A critical component of high-performance EUV lithography source optics is the reflecting multilayer coating. The ideal multilayer will have both high reflectance and high stability to thermal load. Additionally the capping layers must provide resistance to degradations from exposure to an EUV source, and also be compatible with, or enhance, the systems used for cleaning an exposed multilayer coating. We will report on the results of development of C and B4C stabilized Mo/Si multilayers used to increase the as-deposited peak reflectivity (Rp) as well as decreasing the loss of peak reflectivity (Rp) as a function of annealing temperature. Previous results demonstrate that these layers prevent loss of Rp for temperatures up to 600º C. Results on the use of reactively-sputtered oxide capping layers such as SiO2 and ZrO2 will be presented as well, along with results of exposure testing. The deposition is performed in a dual processchamber inline magnetron system, using reactive sputtering for the production of capping layers. The reflectometer and exposure apparatus at the NIST Physics Laboratory is used for evaluation of the performance. Exposure results on the resistance to oxidation in the presence of water vapor will be presented and discussed.
This paper reports on an all-out effort to reduce the intersite gap of the resist outgassing contamination growth in the results obtained under the round-robin scheme. All test sites collaborated to determine the causes of such gaps. First, it was determined that wafer temperature during exposure could impact the amount of contamination growth. We discovered a huge intersite gap of wafer temperatures among the sites by using a wafer-shaped remote thermometer with wireless transmitting capability. Second, whether the contamination-limited regime was attained during testing could have been another primary root cause for such a difference. We found that for one of the model resists whose protecting unit had lower activation energy and molecular weight the contamination-limited regime was insufficient at one test site. Third, the ratio of the exposed area to pumping speed is necessary to equalize contamination growth. We validated the effect of matching the ratio of exposure area to pumping speed on reducing the intersite gap. This study and the protocols put in place should reduce the intersite gap dramatically.
EUV photoresists are considered as a potential source of optics contamination, since they introduce irradiation induced outgassing in the EUV vacuum environment. Therefore, before these resists can be used on e.g. ASML NXE:3100 or NXE:3300, they need to be tested in dedicated equipment according to a well-defined procedure, which is based on exposing a witness sample (WS) in the vicinity of a simultaneously exposed resist as it outgasses. Such an outgassing test infrastructure is available at many sites, but exposure modes on the witness sample and wafer can be significantly different, which potentially could lead to different test results. In this investigation, we first explored in more detail the relationship between resist outgassing as measured by RGA (Residual Gas Analysis) and the carbon growth obtained in the WS test. A good correlation was found by using a timeintegrated and mass-weighted sum of the RGA-measured mass peaks. Next, the impact of the resist exposure mode on the WS contamination result was investigated at imec, where the outgas test setup allows to expose the wafer with EUV irradiation as well as electrons in the same vacuum environment. It was found that minor differences observed in the WS test results, can be explained by adequate characterization of exposure intensity distribution and dose control. Finally the WS test results at imec from the different exposure modes were compared to the test results at NIST. The small differences in contamination that were observed could be explained by differences in test procedure, by using the time dependent RGA approach. From the combined work on outgassing measurements and WS contamination testing we have significantly improved our understanding of the relationship between outgassing and contamination processes induced by EUV photons and electrons. We have also demonstrated how to compare results obtained at different outgas testing sites, which is important in quantifying the potential risk to EUV device manufacturing posed by resist outgassing.
In this work, we discuss several parameters influencing extreme ultraviolet (EUV) resist qualification test results. The witness sample resist qualifications test is implemented on several different instruments world wide. The original protocol for the test developed by the tool manufacturer requires that a 300 mm wafer be exposed to a dose to clear in one hour. Also required is that the carbon contamination rate on the witness sample be mass limited, i.e., that the rate be saturated with respect to the EUV or e-beam intensity. Simple arguments have been presented in the past to show that the thickness of the carbon growth should be inversely proportional to the pumping speed and proportional to the area exposed if dose and duration were held constant. The present experiments demonstrate the real-life limitations of these arguments and provide validation to our area scaling procedure to equate test results done with 200 mm wafers to results expected for 300 mm wafers. In the process of studying the dependence of carbon growth on pumping speed, we encountered the important effect of increased partial pressure on the degree of intensity saturation.
Before being used in an extreme-ultraviolet (EUV) scanner, photoresists must be qualified to
ensure that they will not excessively contaminate the scanner optics or other parts of the vacuum
environment of the scanner. At the National Institute of Standards and Technology we have
designed and constructed a high-throughput beamline on the Synchrotron Ultraviolet Radiation
Facility (SURF III) in order to provide data on the contamination potential of the outgas products
of a candidate resist by simultaneously irradiating a witness substrate and a nearby resist-coated
wafer with EUV radiation, the so called witness sample test that is currently the resist
qualification method required by ASML. We will present results from four sample resists that
were subjected to the test.
Although the witness-sample test based on irradiating the resist with EUV radiation at 13.5 nm
most closely reproduces conditions in a scanner, the limited availability of suitable EUV sources
to conduct such tests has led to development of an alternative method which uses e-beam
irradiation in place of EUV radiation. We will also present the results of a comparison of these
two methods.
We report on optics contamination rates induced by exposure to broad-bandwidth, high-intensity EUV radiation peaked
near 8 nm in a new beamline at the NIST synchrotron. The peak intensity of 50 mW/mm2 allows extension of previous
investigations of contamination by in-band 13.5 nm radiation at intensities an order of magnitude lower. We report nonlinear
pressure and intensity scaling of the contamination rates which is consistent with the earlier lower-intensity
studies. The magnitude of the contamination rate per unit EUV dose, however, was found to be significantly lower for
the lower wavelength exposures. We also report an apparent dose-dependent correlation between the thicknesses as
measured by spectroscopic ellipsometry and XPS for the carbon deposits created using the higher doses available on the
new beamline. It is proposed that this is due to different sensitivities of the metrologies to variations in the density of the
deposited C induced by prolonged EUV irradiation.
We describe a null-field ellipsometric imaging system (NEIS) that provides for the real-time imaging of carbon
deposition profiles on extreme-ultraviolet (EUV) optics in a vacuum system. NEIS has been demonstrated at NIST on a
small chamber that is used for EUV optics lifetime testing. The system provides images of carbon deposition spots with
sub-nanometer resolution thickness measurements that maintain good agreement with those from ex-situ spectral ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS). The system will be implemented on several synchrotron beamlines for real-time monitoring of carbon film growth on optics during EUV irradiation.
The goal of our ongoing optics-contamination program is to estimate the magnitude and scaling laws of the
contamination rates of optics exposed to extreme-ultraviolet (EUV) radiation in the presence of various contaminant
species expected in the EUV-lithography-tool environment by exposing samples to in-band 13.5 nm light from our
synchrotron in the presence of fixed partial pressures of admitted gases. We report contamination rate measurements on
TiO2-capped samples for species observed in separate resist-outgassing measurements (benzene, isobutene, toluene and
tert-butylbenzene) in the pressure range (10-8 to 10-5) Pa. We use two spatially-resolved surface probe techniques,
spectroscopic ellipsometry and X-ray photoelectron spectroscopy, to determine the thickness of deposited carbon. The
correlation and sensitivities of these techniques are discussed. The high sensitivity of ellipsometry shows that
contamination rates for some species have a pronounced non-linear intensity dependence and can be strongly influenced
by admixtures of water vapor, while the rates for other species are linear over the same intensity range and are less
affected by ambient water. Understanding scaling laws is critical when estimating optic lifetimes or cleaning cycles by
extrapolating over the 3-to-6 orders of magnitude between accelerated-testing and tool-environment partial pressures.
The primary, publicly reported cause of optic degradation in pre-production extreme-ultraviolet (EUV) lithography systems is carbon deposition. This results when volatile organics adsorb onto optic surfaces and then are cracked by EUV-induced reactions. Hence the deposition rate depends on the adsorption-desorption kinetics of the molecule-surface system as well as the basic photon-stimulated reaction rates, both of which may vary significantly for different organic species. The goal of our ongoing optics-contamination program is to estimate the contamination rate of species expected in the tool environment by exposing samples to in-band 13.5 nm light from our synchrotron in the presence of fixed partial pressures of admitted gases. Here we report preliminary results of contamination rates on TiO2-capped samples for species observed in resist-outgassing measurements (benzene, isobutene, toluene and tert-butylbenzene) in the pressure range (10-6 to 10-4) Pa which all display an unexpected logarithmic dependence on pressure. This scaling is in agreement with previous EUV exposures of other species at NIST as well as independent measurements of coverage performed at Rutgers University. These results are consistent with a molecular desorption energy that decreases with coverage due to molecular interactions (Temkin model). Use of the proper scaling law is critical when estimating optic lifetimes by extrapolating over the 3-to-6 orders of magnitude between accelerated-testing and tool-environment partial pressures.
The ability to predict the rate of reflectivity loss of capped multilayer mirrors (MLMs) under various conditions of
ambient vacuum composition, intensity, and previous dose is crucial to solving the mirror lifetime problem in an EUV
stepper. Previous measurements at NIST have shown that reflectivity loss of MLMs exposed under accelerated
conditions of dose and pressure can be a very complicated function of these variables. The present work continues this
effort and demonstrates that reflectivity loss does not scale linearly for accelerated exposure doses over the range of
0-350 J/mm2 either for partial pressures of MMA in the range 10-8-10-7 Torr or acetone in the range 10-7-10-6 Torr. We
suggest that this nonlinear scaling may be the result of a varying damage rate as the surface of the growing
contamination layer moves through the EUV standing wave created by exposure of any MLM to resonant radiation. To
further investigate the potential influence of these resonance effects, we report new measurements showing large
variations of the secondary electron yield as a function of thickness of carbon deposited on top of a MLM.
The photon-stimulated emission of organic molecules from the photoresist during exposure is a serious problem for
extreme-ultraviolet lithography (EUVL) because the adsorption of the outgassing products on the EUV optics can lead
to carbonization and subsequent reflectivity loss. In order to accurately quantify the total amount of outgassing for a
given resist during an exposure, we have constructed a compact, portable chamber that is instrumented with a spinning
rotor gauge and a capacitance diaphragm gauge that, unlike the more commonly used ionization gauge or quadrupole
mass spectrometer, provides a direct and accurate measurement of the total pressure that is largely independent of the
composition of the outgas products. We have also developed a method to perform compositional analysis on the outgas
products and, more generally, on any contaminants that might be present in the stepper vacuum. The method involves
collecting the vacuum contaminants in a trap cooled to liquid-nitrogen temperature. Once collected, the products from
the trap are transferred to a system for analysis with gas chromatography with mass spectrometry. We will describe the
workings of the instruments in detail as well as results of initial tests.
Endurance testing of Ru-capped multilayer mirrors (MLMs) at the NIST synchrotron facility has revealed that the damage resulting from EUV irradiation in a water-dominated environment is nonlinear and may be influenced by competing oxidation and carbon-deposition processes. Concurrent results from two different environmental chambers reveal non-intuitive relationships between reflectivity loss and the admitted water-vapor partial pressure, the ambient background-gas composition, the presence or absence of hot filaments in the chamber, the EUV intensity and the irradiation dose. We discuss possible mechanisms and propose further experiments to test them. Determining the MLM lifetime from accelerated tests is a very difficult task. It is crucial that any lifetime testing procedure involves duplicate exposures for consistency, and, if possible, testing in multiple facilities.
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.
The standard silicon (Si) capping layer used for extreme ultra-violet lithography (EUVL) multilayer (ML) mask blanks has some shortcomings, such as low oxidation resistance, low chemical resistance, low etch selectivity in either the SiO2 buffer layer etch to the capping layer or the absorber etch (e.g., TaN) to the capping layer. These performance and process issues with Si capped ML mask blank will reduce the mask lifetime and require tighter process control during EUVL mask fabrication.
Alternative capping materials have been investigated for both EUVL optics and for mask applications.1-5 It has been initially demonstrated that Ru capping layers have high oxidation resistance and high mask process margin as compared to Si ML cap. In this paper, we will present a detailed evaluation of Ru and ion beam deposited (IBD) diamond-like-carbon (DLC) for EUVL mask application. Performance evaluations of the DLC mask blank capping layer and Ru capping layer were made in the area of reflectivity performance, shelf-life, and EUV exposure stability. It has been shown that EUV exposure induced capping layer change depends upon the exposure conditions. However, we found that as long as the induced relative change in the ML cap material are the same (e.g., the same amount of oxidation), regardless of exposure time and exposure conditions, the resulting reflectivity change is about the same. In the case of the two capping layer materials we evaluated, the capping surface reaction with active oxygen is the primary cause for the reflectivity degradation.
There are many obstacles in the path to the commercialization of EUV lithography including optics lifetime and source power. The Photon Physics Group at the National Institute of Standards and Technology has programs to support the extreme-ultraviolet source and optics development community in their efforts to meet the needs of lithography tool makers. Extreme-ultraviolet detector and optics metrology have been key elements of the Photon Physics Group program for over a decade, and this program has expanded recently to allow the calibration of assembled instruments for source development metrology. This program also includes a dedicated beamline for the exposure of multilayer optics in a low-pressure water atmosphere to test the performance of optics under irradiation conditions similar to those expected in a stepper's projection optics box. In this paper we will outline our programs in multilayer optics testing and at-wavelength detector and optics metrology including descriptions of facilities, capabilities and future plans to further support the efforts to commercialize this important technology.
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