In this work we present the status of our high repetition-rate/high power EUV source facility. The masslimited
target concept has demonstrated high conversion efficiencies (CE) previously, with precision solid
state lasers. Currently, experiments are in progress with high power high repetition-rate (3-4 kHz) Qswitched
laser modules. We present a new dedicated facility for the high power EUV source. Also, we
present a precision EUV energy-meter, which is developed for absolute EUV energy measurements.
Spectral measurements of the tin-doped droplet laser plasma are performed with a flat-field spectrometer
(FFS) with a back-illuminated CCD camera. We address the issue of maintaining the calibration of the
EUV optics during source operation at non-optimum intensity at high repetition-rate, which is required for
CE improvement studies. Here we present the unique metrology for measuring EUV energies under a
variety of irradiation conditions without degrading EUV optics, even at high repetition rates (multi-kHz).
Tin-doped droplet target has been integrated with several lasers including high power high repetition rate lasers
and demonstrated high conversion efficiencies for all the lasers. This implies the EUV source power is linearly
increasing as the laser frequency goes higher. The target exhibit very low out-of-band radiation and debris emission.
The drawback of increasing the repetition rate of the target and the laser will be limited. The total amount
of tin consumed for a EUVL source system is also small enough to be operated for a long term without large effort
for recycling of the target materials. We address and demonstrate in this paper the primary issues associated
with long-term high power EUV sources for high volume manufacturing (HVM) using tin-doped droplet target.
Tin is one of the most efficient source materials for both gas discharge plasma sources and laser produced plasma
sources for EUV lithography. Unlike Xenon which was the material commonly investigated for the EUVL source
application, recycling of the target materials is not necessary for tin targets because of its low relative cost.
However, in assessing the benefits of different source architectures, there are large differences in the size of the
tin inventory used, and consequences that ensue. In this paper we make a first attempt to compare these differences,
and assess their impact. Utilizing tin as the radiator at 13.5 nm reduces the total cost of the source system significantly.
Hydrogen-like line emission from lithium has long been considered a candidate for EUV light source for lithography. We have completed the evaluation of the potential of lithium as a laser-plasma source, both theoretically and experimentally. Theoretical calculations show optimum intensity region for lithium for attaining high conversion is close to 5.0 x 1011 W/cm2, with plasma temperature near 50 eV. Experimental studies compare directly, the conversion efficiency and optimum irradiation conditions for both planar tin and lithium solid targets. Best conversion efficiency found in this study is 2% for lithium, while CE measured is better than 4% for tin target at identical experimental conditions.
The 13 nm emission that results from laser plasmas created from tin targets, results from a milliard of transitions occurring in many ions of tin (Sn6+-Sn13+). Understanding the energy manifolds within these multiple states will further our ability to manipulate energy into the narrow emission band demanded by EUV Lithography. A combined experimental theoretical program is underway to measure and interpret the detailed EUV emission spectra from laser plasmas suitable for EUVL, particularly mass-limited droplet laser plasmas. We employ high resolution spectroscopy in the 2 - 60 nm wavelength regions to characterize the emission from the plasma. This is interpreted with the aid of combined hydrodynamic/ radiation transport computer models. The results of this study will have impact on the in-band EUV conversion efficiency, estimation of the out-of-band short-wavelength emission, and in the development of electron temperature plasma diagnostics.
The EUVL collector mirror reflectivity degradation can be measured as erosion of the mirror surface caused by the high energy ion emissions. Characterizing the ion emission permits the analysis of the mechanisms of erosion and provides the capability to reduce the high energy ion emission which directly reduces the erosion rate. The degradation can also be measured as deposition of particulate debris on the mirror surface. The debris particles have sizes of only a few nanometers. We have demonstrated that the use of electrostatic repeller fields mitigates large fraction of the particle transfer. Our microscopic tin-doped droplet target is a mass-limited target and is designed to limit the flux of uncharged particulate matter emanating from the target, with the eventual objective of only generating charged material. The latter then may be inhibited from degrading EUV optics with the use of electrostatic repeller fields and other mitigation schemes.
We present tin-doped droplet target ion emission characteristics in terms of ion energy distribution obtained using our ion spectrometer. Extensive studies on particle generation by controlling plasma conditions and the repeller field effect on individual ion species and particles is also described.
Light sources based on laser plasmas using tin as target material are known to provide high conversion efficiency of laser power to emission in the 13.5 nm spectral region. In addition, laser plasmas produced from microscopic droplet targets enable the utilization of the mass-limited concept which minimizes the effect of target debris produced from the laser plasma interaction. By combining the mass-limited target concept and tin as the choice of target material, we are developing an extreme-ultraviolet (EUV) light source that can supply high power while remaining essentially debris-free. This source uses tin-doped microscopic droplet liquid targets that are generated at high-repetition rates (>30 kHz), which allows convenient upward power scaling when coupled with a high averaged-power laser.
Detailed studies of the radiation from this source have been made using a precision Nd:YAG laser. Broad parametric studies of the conversion efficiency along with in-band spectroscopy of this EUV source have been performed. The parametric dependence of conversion efficiency is established based on measurements made by the Flying Circus diagnostic tool and a calibrated high-resolution flat-field spectrometer. These measurements have been independently confirmed by the Flying Circus 2 team.
A high repetition-rate laser plasma source, possessing distinct radiation and particle emission characteristics, is now a principal candidate light source for the next generation of technology for the fabrication of computer chips. For these sources to satisfy this critical need they will need to meet unprecedented levels of performance, stability and lifetime. We review here some of the principal diagnostics of the EUV radiation that are now being utilized in the metrology, spectroscopy and imaging of these sources.
The most pressing technical issue for the success of EUV lithography is the provision of a high repetition-rate source having sufficient brightness, lifetime, and with sufficiently low off-band heating and particulate emissions characteristics to be technically and economically viable. We review current laser plasma approaches and achievements, with the objective of projecting future progress and identifying possible limitations and issues requiring further investigation.
We are developing a mass-limited, laser plasma target concept that utilizes excited state transitions in tin ions as the source of 13.5 nm radiation, offering in-band conversion efficiencies greater than 1%. The ultimate objective of this EUV source strategy is the utilization of a target that is completely ionized by the laser. To determine the viability of this source for EUVL, we are making extensive measurements of the debris emanating from the target. Here we report on some of these measurements. Also under investigation are various methods of debris mitigation. We have previously shown the effectiveness of electrostatic fields for repelling ions from mass-limited targets, demonstrating improvements in multilayer mirror lifetimes in excess of an order of magnitude, positioning water droplet targets within reach of the EUVL roadmap requirements. Our investigation of debris utilizes various diagnostic techniques including ion collection, ion sputtering and witness-plate capture of particulate debris, and extensive post-mortem microscopic materials analysis.
We have previously reported encouraging results with a new type of laser plasma source. As a radiation source at 13.5nm spectral band, tin has several advantages over xenon, not the least of which is the number of ion species within the plasma that contribute to the in-band emission.
In this paper we report results from spectroscopic measurements of the laser plasma emission from 12 - 19nm from this target, together with hydrodynamic code simulations of the source, towards developing a suitable laser plasma source for EUV lithography.
We have previously proposed the use of mass-limited, tin-containing laser plasma sources for EUV lithography applications. Here we report advances in measurements of the spectral output, conversion efficiency, and debris emission from these sources. We also report progress in the use of repeller field debris inhibition techniques for this source.
One of the key leverage factors in determining the viability of laser-plasma sources for EUVL is the conversion efficiency of laser light to EUV emission in the 13-nm region. We describe experiments and theoretical calculations on a mass-limited laser target design using tin that offers high conversion efficiency.
We describe studies of the debris produced from a high-repetition-rate laser plasma EUVL source based on the mass-limited target concept. In particular, we are developing mass-limited target designs based on complex targets containing tin. Comprehensive analysis of witness-plate detection techniques can reveal many interesting details of the interaction regime, and the impact of the debris. These techniques include Optical Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, Auger Electron Spectroscopy, and Auger Electron Microscopy. We also describe developments of the repeller field concept of debris inhibition. This technique uses electrostatic fields to reduce the flux of plasma ions impinging on the EUV collimating optics. Here, the first measurements of debris mitigation of a tin-doped target are described, and comparisons with earlier measurements of the impact of repeller fields on ion emission from a mass-limited water-droplet target are made.