Major Technical Highlights
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Validated Heat-Flux Sensor Responsivities Using a Spherical Blackbody.
The Division has been researching new methods to accurately determine the
radiative responsivities of heat-flux sensors, widely used in fire and
aerodynamic tests of aerospace vehicles and components.
Figure 1. Spherical blackbody furnace used for the radiative calibration
of heat-flux sensor responsivities to an expanded uncertainty of 2 %
(k = 2). |
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Comparison of radiative heat-flux measurements between various laboratories
using a variety of different techniques reveal that presently claimed
responsivities vary by as much as 15 %. Studies at NIST indicate, however, that
the sensor responsivities are stable and reproducible to better than 1 %
over a period of several years. The calibration method developed by the
Division uses an electrical substitution radiometer to determine the absolute
heat flux at the sensor from a cylindrical heat-pipe blackbody (detector-based
method) and has an expanded uncertainty of 2 % (k = 2).
To validate this detector-based method, the Division has developed a totally
independent source-based calibration procedure in which a sensor is plunged
into a 0.23 m diameter high-temperature spherical blackbody, as shown in
fig. 1, and the heat flux is directly calculated using the
Stefan-Boltzmann equation. This approach also yields a responsivity with an
expanded uncertainty of 2 % (k = 2). Sensors calibrated
using the two approaches give responsivities that agree to within 2 %
(k = 2), validating the two methods. The availability of
accurate heat-flux sensor calibrations from NIST allows sensor users and
manufacturers to better assess the performance of their calibration methods and
offers validated calibration procedures for implementation by other
laboratories. (B. Tsai, D. DeWitt, and M. Annageri) |
- A New Millimeter-to-THz Spectroscopy Tool for Plasma-Etching Chemical
Diagnostics. With support from ATP Intramural funding and building on
expertise developed through a Competence program in Advanced Terahertz (THz)
Metrology, the Optical Technology and Atomic Physics Divisions have
collaborated on the development of a new spectroscopic technique for the
characterization of the complex chemistry that occurs in a semiconductor
plasma-etching reactor. The understanding and the control of this chemistry is
critical to the microelectronics industry as it moves toward more complex
multistep etching processes, larger wafers, and smaller and higher-aspect-ratio
features. The new technique uses millimeter-to-THz linear-absorption
spectroscopy with backward-wave oscillator (BWO) coherent radiation sources to
determine the density and temperature of plasma species, such as radicals, ions,
and molecules, along the radiation propagation path.
Figure 2. Second-derivative submillimeter spectra of an absorption line
of CF2 in a 250 W inductively coupled CF3H plasma in
a Gaseous Electronics Conference (GEC) reference cell at a constant cell
pressure of 1.3 Pa and various flow rates of CF3H. The spectra
demonstrate that the production of CF2 is increased at high flow
rates. |
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The technique is a significant improvement over previous methods as it
provides simple and unambiguous molecular identification and quantification of
a broad range of molecular species. Initial tests (see fig. 2) using a
standard test reactor demonstrated the detection of CF3H,
CF2, CF, CO, and CF2O in a low-pressure, inductively
coupled, CF3H plasma, as might be used, for instance, in the etching
of SiO2 on Si wafers. Decomposition of the CF3H in the
plasma is found to be nearly complete at 90 %. The production of CO and
CF2O is attributed to etching of a quartz (SiO2)
dielectric shielding plate by the CF3H decomposition products or to
reactions of these products with water-vapor impurities in the reactor feed gas.
The latter possibility is presently being addressed by directly examining
submillimeter absorption lines of water vapor in the reactor. Independent
translational, rotational, and vibrational temperature measurements are also
possible with the technique, furnishing additional valuable input data to
plasma models. Efforts are being made to extend the measurement frequency to
above 1 THz using solid-state photomixers to allow the direct measurements of
the HF concentration, an important component of hydrogen-rich,
hydrofluorocarbon-based, etching plasmas. (E. Benck, G. Golubiatnikov,
D. Plusquellic, and G. Fraser) |
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Radiometric Calibration of the NIST Advanced Radiometer (NISTAR) and Earth
Polychromatic Imaging Camera (EPIC) for the TRIANA Satellite. OTD has
completed the radiometric calibration of two instruments, NISTAR (NIST Advanced
Radiometer) and EPIC (Earth Polychromatic Imaging Camera), both planned for
deployment on the Triana satellite. The Triana satellite, after launch by the
Space Shuttle, is to be positioned in an orbit at the Lagrange-1 point to allow
continuous monitoring of the sunlit Earth. NISTAR will measure the absolute
irradiance of the Earth while EPIC will provide hourly, spatially resolved
measurements of cloud properties, ozone concentration, and aerosol levels of
the Earth's atmosphere.
NISTAR, originally designed by NIST, was calibrated at NIST using the
capabilities of the Division's Spectral Irradiance and Radiance Calibrations
with Uniform Sources (SIRCUS) facility. During the tests, NISTAR was
illuminated to mimic its view of the Earth from space. The resulting relative
uncertainties on the calibration are below 1 %. The NISTAR instrument has
since been delivered to NASA's Goddard Space Flight Center and integrated onto
the Triana satellite.
The bulk of the EPIC radiometric calibration was performed during thermal and
vacuum testing of the instrument at Lockheed Martin in Palo Alto, California in
December 2000, with additional, final measurements performed in October 2001 at
the Goddard Space Flight Center. The intense, multi-day calibration effort
required the unique radiometric expertise of NIST staff, specialized
calibration sources and detectors, on-site evaluation of results, and
modifications to the initial calibration plan when circumstances or scheduling
changed. Preliminary results have already been made available to the mission
science team. (T. Early, D. Allen, C. Johnson, J. Rice,
S. Lorentz, S. Brown, and K. Lykke)
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THz Spectra of Biomolecules. As part of a Competence program in Advanced
Terahertz Metrology, the Division has developed a novel THz spectrometer for
the investigation of large-amplitude vibrational motions in biomolecules,
important for their flexibility. The capabilities of the instrument have been
demonstrated by recording THz spectra of two small biological molecules, biotin
and riboflavin, and the protein myoglobin.
THz radiation is produced by difference frequency mixing of two near-infrared
laser beams, separated in frequency from 0.1 THz to 4 THz, in a
solid-state GaAs photomixer. The THz radiation is directed through the sample
and detected by a liquid-He-cooled bolometer. To increase sensitivity and
reduce spectral artifact absorptions, the intense atmospheric water-vapor
absorption present at THz frequencies and the etalon or standing-wave structure
characteristic of this spectral region, were effectively eliminated by placing
the photomixer, liquid-He-cooled sample, and bolometer in a common vacuum
system, with no windows separating the components.
Spectra were recorded with the samples cooled to 4.2 K to reduce
inhomogeneous broadening for increased spectral resolution. The spectra of
biotin and riboflavin reveal a number of low-frequency vibrations at
frequencies less than 4 THz with linewidths of approximately 0.03 THz.
Complementary measurements using Raman spectroscopy have also been performed,
providing additional information on the low frequency vibrations. Efforts are
presently underway to identify these motions with specific nuclear
displacements.
In contrast to biotin and riboflavin, the THz spectra of myoglobin is broad and
structureless Additionally, the transmission significantly increases with
temperature. Previous studies of proteins in this spectral region revealed
significant spectral structure, attributed to standing-wave patterns, which are
effectively removed in our study. Our myoglobin observations challenge the
basis for the application of THz spectroscopy to biological warfare agent
detection, that is, that biological agents will have structured THz spectra
that will aid their detection and identification. (T. Korter,
D. Plusquellic, A. Hight Walker, E. Heilweil, and G. Fraser)
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Radiometric Calibration of the Marine Optical Buoy (MOBY). The Division
is providing radiometric calibration of the Marine Optical Buoy (MOBY) to
ensure the accuracy of the measured down-welling irradiances and ocean-leaving
radiances used to calibrate or validate ocean-color measurement satellite
instruments, such as the Sea-Viewing Wide Field-of-View Sensor (SeaWifs) and
the Moderate Resolution Imaging Spectroradiometer (MODIS). The Division is
supporting the radiometric calibration of MOBY by both traveling to the
deployment site in Hawaii to perform calibrations of the buoy and by developing
standards and methodologies to directly calibrate and test a duplicate MOBY
spectrograph at NIST. This spectrograph, denoted MOS for Marine Optical System,
was characterized at NIST using SIRCUS, in response to concerns about stray or
scattered light causing significant errors in the instrument readings,
particularly at shorter wavelengths (see fig. 3). The success of the
SIRCUS measurements led to the development of a "Traveling" SIRCUS to
allow direct calibration of the MOBY instrument in Hawaii using the Division's
laser-based monochromatic radiance sources. The improved calibration of MOBY is
allowing the correction of the legacy ocean radiance and irradiance data, which
is being made available to researchers dependent on these measurements. The
project has also demonstrated the unique advantages of using SIRCUS for the
calibration of remote sensing instruments. (C. Johnson, S. Brown, and
K. Lykke)
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Figure 3. This plot is the normalized instrument responsivity versus
wavelength for the Marine Optical Spectrograph (MOS) as measured using a
broadband radiation source and a monochromatic source furnished by the
laser-based SIRCUS calibration facility. |
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Traveling SIRCUS. The Division is actively involved in providing
radiometric calibration for remote sensing satellites. It is often desirable to
be able to perform the calibrations at the customer's site, typically an
aerospace company or a NASA or DoD facility, either because of the instruments
large size or fragility, scheduling constraints, or special clean-room or
environmental limitations. To be able to provide the highest accuracy
spectroradiometric calibrations requires that we bring the capabilities of
SIRCUS, i.e., broadly tunable single-frequency radiance and irradiance
standards, to these sites. We are presently developing such capabilities in the
visible and near infrared, which we are calling Traveling SIRCUS. Upon
completion, Traveling SIRCUS will consist of Ti:Sapphire and dye lasers pumped
by a solid-state diode-pumped laser, a small Ar-ion laser, a solid-state
frequency doubler, and integrating spheres, providing a monochromatic source of
continuous spectral radiance from the ultraviolet to the near-infrared, that is,
from 350 nm to 980 nm. While at NIST, the system is used as a
research tool to advance the capabilities of SIRCUS, as the present SIRCUS
instruments are frequently unavailable due to the calibration workload. The
initial deployment of Traveling SIRCUS was made recently at the MOBY site in
Hawaii (see above). (C. Johnson, and S. Brown, K. Lykke)
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Calibration of Light-Pipe Radiation Thermometers for Applications in Rapid
Thermal Processing (RTP). To help improve temperature measurements in
semiconductor RTP, the Division has undertaken extensive research on light-pipe
radiation thermometers (LPRT's) and their calibration. The goal of the RTP
program is to enable the semiconductor industry to achieve temperature
measurements of better than 2 °C at 1000 °C, a goal stated in
the Semiconductor Industry Association roadmap. Improved LPRT measurements are
essential to achieving this goal, as they are the predominant temperature
measurement instruments in RTP. The Division's research has led to a
calibration protocol and a set of recommendations to ensure accurate
temperature measurements with LPRT's.
|
Figure 4. This figure shows the calibration of a light-pipe radiation
thermometer (LPRT) using a sodium heat-pipe blackbody for temperatures between
700 °C and 950 °C. TC refers to thermocouple. |
Expanded uncertainties of 0.6 °C (k = 2) were achieved for the
calibration of LPRT's for radiance temperature near 1000 °C. The radiance
temperature standard was a well-characterized, high-emissivity,
sodium-heat-pipe blackbody, as shown in fig. 4. To be useful in
applications, LPRT's must retain their calibration over a long period of time
to limit the down time of the RTP chamber. Short-term, 10 min variations
in the sensor response were found to be less than 0.1 °C, while long-term,
1 yr variations in the sensor response were less than 1 °C. These
measurements indicate that LPRT's are capable of making temperature
measurements to within 1 °C. (C. Gibson, F. Lovas, and
B. Tsai,)
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Intrinsic Birefringence in CaF2. Deep ultraviolet
photolithography technology has turned to crystalline CaF2 for the
next generation of refractive optical elements because of the poor transmission
of glasses at 193 nm and 157 nm. Despite the cubic symmetry of
CaF2, its crystalline nature introduces spatial-dispersion-induced
birefringence, which results from the finite wave vector of the light.
Measurements made in this Division and the Atomic Physics Division were
supported with theoretical calculations performed in collaboration with the
Electron and Optical Physics Division. We have successfully modeled it in
CaF2, BaF2, and several semiconductor materials from
first principles, obtaining good agreement with all measurements.
Our main results are presented in fig. 5. Our key findings are as follows.
First, the birefringence in CaF2 is about ten times larger than
specifications desired by the semiconductor industry. Second, the birefringence
in CaF2 is opposite in sign from that in BaF2 and many
other materials. Third, a mixed
CaxBa1-xF2 crystal may exhibit
zero birefringence at chosen wavelengths by tuning x. (E.L. Shirley)
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Figure 5. Index difference for two polarizations for light
propagation along the [110] direction. |
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High-level UV Irradiance Calibration. We have calibrated the irradiance
responsivity of a diode array spectrophotometer designed to measure the
irradiance at the exit port of an advanced xenon-lamp-illuminated integrating
sphere source (BFRL). The 2 m diameter-integrating sphere was designed for
accelerated UV aging studies of advanced materials over the spectral range from
290 nm to 400 nm, with irradiance levels at the sphere exit port
calculated to be as high as the equivalent irradiance from 30 suns. A
fiber-coupled diode array spectrophotometer, equipped with an integrating
sphere fore optic, is used to calculate the irradiance at an exit port.
We used a collimated laser source to determine the spectral power responsivity
of the spectrophotometer as several wavelengths over the 300 nm to
400 nm spectral region. We then determined the effective aperture area of
the input fore optic, and averaged the response over the entrance aperture, to
establish the irradiance responsivity. To obtain an estimate of the
responsivity over the entire spectral range from 300 nm to 400 nm, we
measured the output of a Xe arc lamp using a reference standard filter
radiometer, and interpolated the radiance between filter radiometer tie points.
This relative measurement was subsequently scaled at 334 nm to obtain a
measure of the absolute spectral responsivity of the spectrophotometer.
(S. Brown and E. Bryd, with J. Chin, Div. 862)
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Improved Low Background Infrared Facility (LBIR) Broadband Blackbody
Calibrations. The LBIR broadband blackbody calibrations can now be
performed at 1 nW power levels with 1 % Type A (random
contributions) uncertainty. This achievement is a direct consequence of
improvements in background environment stability and the use of the new LBIR
Absolute Cryogenic Radiometer (ACRii), and corresponds to a factor of 10
improvement in power measurement capability. A new refrigerator system with
greater cooling capacity and better temperature stability is partially
responsible for the improved background environment. The calibrations are also
now being performed in the newer Spectral Calibration Chamber (SCC) with its
more efficient cryoshrouds. These improvements have changed the background
environment from 25 K with a relatively common 0.1 K drift in
temperature in a 10 minute period, to a 17 K environment with drifts in
temperature of less that 0.01 K over a 10 minute period. The
electronics-limited 10 pW sensitivity of the ACRii can be fully used in
the new stable environment. This can be compared to the use of the older ACR in
the Broadband Calibration Chamber (BCC) where the noise floor was 100 pW
at best. Figure 6 shows an example of a 97 pW power measurement using
the ACRii in the SCC with the new refrigerator. These improvements have made it
possible to meet most of the recent calibration requirements for the blackbody
sources from the aerospace contractors of the Ballistic Missile Defense
Organization (BMDO). (A. Carter and S. Lorentz)
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Figure 6. The plot shows 97 pW power measurements made using the
LBIR's new Absolute Cryogenic Radiometer (ACRii) in the new Spectral
Calibration Chamber. (1) ACRii heater power without incident optical
radiation. (2) ACRii heater power with incident optical radiation. |
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Figure 7. Photograph of the NIST integrating sphere for absolute
infrared spectral transmittance and reflectance.
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Development of Monte-Carlo Integrating Sphere Reflectometer Model. The
Division has developed a highly efficient code based on Monte Carlo methods and
ray-tracing to study the performance of an integrating sphere designed for
hemispherical-directional reflectance measurement. This code has been used to
perform a comparative analysis of different sphere designs, with variation of
critical sphere parameters including sphere wall reflectance and sample
scattering characteristics. Integrating spheres have been used for nearly a
century for reflectance measurements. However, due to the inherent difficulties
of accurate uniform collection of all reflected or transmitted light from a
diffuse sample, significant measurement errors and uncertainties are common. A
significant contributor is the complexity of analytical approaches to sphere
analysis that ultimately necessitates the use of approximations. Numerical
modeling approaches do not require approximations, but the long computation
times even with the fastest computer processors, limit the accuracy of the
results.
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Figure 8. This plot demonstrates the effect of different design options,
for integrating spheres, on the accuracy of reflectance measurements. |
| |
Our code incorporates a number of significant time-saving features, that enable
us to reduce statistical uncertainties to less than 0.1%, even for very low
efficiency spheres. Numerical modeling methods offer the most promise for
sphere analysis, design, and error reduction. For example, the effect of
different design options including baffling arrangements on the measurement
accuracy of the sphere was analyzed with results shown in fig. 8. The
studies have resulted in a specific sphere design to be built for near-infrared
reflectance and non-contact temperature measurements. (L. Hanssen) |
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LED Sphere. Currently, typical light sources used for radiometric,
photometric, and colorimetric calibrations use broad-band, incandescent or
Xenon-arc lamps. Most of the radiation from these sources lies in the infrared
spectral region, and is consequently not useful for photometric and
colorimetric calibrations. Colorimeters and photometers often measure sources
(such as displays) with significantly different spectral distributions from the
calibration source. The errors in these measurements are often unknown, and can
be quite large. For example, colorimeters can be calibrated against an
incandescent source with minimal errors of 0.001 or less in x,y
chromaticity. However, errors in measurements of displays using these
colorimeters can be larger than 0.01 in x,y. Development of a stable,
spectrally tunable, radiometric source could positively impact a variety of
photometric, colorimetric, and radiometric programs. Recent advances in LED
materials and their manufacturing processes have resulted in the commercial
availability of LED's with colors spanning the entire visible spectrum. We are
exploiting this newly enhanced technology in the design of a tunable spectral
distribution integrating sphere source (ISS).
Figure 9. Picture of LED sphere showing red (upper left), green
(upper right), blue (lower right), and white (center) outputs.
We have developed a prototype tunable LED-based ISS, shown in fig. 9. The
integrating sphere is equipped with 10 different LED's with spectral
distributions ranging from the blue to the red. By varying the drive current of
the individual LED's, the sphere spectral radiance can be tuned over a broad
wavelength range enabling calibrations of colorimeters and spectroradiometers
against the ISS with different spectral distributions. (S. Brown and
G. Eppeldauer)
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Detector Damage/Depth Dependence. A new detector measurement capability,
illustrated in fig. 10 has been added to the UV radiometric beamline at
the Synchrotron Ultraviolet Radiation Facility (SURF III). The beamline is
capable of calibrating photodetectors from 130 nm to 600 nm.
Figure 10. Schematic of the NIST quantum efficiency measurement setup.
The new setup allows simultaneous measurement of the spectral reflectivity and
the spectral power responsivity of the detector. With the measurement of both
quantities, the important detector quantity of internal quantum efficiency, the
response of the detector per total amount of the radiation absorbed by the
detector, can be deduced. The internal quantum efficiency of a detector depends
only on the internal mechanism of converting photons to electrons and the
collection of electrons because the variation in radiation loss due to detector
surface reflectivity is eliminated during the calculation of internal quantum
efficiency. In the visible, the internal quantum efficiency of a working
standard silicon photodiode is close to 100 % and was modeled to provide a
detector calibration curve by extrapolating to all wavelengths from visible to
near infrared. The new setup measures both reflectivity and power responsivity
simultaneously to reduce the uncertainty in deriving the internal quantum
efficiency caused by effects like positioning of the detector and contamination
of the detector surface when measurements are performed separately. We have
studied the internal quantum efficiency of a variety of photodetectors,
especially UV detectors with potential application for the photolithographic
industry. Our measurements of the internal quantum efficiency have provided us
with insight into the photon detection mechanism. We were able to model the
detector internal quantum efficiency and found clear evidence of interface trap
states inside some of the detectors. We also found evidence that these trap
states were formed when a detector is damaged by UV radiation. The capability
of studying detector internal quantum efficiency has become an important tool
for detector characterization. (R. Gupta and P. Shaw)
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Mutual Recognition Arrangement. A Mutual Recognition Arrangement (MRA)
was drawn up by the International Committee of Weights and Measures (CIPM),
under the auspices of the Meter Convention. At a meeting held in Paris on
14 October 1999, the directors of the national metrology institutes (NMIs)
of 38 Member States of the Meter Convention and representatives of two
international organizations signed the MRA.
This Mutual Recognition Arrangement is a response to a growing need for an open,
transparent, and comprehensive scheme to give users reliable quantitative
information on the comparability of national metrology services and to provide
the technical basis for wider agreements related to international trade,
commerce, and regulatory affairs. The eventual outcome of the MRA will be
statements of the measurement capabilities of each NMI in a web-accessible
database, known as the key comparison and calibration database (KCDB),
maintained by the BIPM.
The OTD has been involved in a number of international intercomparisons
including spectral responsivity (from 200 nm to 1600 nm), spectral
irradiance, aperture area, luminous intensity and luminous flux, spectral
regular transmittance, and diffuse spectral reflectance. These intercomparisons
form the underpinning of the MRA verification process and are listed in
Appendix C.
The acceptance of NIST's capabilities for Thermometry and Photometry and
Radiometry are anticipated for spring of 2002. Additional information on the
MRA and the KCDB can be found at
http://kcdb.bipm.org/BIPM-KCDB/
(C. Gibson, T. Early, Y. Ohno, and S. Bruce)
- Determining the Molecular Basis of Adhesion at Buried Interfaces.
Adhesion between dissimilar materials is of critical importance to a variety of
industrial products and processes, e.g., coatings, fiber-reinforced composites,
multilayer electronic devices, and integrated circuit packaging. Until recently,
a technique to adequately characterize buried interfaces, and in particular, to
determine the molecular structure at such an interface has been lacking. We
have been developing novel state-of-the-art non-linear optical spectroscopies,
such as Sum Frequency Generation (SFG), to explore molecular structure at
material surfaces and buried interfaces for a variety of applications.
Figure 11. Vibrationally resonant SFG spectra illustrating changes in
molecular orientation which correlate with atomic oxygen treatment of the
substrate. |
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We have extended our previous studies of free polymer surfaces [J. Phys. Chem.
B 105, 2785 (2001)] to investigate the molecular structure of buried
polymer interfaces, specifically, the polystyrene(PS)/spin-on-glass(SOG)
interface. Thin films of PS and SOG of specific thicknesses are used to create
optical interferences that modulate the signal coming from the interface of
interest. The SOG substrate is comprised of a hydrogen silsesquioxane inorganic
polymer film, which, when properly cured, has optical and chemical properties
similar to glass. However, the untreated surface of a SOG thin film is
terminated by silicon-hydrogen (Si-H) bonds, resulting in a low free energy,
hydrophobic surface. Increasing exposures of this native SOG surface to UV
activated oxygen species decreases the number of Si-H bonds in favor of
silicon-hydroxyl (Si-OH) bonds, which form a high free energy, hydrophilic
surface. Figure 11 shows the vibrationally resonant SFG spectra of the
PS/SOG interface for a series of PS/SOG samples with varying exposure of the
SOG substrate to activated oxygen (Oa). Analysis of the SFG spectrum
for the native SOG substrate (i.e., 0 min Oa treatment), gives
an absolute orientation of phenyl groups pointing away from the PS film (i.e.,
towards the SOG substrate) with an absolute molecular orientation similar to
that observed previously for phenyl groups at the free PS surface. Changes in
the SFG spectra for the Oa treated SOG substrates correlate with
orientational changes in the tilt and twist angles of the pendant phenyl groups.
The observed spectral changes suggest the phenyl groups alter their orientation
to enable more favorable chemical interactions with surface Si-OH species. |
The changes in molecular orientation observed with SFG have been correlated
with the adhesive strength of the polystyrene thin-film/glass interface. PS
films peel off the untreated, low surface energy, hydrophobic SOG substrates;
whereas PS films adhere to the higher surface energy, hydrophilic SOG
substrates. This work paves the way for critically addressing the issues of
molecular based adhesion between dissimilar materials in a variety of
disciplines. (K. Briggman, J. Stephenson, with P. Wilson and
L. Richter, Div. 837)
- Scattering by Small Metallic Spheres. Small (50 nm to
200 nm) polystyrene latex (PSL) spheres are used by the semiconductor
industry to calibrate the particle-sizing function of light-scattering-based
scanning surface inspection instruments. However, these spheres are not typical
of real-world particles encountered on production lines. The industry needs
accurate models for scattering by real-world particles to design instruments
and to provide the basis for particle size and particle identification using
the instrument.
In 1986, Bobbert and Vlieger described the solution to the scattering of light
by a sphere on a flat substrate. However, it was found that for metallic
particles on a highly reflecting silicon substrate, numerical instabilities
caused their implementation to fail long before convergence was achieved.
Painstaking efforts were made to find the roots of the instabilities, resulting
in a final implementation of the theory, which is efficient, robust, and
accurate. This solution was extended to account for uniform coatings on the
substrate and the sphere.
In collaboration with NIST's Building and Fire Research Laboratory and the
University of Maryland, methods were developed in the Division to generate
size-monodisperse copper spheres and to deposit them onto silicon wafers.
Polarized light scattering measurements, performed at a wavelength close to the
copper plasma frequency, were performed on these samples. The results yielded
excellent agreement with the theoretical calculations (see fig. 12). The
implementation of the theory, made publicly available through NIST's SCATMECH
library of scattering codes, can act as a benchmark by which approximate codes,
suitable for more complex particles, can be compared. (T. Germer)
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Figure 12. Light scattering parameters are plotted [differential
scattering cross-section (DSC), the degree of polarization (P), the
normalized degree of circular polarization (PC/P), and
the principal angle of the polarization (η)]
for 96 nm (circles), 113 nm (triangles), and 158 nm (squares)
copper spheres on silicon wafers measured in the plane of incidence using 45°
polarized, 633 nm light incident at an angle of 60°. The solid curves
represent the predictions of the Bobbert-Vlieger theory. |
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Measuring the Dynamics of Individual RNA Molecules. We have measured the
rotational dynamics of single RNA molecules tethered to a glass substrate.
Recent single molecule experiments and many gene-chip technologies rely on the
ability to tether a molecule to a surface without changing its activity. An
assumption is often made that the molecules are unaffected by the presence of
the surface. Here we explore this assumption on a single-molecule basis. A
common scheme for tethering nucleic acids to surfaces exploits the high
affinity bond of biotin with streptavidin (fig. 13). Here we label a
single stranded RNA molecule on its 3' end with a dye called
Cy3TM. At the 5' end, a single biotin molecule is attached to the
RNA via a flexible tether. Streptavidin links the biotinylated RNA molecule to
a coverslip coated with biotinylated bovine serum albumin (BSA).
(L. Goldner, K. Weston, W. Heinz, and A. Bardo)
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Figure 13. Tethering RNA to a surface. |
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Figure 14. Infrared absorption at 2000 cm-1 for Si-H
surface species in a 4×4 CVD film array (progression from dark gray to
white denotes increasing Si-H density and infrared absorption). |
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Carrier Lifetimes of Low-Temperature GaAs Measured for Terahertz Antenna
Applications. To better understand the broadest attainable frequency
bandwidths of terahertz (THz) generators and detectors, Division researchers
teamed with NIST's EEEL to measure carrier lifetimes in low-temperature GaAs
films (LT-GaAs). Improving and controlling growth conditions for LT-GaAs films
is critical for obtaining optimum performance in high-frequency THz devices.
Charge mobility in antenna semiconductor substrates directly affects the output
pulse duration and speed, thus producing varying terahertz frequency bandwidths
in spectroscopic or imaging applications.
Figure 15. Transient reflectivity recovery dynamics for two samples of
1 µm thick MBE-grown LT-GaAs on GaAs substrates. The 365 fs
carrier decay time is for EEEL material: while the slower 1.9 ps decay
time is for a commercial sample. |
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LT-GaAs films are known to yield several picosecond carrier lifetimes compared
to GaAs or related semiconductors (100's of ps). High electron and hole
mobilities correlate with shorter lifetimes - achieving short carrier lifetime
films is preferred. Lifetimes for 1 µm thick chemical vapor deposited
LT-GaAs films were measured using an amplified femtosecond Ti:Sapphire system
with 50 femtosecond, 800 nm pulses. A reflection pump-probe apparatus
directly monitored the time-dependent carrier reflectivity change after pump
excitation. Figure 15 shows transient reflectivity results for films
produced under ostensibly similar pressure and temperature growth conditions by
a commercial vendor (decay time 1.9 ps) and in NIST's EEEL Division
(365 fs decay). The carrier decay time fit (solid line) is approximately
five times longer for the commercial film than for the EEEL material. We
measure ~2 THz bandwidths for strip-line antenna systems using the
commercial material and, from the lifetime decrease, bandwidths of up to
~12 THz may be achievable with the EEEL material. These measurements
demonstrate the critical need to monitor temperature during deposition
(achieved in the EEEL apparatus) and that measured lifetimes correlate with
desired frequency response for THz antenna systems. (E. Heilweil and
A. Migdall) |
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Colorimetric Characterization of Special-Effect Pigment Coatings. The
Division, with ATP support, is developing methods for the quantitative
characterization of special-effect pigment coatings, such as pearlescent
coatings, important for the appearance of many commercial products, including
automobiles, cosmetics, and various consumer goods. The complexity of these
coatings increases the need for better appearance measurements for process and
quality control.
Pearlescent coatings typically consist of thin metal-oxide-coated transparent
mica platelets (see fig. 16). Constructive and destructive interference of
light from the front and back surface reflections of these platelets are
responsible for the chroma, hue, and brightness variation with the angles of
incidence and viewing.
To quantify the properties of the pearlescent coatings, a series of samples
supplied by manufacturers were examined using the Division's Spectral
Tri-function Automated Reference Reflectometer (STARR). STARR measured the
reflectance of the samples for incident wavelengths from 380 nm to
780 nm in 10 nm increments and incident angles of 15°, 25°,
45°, 65°, and 75°. Viewing angels were chosen from –80° to
80° in 5° steps. For each pair of incident and reflected angles,
colorimetric values of lightness, a, b, hue, and chroma were
determined. The STARR-determined colorimetric quantities validated the
qualitative expectations by exhibiting a strong dependence on the incident and
reflected angles. Empirically, it is found that measurements at a subset of the
reflectance angles, 15°, 35°, 45°, 70°, and 85°, for
each incident angle, provide a complete characterization of the coatings.
Future efforts are directed at developing improved light scattering models to
predict the optical properties of special-effect coatings, using recent
experimental data on idealized coated samples for validation. (M. Nadal
and T. Germer)
| Type of pigment |
Absorption |
Metallic |
Pearlescent |
|---|
Optical principle of pigments
(first order) |
Absorption and/or difuse scattering |
Specular reflection |
Thin-film interference |
| Perceived chroma and hue |
Independent of geometry |
Independent of geometry |
Dependent on geometry |
| Perceived brightness |
Independent of geometry |
Dependent on geometry |
Dependent on geometry |
| Measurement geometry |
0°/45° or ~0°/d |
45°/15°, 45° & 110° |
Under study |
Figure 16. Illustration of scattering mechanisms, perceptions, and
measurement geometries for various types of common pigmented coatings.
Division Overview | Program Directions |
Major Technical Highlights
|
