Optical Technology Division Activities at
Synchrotron Ultraviolet Radiation Facility (SURF III)
Blackbody radiation and synchrotron radiation are the only two known sources of
absolutely calculable radiation. Knowledge of a few simple parameters
(thermodynamic temperature and spectral emissivity for a blackbody; storage
ring electron energy, magnetic field and circulating current for synchrotron
radiation) completely determine the spectral and spatial distribution of
radiation emitted by these sources, making them suitable as absolute
radiometric standards.
In the UV and even shorter wavelength regions, synchrotron radiation stands
out as the only standard source available to date since this wavelength region
is out of reach to the widely used blackbody source standard.
Figure 1 below compares the spectral radiance of a
typical low-energy synchrotron radiation machine such as SURF III at a
typical 200 mA storage ring current to 1000 K and 3000 K thermal
sources. Synchrotron radiation is much brighter - and has an enormously greater
useful spectral range - than any practical blackbody source.
Figure 1. SURF
(200 mA beam current) and blackbody radiance |
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As most of the modern synchrotron radiation machines are competing with each
other in the soft to hard x-ray spectral range, SURF III is unique in that
it is one of the few low electron-energy machines in the world that is
dedicated to UV radiometry. It has a text-book style circular electron beam
orbit with the energy of the electron beam operated anywhere between
10 MeV to 380 MeV. Such low electron energy machine is ideal for UV
work because of the dramatically reduced damaging x-ray radiation associated
with typical modern synchrotron radiation. The design of SURF III ensures
that all the fundamental parameters can be measured with highest accuracy and,
therefore, makes the radiation from SURF III a broad-band absolute
national standard. In addition, the relatively small size of SURF III
provides great flexibility for its users in terms of scheduling and the
condition of operation. |
The low absolute uncertainties in SURF III as a radiometric source permit
substantial improvements in the NIST scales of spectral radiance and irradiance,
especially in the ultraviolet and infrared spectral regions where existing
source-based and detector-based standards have relatively large uncertainties.
SURF III also permit unification of disparate scales scattered across the
spectrum based on different sources and technologies.
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Figure 2 summarizes some of the radiometric applications of SURF III.
Particularly important in SURF III is the substantial improvements in
ultraviolet radiometry in support of such diverse and critically important
applications as standards for accurate long term monitoring of solar
ultraviolet radiation, and improved standards, sources and detectors for
control of UV curing and semiconductor lithography (continually moving to
shorter wavelengths to reduce feature size). While the principal goal of
SURF III is to improve the radiometric accuracy of the source, an
additional benefit is the operation at higher electron energy of 380 MeV,
resulting in dramatic soft x-ray flux increases, including the biologically
important water window (water is transparent in the 2.3 nm to 4.4 nm
spectral range) for use in x-ray microscopy. |
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Figure 2. SURF III radiometric applications |
The Optical Technology Division, in collaboration with the Electron and Optical Physics Division,
operates two radiometric beamlines, beamline 3 and beamline 4, at
SURF III. Beamline 3 is a national source standard based on
synchrotron radiation whereas beamline 4 is a detector-based calibration
facility with its primary scale derived from a helium-cooled cryogenic
radiometer.
Beamline 3, or the Facility for Irradiance Calibration Using Synchrotrons
(FICUS), is a white light beamline which directs the calculable SURF III
radiation to the user station with minimum obstruction from optical components
for spectral irradiance calibration of user sources such as deuterium lamps.
Calibration of sources directly against SURF III yield results with much
reduced measurement uncertainty. In the case of deuterium lamps, which are
commonly used in industry as a transfer standard, we achieved an uncertainty of
1.2% (k=2) from 200 nm to 400 nm. In response to requests from
industry, the beamline is currently undergoing upgrade to extend the spectral
range to vacuum UV and further reduce the uncertainty.
Synchrotron radiation from beamline 3 for irradiance calibration.
Beamline 4 is a general purpose radiometric beamline consists of a
2 m monochromator to disperse radiation from SURF III. An Absolute
Cryogenic Radiometer (ACR) at the end of the beamline accurately determines the
radiant flux of the dispersed beam which, in turn, is used to calibrate the
response of photodetectors. We have achieved an uncertainty of less than 1%
(k=2) from 135 nm to 325 nm in measuring the spectral response of a
typical silicon detector. In addition to detector power response calibration,
the flexible design of the beamline enables a wide range of measurements such
as reflectivity, internal quantum efficient, irradiance responsivity and
radiation degradation of photodetectors as well as reflectivity and
transmission of optical materials. Such works allow us to understand and model
the behavior of various solid-state photodetectors and resulted in great impact
on accurate UV detection for industries such as UV curing and semiconductor
photolithography.
Photograph of SURF III beamlines.
References
- "Stability of photodiodes under irradiation with 157 nm pulsed
excimer laser," P.S. Shaw, R. Gupta, and K.R. Lykke, Appl.
Opt. 44, 197-207 (2005).
- "Damage to solid-state photodiodes by vacuum ultraviolet
radiation," U. Arp, P.S. Shaw, R. Gupta, and
K.R. Lykke, J. Electron. Spectrosc. Relat. Phenom. 144-147 (2005)
1039.
- "Forty years of metrology with synchrotron radiation at SURF,"
U. Arp, A.P. Farrell, M.L. Furst, S. Grantham,
E. Hagley, S.G. Kaplan, P.S. Shaw, C.S. Tarrio, and
R.E. Vest, Synchrotron Radiat. News 15(5), 30 (2003).
- "A SURF beamline for synchrotron source-based absolute
radiometry," P.S. Shaw, U. Arp, H.W. Yoon,
R.D. Saunders, A.C. Parr, and K.R. Lykke, Metrologia 40,
S124-127 (2003).
- "Characterization of an UV and VUV irradiance meter with synchrotron
radiation," P.S. Shaw, R. Gupta, and K.R. Lykke, Appl. Opt.
41, 7173 (2002).
- "The new beamline 3 at SURF III for source-based radiometry,"
P.S. Shaw, D. Shear, R.J. Stamilio, U. Arp, H.W. Yoon,
R.D. Saunders, A.C. Parr, and K.R. Lykke, Rev. Sci. Instrum.
73, 1576 (2002).
- "Characterization of UV detectors at SURF III," P.S. Shaw,
T.C. Larason, R. Gupta, and Keith R. Lykke, Rev. Sci. Instrum.
73, 1625 (2002).
- "The new ultraviolet spectral responsivity scale based on cryogenic
radiometry at Synchrotron Ultraviolet Radiation Facility III,"
P.S. Shaw, T.C. Larason, R. Gupta, S.W. Brown,
R.E. Vest, and K.R. Lykke, Rev. Sci. Instrum. 72, 2242,
(2001).
- "SURF III - An improved storage ring for radiometry," U. Arp,
R. Friedman, M.L. Furst, S. Makar, and P.S. Shaw,
Metrologia, 37, 357 (2000).
- "Characterization of Materials using UV radiometric beamline at
SURF III," P.S. Shaw, R. Gupta, T.A. Germer,
U. Arp, T. Lucatorto, and K.R. Lykke, Metrologia, 37, 551
(2000).
- "UV radiometry using synchrotron radiation and absolute cryogenic
radiometer," P.S. Shaw, K.R. Lykke, R. Gupta,
T.R. O’Brian, U. Arp, H.H. Hunter, T.B. Lucatorto,
J.L. Dehmer, and A.C. Parr, Appl. Opt. 38, 18 (1999).
- "New UV radiometry beamline at the synchrotron ultraviolet radiation
facility at NIST," P.S. Shaw, K.R. Lykke, R. Gupta,
T.R. O’Brian, U. Arp, H.H. Hunter, T.B. Lucatorto,
J.L. Dehmer, and A.C. Parr, Metrologia, 35, 301 (1998).
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