Extreme Ultraviolet Radiation Metrology:
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 2 nm and
200 nm, 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 primary national
radiometric standards in the EUV region. 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 key candidates for
next-generation semiconductor lithography is an EUV light source, since its
short wavelength (13 nm vs. 193 nm for present ultraviolet production
lithography) enables diffraction-limited imaging of features with smaller
critical dimensions. We are working actively with the semiconductor industry to
develop "optical bench" capabilities for characterizing EUV imaging
systems.
The Division's key tool for EUV optical metrology is 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, since it does not
produce the hard x-ray radiation of higher-energy sources and 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
Metrology for Extreme-Ultraviolet Lithography
Figure 7. NIST Synchrotron Ultraviolet Radiation Facility (SURF III)
and Beamlines 1-5, with principal users and staff.
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Figure 8. Results of the international comparison of reflectivity and
wavelength of maximum reflectivity of a test EUV multilayer mirror, from the
Center for X-Ray Optics, (Berkeley, CA), the Physikalisch-Technische
Bundesanstalt (Berlin), the Association of Super-Advanced Electronics
Technologies (ASET, Japan), and NIST. |
New semiconductor industry requirements for highly accurate EUV reflectivity
measurements have provided the impetus for an international comparison of
results of measurements at four different EUV facilities: the
Advanced Light
Source at Lawrence Berkeley National Laboratory (CXRO), the PTB instrument
at the BESSY
storage ring in Berlin, the ASET instrument in Japan, and
SURF III at NIST.
Each lab provided at least one multilayer mirror, which they measured before
and after the entire set had been measured at the other facilities. (This was
to account for the aging of the mirrors.) For each mirror, the object was to
determine the wavelength of peak reflectivity, and the reflectivity at that
wavelength.
Figure 8 shows the results for one sample that is typical of the set. From
these results we are confident that the NIST-quoted uncertainty of 0.3 %
is a reliable estimate. This uncertainty represents a factor-of-ten improvement
over earlier instrument performance, before a comprehensive set of improvements
were made in response to more stringent industry demands.
Absolute Radiometry at SURF III
The NIST Synchrotron Ultraviolet Radiation Facility (SURF III) serves as
the Nation's primary standard for absolute source-based optical radiometry from
the visible through the extreme-ultraviolet (EUV) spectral regions. It supports
a variety of scientific and measurement missions, primarily in the EUV.
Figure 9. White light from SURF III (right) is reflected by
diffraction grating (foreground), to display spectrum on screen. |
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Figure 10. The new white-light radiometry endstation on Beamline 3. |
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Figure 11. SURF operations at high current for EUV optics
characterization. |
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Figure 12. SURF at low current for EUV spectrometer calibration. |
One of SURF's main customers is NASA, which calibrates virtually all its
spaceborne EUV spectrometers at the spectrometer calibration facility on SURF
Beamline 2. Recent calibrations include instruments flown on the
Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission, the
EOS SORCE SOLSTICE A and B missions (Earth Observing System, Solar Radiation
Climate Experiment, Solar Stellar Irradiance Comparison Experiment), and the
SETI/UCSB (VUV Stimulated Mid-IR Experiment) mission.
We have completed an upgrade of the accelerator to improve the radiometric
performance, both in accuracy and spectral range. Absolute flux uncertainties
are better than 1 % from 2 nm to beyond 400 nm. The storage ring
now operates typically at an electron energy of 380 MeV, rather than the
284 MeV energy of SURF II. This results in a significant increase in
the flux at soft x-ray wavelengths. Testing of the magnet system has shown the
capability of operating from 73 MeV to 417 MeV.
SURF can be run under a wide variety of operating conditions according to the
needs of different users. Figures 11 and 12 show the quite different
conditions required for EUV optics metrology and satellite spectrometer
calibration.
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