the development of metrology for extreme-ultraviolet (EUV) optics, the maintenance of national primary standards for radiometry in the EUV and adjoining spectral regions, and the operation of national user facilities for EUV science and applications.
INTENDED OUTCOME AND BACKGROUND
The intended outcomes of this program are: maintenance and continuous improvement of the national primary measurement standards for extreme ultraviolet radiation (EUV: wavelengths between 4 nm and 250 nm, i.e., from soft x rays to vacuum ultraviolet), development of techniques for fabricating and characterizing EUV optical systems, and the development of a synchrotron-based, national primary standard for source-based optical radiometry.
The Division has longstanding responsibility for the national primary radiometric standards in the EUV region of the spectrum. EUV radiation is an important tool for determining the electronic structure of materials, diagnosing plasmas, measuring dynamics of the upper atmosphere, and probing the structure and dynamics of astrophysical objects.
One of the top candidates for next-generation semiconductor manufacturing technology is an EUV micropatterning tool, since operation at this short a wavelength (13 nm vs. 193 nm for present, production ultraviolet lithography) enables diffraction-limited imaging of features with smaller critical dimensions. We are working actively with the semiconductor industry to develop new metrology and testing capabilities as needs arise in their effort to commercialize this next-generation lithography.
The Division’s key tool for EUV metrologyis the NIST Synchrotron Ultraviolet Radiation Facility (SURF III). SURF III, the successor to the world’s first dedicated source of synchrotron radiation, is a low-energy (<400 MeV), high beam-current (up to 1 A), perfectly circular electron storage ring. Its operational characteristics are ideal for EUV metrology. It does not produce the hard x-ray radiation of higher energy sources, and it can be operated over a wide range of beam energies to match the spectral response of systems of interest. As a calculable source of radiation from the far infrared through EUV spectral regions, SURF is also used as a primary standard for source-based radiometry throughout the optical spectrum.
Accomplishments
Calibration of the EUV Variability Experiment for NASA’s Solar Dynamics Observatory Mission
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Figure 1. The EVE package in the course of a SuRF III calibration run. Here it is being loaded into a large vacuum chamber on SuRF III beamline 2, which is enclosed in a clean room environment. |
The SURF III facility has calibrated every extreme ultraviolet (EUV) spectrometer flown on NASA missions since the 1970s. In 2008, the first mission of NASA’s Living With a Star program is scheduled for launch: the Solar Dynamics Observatory (SDO). The SDO mission will provide measurements and models of the solar radiation and dynamics that can disturb Earth’s space weather environment.
SDO contains three instrument packages, one of which is the EUV Variability Experiment (EVE), built by the Laboratory for Atmospheric and Space Physics (LASP) of the University of Colorado. EVE measures solar EUV irradiance with unprecedented spectral resolution, temporal cadence, accuracy, and precision. The EVE investigation program incorporates physics-based models of the solar EUV irradiance to advance the understanding of the solar EUV irradiance variations based on the activity of the solar magnetic features. Such variations have been found to have major effects on satellite drag and radio propagation in the ionosphere, due to the complete absorption of EUV radiation in the earth’s upper atmosphere.
The EVE package was calibrated at SURF III before its transfer to NASA for incorporation into SDO. Such calibration is essential for integrating EVE results into the historical record of solar EUV variability, and SURF III is presently the only facility in the world that can provide this service.
Electron Emission Properties of Graphene and Carbon Nanotube Devices
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Figure 2. (a) PEEM and (b) field-emission micrographs of a carbon nanotube electron emission device. |
Carbon nanotubes and nanosheets (graphene) are promising candidates for low-field, cold-electron emission devices, such as efficient electron sources
and displays. Low threshold fields are required to initiate electron emission in these systems, on the order of 1 mV/nm to 10 mV/nm.
This field strength is roughly the same as that used in photoemission electron microscopy, so we investigated the field-emission and photoemission
properties of carbon nanotubes and nanosheets, with emphasis on the threshold field and the uniformity of emission. Typical nonimaging
characterization focuses on the total emitted current. However, to optimize device performance, the entire emitter structure must be active.
This motivated our spatially resolved photoelectron emission microscopy (PEEM) studies of prototype devices produced by Prof.
Brian Holloway at the College of William and Mary. These studies were carried out in collaboration with Prof. Martin Kordesch of Ohio University,
while he was on sabbatical leave at NIST.
Devices prepared for photoelectron emission imaging and field emission microscopy were observed in both
imaging modes with submicron spatial resolution.
It was found that low-current emission was uniform and correlated with the photoelectron (PEEM) image. Hot spots were observed on some specimens that
could be imaged simultaneously in PEEM and field emission microscopy. Metallized graphene sheets were found to be very good emitters, and the nature
of their emission enhancement is currently under study.
Record Beam Currents, Autonomous Operation, and Royal Visit at SURF III
In late 2006, it became possible for the first time to inject currents in excess of an ampere into the SURF III electron storage ring.
When SURF II first came on line in the mid1970s, its maximum injection currents were 10 mA. These were increased to about 300 mA by the time of the
conversion to SURF III in 1997, but the maximum currents then could not be generated reproducibly. A wide range of improvements in the injection and
RF control systems, carried out over the past five years, have led to deterministic injection conditions, which generate initial currents that are as
large as those used in any other synchrotron radiation source.
Furthermore, in 2007 we were able to institute regular autonomous operation of the facility, whereby it automatically re-injects itself when the
beam current decays to a predetermined target level. (Such decay occurs due to electrons being lost from the beam due to collisions.)
This allows for round-theclock operation, which has been necessary to accommodate growing user demand, particularly that associated with lifetime
testing of EUV optical components, which is discussed below.
The synchrotron radiation research community is tight-knit, and members of the SURF III staff frequently interact with their counterparts at other
synchrotron facilities worldwide. In May 2007 an unusual engagement of this type occurred in the form of a visit by a royal personage,
HRH Princess Sumaya of Jordan. The Princess is the head of the Royal Scientific Society of Jordan, which contains the Jordanian counterpart of NIST,
and is also the sponsoring organization of a synchrotron radiation facility, SESAME, which is under construction there. During her tour of SURF III,
Princess Sumaya was offered the opportunity to inject a beam, and carried out the procedure quite successfully.
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Figure 3. SURF III beam current as a function of time,
showing re-injections during the night in the autonomous mode of operation.
Figure 4. HRH Princess Sumaya of Jordan receives a certificate of proficiency in synchrotron facility operation from Dr. Katharine Gebbie.
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Predicting the Lifetime of Extreme Ultraviolet Optics
To help the semiconductor industry meet its goal of achieving extreme-ultraviolet lithography (EUVL) production by 2010, we have established a dedicated beamline at SURF III for durability testing of multilayer mirrors, an essential underlying technology. The new beamline is devoted to accelerated testing, and we have added a second branch to a preexisting beamline to provide broadband illumination (wavelengths of approximately 11 nm to 50 nm) onto a single spot at approximately 100 times the intensity attainable before.
To determine how damage scales with various parameters, we recently exposed EUVL mirrors (provided by SEMATECH from work it co-funded) to varying levels of light intensity, water, and hydrocarbon concentrations. Contrary to expectations, we found that increasing amounts of water vapor caused less mirror damage, which may be due to a simultaneous increase in the ambient hydrocarbon levels. Subsequent experiments have shown that deliberately introducing trace amounts of a simple hydrocarbon like methanol can mitigate significantly the water-induced damage.
Nanoscale Chemical Imaging with Electron Beam Tomography
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Figure 5. (a) A synthetic photonic band gap structure, and (b) its reconstruction using Bayesian tomography in the multiple scattering regime. |
Tomography became an important field about 40 years ago with the application of x rays to medical imaging. The practice quickly spread to electron microscopy. The principal contrast mechanism in x-ray scattering is absorption which follows Beer’s Law, i.e., the rule of exponential attenuation. Although it was necessary to develop radically different algorithms for tomography using magnetic resonance imaging or ultrasound, the electron microscopy community imported the assumption of a probe traveling in a straight line through a sample with exponential attenuation.
This assumption is valid for thin samples, but for thick samples multiple electron scattering renders it invalid. Our theoretical analysis found that near the onset of multiple scattering (as the sample thickness under consideration increases), there is a regime in which the projective assumption remained valid, but the transmission as a function of thickness deviated significantly from Beer’s Law. Extensive numerical simulations confirmed this, attaining excellent reconstructions of an 8 μm square sample of a photonic band gap material using the multiple scattering transmission function.
We used a Bayesian approach known as the generalized Gaussian Markov random field, and extended it to treat systems with multiple scattering. The principal features of this formalism are a prior distribution based on correlations of neighboring pixels (or voxels in 3D) in which the smoothness of the reconstruction may be adapted to the sample, and a quadratic approximation to the log of the likelihood derived from Poisson statistics. In its original formulation, Beer’s Law was also assumed. We made a more general assumption: that the transmission is any known function, with sufficient differentiability, of a linear combination of the material parameters of the sample. This has enabled us to get highly satisfactory reconstructions of three-dimensional materials structure from limited angular sampling data.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents |
