Obert R. Wood

A dual-mode actinic EUV mask inspection tool

To qualify the performance of non-actinic inspection tools, a novel EUV mask inspection system has been installed at the Advanced Light Source (ALS) synchrotron facility at Lawrence Berkeley National Laboratory. Similar to the older generation actinic mask inspection tool1, the new system can operate in scanning mode, when mask blanks are scanned for defects using 13.5-nm in-band radiation to identify and map all locations on the mask that scatter a significant amount of EUV light. By modifying and optimizing beamline optics (11.3.2 at ALS) and replacing K-B focusing mirrors with a high quality Schwarzschild illuminator, the new system achieves an order of magnitude improvement on in-band EUV flux density at the mask, enabling faster scanning speed and higher sensitivity to smaller defects. Moreover, the system can also operate in imaging mode, when it becomes a zone-plate-based full-field EUV microscope with spatial resolution better than 100 nm. The microscope utilizes an off-axis setup, making it possible to obtain bright field images over a field-of-view of 5x5 um2.

The erosion of materials exposed to a laser-pulsed-plasma (LPP) extreme ultraviolet (EUV) illumination source

A critical issue in the realization of EUV lithography (EUVL) as a production technology is the lifetime of the condenser, the optic in closest proximity to any compact, high-power EUV source. During operation of the Engineering Test Stand (ETS), a full-field, high-power EUVL alpha tool, the silicon/molybdenum multilayer mirrors used as a condenser were eroded by extended exposure to the LPP source. The erosion rate varied considerably, and diagnostic instrumentation on the ETS was not intended to address this issue, so the cause of this erosion was not determined at the time. We present here the results of experiments in which samples of gold, molybdenum, and silicon were exposed to an LPP using a liquid xenon jet as the target. The measured erosion rates suggest a sputtering mechanism. Observations of the plasma environment at the condenser position show the presence of fast ions, which, if they are xenon, have kinetic energies of tens of keV. Such ions would contribute significantly to condenser erosion.

Actinic inspection of multilayer defects on EUV masks

The production of defect-free mask blanks, and the development of techniques for inspecting and qualifying EUV mask blanks, remains a key challenge for EUV lithography. In order to ensure a reliable supply of defect-free mask blanks, it is necessary to develop techniques to reliably and accurately detect defects on un-patterned mask blanks. These inspection tools must be able to accurately detect all critical defects whilst simultaneously having the minimum possible false-positive detection rate. There continues to be improvement in high-speed non-actinic mask blank inspection tools, and it is anticipated that these tools can and will be used by industry to qualify EUV mask blanks. However, the outstanding question remains one of validating that non-actinic inspection techniques are capable of detecting all printable EUV defects. To qualify the performance of non-actinic inspection tools, a unique dual-mode EUV mask inspection system has been installed at the Advanced Light Source (ALS) synchrotron at Lawrence Berkeley National Laboratory. In high-speed inspection mode, whole mask blanks are scanned for defects using 13.5-nm wavelength light to identify and map all locations on the mask that scatter a significant amount of EUV light. In imaging, or defect review mode, a zone plate is placed in the reflected beam path to image a region of interest onto a CCD detector with an effective resolution on the mask of 100-nm or better. Combining the capabilities of the two inspection tools into one system provides the unique capability to determine the coordinates of native defects that can be used to compare actinic defect inspection with visible light defect inspection tools under commercial development, and to provide data for comparing scattering models for EUV mask defects.

XCEED: XTREME commercial EUV exposure diagnostic experiment

The XCEED chamber was designed to allow diagnostic access to the conditions experienced by collecting optics for a discharge produced plasma (DPP) source. The chamber provides access for EUV photodiodes, sample exposure tests, Faraday cup measurements, and characterization of the ion debris field by a spherical sector energy analyzer (ESA). The Extreme Ultraviolet (EUV) light source creates a xenon z-pinch for the generation of 13.5 nm light. Typical EUV emission is characterized though a control photodiode. The chamber also allows characterization of optic samples at varying exposure times for normal and grazing incidence reflection angles during tests lasting up to 40 million pulses. The principal investigation is characterization of the debris field and the erosive effects on optics present. Light emission from the z-pinch is followed by ejection of multiply-charged ions which can significantly damage nearby mirror surfaces. Characterization of the ejecta is performed with an ESA that diagnoses fast ion species by energy-to-charge ratio using ion time of flight (ITOF) analysis. The ITOF-ESA is used to characterize both the energy and angular distribution of the debris field. In the current paper, the ESA is applied only to the ion debris emitted from the source. The effects of total particle flux on mirror samples are investigated through exposure testing. Samples are exposed to the source plasma and surface metrology is performed to analyze erosion and deposition effects on mirrors within the source chamber.

Scaling studies of capping layer oxidation by water exposure with EUV radiation and electrons

Silicon capped [Mo/Si] multilayer mirrors (MLM’s) can undergo oxidation by the combined effects of radiation (Extreme Ultraviolet [EUV], electron) and water vapor. This parametric study provides silicon-capped MLM oxidation rate data. The goal of this study was to determine the dependence of silicon oxidation on water vapor pressure and radiation flux density over three orders of magnitude. Previous work1 has shown that electron and 95.3 eV EUV exposures produce similar oxidation. The present study verifies that correlation and examines the effects of EUV and electron flux on the oxidation rate of the Si-capping layer. E-beam and EUV exposed areas on silicon-capped MLM samples were analyzed following radiation exposure by Auger depth profiling to determine the thickness of the oxide grown. A ruthenium (Ru) capped MLM was also exposed for 4-hours, however it showed very little oxidation under the most extreme conditions of our test matrix. Also the effect of varying the primary e-beam voltage (0.5-2.0 keV) on Si-capped MLM was examined, which showed that exposures in the 1-2 keV range produce similar results.

EUV testing of multilayer mirrors: critical issues

Recently, while performing extensive EUV irradiation endurance testing on Ru-capped multilayer mirrors in the presence of elevated partial pressures of water and hydrocarbons, NIST has observed that the amount of EUV-induced damage actually decreases with increasing levels of water vapor above ~5x10-7 Torr. It is thought that the admitted water vapor may interact with otherwise stable, condensed carbonaceous species in an UHV vacuum system to increase the background levels of simple gaseous carbon-containing molecules. Some support for this hypothesis was demonstrated by observing the mitigating effect of very small levels of simple hydrocarbons with the intentional introduction of methyl alcohol in addition to the water vapor. It was found that the damage rate decreased by at least an order of magnitude when the partial pressure of methyl alcohol was just one percent of the water partial pressure. These observations indicate that the hydrocarbon components of the vacuum environment under actual testing conditions must be characterized and controlled to 10-11 Torr or better in order to quantify the damage caused by high levels of water vapor. The possible effects of exposure beam size and out-of-band radiation on mirror lifetime testing will also be discussed.

Investigation of plasma-induced erosion of multilayer condenser optics

Experiments are presented that investigate the mechanistic cause of multilayer erosion observed from condenser optics exposed to EUV laser-produced plasma (LPP) sources. Using a Xe filament jet source excited with Nd-YAG laser radiation (300 mJ/pulse), measurements were made of material erosion from Au, Mo, Si and C using coated quartz microbalances located 127 mm from the plasma. The observed erosion rates were as follows: Au=99nm/106 shots, Mo= 26nm/106 shots, Si=19nm/106 shots, and C=6nm/106 shots. The relative ratio Au:Mo:Si:C of erosion rates observed experimentally, 16:4:3:1 compares favorably with that predicted from an atomic sputtering model assuming 20 kV Xe ions, 16:6:4:1. The relative agreement indicates that Xe-substrate sputtering is largely responsible for the erosion of Mo/Si multilayers on condenser optics that directly face the plasma. Time-of-flight Faraday cup measurements reveal the emission of high energy Xe ions from the Xe-filament jet plasma. The erosion rate does not depend on the repetition rate of the laser, suggesting a thermal mechanism is not operative. The Xe-filament jet erosion is ~20x that observed from a Xe spray jet. Since the long-lived (millisecond time scale) plasma emanating from these two sources are the same to within ~30%, sputtering from this long-lived plasma can be ruled out as an erosion agent.

The effect of debris on collector optics, its mitigation and repair: next-step a gaseous Sn EUV DPP source

The critical issue related to advanced fuel plasma EUV sources is collector lifetime. The Illinois Debris-mitigation EUV Applications Laboratory (IDEAL) is continuing research with a dense plasma focus (DPF) light source. The IDEAL DPF electrodes have been redesigned in order to allow for advanced fuel testing, better pinch operation and increased debris generation. The DPF light source operates at negative polarity, 50 Hz, 3 kV and 7.5 Joules of energy per pulse with tetramethyltin [(CH3)4Sn] as an advanced fuel source. EUV output power is measured with filtered photodiodes and results from a gridded energy analyzer still show two primary ion components with a high-energy peak near 6keV. A Faraday-shielded immersed RF antenna provides a 2kW secondary discharge near the DPF for both pre-ionization and mitigation of the debris with a foil trap (>90%). In addition the Surface Cleaning of Optics by Plasma Exposure (SCOPE) facility has been constructed where evaporated and/or ion implanted metals can be deposited on and removed from EUV mirrors. In SCOPE metals were evaporated on to mirror samples held at various temperatures. A metal ion beam was also added to simulate the energetic erosive flux and a helicon plasma was used in situ to study plasma cleaning. Reactive ion etching of tin by chlorine and other gases has shown 500:1 selectivity factors and very high etch rates suitable to refresh an optical mirror surface within a few seconds. Mirror samples were analyzed at the Center for Microanalysis of Material where the diffusion and transport of the metals and surface roughness were studied for lifetime estimation. Lastly, the Xtreme Characterization EUV Experiment Device (XCEED) was used for characterization of the debris has been accomplished by use of an energy sector analyzer in combination with ion time-of-flight. This diagnostic has been designed to measure velocity, mass and charge states of the incoming ions and neutrals, giving discrete debris spectra while in negative polarity operation. Latest results will be presented based on this work.

Characterization of collector optic material samples before and after exposure in LPP and DPP EUV sources

The University of Illinois at Urbana-Champaign (UIUC) and several national laboratories are collaborating on an SEMATECH effort to characterize xenon plasma exposure effects on EUV condenser optics. A series of mirror samples provided by SEMATECH were exposed for 10M shots in an Xtreme Technologies XTS 13-35 commercial EUV discharge plasma source at UIUC and 5M at the high-power TRW laser plasma source at Sandia National Laboratories. Results for both pre and post-exposure material characterization are presented, for samples exposed in both facilities. Surface analysis performed by the Center for Microanalysis of Materials at UIUC investigates mirror degradation mechanisms by measuring changes in surface roughness, texture, and grain sizes as well as analysis of implantation of energetic Xe ions, Xe diffusion, and mixing of multilayers. Materials characterization on samples removed after varying exposure times in the XTS source, together with in-situ EUV reflectivity measurements, identify the onset of different degradation mechanisms within each sample over 1M-100M shots. Results for DPP-exposed samples for 10 million shots in our XCEED (Xtreme Commercial EUV Exposure Device) experiment showed, in general, that samples were eroded and the surfaces were roughened with little change to the texture. AFM results showed an increase in roughness by a factor of 2-5 times, with two exceptions. This was confirmed by x-ray reflectivity (XRR) data, which showed similar roughening characteristics and also confirmed the smoothening of two samples. SEM pictures showed that erosion was from 4-47 nm, depending on the sample material and angle of incidence for debris ions. Finally, microanalysis of the exposed samples indicated that electrode material was implanted at varying depths in the samples. The erosion mechanism is explored using a spherical sector energy analyzer (ESA) to measure ion species and their energy spectra. Energy spectra for ions derived from various chamber sources are measured as a function of the Argon flow rate and angle from the centerline of the pinch. Results show creation of high energy ions (up to E = 13 keV). Species noted include ions of Xe, the buffer gas, and various electrode materials. The bulk of fast ion ejection from the pinch includes Xe+ which maximizes at ~8 keV followed by Xe2+ which maximizes at ~5 keV. Data from samples analysis and ESA measurements combined indicate mechanism and effect for debris-optic interactions and detail the effectiveness of the current debris mitigation schemes.

Predicting overlay performance for electron projection lithography masks

Minimizing mask-level distortions is critical to the success of electron projection lithography (EPL) in the sub-100-nm regime. A number of possibilities exist to reduce mask-fabrication and pattern-transfer distortion including subfield correction, dummy patterns, pattern splitting, and film stress control. Finite element modeling was used to illustrate the advantages and capabilities of these correction schemes for a 100-mm stencil mask with 1-mm×1-mm membrane windows. Static-random-access-memory-type circuit features, including both the interconnect and contact levels, were used, to simulate realistic circuit layouts with both cross-mask and intra-membrane pattern density gradients. With such correction techniques, it is possible to reduce the EPL mask-level distortions for worst-case mixed pattern types to less than 1.0 nm.