Radiometric Physics Division
name changed to
1995/1996 Technical Highlights
- Optical Metrology for Photolithography.
Smaller, faster microelectronic devices can be
made by using shorter wavelengths for photolithography,
but creating the next generation of
photolithographic tools operating at 193 nm poses
unprecedented challenges for the purity and
characterization of optical materials. The Division
participates in an extensive collaborative effort,
involving several NIST laboratories, the National
Semiconductor Metrology Program, SEMATECH,
materials suppliers, stepper manufacturers, and
MIT Lincoln Laboratory to provide measurements
and support for characterization of the optical
properties of fused silica and calcium fluoride.
The Division activity is focused in three areas:
Critical dimensions at this shorter wavelength
require smaller focus budgets and tighter control
over the lens design, which are not tenable
without accurate knowledge of the refractive index
and the thermal coefficient of the refractive index
of fused silica and calcium fluoride. We have
developed a UV reflectometer capable of
measuring the refractive index at 193 nm with an
accuracy of 10-5, which is sufficient to meet the
industry's immediate need. For the longer term
and for shorter wavelengths, we are developing
interferometric methods capable of higher
accuracy. (R. Gupta)
Optical scatter at 193 nm from surfaces and
from within lenses degrades photolithographic
images, and complicates critical transmission and
absorption measurements performed on the
materials. We have performed total integrated
scatter measurements that show that the intrinsic
Rayleigh scatter coefficient in fused silica is on
the order of 0.001 cm-1 at 193 nm, a value
considered to be the maximum tolerable level for
this application. The scatter in calcium fluoride,
which is a candidate to replace fused silica as
photolithography moves to shorter wavelengths,
was substantially lower, as expected for a crystalline
material. (C.C. Asmail and T.A. Germer)
Accurate reflection, transmission, and
absorption measurements are essential to optical
design since absorption modifies the temperature-dependent
refractive index. NIST has developed
an instrument capable of making these
measurements, and has assisted Lincoln Laboratory
researchers in the characterization of their
transmittance instrument. (D.J. Dummer)
- Intrinsically Absolute Metrology Using Correlated Photons.
The Division is investigating the
application of correlated photon techniques to
metrological problems. Using correlated photons,
we have developed methods to measure absolute
detector responsivity without externally calibrated
standards. We have also developed methods to
measure infrared source radiance in a direct
intrinsically absolute manner. An additional
technique that we are developing uses correlated
photons to determine absolute polarization mode
dispersion in optical materials. In this
application, pairs of correlated photons propagate
collinearly but with orthogonal polarizations.
Since photons of a pair are created
simultaneously, any shift in the timing between
the two is indicative of polarization mode dispersion
in intervening material. The accuracy with
which the time shift is measured determines the
accuracy of the polarization mode dispersion
measurement. The potential accuracy of the
method is better than 0.1 fs.
he correlated pairs of photons are produced
by an optical parametric down-conversion source
and detected via coincidence circuitry. An optical
arrangement with multiple paths allows for
coincidences via multiple indistinguishable paths,
leading to quantum interference that is seen as a
modulation of the coincidence rate. When the two
coincidence paths differ by more than a coherence
length of the photons, the paths become
distinguishable, producing no interference, and
the coincidence rates of the two paths are directly
summed. When the paths differ by less than a
coherence length, the two paths become
indistinguishable, resulting in destructive
interference that causes the coincidence rate to
drop to near zero. This dip in the coincidence rate
acts as a marker indicating a precise delay
between the two photons. When a sample is
added to the optical path, the change in the delay
between the photons is encoded in the shift of this
dip marker. A new optical arrangement with
additional paths has the potential for higher
ultimate measurement accuracy. That
arrangement fills the previously described dip
with high frequency oscillations (see figure on the
cover of this section of the annual report), which
may allow the shift of the dip to be determined
with even higher resolution. (A.L. Migdall)
- Near-field scanning optical microscopy
(NSOM): nanometer-scale characterization of
optical fields. NSOM has great potential for
noninvasive optical characterization of nanostructured
materials, but the application of NSOM
as a metrological tool is limited because contrast
and resolution are poorly understood or ill
defined. To address this problem, we are studying
the fundamental mechanisms in NSOM which
generate contrast and determine resolution for
different materials.
Figure 1. NSOM image of the optical fields
produced by a nanochannel glass array. The
image reveals local optical modes of the array
which depend on crystal geometry and composition.
In collaboration with the University of Virginia
and the Naval Research Laboratory, we have used
NSOM to image a nanochannel glass array. The
array is a two-dimensional "photonic crystal"
composed of two glasses with slightly different
indices of refraction. An array of glass cylinders
(the channel glass) is embedded in another glass
(the matrix glass). The channel glass elements are
745 nm in diameter, with a center-to-center
separation of approximately 1100 nm. The NSOM
image shown in Fig. 1 was recorded by
illuminating the sample with a fiber-optic probe
positioned about 10 nm above the surface. The
probe is scanned across the sample surface and
transmitted light is collected on the opposite side
of the sample. This image is not strictly analogous
to a microscope image, but contains additional
information about the optical modes supported by
the sample. The NSOM measurements, supported
by quantitative models, show sensitivity to the
local density of photon states in the array, which
in turn depends on such properties as the index
difference between the two glasses, the geometry
of the crystal, and the composition of an interdiffusion
layer between the two glasses. By varying
the photon energy and the numerical aperture of
the collection optic, contributions are observed
from different optical modes of the array. The
results demonstrate that NSOM can be used with
detailed models to determine quantitatively nanometer-scale material properties,
such as the spatial variation of the index of refraction, that cannot be
measured by other techniques. (L.S. Goldner and E.L. Shirley with
G.W. Bryant and P.S. Julienne of Div. 842)
- Femtosecond Laser-Induced Desorption:
Theory and Experiment. Fundamental events
which occur at surfaces and interfaces underlie
many technologically important processes. For
example, the exchange of energy among electrons
in a metal and molecules at the metal's surface
underlies catalysis, the non-thermal interaction of
light at materials surfaces is responsible for
photoetching in semiconductor device fabrication,
and the scattering of carriers from interfaces
influences the operation of devices. In
collaboration with the Chemical Science and
Technology Laboratory, we have used
femtosecond lasers to measure characteristics of
these interactions at surfaces. We studied the
chemistry of carbon monoxide, CO, adsorbed on
a copper single crystal, Cu(100), irradiated by
ultrafast laser pulses. This system was chosen
because unique state-of-the-art ab initio
calculations for CO/Cu have just become available (from
Lucent Technologies and University of California,
Berkeley). Also, this system has been
well characterized by many other experimental
techniques that provide key information about
bond energies, adsorbate vibrations, and substrate phonons,
making it an ideal test of the theory.
Laser pulses of 160 fs duration impinged on
a Cu surface, which was initially at a temperature
TS=100 K and covered with an ordered
half-monolayer (0.5 ML) of CO; each CO is bound
directly on top of a Cu atom, C toward the
surface. The laser pulse initially excites electrons
in the bulk metal within 20 nm of the surface,
creating hot electrons with a temperature
TE=3000 K. Over the next several picoseconds,
these hot electrons transfer energy to the metal
bulk phonons, as well as to adsorbate vibrations
such as the C-O stretch and the Cu-CO stretch
(the molecule-metal bond). When the latter bond
gains sufficient energy (about T=420 K), the CO
molecules desorb, and are detected in the gas
phase by another laser, which determines the
vibrational (TV), rotational (TR), and kinetic
temperature (TT) of the desorbed CO. Since the
electrons and the various vibrations all have
different temperatures for a brief time (2 ps), this
type of experiment allows the relative importance
of the different excitations to be assessed.
Figure 2. Yield (Y) of CO from a Cu(100) surface as a function of
time delay (td) between two laser pulses of equal intensity (I)
which cause CO desorption. Since Y scales as I8, Y is strongly
peaked at td=0. The 3 ps width is well fit by a model (solid
line) developed at NIST. Inset shows autocorrelation of the 140 fs laser
pulses.
The results for desorbed CO were TR=225 K,
TV=1330 K, and TT=215 K. The desorption yield
varied nonlinearly with laser fluence
(Y=F 8). These
data agreed well with the ab initio theory in which energy is
exchanged between hot electrons and vibrations though electronic frictions.
The results also agreed with an empirical model developed at NIST, in which
readily available data (i.e., desorption rate parameters and vibrational
damping times measured under thermal low-temperature conditions) was used to
predict the reactions of this system under the experimental conditions which
were far from equilibrium. The agreement with both the theory and the model is
very encouraging, providing insight and a method that can be used to predict
results of other surface processes, whether or not induced by femtosecond
lasers. Since the interaction of ultrashort laser pulses with materials is
becoming common in communications, in measurement technologies, and in other
fields, validation of such models is increasingly important. These results
have been accepted for publication in Physical Review Letters.
(J.C. Stephenson with Division 837 researchers L.M. Struck,
L.J. Richter, S.A. Buntin, and R.R. Cavanagh)
- Haze on Silicon Wafers. Measurements of
optical scatter are often employed in production
line diagnostics for surface roughness of silicon
wafers. However, the geometry of the optical
scatter instrumentation lacks standardization,
making if difficult to compare values obtained by
instruments made by different manufacturers.
The Bidirectional Reflectance Distribution Function
(BRDF), on the other hand, is a well-defined
quantity, and under conditions usually met with
bare silicon wafers, can be related to the power
spectral density (PSD) of the surface roughness.
We have developed an approach for characterizing
low-level optical scatter instrumentation using a
spatial frequency response function. The function
gives the sensitivity of an instrument with a
specified geometry to microroughness on different
length scales, allowing the haze signal to be
treated as an integration of the PSD with the
response function. Algorithms were developed for
calculating this response function for different
geometries, and a computer program will be made
available which will allow instrument
manufacturers to calculate the response function
for each of their products. This methodology is
being incorporated into ASTM documents
describing the standard practice for calibration of
scanning surface inspection systems. (C.C. Asmail and
T.A. Germer)
- Large Aperture Blackbody Calibrations at the
NIST Low Background Infrared Calibration
Facility (LBIR). An upgrade to the LBIR facility
has been completed with the addition of an ante-chamber
to the broadband calibration chamber.
There has been a demand from infrared space-based
missions to calibrate higher temperature
and larger aperture blackbodies than the facility
was able to accommodate in its original design.
This new addition was developed in collaboration
with Los Alamos National Laboratory for the
calibration of two blackbodies which have 10 cm
and 18 cm diameter apertures and operating
temperatures from 250 K to 350 K. These blackbodies
will be used as sources in a new satellite
sensor calibration facility under development at
Los Alamos. At NIST, they will be operated in the
evacuated ante-chamber with shrouds cooled by
liquid nitrogen. The ante-chamber is attached at
the source end of the LBIR chamber with a
precision aperture. The typical flux levels at the
radiometer aperture, which is located one meter
away in the LBIR chamber, will be in the range of
2 µW to 10 µW. A two-axis translation stage is
also available to allow spatial measurements of
the radiance temperature. This new capability
removes many of the constraints on customer
blackbodies that were to be operated in the
original cryogenic chamber, such as size and total
power dissipation. (S. Lorentz)
- Medium Background Infrared Facility. Infrared
radiometry has an important role in space-based
civilian, defense, and industrial applications. The
growing realization among users of infrared
radiometers of the critical role for calibration and
characterization of these devices has led to the
development of a new Medium Background
Infared (MBIR) Facility in the Division. This
facility will be used to maintain an infrared scale
for specialized applications that involve radiometric
instruments that need to operate in a
vacuum environment surrounded by a light-tight
shroud cooled to as low as 80 K with liquid
nitrogen. NIST has had facilities for infrared
radiometric measurements in ambient environments and
at the 20 K Low-Background Infrared
(LBIR) Facility, but had lacked a facility for the
increasingly important medium (80 K) background applications.
In particular, the capability will be established for measurements on
large-area, vacuum-operational, blackbody sources
operated from 200 K to about 400 K, which are
traceable to NIST via infrared radiometry through
the radiance temperature of the source. An
example of the type of scientific activity that the
MBIR facility will support is the use of earth-orbiting
satellites for the determination of temperature of the
earth's surface and atmosphere by
radiance measurements. These measurements are
the basis for the study of global warming. To
establish radiometric traceability of satellite
instruments to NIST, the Division is developing a
liquid-nitrogen cooled, portable infrared
radiometer. It will be used to intercompare large-area
blackbody sources at contractors' facilities in
NASA's Mission to Planet Earth Project. The MBIR
facility will be used to characterize and maintain
the NIST calibration of this portable radiometer. (J. Rice)
- Modeling Optical Properties of Materials. The
Division is developing a capacity to model and
predict optical properties of materials, using
state-of-the-art first-principles methods, as part
of its long-term strategy to facilitate improved
optical metrology. Optical properties such as a
material's refractive index and absorption spectra
depend on the description of electron-electron
interactions at a level of detail which challenges
current algorithms, models, and computers.
Present activities are focused on the description of
such interactions and their effects on x-ray
absorption spectra extremely close to atomic x-ray
edges (within ~10 eV of an edge). In such energy
regions, standard methods used to describe
absorption at higher energies are not suitable
because of their incomplete description of electron
interactions. Planned activities for the coming
year include an assessment of techniques in a
wider range of materials than those studied thus
far (e.g., hexagonal boron nitride, Fig. 3), and
adaptations of the techniques to the more difficult
problem of visible and soft ultraviolet absorption
spectra of semiconductors and insulators, as well
as seeking improvements in the first-principles
modeling of infrared absorption because of atomic
vibrations. (E.L. Shirley and R.U. Datla)
Figure 3. First-principles x-ray absorption of hexagonal boron nitride,
including electron interactions (solid line), and omitting such interactions
(dashed line). Including electron interactions dramatically improves agreement
with experiment.
- High-Contrast Broadband Infrared Polarizer.
The Optical Technology Division has constructed and tested a linear
polarizer for use with a broad range of visible and infrared radiation.
The device works on Brewster angle reflections from four germanium plates
arranged in a chevron geometry. Tests with 0.633 µm and
3.39 µm wavelength laser radiation have shown extinction ratios
(defined as the ratio of the transmittances of p and s polarized light) of
4 × 10-6 and 3 × 10-7,
respectively. The extinction ratio is expected to be less than 10-6
for wavelengths up to at least 25 µm. Development of polarization
metrology in the infrared at NIST is being driven by the increasing importance
of polarized light measurement capability in such diverse fields as optical
communication, pharmacology, and infrared imaging. These applications depend on
the quality and calibration of polarization components. The high-quality linear
polarizer that has been developed is expected to find use directly in
specialized applications, and as a calibration standard. (D. Dummer,
S. Kaplan, A. Pine, and L. Hanssen)
- New THz Source Developed for Spectroscopic
Studies. A new scheme involving generation of
coherent, tunable, far-infrared radiation by
mixing two visible laser beams in an ultra-high-speed
photoconductor has resulted in the development of a
new THz spectrometer. This work has
been carried out in collaboration with MIT Lincoln
Laboratory. The ultrafast photomixers were
fabricated at Lincoln Laboratory using an
epitaxial layer of low-temperature-growth (LTG)
GaAs on a semi-insulating GaAs substrate. The
LTG GaAs material has subpicosecond recombination
lifetimes, enabling a frequency response to
several THz. Microscopic interdigital electrodes
driving a broadband self-complementary spiral
antenna are deposited on the material using
lithographic techniques. The THz radiation is
coupled out of the GaAs photomixer into free-space
using a high-index Si aplanatic lens and is
detected using a conventional liquid-He-temperature
bolometer. The new spectrometer has been
used to record the rotational spectrum of SO2 in
the 0.1 THz to 1.2 THz region. The new spectrometer
has also been used to obtain pressure-broadening
parameters for these SO2 transitions. (A.S. Pine and
R.D. Suenram)
- Calibration of Night-Vision Equipment. The
available radiometric calibration methods for
night-vision transfer-standard detectors and
goggles have been limited to an uncertainty of
approximately 10%, which is inadequate for most
purposes. This limited accuracy exists because
the calibration chains are long and the precision
of the calibrating equipment is poor. In order to
improve accuracies and calibration techniques in
DOD laboratories and in the night- vision
industry, standard detectors must be calibrated
for both radiance and irradiance responsivities
traceable to high-accuracy standard detectors.
NIST has developed a facility to calibrate night-vision
transfer-standard detectors in spectral
radiance and irradiance response modes against
NIST detector-based radiometric scales. A large
output area, monochromatic sphere source has
been developed for the visible and near infrared
wavelength ranges. Standard-quality silicon
irradiance and radiance detectors have been
designed and fabricated. Calibration equipment
has been designed and realized with precision
geometry in order to make an accurate flux-measurement
transfer between the different
radiometric calibration modes. A detector-based
spectral radiance and irradiance response calibration
system is now available at NIST for
accurate and uniform calibration of night-vision
transfer-standard radiometers. (G. Eppeldauer)
- New Photometry Capabilities. The Division is
responsible for the realization of the candela, one
of the SI base units, and other photometric units
for luminous flux, illuminance, luminance, and
color temperature. The NIST photometric units
are based on standard photometers which are
traceable to the Division's High Accuracy Cryogenic
Radiometer (HACR), which has a combined
relative standard uncertainty of 0.02%. Using
detector-based methods to realize the photometric
scales at NIST has reduced uncertainties of
photometric calibrations and has resulted in the
availability of additional photometric calibration
services at NIST. These improved services impact
on a variety of industries. Examples of products
that rely on photometric standards are lighting,
display, optical instrumentation, and illuminated
safety devices for the automotive and aircraft
industries.
Recently, significant advances have been
made in the Division's photometry capabilities. In
1995, a new luminous flux unit was realized
using an innovative integrating sphere method.
Using this method, the NIST lumen is now also
traceable to the HACR. This new unit has been
disseminated to customers since January 1996.
Standard lamps for luminous intensity and
color temperature were made available in 1994
and calibration of linear fluorescent lamps
for luminous flux was added in 1995. A high
illuminance source was developed in 1996,
making possible illuminance calibration up to
100,000 lux. In addition to these recent developments,
the Division accepts various artifacts for
calibration, such as illuminance meters, luminance
meters, standard photometers, sphere
sources, opal glass, as well as various types of
transfer standard lamps for luminous intensity
and luminous flux.
A new publication, Photometric Calibrations (SP 250-37), describes these
expanded capabilities. It replaces the previous SP 250-15 (1987) and
provides extensive information on the realization of the NIST detector-based
photometric units and the new calibration procedures for luminous intensity,
illuminance, luminance, luminous flux, and color temperature.
Work is in progress to establish illuminance measurement standards for flashing
lights. Accuracy of the measurement of flashing anti-collision lights is
critical to the aircraft industry. The Division plans to offer a calibration
service for flashing-light meters in late 1997.
Another new activity involves the measurement
of color. Colorimeters are often used during
the manufacture of commercial display units
(television and computer monitors) to calibrate
color. However, it is often difficult and expensive
to calibrate these instruments correctly. It is
especially important in the computer industry to
accurately calibrate colorimeters, since this
industry puts a premium on correctly calibrating
the appearance of computer displays. To answer
these needs, the Division is establishing a project
for colorimetry of displays and imaging devices
with the goal of ultimately offering calibration
services in this area. Research will be focused on
the characterization of various color displays
including flat panel displays and color reproduction
through color imaging devices. (Y. Ohno)
- Calibration Quality Program. In response to
NIST customers' requests and to help lead the
nation into the future of laboratory accreditation,
an effort has been established in the Optical
Technology Division to document its quality
system for the calibration services it offers. The
calibration services participating in the effort are:
Radiance Temperature Measurements, Spectroradiometric Source Measurements,
Optical Properties of Materials Measurements,
Photometric Measurements, and Spectroradiometric
Detector Measurements. The quality system is based on ANSI Z540-1-1994, the
American National Standard for Calibration and
Testing Laboratories. Several key sections of the
standard have been implemented. Quality manuals have
been written for each calibration service
offered by the Division. Computer software and
calibration methods have been uniformly
documented in the quality system. Uniform test-report
formats are issued by the Division's
calibration laboratories. In addition, the Division
has adopted a standard procedure for handling
customer complaints. An internal audit was
completed in FY 95. In FY 96 the Division's
quality manager and the deputy quality manager
completed both internal auditor training from
NVLAP and lead assessor training approved by
Registrar's Accreditation Board and the International
Register of Certified Auditors. Compliance
with the ISO quality standard and the effort to
document the quality system will benefit the
services offered by the Division. (T. Larason and S. Bruce)
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