that differences between the corresponding signal profiles, obtained using both detectors, were not very significant. Moreover, it was demonstrated that the duration of the EUV emission from plasmas, created in different gases, were comparable with the duration of the driving EUV pulse. The longest EUV emission was observed for Kr plasmas, approximately 1.5 times the full width half maximum of the driving EUV pulse.

Spectroscopic characterization of Si/Mo thin-film stack at extreme ultraviolet range.

A noninvasive method for characterizing Si/Mo thin-film stack thickness and its complex transfer function using common-path optical coherence tomography is proposed, analyzed, and experimentally demonstrated. A laser-produced plasma (LPP)-based extreme ultraviolet (EUV) source was excited by a four-stage nanosecond Yb:fiber laser amplifier with a pulse energy of 1.01mJ. The tabletop LPP EUV source was compact and stable for generating the EUV interference fringes. The measured complex transfer function of the Si/Mo stack was verified near the pristine 13.5-nm wavelength range.

Low temperature plasmas induced in SF6 by extreme ultraviolet (EUV) pulses

In this work, a comparative study of extreme ultraviolet (EUV) induced low temperature SF6-based plasmas, created using two different irradiation systems, was performed. Both systems utilized laser-produced plasma (LPP) EUV sources. The essential difference between the systems concerned the formation of the driving EUV beam. The first one contained an efficient ellipsoidal EUV collector allowing for focusing of the EUV radiation at a large distance from the LPP source. The spectrum of focused radiation was limited to the long-wavelength part of the total LPP emission, λ > 8 nm, due to the reflective properties of the collector. The second system did not contain any EUV collector. The gas to be ionized was injected in the vicinity of the LPP, at a distance of the order of 10 mm. In both systems, energies of the driving photons were high enough for dissociative ionization of the SF6 molecules and ionization of atoms or even singly charged ions. Plasmas, created due to these processes, were investigated by spectral measurements in the EUV, ultraviolet (UV), and visible (VIS) spectral ranges. These low temperature plasmas were employed for preliminary experiments concerning surface treatment. The formation of pronounced nanostructures on the silicon surface after plasma treatment was demonstrated.

Spectral lines and characteristic of temporal variations in photoionized plasmas induced with laser-produced plasma extreme ultraviolet source

Abstract Spectral lines for Kr/Ne/H2 photoionized plasma in the ultraviolet and visible (UV/Vis) wavelength ranges have been created using a laser-produced plasma (LPP) EUV source. The source is based on a double-stream gas puff target irradiated with a commercial Nd:YAG laser. The laser pulses were focused onto a gas stream, injected into a vacuum chamber synchronously with the EUV pulses. Spectral lines from photoionization in neutral Kr/Ne/H2 and up to few charged states were observed. The intense emission lines were associated with the Kr transition lines. Experimental and theoretical investigations on intensity variations for some ionic lines are presented. A decrease in the intensity with the delay time between the laser pulse and the spectrum acquisition was revealed. Electron temperature and electron density in the photoionized plasma have been estimated from the characteristic emission lines. Temperature was obtained using Boltzmann plot method, assuming that the population density of atoms and ions are considered in a local thermodynamic equilibrium (LTE). Electron density was calculated from the Stark broadening profile. The temporal evaluation of the plasma and the way of optimizing the radiation intensity of LPP EUV sources is discussed.

Solid-state seeded nano-second pulse CO2 oscillator for extreme ultraviolet lithography

This Letter describes in some detail a novel, solid-state seeded, nano-second pulse, multi-line CO2 oscillator designed and built for the extreme ultraviolet (EUV) laser-produced-plasma (LPP) source. The oscillator employed quantum cascade laser (QCL) seeders, a spectral beam combiner and a diffusion-cooled, radio-frequency-discharge excited, slab-waveguide CO2 amplifier in a multi-pass and compact regenerative amplifier configuration. The output spectrum consisted of individually controlled laser lines of 0001-1000 band of a CO2 molecule, P18, P20 and P22 with a target 4-line operation (P18-P24). The resistance to self-seeding of more than 40dB and to the parasitic back-seeding of more than 55dB was realized. The oscillator produced up to 20W of average output power at repetition rate of 100kHz in a near-diffraction limited beam of M2 lower than 1.3 and RMS pointing stability below 50 microradians. Full Text: PDF References V. Bakshi, "EUV lithography", vol. 178 (SPIE Press, 2009). T. Ohta, K. M. Nowak, T. Suganuma, H. Kameda, M. Moriya, T. Yokoduka, Y. Kawasuji, J. Fujimoto, and H. Mizoguchi, "Development of the reliable high power pulsed carbon dioxide laser system for LPP EUV light source", Proc. SPIE 8322, 832220 (2012) CrossRef D. C. Brandt, I. V. Fomenkov, M. J. Lercel, B. M. La Fontaine, D. W. Myers, D. J. Brown, et al. "Laser produced plasma EUV sources for device development and HVM", Proc. SPIE 8322, 832246 (2012) CrossRef K. M. Nowak, T. Ohta, T. Suganuma, J. Fujimoto, H. Mizoguchi, and A. Sumitani, "CO2 laser drives extreme ultraviolet nano-lithography ? second life of mature laser technology", Opto-Electronics Review 21, 52 (2013). DirectLink K. M. Nowak, T. Ohta, T. Suganuma, J. Fujimoto, and H. Mizoguchi, "Multiline short-pulse solid-state seeded carbon dioxide laser for extreme ultraviolet employing multipass radio frequency excited slab amplifier", Opt. Lett. 38, 881 (2013). CrossRef K. M. Nowak, T. Ohta, T. Suganuma, T. Yokotsuka, J. Fujimoto, H. Mizoguchi, and A. Endo, "Spectral characteristics of quantum-cascade laser operating at 10.6 ?m wavelength for a seed application in laser-produced-plasma extreme UV source", Opt. Lett. 37, 4765 (2012). CrossRef K. M. Nowak, T. Ohta, T. Suganuma, T. Yokotsuka, J. Fujimoto, and H. Mizoguchi, "Wavefront measurement of single-mode quantum cascade laser beam for seed application in laser-produced plasma extreme ultraviolet system", Opt. Lett. 37, 4886 (2012). CrossRef

Photo-ionized neon plasmas induced by radiation pulses of a laser-plasma EUV source and a free electron laser FLASH

AbstractIn this work, a laser-produced plasma extreme ultraviolet source and a free electron laser were used to create Ne photo-ionized plasmas. In both cases, a radiation beam was focused onto a gas stream injected into a vacuum chamber synchronously with the radiation pulse. Extreme ultraviolet radiation from the plasma spanned a wide spectral range with pronounced maximum centered at λ = 11 ± 1 nm while the free electron laser pulses were emitted at a wavelength of 32 nm. The power density of the focused plasma radiation was approximately 2 × 107 W/cm2 and was seven orders of magnitude lower compared with the focused free electron laser beam. Radiation fluences in both experimental conditions were comparable. Despite quite different spectral characteristics and extremely different power densities, emission spectra of both photo-ionized plasmas consist of the same spectral lines within a wavelength range of 20 to 50 nm, however, with different relative intensities of the corresponding lines. The dominating spectral lines originated from singly charged ions (Ne II); however, Ne III lines were also detected. Additionally, computer simulations of the emission spectra, obtained for photo-ionized plasmas, driven by the plasma extreme ultraviolet source, were performed. The corresponding measured and calculated spectra are presented. An electron temperature and ionic composition were estimated. Differences between the experimental spectra, obtained for both irradiation conditions, were analyzed. The differences were attributed mainly to different energies of driving photons.

CO2 laser drives extreme ultraviolet nano-lithography — second life of mature laser technology

AbstractIt was shown both theoretically and experimentally that nanosecond order laser pulses at 10.6 micron wavelength were superior for driving the Sn plasma extreme ultraviolet (EUV) source for nano-lithography for the reasons of higher conversion efficiency, lower production of debris and higher average power levels obtainable in CO2 media without serious problems of beam distortions and nonlinear effects occurring in competing solid-state lasers at high intensities. The renewed interest in such pulse format, wavelength, repetition rates in excess of 50 kHz and average power levels in excess of 18 kiloWatt has sparked new opportunities for a matured multi-kiloWatt CO2 laser technology. The power demand of EUV source could be only satisfied by a Master-Oscillator-Power-Amplifier system configuration, leading to a development of a new type of hybrid pulsed CO2 laser employing a whole spectrum of CO2 technology, such as fast flow systems and diffusion-cooled planar waveguide lasers, and relatively recent quantum cascade lasers. In this paper we review briefly the history of relevant pulsed CO2 laser technology and the requirements for multi-kiloWatt CO2 laser, intended for the laser-produced plasma EUV source, and present our recent advances, such as novel solid-state seeded master oscillator and efficient multi-pass amplifiers built on planar waveguide CO2 lasers.

Out-of-band radiation mitigation at 10.6 μm by molecular absorbers in laser-produced plasma extreme ultraviolet sources

Out-of-band radiation in extreme ultraviolet (EUV) exposure tools remains one of the critical issues that must be addressed before the implementation of this lithography technique for high-volume manufacturing. The out-of-band spectrum at the intermediate focus of EUV sources that use a CO2 laser-produced plasma is dominated by scattered radiation from the drive laser, which operates near 10.6-μm wavelength. To reduce the unwanted heating of optical components in the exposure tool caused by the infrared wavelength, a 10.6-μm wavelength-absorbing gas can be used to reduce the number of photons at this wavelength reaching the intermediate focus. Gaseous sulfur hexafluoride (SF6), whose υ3 infrared active mode is vibrationally excited by radiation around 10.6-μm wavelength, can be used to function as a molecular absorber and thus, mitigate part of the infrared radiation. In this work, the optical absorption of gaseous SF6 is experimentally investigated at the CO2 laser wavelengths close to 10.6 μm as well as the EUV wavelength. Various considerations affecting the use of a gas-based spectral filter are also discussed in this paper.

Deposition and sputtering yields on EUV collector mirror from Laser Plasma Extreme Ultraviolet Sources

Based on the self-similar solution of gas dynamic equations, spherical expansion of the highly ionized plasma with limited mass into a vacuum is investigated for the droplet target laser-produced plasma extreme ultraviolet (LPP-EUV) sources. Using partially numerical and partially analytical technology, the velocity, the temperature and the density profiles in the plume versus ionization degree, adiabatic index and initial conditions are presented. Furthermore, the spatial thickness variations of the deposited substrate witness and ion sputtering yields for Ru, Mo, and Si under Sn ion bombardment are theoretically calculated, which can be useful to enable LPP-EUV sources suppliers to estimate collector lifetime and improve debris mitigation systems.

Rare-earth plasma extreme ultraviolet sources at 6.5–6.7 nm

We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5–6.7 nm region based on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these resonant lines around 6.7 nm (in-band: 6.7 nm ±1%) suggest that the in-band emission increases with increased plasma volume by suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in maximized emission.

Modeling of radiative properties of Sn plasmas for extreme-ultraviolet source

Atomic processes in Sn plasmas are investigated for application to extreme-ultraviolet (EUV) light sources used in microlithography. We develop a full collisional radiative (CR) model of Sn plasmas based on calculated atomic data using Hebrew University Lawrence Livermore Atomic Code (HULLAC). Resonance and satellite lines from singly and multiply excited states of Sn ions, which contribute significantly to the EUV emission, are identified and included in the model through a systematic investigation of their effect on the emission spectra. The wavelengths of the 4d−4f+4p−4d transitions of Sn5+ to Sn13+ are investigated, because of their importance for determining the conversion efficiency of the EUV source, in conjunction with the effect of configuration interaction in the calculation of atomic structure. Calculated emission spectra are compared with those of charge exchange spectroscopy and of laser produced plasma EUV sources. The comparison is also carried out for the opacity of a radiatively heated Sn sample. A reasonable agreement is obtained between calculated and experimental EUV emission spectra observed under the typical condition of EUV sources with the ion density and ionization temperature of the plasma around 1018 cm−3 and 20 eV, respectively, by applying a wavelength correction to the resonance and satellite lines. Finally, the spectral emissivity and opacity of Sn plasmas are calculated as a function of electron temperature and ion density. The results are useful for radiation hydrodynamics simulations for the optimization of EUV sources.

Angular distribution of the ion emission from a tin-based laser-produced plasma extreme ultraviolet source

The charge state and energy distributions of ions emitted from a Sn-based laser-produced plasma extreme ultraviolet source, formed at a power density of ≈4 × 1011 W cm−2, have been recorded as a function of angle. Measurements were recorded from 20° to 80° with respect to the target normal. For each individual ion stage detected, more energetic ions showed preferential emission close to the target normal whereas the less energetic ions exhibited peak emission at larger angles. The peak in the observed ion emission moved to higher energies with increasing ionization at all angles. The sum of the emission, for each ion stage recorded, at each observed kinetic energy, decreased with increasing angle from the target normal.

Auto-cleaning of deposited tin debris in a laser-produced plasma extreme ultraviolet source by using a liquid jet target containing tin chloride

We have demonstrated auto-cleaning of deposited tin debris produced in a laser-plasma extreme ultraviolet (EUV) source by using a liquid jet target containing tin chloride solution. The use of double laser pulse irradiation improved the EUV conversion efficiency of 1.1%, which was 3.3 times as large as that obtained with single laser pulse irradiation. At an appropriate concentration of tin chloride, the amount of deposited debris was balanced out with that of sputtered and/or etched debris, resulting in a thickness of deposited debris less than 1 nm after 40 000 laser pulse irradiations.

Deposited debris characteristics and its reduction of a laser-produced plasma extreme ultraviolet source using a colloidal tin dioxide jet target

Debris characteristics and their reduction have been investigated for a laser-produced plasma extreme ultraviolet source by using a colloidal jet target containing tin dioxide nanoparticles. The amounts of deposited debris on a witness plate were determined by total laser energy irradiated onto a target. In situ low-temperature (100°C) heating of a plate was effective to reduce the amounts of deposited debris, since colloidal debris was easily vaporized by the heat.

Debris Characteristics of a Laser-Plasma Colloidal Target Containing Tin Dioxide Nanoparticles

ebris characteristics and theireduction have been investigated for a laser-produced plasma extreme ultraviolet source by using a colloidal jet/droplet target containing tin dioxide nanoparticles. The amount of deposited debris on a silicon witness plate was determined by totalaser energy irradiated onto the target. Double laser pulse irradiation was not effective for deposited debris reduction. Incontrast, in situ low-temperature heating of the witness plate was effective inreducing the amount of deposited debris. Room-temperature photon processing using an incoherent vacuum ultraviolet excimer lamp at 126 nm deoxidized a deposited tin oxide layer

Low-debris, efficient laser-produced plasma extreme ultraviolet source by use of a regenerative liquid microjet target containing tin dioxide (SnO2) nanoparticles

We demonstrated a low-debris, efficient laser-produced plasma extreme ultraviolet (EUV) source by use of a regenerative liquid microjet target containing tin-dioxide (SnO2) nanoparticles. By using a low SnO2 concentration (6%) solution and dual laser pulses for the plasma control, we observed the EUV conversion efficiency of 1.2% with undetectable debris.

Extreme ultraviolet light sources for use in semiconductor lithography—state of the art and future development

This paper gives an overview of the development status and plans of extreme ultraviolet (EUV) light sources at XTREME technologies, a joint venture of Lambda Physik AG, Göttingen and JENOPTIK LOS GmbH, Jena, Germany. Results for gas discharge-produced plasma (GDPP) and laser-produced plasma (LPP), the two major technologies in EUV sources, are presented.The GDPP EUV sources use the Z-pinch principle with efficient sliding-discharge pre-ionization. First prototypes of commercial gas discharge sources with an EUV power of 35 W in 2π sr have already been integrated into EUV microsteppers. These sources are equipped with a debris-filter which supports an optics lifetime exceeding 100 million pulses at 1 kHz repetition rate. The same lifetime was achieved for the components of the discharge system itself.The progress in the development of high-power discharge sources based on xenon resulted in an EUV power of 200 W into a 2π sr solid angle, in continuous operation, at 4.5 kHz repetition rate, by implementation of porous-metal cooling technology. The available intermediate focus (IF) power is 22 W taking into account experimentally verified losses in a 1.8 sr source collector module. The usable IF power depends on the etendue of the optical system of the EUV scanner. For the current size of the EUV emitting plasma the etendue acceptance factor may be below 0.5. The currently usable IF power with 1.8 sr collector mirror may therefore be about 10 W.Z-pinch discharge sources with Sn as the emitter have been developed as a more efficient alternative to xenon fuelled sources. Tin sources showed a conversion efficiency (CE) that was double that of xenon. EUV power of 400 W in 2π sr has been generated at only 4.5 kHz repetition rate. The available IF power is 44 W. Estimates evaluating the tin source performance reveal the potential for achieving high-volume manufacturing (HVM) power specification by using existing technology.Because of their small plasma size and the rather simple thermal management in the EUV generator the LPP EUV sources are investigated as alternatives to GDPP sources to achieve sufficient power for HVM with EUV lithography. These sources use xenon-jet target systems and high-power pulsed lasers as plasma excitation drivers developed at XTREME technologies. The maximum CE from laser power into EUV in-band power is 1.0% into a solid angle of 2π. Experimentally, 7 W EUV radiation is generated at 13.5 nm in a 2π sr solid angle with 0.7 kW laser power on the target. The small source volume of <0.5 mm diameter will allow large collection angles of 5 sr. The corresponding usable IF power is estimated to be 2.3 W. With the full power of the installed 1.2 kW laser driver 10 W EUV power in 2π sr is expected. LPP sources with tin targets are estimated to achieve nearly 10 W IF power with existing driver laser technology.GDPP and LPP sources still compete for the technology of HVM sources for EUV lithography. Each of these technologies has its challenges. The optimization potential of the etendue of the optical system of EUV scanners will certainly influence any decision for a HVM source technology.

Candidate plasma-facing materials for extreme ultraviolet lithography source components

Material selection and lifetime issues for extreme ultraviolet (EUV) lithography are of critical importance to the success of this technology for commercial applications. This work reviews current trends in production and use of plasma-facing electrodes, insulators, and wall materials for EUV-type sources. Ideal candidate materials should be able to: withstand high thermal shock from the short pulsed plasma; withstand high thermal loads without structural failure; reduce debris generation during discharge; and be machined accurately. We reviewed the literature on current and proposed fusion plasma-facing materials as well as current experience with plasma gun and other simulation devices. Both fusion and EUV source materials involve issues of surface erosion by particle sputtering and heat-induced evaporation/melting. These materials are either bare structural materials or surface coatings. EUV materials can be divided into four categories: wall, electrode, optical, and insulator materials. For electric discharge sources, all four types are required, whereas laser-produced plasma EUV sources do not require electrode and insulator materials. Several types of candidate alloy and other materials and methods of manufacture are recommended for each component of EUV lithography light sources.