The spectral emission properties of a droplet-based laser-produced plasma are investigated in the vacuum ultraviolet (VUV) range. Measurements are performed with a spectrograph that operates from 30 to 180 nm with a spectral resolution of 0.1 nm. The emission spectra are recorded for different metal droplet targets, namely tin, indium, and gallium. Measurements were performed at different pressure levels of the background gas. Several characteristic emission lines are observed. The spectra are also calibrated in intensity in terms of spectral radiance to allow absolute emission power estimations from the light source in the VUV region. The presented experimental results are relevant for alternative light sources that would be needed for future wafer inspection tools. In addition, the experimental results help to determine the out-of-band radiation emission of a tin-based extreme ultraviolet (EUV) source. By tuning the type of fuel, the laser energies, and the background gas, the laser-produced plasma light source shows good capabilities to be operated as a light source that covers a spectral emission range from the EUV to the sub-200 nm range.
EUV sources with high brightness and stability are required for actinic photomask inspection. High availability and cleanliness after IF are additional stringent requirements. EUV lithography is only production ready, if these tools are available with HVM specifications. At the Laboratory for Energy Conversion, ETH Zurich, droplet-based EUV LPP sources have been designed, developed and tested at the system level for the last 8 years and has been commercialized by Adlyte AG. The most advanced facility, namely ALPS II, has been operated as a prototype source for hundreds of hours. In the present work, the EUV plasma is imaged with the help of a pinhole camera. The dimension of the plasma in the direction of the laser axis and the direction of the train equal 60 μm and 70 μm, respectively. The plasma is also imaged using an ICCD with an exposure time of 5 ns. The observed droplet plasma has a characteristic kidney shape. The ICCD is a valuable diagnostic as inspection tools require high pulse-to-pulse reproducibility that cannot be assessed to the full extend using a EUV pinhole camera. Various collector configurations, using either NI or GI, have been integrated into the source. The measurements of the emission characteristics at IF for a GI collector configuration reveal a Gaussian spot shape at IF and a pulse-to-pulse stability of 6.8 % (σ), which matches previous stabilities at the source level. The debris mitigation system employs a three layer strategy between the plasma and IF. Introducing a high momentum flow as a first layer of debris mitigation, the load of tin spots on the collector could be reduced by a factor of 9, hence a significant increase of source life-time is obtained. A quantification by Adlyte of IF cleanliness after 24 hours source operation revealed no relevant contamination with respect to the requirements for Blank Inspection Cleanliness after IF.
In this work, the spectral emission proprieties of a droplet-based laser-produced plasma are investigated in the VUV range. These studies are performed with a spectrograph operating from 30 nm to 180 nm at a spectral resolution of 0.1 nm. The emission spectra are recorded for different droplet-based metal fuels such as tin, indium and gallium in the presence of different background gas pressure levels. The experimental results are relevant for alternative light sources that would be needed for future wafer inspection tools. In addition, the experimental results help to determine the Out- Of-Band (OOB) radiation emission of the EUV source. By tuning the type of fuel, the laser energies and the background gas, the LPP light source shows good capabilities to be operated as a tunable light source that covers a spectral emission range from the EUV to the sub-200 nm range.
At the Laboratory for Energy Conversion, ETH Zurich a new tin droplet-based laser-produced plasma source with application in EUV lithography is operational since Q3 2013. The EUV source ALPS II is equipped with a large capacity droplet dispenser and a high power (kW), high repetition rate (>6 kHz) Nd:YAG laser. The new source should address the requirements of high volume manufacturing for different inspection and metrology applications found in EUV lithography. The average source brightness is equal to 350 W/mm2sr. Individual droplet tracking in time and space, which is coupled to a droplet positioning and triggering system helps to increase the pulse-to-pulse EUV emission stability of the source. The lateral droplet stability is on the order of 10-15% of the droplet diameter. The individual droplet triggering yields deviations between the laser trigger and the droplet passage time at the irradiation site of less than 1 us, even for large droplet timing fluctuations (>5%). The in-band EUV radiation is measured with an energy monitor, which is coupled to a fast analog hardware-based integrator. The pulse-to-pulse EUV energy stability for high stability data equals 3% (σ). In the case of window-averaged (0.1 s) data, the EUV stability equals 0.86% (σ). Low stability data is also reported. The large brightness of the presented LPP-based light source can be tuned to adjust the EUV light stability that is required by the inspection tool.
At the Laboratory for Energy Conversion (LEC), ETH Zurich, droplet-based LPP-EUV light sources have been developed since 2007. The main LPP source is ALPS II, which is fully operational since more than one year. The facility is an engineering test stand for long-term e↵ect studies. In order to improve the debris mitigation techniques, it is essential to investigate the droplet plasma dynamics in time and space. Recently a new diagnostic tool based on a multiple array of motorized Langmuir probes has been constructed for this purpose. The detector has been used to map the angular and radial distribution of the ion and electron
dynamics around the droplet target. In this paper, some of the experimental results obtained with the new detector are reported. The angular and radial distribution of the ion flux and kinetic energy of the droplet plasma reveals an anisotropic expansion of the ions in terms of kinetic energy and amount of ion charge around the droplet target. These results have been obtained during continuous source operation and for the first time on droplet-based laser produced plasmas.
High-brightness extreme-ultraviolet light sources are required for mask inspections and metrology, including mask blank inspection, actinic pattern inspection, and aerial image measurement system to improve yield and lower cost of ownership. Laser-produced plasma (LPP) light sources have the highest potential to achieve the brightness requirements for all the range of mask inspection tools currently foreseen. High brightness of LPP sources (100 to 1000 W/mm2 sr) is the result of a smaller source size ( ∼ 0.1 mm) than that of competing technologies. Since brightness is inversely proportional to the area of the source, smaller source size corresponds with greater brightness and hence greater inspection throughput.
At the Laboratory for Energy Conversion of ETH Zurich, a fully operational continuous-running multi-kHz LPP light source has been developed over the last five years and is now undergoing system optimization. Adlyte, a spin-off of ETH Zurich, is working with industry leaders to commercialize this LPP source. Individual subsystem configuration and the physical boundary conditions and limitations that affect power, brightness, stability, and lifetime management are discussed. This integrated system produces a measured brightness of 259 W/mm2 sr. Outlook for the future growth and integration of the source in high-volume manufacturing tools is then discussed.
The tin droplet generator is a key component of EUV LPP sources. Small tin droplets, when combined with a high power
laser, form a regenerative target with high CE. A major challenge associated with today's EUV sources is energy
stability, which directly correlates with the stability of the fuel delivery system. The LEC droplet dispenser is now in
its 5th generation design, with several years of development, including studies of different nozzle types, excitation
mechanisms, thermal management approaches and contamination control systems. The dispenser produces droplets in
the frequency range required for both metrology and HVM EUV sources. The two relevant instability modes are drop-to-drop
jitter and lateral instabilities. The low frequency content of the lateral droplet displacement is compensated by a
newly implemented dispenser positioning system. The drop-to-drop jitter, which is studied over 2000 s, equals 11.2%
(3σ) of the mean droplet spacing, which makes individual droplet laser triggering necessary. The lateral instabilities,
which are mainly relevant in the plane perpendicular to the laser axis, are determined to be in the range of 7.1% (3σ;) of
the droplet diameter. The lateral displacements are recorded over 2.2 hrs. The related EUV temporal energy stability
(open-loop) is estimated to be 0.35% (3σ) for the worst case scenario, a laser spot size which matches the droplet
A key component of EUV LPP sources is the droplet generator. Small tin droplets, when combined with a igh power
laser, deliver a regenerative target with high CE. This is mandatory for long-term operation in an EUV source. The
overall source stability directly correlates with the stability of the fuel delivery system. In this work, droplets are imaged
directly at the irradiation site. The droplet diameter and position are extracted from recorded droplet train images. Tin
droplets are successfully generated at diameters of 35-58um, with droplet velocities ranging from 8 to 12m/s. The
obtained droplet sizes limit the amount of neutral atoms and residual tin at the plasma formation site. The droplet
velocities lead to droplet spacings of up to 8 droplet diameters. The resulting spacing helps to minimize the plasma-droplet
train interaction. The droplet generator frequency range fulfills the requirements for low power metrology tools
and high power HVM sources. Stability of the droplet generator is studied at 20kHz. The droplet diameter stability
presents no significant fluctuations. The lateral droplet stability is in the range of 1.8% (3σ) of the droplet diameter, so
no influence on source stability is expected. The variations in drop-to-drop distance, which go up to 7.3% (3σ) can
influence source stability for the case of constant laser triggering.
The life-time of normal incidence collectors used in LPP EUV sources has been computationally investigated. A two-dimensional/
axisymmetric hydrodynamic-particle code is used to model the plasma expansion from the laser-droplet
interaction up to the collector optic. The plasma is formed from the interaction of a Nd:YAG laser, operating at the
fundamental frequency, with 50μm tin droplets. The simulation results show non-uniform mass-density distributions at
the end of the laser pulse. As the expansion continues up to the collector, the non-uniformities continue to develop. Sn5+
is the most energetic ion impinging on the collector, with kinetic energies up to 7keV. The sputtering yields for Sn ions
onto Mo and Si show a strong dependence on both the ion energy and their impact angle. The deposition of neutral tin
atoms on the collector has also been assessed with a large scale hydrodynamic simulation. These results are used to
investigate the build-up of tin vapor at the irradiation site.
The life-time of the collection optics of an LPP EUV source is computationally studied. The near-field (radiating layer,
micrometer scale) and far-field (optics, meter scale) radiation and particle dynamics are investigated with a twodimensional/
axisymmetric coupled hydrodynamic-particle code, which is used together with an atomic physics code to
predict the laser-plasma processes. The droplet target is found to have a conversion efficiency of 2.2%. The nonuniformity
of the initial plasma expansion is detailed. In the far field study, the neutral and ion distributions are projected
on a normal incidence mirror. Ions up to Sn4+ reach the mirror. Fast neutrals mostly deposit in the central region of the
mirror, while ions erode the outer region. The simulated ion kinetic energies, which are in the range of a few keV volts
match experimental values. The local time durations for a reflectivity drop from 70% to 60% are in the range of 2.5 to 4
hours. The extension of the life-time of the collection optics up to 30'000 hours requires either a 4 order of magnitude
reduction of the ion flux or a 5-fold reduction of the ion kinetic energies. In order to fulfill the EUVL source requirement
of continuous operation, an effective mitigation scheme for fast ions and neutrals is mandatory.
The conversion efficiency and potential for debris of planar and spherical targets of laser-produced tin-plasmas for use in
a high-volume manufacturing EUVL source collector module are computationally examined. A Nd:YAG laser beam is
used to irradiate the targets under different irradiances and pulse durations. A two-dimensional/axisymmetric
hydrodynamic code, an atomic physics code and an analytical model are used to perform simulations of the laser-plasma
processes. The predicted conversion efficiencies are in good agreement with data reported from experiments. The
optimum laser parameters yield maximum conversion efficiencies of 1.86% and 1.45%, respectively for the planar and
spherical targets. However, the spherical target is best suited for low cost-of-ownership, as it has significantly lower
neutral debris compared to the planar target. The key finding of this work is that the laser produced plasmas of both
planar and spherical targets are non-uniform. These non-uniformities must be accounted for in the design of collection
optics and debris mitigation schemes.
A mass spectrometry and time-resolved shadowgraphy study is presented, focusing on the radiators/debris transport
process for an LPP-EUVL source, under worst-case scenarios such as solid target, wide angular dispersion, etc. The solid
angle of ion debris ejection increases as a function of plume lifetime, and up to 0.5 μs (approx. 15 mm above target) does
not exceed 0.3 πsr. around the target normal. The plume expansion is hypersonic (15-20 km/s) with the formation of a
bow shock with an initial length scale of 5-8 mm that gradually reduces its aspect ratio. Primary (low energy) ion debris
is induced directly during laser-surface interaction, whereas almost 90% of these primary ions are converted to Z=6 -12
secondary fast ions by means of laser-plasma interaction. At very high laser power, the fraction of primary ions
converted to high charge ions is diminished to 70%, possibly due to increased LPP shielding. Hence, the external atomic
degrees of freedom (kinetic energy) and the internal degrees of freedom (electronic configuration) are pumped
concomitantly, which presents a tradeoff to combine fast ion debris minimization and efficient radiators' maximization.
Nevertheless, the radiating body is observed fairly static with time, with a length-scale of a few mm's. Hence, the
decoupling of the mass flux and radiation flux may be achieved either in the short (10< t < 100 ns) or in the very long (t
> 500 ns) time scales. Considering the EUV emission time scale constraints, the former is the only viable solution for
enabling LPP-based EUVL.