Technical Highlights
- Spectroscopic Measurements for Lighting Applications. About 20% of
all electricity generated in the United States is used for lighting, and
improvement of the energy efficiency of lighting systems is a national
priority. We have set up an experiment to measure the radiant output of
selected spectral lines in the visible and ultraviolet regions from discharges
of importance for lighting, and we determine how light output of these spectral
lines varies when the discharge conditions in the lamp are changed
systematically. These data are used to test and validate computer models of
lighting discharges being developed by commercial manufacturers of lamps. In a
first set of experiments we have observed discharges of mercury, which is a
primary constituent in both fluorescent and high-intensity discharge lamps.
Also as part of this program, we are using our high resolution Fourier
transform spectrometer to study the spectra of rare earth elements used in high
intensity discharge lamps to improve their light output and to achieve better
color rendering. We have extended the spectral analysis of neutral and
singly-ionized dysprosium, classifying over 400 lines as transitions to 43 new
energy levels of Dy I and 22 new energy levels of Dy II. We have also
measured branching fractions in neutral and singly-ionized holmium, which will
be combined with previously-published lifetimes to obtain transition
probabilities. Computer models of such lamps require the kind of comprehensive
atomic data our experiments provide to realistically simulate the performance
of new lamp designs. (G. Nave, C. Sansonetti, and J. Reader)
- Image Plates for Registration of Far UV Spectra. In recent years manufacturers of
photographic emulsions have discontinued production of the special photographic
plates used for the far UV (100 Å to 500 Å). This presents a
significant obstacle to observing spectra of interest for fusion research and
space astrophysics. In response, we have investigated the use of
autoradiographic photo-stimulable phosphor storage plates (image plates) to
record such spectra. This has not been previously attempted. The image plate is
first exposed to high intensity visible radiation to erase any latent image on
the plate, and then placed in the spectrograph and exposed. The exposed plate
is read by a He/Ne laser in a computer controlled scanner. Fluorescence of
exposed areas occurs and is detected by a photomultipler and stored as a
two-dimensional map. The plate can then be erased and used again. A tracing of
a spectrum in the far UV recorded with an image plate is compared with that
from a photographic plate in Fig. 1.
(J. Reader, C. Sansonetti, and R. Deslattes)
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![[Link to Figure 1]](Images/fig1T.gif)
Figure 1. |
Figure 2. Total ionization cross section of He.
T = incident electron energy; BED = present theory; Rapp,
Montague, and Shah = Experiments; Bray = convergent close
coupling theory.
Comparison of
electron-impact cross section for ionization of the helium atom as a function
of the incident electron energy.The comparison covers the range of incident
electron energy T from the threshold at 24.59 electron-volts to 10 keV.
The theoretical cross section based on the Binary-Encounter-Dipole (BED) model
agrees within ±5 % in the entire range shown with the best available
experimental data measured by Rapp and Englander-Golden, by Montague
et al., and by Shah et al., while the theoretical data calculated
by Bray et al., using the convergent close coupling method begins to
fall below the BED model and the experimental data at high T above
500 eV.
- New Chapters for 3rd Edition of A Physicist's Desk Reference.
Two chapters have been prepared for the new edition of the popular book A
Physicist’s Desk Reference, to be published by Springer-Verlag for the
American Institute of Physics under the editorship of D.R. Lide. The
chapter on "Atomic Spectroscopy," was updated and in large measure
rewritten by W.L. Wiese. An entirely new chapter on "Optics" was
written by J. Reader. (J. Reader and W. Wiese)
- Atomic Interactions and Collisions of Cold, Trapped Atoms. The
control of atomic interaction parameters and collision rates by magnetic or
optical fields is an important goal of research on cold atoms. Applications
include manipulation of quantum degenerate Bose and Fermi gases, cold molecule
formation, and quantum logic gates. We have evaluated all existing data on cold
Rb ground state collisions to help resolve discrepancies concerning collisional
shifts in the Rb atomic fountain clock. Our calculations for the properties of
magnetically or optically induced scattering resonance states near zero
collision energy quantitatively explain the effects of such resonances observed
in a sodium Bose-Einstein condensate. We also developed methods to solve the
time-dependent Gross-Pitiaevskii equation in three-dimensions and used them to
predict condensate coherence properties and nonlinear four-wave mixing of
matter wavepackets generated from Bose-Einstein condensates. Our calculations
agree well with recent measurements at NIST. (P. Julienne,
C. Williams, J. Burke, E. Tiesinga, P. Leo, F. Mies,
M. Doery, M. Trippenbach, and Y. Band)
- Complex Quantum Nanostructures. Quantum nanostructures are being
studied by many labs to realize their promise of enhanced optoelectronic
devices. We have implemented realistic empirical tight-binding models in our
theory for quantum dot structures and used these models to study CdS/HgS and
CdS/ZnS quantum dot quantum wells and Ge nanocrystallites. Our atomistic models
allow us to study quantum nanostructures down to the smallest sizes, such as
quantum dot quantum wells with layers as thin as one monolayer and tightly
confined systems with indirect gaps (Ge) or strong valence-band/conduction-band
mixing (InAs). Comparison with optical experiments on these systems allows us
to provide a detailed understanding of the spectroscopy of these structures.
These models have been extended to consider quantum dot solids, which are
ordered arrays of quantum dots. (G. Bryant and W. Jaskolski)
- Theory of Near-Field Optical Microscopy. Near-field scanning optical
microscopy (NSOM) offers optical resolution much better than the diffraction
limit. Detailed theory and modeling is needed to interpret and analyze
near-field images. We have continued development of our coupled dipole method
and implemented a finite-difference time domain method for nano-optics modeling.
Our simulations show how NSOM can provide nanoscale local optical metrology of
optical waveguides. Our modeling of recent NSOM measurements on thin film
polymers and semiconductors revealed how film thickness and defect scattering
determine image contrast. Nano-optics modeling applied to optical resonators and
quantum dots embedded in nanoscale semiconductor pyramids has shown how optical
near-fields can be engineered to provide novel atom traps and to enhance
optical coupling to quantum nanostructures. (G.W. Bryant,
P.S. Julienne, A. Rahmani, S.-T. Chu)
- High-Accuracy 157 nm Index of Refraction Measurements. Over the
past year the semiconductor microelectronics industry has pursued the
development of 157 nm optical lithography as a technology to enable
integrated circuit feature size reduction to below 130 nm, as set forth in
the International Technology Roadmap for Semiconductors, 1999 Edition.
Designing these systems requires accurate values for the optical properties of
the lens materials, such as the refractive index, which had not been accurately
known for any material near 157 nm. We have made the first and to date the
only measurements of the absolute index of refraction, as well as its
dispersion and temperature dependence, near 157 nm for calcium fluoride,
the principal candidate 157 nm material. These values have gone into the
designs of the steppers from all major stepper manufacturers. Furthermore, in
the past year new grades of fused silica have been developed that transmit
sufficiently near 157 nm to be considered as optical materials for
157 nm photomasks and other optics. We have also made the first and only
index measurements of these new materials. (J. Burnett and U. Griesmann,
Div 842; and R. Gupta, Div 844).
- GEC-ICP RFGEC-ICP RF Plasma Source. High-density,
low-pressure plasma sources are becoming increasingly important to meet the
demands of reducing the critical dimensions of etched structures in the
semiconductor industry. As the wafer diameters used in etching increase,
monitoring and control of plasma uniformity becomes increasingly important. A
new fiber optic based optical tomography sensor has been built to measure
2D plasma uniformity. Optical tomography derives the two dimensional
distribution of plasma species in a plasma from line-integrated measurements,
such as optical emission measurements or laser absorption measurements, without
assuming radial symmetry of the plasma. This new sensor simultaneously acquires
82 different optical emission measurements through two small windows,
significantly reducing the required data acquisition time (<1 s) from
the previous single detector tomography sensor (>30 min). Initial
2D plasma distributions have been obtained with this detector from the
GEC-ICP RF Plasma Source. Pulsed power operation of the GEC-ICP RF Plasma
Source has also been investigated and found to be very beneficial. By
momentarily interrupting the power to the inductive coil, the properties of an
inductively coupled plasma are significantly altered. Sheaths within the plasma
collapse when the rf power is removed, allowing neutralization of surface
charges on wafers to occur. The neutralization of surface charge reduces
problems with aspect ratio dependent etching rates, notching due to electron
shading, and surface damage. A variety of different time-resolved optical
(optical emission, diode laser absorption, intensified CCD imaging) and electrical
diagnostics (Langmuir probe, coil current and voltage, reflected power) have
been applied to study this new mode of discharge operation and are being
investigated as potential process control parameters. With electronegative
gases, such as O2 and CF4, the GEC-ICP RF Reference Cell
exhibits an exceptionally long capacitive or E mode plasma when the rf
power to the coil is resumed. This enables detailed studies of the processes
responsible for the E (capacitive) to H (inductive) mode transition.
(E. Benck, M. Edamura, and K. Etamadi)
- In Situ Observation of Highly Charged Ion Induced Features in
Graphite. Nanoscale features have been created on graphite surfaces by
impact of highly charged ions and observed using an ultra high vacuum scanning
tunneling microscope. A single Xe44+ ion makes a protrusion
6 nm wide and 1 nm high. Similar features have been observed in mica
after highly charged ion impact and exposure to air. It was hypothesized that
the impact leaves the surface flat (or possibly with a crater) but with many
broken chemical bonds, and that the protrusions were due to adsorption of
atmospheric contaminates to these broken bonds. This is the first time that
highly charged ion induced surface features have been imaged in situ,
without the possibility of such contamination in air. The fact that we observe
protrusions, rather than craters, rules out this contamination hypothesis, at
least for graphite surfaces. Furthermore, we performed these experiments with
two charged states of xenon, whose potential energies differ by nearly a factor
of two, but with the same kinetic energy. We found that the widths of the
features increased by a factor of two with increased potential energy
indicating that the features are due, at least in part, to the projectile’s
potential energy which, compared to its kinetic energy, is deposited in a very
small volume near the surface. (L.P. Ratliff, R. Minniti, and
J.D. Gillaspy)
- New High Resolution, Broad Band X-Ray Detector Deployed on EBIT. The
NIST electron beam ion trap (EBIT) team, in collaboration with a team from the
Harvard-Smithsonian Astrophysical Observatory (SAO), performed x-ray
measurements on highly charged ions using a newly developed x–ray
microcalorimeter. In a preliminary study, the two teams made 7 eV
resolution measurements of the x-ray spectra of highly charged nitrogen,
oxygen, neon, argon, and krypton, which have x-ray energies that differ by more
than a factor of 10. The preliminary work represents one of the first
applications utilizing recent advances in single photon calorimetry. Originally
designed for space bound x-ray observatories, the Harvard/SAO microcalorimeter
is ideal for studying the highly charged ions available in an EBIT. The
detector has both high resolution and broadband sensitivity, a property not
found in any other x-ray detectors. The EBIT, in turn, provides a well
controlled environment for the study of the high energy atomic processes
occurring in many astrophysical environments and in a wide variety of earth
based plasmas such as tokamaks. The combination of the EBIT and
microcalorimeter will make possible several measurements which are currently
very difficult or not possible using other techniques, for example the
simultaneous intensity and wavelength calibrated measurement of L-shell
and K-shell transitions in a particular ion species. Future experiments will
continue to provide atomic data of interest for plasmas and will be extended to
include the study of highly charged ion/surface interactions. (J.V. Porto,
E. Takacs, and J.D. Gillaspy)
- Fourier Transform Spectrometer Operates in VUV. We have upgraded the FT700
Fourier transform spectrometer at NIST to work in the vacuum ultraviolet (VUV)
at wavelengths as low as 133 nm. This is, to our knowledge, the shortest
wavelength that was ever measured with a scanning Fourier transform
spectrometer. The upgraded FT
spectrometer was used to solve a number of problems. For example, we have made
measurements of branching fractions and transition probabilities in Kr III,
Xe II, Xe III and Mn II (with partial support by NASA) in the
UV and VUV. The measurements provide
data to test recent sophisticated atomic structure calculations and are needed
for the diagnostics of stellar plasmas. We have also measured the refractive
index of nitrogen gas between 270 nm and 145 nm. Our measurements
resolved a long standing discrepancy between earlier measurements of the
nitrogen index in the VUV. The refractive index of nitrogen in the VUV,
specifically at 157 nm and 193 nm, is an important design parameter
for future semiconductor lithography systems which may use nitrogen as a purge
gas. (U. Griesmann, R. Kling, J.H. Burnett, and
L. Bratasz)
- X-Ray Spectroscopy on
EBIT. In collaboration
with researchers from Australia, we have completed a measurement of the x–ray
emission from helium-like vanadium ions. With an absolute accuracy of better
than 30 parts-per-million, these are the most precise measurements of the
helium-like resonance lines in the Z=19 to Z=31 range of atomic numbers.
Previous measurements by other groups in this range showed a disagreement with
predictions and were interpreted as evidence for needed corrections to the
calculations. In contrast, our results show excellent agreement with the
predictions. Our work takes into account corrections for systematic effects not
fully included in previous measurements. We have subsequently developed, and
begun to deploy, a new type of x-ray spectrometer that consists of two curved
crystals that are displaced by a fixed and precisely determined amount. This
system produces a pair of spectral lines for each emission wavelength,
a feature that can be used to produce an internally calibrated system that
obviates the need for repeated use of external reference lines. The curved
crystal geometry, never implemented before in such a double crystal
configuration, makes the system highly efficient and particularly appropriate
for use on EBIT-type light sources. (E. Takacs, L. Hudson,
J.D. Gillaspy, and R.D. Deslattes).
- Highly Directional Atom Laser. We have demonstrated a highly
directional, quasi-continuous atom laser using a new technique of output
coupling. This technique relies on stimulated, optical Raman transitions to
change the internal state of the Bose-Einstein condensate atoms from one
confined by a magnetic trap to one unconfined. The simultaneous absorption and
stimulated emission of photons in the Raman process gives the atoms a kick
which sends them along a particular direction away from the remaining trapped
atoms. By choosing the orientation of the laser beams, both the magnitude and
direction of the momentum of the extracted beam of atoms can be varied. Hence,
unlike any other demonstrated technique, this output coupler does not rely on
gravity. This scheme of output coupling has the additional advantage that the
extracted beam of atoms has near diffraction limited divergence, where the
transverse spread in momentum is determined by the corresponding spatial extent
of the condensate. In the other atom lasers the divergence or spreading of the
beam was determined primarily by the repulsive interaction between the atoms.
In our experiment the atoms receive a big momentum kick and spend little time
in the presence of the remaining trapped atoms. Hence the beam experiences less
of an increase in transverse velocity due to interactions with the remaining
atoms.
Figure 3. Demonstration of a highly directional, well collimated atom
laser. Bose-Einstein condensed atoms in the m = -1 magnetic
sublevel are confined in a magnetic trap. A stimulated optical Raman transition
is used to couple atoms to the m = 0 magnetic sublevel which
does not feel the effect of the magnetic fields. The two photon Raman process
imparts a 2 k
momentum kick to the transferred atoms causing them to leave the region of the
condensate in a direction determined by the orientation of the laser beams.
Using a fast pulse rate, we achieve a substantial overlap of the pulses of
transferred atoms, and the atom laser beam appears continuous.
Our Raman output coupling can be pulsed or continuous, similar to rf output
coupling. In our experiment the output coupling was pulsed due to the presence
of time varying magnetic fields; however, the repetition rate of the pulses was
fast enough such that there was substantial overlap of the clouds of outcoupled
atoms from pulse to pulse. The result was a continuous beam of atoms directed
perpendicular to gravity (see Fig. 3). We refer to our atom laser as a
quasi-continuous source since, like all other atom lasers demonstrated, there
is no replenishment of atoms into the condensate from an external source. The
atom laser would cease to emit its beam once the condensate was depleted of
atoms, which in our case is a few million. (K. Helmerson, M. Kozuma,
W.D. Phillips, S.L. Rolston and J. Wen with L. Deng and
E.W. Hagley of Div. 841)
- Coherent Atom Optics – the Talbot Effect. We have demonstrated a new
matter wave manifestation of the Talbot effect using a short-pulsed phase
grating to diffract a Bose-Einstein condensate. In the optical Talbot effect,
coherent light passing through a periodic grating will form an
"image" of the grating at a characteristic distance known as the
Talbot length. For a phase grating, this "image" corresponds to the
initial intensity distribution of the light with the phase distribution of the
grating. Unlike light, however, atoms can be initially at rest, and the
"reimaging" of the phase grating occurs at integer multiples of the
Talbot time. Also, unlike light, atoms can be exposed to a pulsed phase grating,
which leads to a unique manifestation of the Talbot effect.
In the experiment, a short-pulse, phase grating for matter waves is realized by
a short-pulse, optical standing wave, which "writes" a sinusoidal
phase variation onto the condensate wavefunction. A second identical
diffraction pulse is applied after a variable delay to analyze the temporal
evolution of the resulting wavefunction. We observe that the initial phase
distribution reimages itself at integer multiples of
t = 10 µs, the Talbot time for our parameters. When the
second pulse is applied at odd multiples of half the Talbot time, self imaging
of the condensate in momentum space is observed. Intermediate delays produce
more complicated momentum-space patterns that are in excellent agreement with
theory. The coherent property of the condensate provides signals of very high
contrast. In addition, we observe that the dynamics of the short pulse is
different from that of a static grating because it has a broad frequency
spectrum and hence can add energy to the system. It is the dispersion relation
of matter waves, not the path length difference as in the case of static
gratings, that results in this new and unique Talbot effect.
(J. Denschlag, K. Helmerson, W.D. Phillips, S.L. Rolston, and
J.E. Simsarian with C.W. Clark, L. Deng, M.A. Edwards, and
E.W. Hagley of Div. 841)
- Phase Dispersion of a Condensate. We have studied the coherence
properties of a Bose-Einstein condensate (BEC) using an interference technique.
Two optical standing wave pulses of duration 100 ns and separation time
Δt are applied to the condensate. Each standing-wave phase grating
diffracts the condensate, making small "copies" of the condensate
displaced in momentum space by twice the momentum of a single photon. As the
first copy moves away from the condensate its phase is evolving at
4ER/, where ER is
the single photon recoil energy. (For sodium atoms with an excitation
wavelength of 589 nm, ER/h is 25 kHz.) After the
second copy is created at a time Δt later, the phases of both copies then
evolve at nominally the same rate. The quantum mechanical amplitudes of each
copy interfere, and the total number of atoms coupled out of the condensate by
the two pulses is measured. The resulting interferogram oscillates at the
expected 100 kHz phase evolution of the first copy with respect to the
second copy. The decay of the envelope of the interferogram is due to both the
spatial overlap of the two copies (since the first copy has moved during
Δt due to the momentum kick) and to the initial spatial phase
variations across the condensate.
When the coherence measurement is made on a condensate held in a confining
potential, we obtain an interferogram whose envelope decays essentially as the
spatial overlap of the two outcoupled copies. The results are consistent with
the trapped condensate having a uniform spatial phase. Hence we have
experimentally verified that the trapped BEC, despite being spatially expanded
due to the mean-field interaction between the atoms, is limited in its spatial
extent and momentum spread due to the Heisenberg uncertainty principle.
Alternatively, a released BEC exhibits large phase variations across the
condensate as the mean-field interaction is converted into kinetic energy. This
is apparent in our measurements where we obtain an interferogram with an
envelope that decays much faster than the spatial overlap of the two copies.
Our measurements also show that the successive, Raman output coupled pulses of
atoms in our atom laser are fully coherent. (Y.B. Band, M. Doery,
K. Helmerson, P.S. Julienne, M. Kozuma, W.D. Phillips,
S.L. Rolston, and M. Trippenbach with L. Deng,
M.A. Edwards, and E.W. Hagley of Div. 841)
- Sodium Cold-Collisions Experiments. During the past year the cold
collisions experimental program has undertaken an investigation of the time
dependence of the photoassociative ionization process by performing
pulsed-laser photoassociation experiments. Photoassociation is the process by
which two colliding atoms absorb a photon and form a bound, excited state of a
diatomic molecule. The transition is detected by ionizing the molecule and
detecting the ion, or by the loss of atoms from the trap. The
frequency-dependence of this process allows one to investigate the spectroscopy
of the molecule and to learn about the properties of the cold atoms undergoing
the collisions. The time dependence of the photoassociation, followed by a
secondary excitation/autoionization step, can be examined using a pump-probe
technique. The slow motion of the laser-cooled atoms and the ability of the
photoassociation process to produce states near the molecular dissociation
limit, where the energy level spacing is small, makes the dynamics of this
reaction slow enough to be studied with picosecond laser pulses. We perform
pump-probe experiments much like those performed with femtosecond lasers, but
on a much slower time scale. The laser pulse excites a superposition of
molecular states, creating a wavepacket of population that travels on the
molecular potential. The population travels on an attractive excited state
potential, and this allows the colliding atoms to get past the angular momentum
barriers on the ground state potential that would prevent the reaction from
occurring. The experimental ionization signal is seen to rise with pump-probe
delay time to delays of 3 ns to 4 ns, and then fall off gradually.
Theoretical simulations confirm the above ideas. (F. Fatemi,
K. Jones, and P. Lett)
- Creation of an Ultracold Neutral Plasma. We have created the coldest
neutral plasma ever formed by photoionizing a sample of laser-cooled xenon
atoms with a laser, tuned just above the ionization limit. We observe the
plasma formation by monitoring electrons extracted by an applied electric
field, and developed a model that quantitatively fits our experimental data. We
can produce plasmas with temperatures of less than 1 Kelvin and densities
as high as 1010 cm-3. In this regime, both the
electrons and ions form strongly-coupled plasmas (the Coulomb energy dominates
the thermal energy). Although estimates of three-body recombination suggested
that ultracold plasma formation would be impossible, we see lifetimes of many
microseconds, limited by expansion due to a small residual charge imbalance.
From this lifetime we can set a limit on the three-body recombination rate of
at least 4 orders of magnitude below the classical estimate. This work
allows the investigation of a previously unexplored regime of plasma physics.
(S. Rolston, S. Kulin, T. Killian, C. Orzel, and
S. Bergeson)
- Calibration of the Chandra X-Ray Observatory. The Chandra X-Ray
Observatory, formerly known as the AXAF (Advanced X-Ray Astronomy Facility), is
the largest and most sensitive x-ray telescope ever built. Chandra’s x-ray
optics and detectors were calibrated by NASA scientists using a double crystal
monochromator designed, built, and tested in the Quantum Metrology Group. The
unique NIST monochromator was an essential component in defining the
sensitivity and energy response of the telescope to incoming photons. Because
Chandra is in an orbit (perigee of ≈ 10,000 km) beyond any
retrieving capability of the space shuttle, testing was much more demanding
than for the Hubble Space Telescope. When this observatory took its first
images in August 1999, the world’s x-ray astronomers were astounded and
delighted at the quality of the data. The calibration obtained from the NIST
monochromator is a critical element in the analysis of the Chandra X-Ray
Observatory data. (J.-L. Staudenmann, L. Hudson, and
R. Deslattes)
- Binding Energy of Light Nuclei and Gamma-Ray Energy Standards. Our
program on precision gamma-ray wavelengths was awarded about eight weeks of
beamtime at the high-flux reactor of the Institut Laue
Langevin in Grenoble, France. The uniqueness of this program is the
linking of the gamma-ray and optical wavelengths through the lattice spacing of
the diffraction crystals. The efforts this year include high-energy
measurements to determine the binding energy of a light nucleus and low-energy
measurements that form the basis of recommended gamma-ray energy standards. The
wavelengths of gamma-rays produced in the reaction n + 32S
→ 33S + γ (8.6 MeV) were measured
and the binding energy of 33S, Sn(33S),
was determined by summing gamma-rays at 841, 2380, and 5420 keV. The
relative uncertainty of these measurements is ≈ 3 ×
10-7. The sulfur measurements along with earlier Cl binding
energy measurements are now of sufficient accuracy that they impact precision
atomic mass measurements as can be seen by expressing the
above reaction in atomic masses: Sn(33S) =
m(n)-[m(33S)-m(32S)]. The low energy measurements used
two long lived sources (198Au and 192Ir) and included the
412 keV line (198Au) and the 296, 308, 317, 468, 604, and
612 keV lines (192Ir). Because the sources are long lived, some
of the measurements were recorded between reactor cycles. The relative
uncertainty of these measurements is ≈ 3 ×
10-7. When these measurements are combined with relative
curved-crystal and Ge-detector data, an array of about 260 gamma-ray
standards from 50 radionuclides and 2 (n,γ)
reactions covering the range of 100 to 6000 keV is obtained. All of these
standards are linked to the optical wavelength region via the NIST precision
gamma-ray measurements. (E. Kessler and R. Deslattes with
M.S. Dewey of Div 846)
- X-Ray Reflectivity Analysis of Semiconductor Materials. While
technical improvements to our x-ray reflectivity and scattering resources have
continued, the past year showed an increasing number of applications covering a
wider variety of materials and increasing stack complexity. In terms of
thickness parameters, samples ranged from below 2 nm for silicon
oxynitride gate dielectics to above 2000 nm for some ultra-low dielectric
constant materials. In addition we successfully analyzed complex metal
interconnect stacks of copper, tantalum and tungsten nitride including accurate
determination of thin tungsten layers deeply buried by electro-deposited copper.
We saw evidence of spontaneous polymorphic transitions during film growth of
tantalum nitride (a copper diffusion barrier) and demonstrated unexpected
structural complexity in tantalum pentoxide. As noted under "Industrial
Applications" we are working with a number of individual companies but
primarily doing so through SEMATECH
coordination. (R. Deslattes)
- Laser System for Sub-atomic Position Control. By their very nature,
displacement measurements based on laser metrology are limited by the accuracy
of the laser wavelength standard. The iodine-stabilized laser, while providing
absolute accuracy of 12 kHz (2.5 parts in 1011), has
short-term frequency excursions over 100 times larger. We have now
constructed a laser system for displacement metrology in which the absolute
accuracy of the iodine-stabilized laser is transferred to a bank of
"flywheel" lasers with higher power and far greater short-term
stability. The performance of the system is such that the "flywheel"
lasers exhibit the accuracy of the iodine-stabilized laser on all time scales,
even very short ones. This is a crucial element to making real-time
interferometric measurements for active servo control. (J. Lawall,
M. Pedulla, Y. LeCoq, and R. Deslattes)
- Ultra-Quiet Research Facility in a Normal Laboratory Environment.
Accurate measurement and control of motion at the atomic scale presupposes
near-perfect control of vibration. We have designed and built a facility for
the prototyping of interferometers and position control systems in vacuum
offering remarkable performance in a normal laboratory environment. A two-stage
passive vibration isolation system in conjunction with a specially designed
vacuum chamber pumped by a magnetically levitated turbo pump provides
exceptional performance and ease of use. We have compared our system to two
other ultra-quiet research facilities at NIST-Gaithersburg. The first was an
older single-stage isolation system used for x-ray interferometry located in a
benign underground environment. The second was a very large-scale,
well-engineered platform in current use for neutron interferometry operating in
the less benign environment of a large experimental hall. The results of these
comparisons showed that the total noise at all frequencies above 5 Hz was
at least an order of magnitude lower in our new system than in either of the
other ultra-quiet environments. While the active isolation system of the
neutron interferometer provides better performance for very slow (<3 Hz)
disturbances, our work is largely immune to such effects, and the new system is
a key element to our progress in ultra-accurate interferometric positioning
systems. (J. Lawall and E. Kessler)
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