Technical Highlights
- High Resolution FTS Upgrade Completed. Operating characteristics of
our high resolution Fourier transform spectrometer (FTS) have been greatly
improved by replacement of a number of electronic and mechanical components.
Our new data acquisition system has been completely redesigned, tested, and
installed on the spectrometer. This completes the modernization of all
components of the electronic systems of the instrument. We have also replaced
the linear motors and motion control system, giving us a simpler and more
easily maintained system that eliminates one of the auxiliary interferometers
with no reduction in performance. These instrument upgrades in conjunction
with software improvements have produced a significant improvement in
signal-to-noise ratios and have reduced ghosts in the system to a negligible
level. (G. Nave, U. Griesmann, and R. Kling)
- Rare Earth FTS Spectroscopy for Lighting Applications. We have used
our high resolution FTS to measure branching ratios for over 300 lines of
Dy I and II in the range
400 nm to 2500 nm. This work has been combined with complementary
work at the University of Wisconsin for a comprehensive set of branching
ratios in these spectra. The results are important for the development of more
efficient commercial lighting. Rare earth admixtures in high-pressure lamps
are being utilized by the lighting industry both to increase luminosity as
well as to achieve better color rendering, and atomic data for rare earth
spectra are needed for the modeling of future lamp designs. We have also taken
spectra of a Mn hollow cathode lamp in the VIS/IR region. This will be
combined with UV data to obtain branching ratios for
Mn II lines of interest to space astronomy groups.
(G. Nave, R. Kling, and U. Griesmann)
- High Resolution Spectroscopy for Space Astronomy. We made new
observations of the spectrum of singly-ionized mercury
(Hg II) in the visible and near infrared with a
pulsed radio-frequency discharge on our 10.7 m air Eagle spectrograph.
These observations provide the first accurate wavelengths for
Hg II in this region. We have now classified
nearly 500 lines as transitions between 90 energy levels. Some of these are
transitions in the visible that originate from levels of the
5d96s5f configuration lying well above the ionization limit and
that terminate on levels of 5d96s6d lying just below the limit, a
most unusual occurrence. From our new level values an accurate value for the
ionization energy was determined. All of our results for
Hg II are being assembled into a comprehensive
report that will contain a complete quantum mechanical interpretation of the
Hg II level structure. This report, which will
also provide calculated transition probabilities, will constitute the first
modern description of the spectrum and energy levels of this important atomic
ion.
A portion of our results for Hg II have been
incorporated into a collaborative report with several astronomers that
analyzes the abundance of mercury in the chemically peculiar stars chi Lupi
and HR7775. These stars have an abundance of Hg about 105 times the
solar abundance. They also exhibit isotopic abundance anomalies. In chi Lupi,
for example, the observed Hg is all in the form of isotope 204, the
heaviest stable isotope, which comprises only 7% of terrestrial Hg. Many of
our results for Hg II as well as for
Bi I, II, and
III, Hg III,
Pb III, Zr II and
III, Y III, and
Sb II have been used in an atlas of observations
of chi Lupi from the Goddard High Resolution Spectrograph prepared by Hubble
Space Telescope scientists. (C. Sansonetti and J. Reader)
- Quantum Electrodynamic Effects in Low Energy Levels of Helium.
Accurately determined ionization energies for the low 1sns and
1snp (nS and nP) levels of helium furnish excellent tests
of calculations for this important three-body system, including two-electron
quantum-electrodynamic (QED) effects. By combining a variety of high-accuracy
measurements of transitions in helium, we derived ionization energies for
several low nS and nP levels. The uncertainties for the
n = 1 and n = 2 levels vary from 8 parts
in 109 for the 1S ground level to 5 parts in
1011 for the 2 3S level. Corresponding
theoretical energies including QED shifts of order α3 atomic
units and higher were calculated by Gordon Drake of the University of Windsor,
Canada, except for the three lowest levels. For these most accurate levels the
main part of the QED shift of order α3 was based on a
preliminary calculation of the Bethe logarithm which was completed in final
form by an NRC Postdoctoral Associate at NIST. This particular calculation has
been the object of theoretical studies for over 40 years. This latest
calculation provides the definitive result, with an improvement in accuracy
over previous calculations of about four orders of magnitude for the Bethe
logarithm and two orders of magnitude for the energy levels.
Comparisons of the experimental energies with the less accurate calculated
values for the seven 1S, 2S, and 2P levels give
agreements well within the estimated theoretical uncertainties of 1 to
3 parts in 108. The results verify the usefulness of the
Kabir-Salpeter formalism for calculating QED shifts at least up to order
α4. Much work is still needed, however, to obtain a
two-electron theory of higher-order relativistic and QED contributions
approaching the accuracy in hydrogen. (W. Martin and J. Baker)
- 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 the properties of Bose-Einstein condensates, cold
molecule formation, and quantum computing. We have started new calculations
for the properties of magnetically or optically induced scattering resonance
states near zero collision energy. Our calculations quantitatively explain the
strength and width of several such resonances recently measured in a sodium
Bose-Einstein condensate. We also have set up the time-dependent
Gross-Pitiaevskii equation and have predicted nonlinear four-wave mixing of
matter wavepackets generated from Bose-Einstein condensates, an effect that
has now been observed experimentally at NIST (see also the division's cover
picture and the highlight on "nonlinear matter wave optics with
Bose-Einstein Condensates" which contains another figure). Calculations
of the matter wave coherence agree with experiment and other recent
calculations. (C. Williams, P. Julienne, E. Tiesinga,
P. Leo, F. Mies, M. Doery, M. Trippenbach, and
Y. Band)
- Electron Impact Ionization Cross Sections. The semiconductor
industry is shifting toward theoretical modeling of etching by plasma
processing to save time and expenses in designing new chips. One critical need
for such modeling is the ionization cross section of halogen molecules used in
etching. We have developed a Binary-Encounter-Bethe model (BEB) for
calculating such cross sections. It is the only ab initio theory in the
world that can distinguish reliable experimental data from less reliable data
for large neutral molecules of interest to the semiconductor industry. The BEB
model has also been found to be effective for atomic and molecular ions of low
charge states. A new collaboration with a quantum chemistry group at NIST has
been initiated in addition to the existing collaboration with researchers at
CalTech and NASA Ames Research Center to predict reliable ionization cross
sections for large and complex molecules. New results are being posted on the
Physics Laboratory Web site (/ionxsec) as they are
published. Modelers of plasma processing in the semiconductor industry (e.g.,
Intel, Motorola, Phillips) have started to request theoretical cross sections
for molecules of interest to them. (Y.-K. Kim, M. Ali)
- 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 of quantum dot structures and used these models to study CdS/HgS and
CdTe/ZnTe 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), where effective mass models are
expected to break down. We have also extended our theory of T-shaped quantum
wires to include the effects of magnetic fields. This has allowed us to
explain recent magneto-photoluminescence experiments on these systems and
provides a compelling description of confinement effects in these structures.
(G. Bryant and P. Julienne)
- Theory of Near-Field Optical Microscopy. Near-field microscopy
offers optical resolution much better than the diffraction limit. Detailed
theory and modeling is needed to interpret and analyze near-field images. We
developed a coupled dipole theory for imaging with transmission near-field
optical microscopy (NSOM) and applied it to accurately model experimental NSOM
images of Au nanoparticles. It is critical to model the entire imaging process
because it allows us to clearly identify the contribution of the near-field
optical excitation source, the coupling to the sample local fields, and the
collection optics to NSOM image formation. We find that field enhancement
under the metal cladding of the NSOM probe critically determines the structure
in the NSOM images. We also modeled the near-field nonlinear optical response
of nanoscale structures to understand how near-field optics can be used to
extend the capabilities of nonlinear optical spectroscopy. (G. Bryant and
P. Julienne)
- Comprehensive Spectra Database on the World Wide Web. The Atomic
Spectra Database (ASD) interactive Web server has become accessible at the
NIST Physics Laboratory Web site: physics.nist.gov/asd. The
new version 2.0 of ASD contains significantly more extensive coverage than
previous versions. It contains data on about 950 spectra, with about 70,000
energy levels and 90,000 lines from 1 Å to 200 µm, 40,000 of
which have transition probabilities with estimated accuracies. Wavelengths of
observed transitions are included for the first 99 elements in the
periodic table. ASD offers a comprehensive range of user-specified options and
selection criteria and includes a "Help" file, which also serves as a users
manual. (P. Mohr, D. Kelleher, W. Martin, J. Fuhr,
A. Robey, and W. Wiese)
- Fundamental Constants - Toward a Better Rydberg constant. Recently,
there has been a dramatic increase in the frequency metrology of hydrogen, and
it is expected that the 1S-2S transition in hydrogen will
eventually be measured to 1 Hz, a relative uncertainty below
5 × 10-16, possibly using trapped hydrogen atoms. In
order for the anticipated improvement in experimental precision to provide
better values of the fundamental constants, there must be a corresponding
improvement in the precision of the theory of the energy levels in hydrogen,
particularly in the Lamb shift. As a first step toward this goal, we have
carried out a numerical calculation of the one-photon self energy of the
1S state. Numerical convergence acceleration techniques were developed to
decrease the substantial computation time by about three orders of magnitude.
The result is the first complete calculation of the self energy in hydrogen
and provides a value that contributes an uncertainty of about 0.8 Hz. The
result is a step toward an improved value of the Rydberg constant and possibly
toward the use of hydrogen as a frequency standard over a wide range of
frequencies. The calculation was done in collaboration with the Technical
University of Dresden, Germany. (P. Mohr and U. Jentschura)
- Critical Compilations Uncover Serious Problems for Calculated
Transition Probabilities. The vast majority of transition probability data
for atoms and ions are computed. In comparing the sophisticated atomic
structure codes (there are about half-a-dozen in existence), we have found
that the agreement is usually excellent for the strongest transitions, but
that disagreements typically become greater than 50% for oscillator strengths
smaller than 0.1 and increase to one or more orders of magnitude with further
decreasing strengths. Figure 1 shows an example of the severity of the
problem.
 |
Figure 1. Comparison of two different theoretical data sources for
oscillator strengths, showing order of magnitude discrepancies.
|
We have alerted the data generators to the seriousness and extent of this
problem. Also, we organized a special session on this problem at the Sixth
International Conference on Atomic Spectra and Oscillator Strengths, August
1998, in Victoria, B.C. This has sparked renewed and more critical work at
several institutions focusing on spectra and transitions that we recommend.
The first new high-accuracy computations in response to our requests are
already producing promising data for noble-gas-like spectra.
(D. Kelleher, J. Fuhr, and W. Wiese)
- Atomic Spectral Line Broadening Bibliographic Database Issued. The
first bibliographic database on atomic spectral line broadening has been
completed and made available on the NIST Physics Lab Website. This database
contains approximately 850 recent references for the time period 1993 to 1998,
all collected after the last published NIST bibliography: [NIST Special
Publications 366, Supplement 4, 1993]. The papers listed in the database
contain either numerical data or general information, comments, and review
articles and are part of the collection of the Data Center on Atomic Line
Shapes and Shifts at NIST. The following search categories are included:
chemical element, stage of ionization, broadening mechanism, experiment,
theory, word in title, author, and year of publications. This database is
patterned after the existing NIST Web-based bibliographic database on atomic
transition probabilities. Our plan is to add all 5000 earlier references from
the Data Center collection to the database in order to provide a complete set.
(J. Fuhr and H. Felrice)
- X-ray Spectroscopy on EBIT. In collaboration with Russian
researchers who have developed expertise in fabricating high quality
spherically curved crystals of mica and quartz, we have deployed these
crystals as the heart of a new type of x-ray spectrometer for use on an
Electron Beam Ion Trap (EBIT). This spectrometer has an advantage over all
other x-ray spectrometers previously used on an EBIT: the ability to acquire
spectra with both high light collection efficiency and relative insensitivity
to source position. This simultaneously addresses the two main factors that
have limited the precision of previous measurements on EBIT's-photon statistics
and calibration systematics. Demonstration spectra from neon-like barium
(Ba46+) and helium-like argon (Ar16+) were obtained,
paving the way for future high accuracy measurements. In parallel with this
work, progress has been made using traditional NIST x-ray spectrometers to
determine wavelengths in hydrogen-like and helium-like vanadium ions with an
absolute accuracy that rivals the best previous measurements in this region of
the one- and two-electron isoelectronic sequences. With an accuracy of 20 to
30 parts per million, these results critically challenge calculations of the
atomic structure of highly charged ions, particularly considering recent
significant revisions involving higher order quantum electrodynamic
corrections. Our work is proceeding in collaboration with researchers from
Australia; a preliminary report was recently submitted for publication, and a
final report is under preparation. (J. Gillaspy and L. Hudson)
- Highly Charged Ions Used to Pattern Surfaces. Masked ion beam
lithography using highly charged ions (Xe44+) was demonstrated by
the EBIT team by exposing a silicon wafer coated with a commercial resist
material (PMMA). Subsequent chemical development of the resist revealed the
imposed pattern -- a regular array of hundreds of 1 micrometer wide squares
with better than 100 nm edge resolution. Atomic force microscopy was also
used to image single ion impact sites, which appear as 24 nm wide holes
in the surface. Although PMMA is widely used in the community of ion-beam
lithography researchers, this is the first time that highly charged ions have
been used and that atomic force microscopy has been deployed to reveal the
effect of a single ion on this material. Some related work has just been
completed using Xe44+ ions to pattern an advanced ultrathin resist
consisting of self-assembled monolayers of alkanethiolates. (L. Ratliff,
J. Gillaspy, and R. Minniti)
|
|
Figure 2. Portion of an array of squares produced using highly charged
ions to expose a self-assembled monolayer resist.
|
- Characterization of the GEC-ICP RF Plasma Source. This new class of
high-density, low-pressure plasma sources is 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
increases, monitoring and control of plasma uniformity become increasingly
important. A new diagnostic technique for plasma uniformity measurements based
on 2D optical tomography has been developed for vacuum chambers with
restricted optical access. Optical tomography determines 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 technique has been
applied to optical emission measurements from the GEC-ICP RF Plasma Source.
Several conditions creating radially asymmetric plasmas have been identified,
such as gas flow rate, proximity to the induction coil, and feed gas
composition.
Operation of the GEC-ICP RF Plasma Source with pulsed RF power has been
investigated. By momentarily interrupting the power to the inductive coil, the
properties of an inductively coupled plasma can be significantly altered. With
electronegative gases commonly used in commercial etching reactors,
interruption of the RF power results in a rapid loss of electrons creating a
decaying plasma composed of only positive and negative ions. The resulting
ion-ion plasmas have the potential to improve etching performance and reduce
surface damage on wafers. The decay and growth of the plasma during pulsed
power operation of the GEC-ICP RF Plasma Source has been measured using a new,
intensified CCD camera. In argon/oxygen mixtures, when the RF power is turned
back on, the plasma first ignites as a dim capacitive discharge before
switching back into a bright inductive discharge. (E. Benck and
J. Roberts)
- Plasma Radiation. We have recently completed the rebuilding of the
NIST FT700 ultraviolet Fourier transform spectrometer. The FT700 spectrometer
is a unique resource. It has a wavelength coverage from the visible down to
approximately 200 nm and a resolving power of 106. (An upgrade
of the interferometer optics is currently underway to extend the range to
140 nm.) We have used the new spectrometer to make accurate measurements
of spectral line intensities and branching ratios in the ultraviolet in
Kr II, Mn II, and
Xe II. The measurements provide data to test
recent, sophisticated, atomic structure calculations and are needed in the
diagnostics of laboratory and stellar plasmas. (U. Griesmann,
K. Dzierzega, R. Kling, and W. Wiese)
- High-Accuracy DUV and VUV Index of Refraction Measurements. As part
of a collaborative project with MIT Lincoln Laboratory, SEMATECH, and the NIST
Optical Technology Division, we have made the highest accuracy
(~7 × 10-6) measurements of the index of refraction,
its dispersion, and its temperature dependence, of fused silica and calcium
fluoride near 193 nm. These numbers are being used by the semiconductor
electronics industry for the design of the transmissive optics of
photolithographic steppers using 193 nm ArF excimer laser excitation.
These will be used for the fabrication of 0.18 µm minimum-feature-size
integrated circuits, scheduled for large-scale production by the U.S.
semiconductor industry beginning in 2001. For future-generation integrated
circuits, stepper manufacturers are designing steppers based on 157 nm
F2 excimer laser excitation and calcium fluoride optics. Responding
to requests from all major stepper manufacturers, we have made the only
measurements of index of refraction (to 7 × 10-6), its
dispersion, and its temperature dependence, of calcium fluoride near
157 nm. [J. Burnett, U. Griesmann, and R. Gupta (Div 844)]
- Ultracold Collisions in Metastable Xenon. We have investigated the
effects of spin polarization and quantum statistics on ultracold inelastic
collisions in metastable xenon. We found that, contrary to expectations, the
rate of inelastic ionizing collisions was not at all suppressed by
spin-polarizing the sample of atoms. The spin selection rules that might be
expected to apply are voided by a molecular effect where the spins
"lock" to the molecular axis instead of the laboratory axis. The
atoms strongly depolarize during the collision, so that the initial
polarization has no effect on the outcome. This result calls into question the
likelihood of using metastable rare gases other than helium for Bose-Einstein
condensation. We also measured the spin-polarized collision rates for fermionic
and bosonic isotopes of xenon, and found a significant decrease in the
collision rate of the fermions at low temperatures. This is directly ascribable
to quantum statistics and the Pauli exclusion principle, which prevents two
identical fermions from occupying the same state. This measurement is the first
clear observation of quantum statistical suppression in cold collisions.
Because ultracold atoms move so slowly, it is possible to observe the temporal
dynamics of collisions. By preparing excited state atoms with a short pulse of
laser light and measuring the arrival time of the ions produced in collisions,
we were able to study the collision process in detail. We have observed the
acceleration of the atoms on the attractive intermolecular potential and have
clearly observed collisions that include the decay of the excited atom to the
ground state during the collision. We have also been able to time-resolve the
optical shielding process, where light excites a pair of atoms onto a
repulsive molecular potential, preventing a short range, ionizing collision
from occurring. (S. Rolston, C. Orzel, and S. Kulin)
- Large Bose-Einstein Condensation of Sodium in a TOP Trap. We have
created a large Bose-Einstein condensate (BEC) of sodium atoms in a
time-averaged orbiting potential (TOP) trap. A TOP trap is a magnetic trap
consisting of a quadrupole magnetic field and a constant magnitude, rotating,
bias field. The arrangement of our fields produces a tri-axial potential that
is well matched for loading from the nearly spherical clouds of laser cooled
atoms. We have developed two new strategies for evaporatively cooling atoms to
Bose-Einstein condensation. The first strategy involves evaporative cooling
using rf, with the atoms initially trapped in a quadrupole field. This is then
followed by rapidly transferring them into the TOP trap and further cooling of
the sample to condensation, again using rf-induced evaporation. The second
strategy involves starting with atoms in the TOP trap and evaporatively
cooling the atoms with the "circle-of-death" (the zero field region rotating
around the center of the trap) all the way to condensation. Both strategies
produce approximately the same number of final condensate atoms, about
3 × 106, at a BEC transition temperature of 1.2 µK.
(L. Deng, E.W. Hagley, K. Helmerson, M. Kozuma,
R. Lutwak, J.-H. Müller, W.D. Phillips, S.L. Rolston,
and J. Wen)
- Bragg Diffraction of a Bose-Einstein Condensate. We have coherently
split and deflected a Bose-Einstein condensate (BEC) of sodium atoms using
Bragg diffraction by a moving, optical standing wave, comprised of two
counterpropagating laser beams with a frequency difference. The condensate
atoms, initially at rest, will simultaneously absorb photons from the higher
frequency laser beam and be stimulated to emit photons into the lower
frequency beam acquiring several units of photon momentum in the process.
Hence the momentum transfer is uni-directional and coherent. The increase in
kinetic energy of the Bragg diffracted atoms comes from the energy difference
between the absorbed and emitted photons from the two different frequency
laser beams.
|
In our experiments we start with an adiabatically expanded BEC with no
discernable thermal fraction present. The momentum spread of the condensate
atoms released from the trap is much less than the momentum of a single
photon. We then expose the atoms to a short pulse of the moving, optical
standing wave while they are either still in the TOP trap or shortly after
releasing them from the trap. We detect the momentum transferred to the atoms
from the diffraction process by taking an absorption image after a sufficient
time delay, such that the various atomic wave-packets with different momenta
have spatially separated. Figure 3 shows first, second and third order
Bragg diffraction of Bose condensed atoms, corresponding to momentum transfer
of 2, 4 and 6 times the single photon momentum. We have observed up to
6th order Bragg diffraction. The direction of the momentum transfer can be
reversed by changing the sign of the frequency difference. We have observed
first order Bragg diffraction of 100% of the condensate atoms. (L. Deng,
E.W. Hagley, K. Helmerson, M. Kozuma, R. Lutwak,
W.D. Phillips, S.L. Rolston, and J. Wen)
|
|
Figure 3. 1st, 2nd and 3rd order
Bragg diffraction of a BEC by a moving, optical standing wave.
|
- Non-Linear Matter-Wave Optics with Bose-Einstein Condensates. Due
to the relatively strong influence of the atom-atom interactions in a
Bose-Einstein condensate, non-linear effects in matter-wave optics can occur.
These non-linear effects are analogous to non-linear optical wave phenomena.
Specifically, the theory predicts that an interacting condensate is analogous
to optical waves interacting with a third order, non-linear medium. The
resulting process from such an interaction is 4-wave mixing. In 4-wave mixing,
three waves are sent into a non-linear medium and a fourth wave emerges. We
have observed a similar phenomenon with matter-waves, where the non-linear
medium is the interacting atoms themselves. (See also the Division's cover
picture.)
Figure 4. Image of the distribution of atoms resulting from 4-wave
mixing of matter waves.
|
|
In order to observe the generation of a fourth wave due to 4-wave mixing,
the three incident waves must have the appropriate momenta to satisfy energy
and momentum conservation. We use two Bragg diffraction pulses to produce
condensates in the three appropriate momentum states to observe 4-wave mixing
of matter-waves. The pulses are applied rapidly enough that the atoms in the
three momentum states still overlap. The non-linear interaction between the
atoms produces a fourth state with a different momentum. Figure 4 is an
image of the distribution of atoms resulting from 4-wave mixing of
matter-waves, taken after the different momentum states have spatially
separated. This represents the first example of non-linear atom optics. The
smallest peak is the fourth matter-wave, generated by the 4-wave mixing
process. We have observed up to 12% of the initial condensate atoms appearing
in the fourth wave. We have also confirmed that the process depends on the
product of the densities of atoms in the three initial momentum states.
(L. Deng, E.W. Hagley, K. Helmerson, W.D. Phillips,
S.L. Rolston, and J.E. Simsarian)
|
- More Precise Value of the Neutron Mass. The absolute wavelength of
the gamma-ray produced in the reaction n+p→d+γ
(2.2 MeV) was measured with a relative uncertainty of
2 × 10-7 using the NIST ILL GAMS4 crystal diffraction
facility at the Institut Laue-Langevin in Grenoble, France. This wavelength
measurement, expressed in energy units and corrected for recoil, is the binding
energy of the neutron in deuterium. A previous crystal diffraction measurement
of the deuteron binding energy has an uncertainty 5 times larger than this
new result. The neutron mass follows directly from the reaction expressed in
atomic mass units: m(n) = m(2H) -
m(1H) + S(d) where S(d) is the
separation energy of the neutron in deuterium. The uncertainties of the atomic
mass difference, m(2H) - m(1H),
and the new determination of S(d) are
0.71 × 10-9 u and
0.42 × 10-9 u, respectively, where u is unified
atomic mass unit. The new, more precise value for the neutron mass,
m(n) = 1.008 664 916 37(82) u, has an
uncertainty which is &asymp 2.5 times smaller than the previous best
value. [E. Kessler and M.S. Dewey (Div 846)]
- New High-flux X-ray Diffractometer/Reflectometer. Using a novel
optical design, a new x-ray analysis facility has been built which provides a
peak count rate of 107 photons/s for the study of materials of
interest to the semiconductor industry. Industry standard 200 mm wafers
can now be examined using the new instrument. Films as thin and light as
1.5 nm of Si3N4 and as thick and dense as
100 nm of Pt have been successfully characterized by this facility. The
high counting rate also allows large batches of films to be studied in short
order. Recent materials of interest characterized for various industrial
partners include SiOxNy,
Ba(1-x)SrxTiyO3 (Fig. 5),
Ta2O5 and TaNx. (S. Owens,
J. Pedulla, and R. Deslattes)
Figure 5. Grazing incidence x-ray reflectivity data and modeled fits
for Ba(1-x)SrxTiyO3 thin films.
- Development of Thin Film Reference Materials. Accurately
characterized, highly uniform thin films are in great demand for fluorescence
measurement calibration. Using a Dual Ion Beam Assisted Deposition facility, a
wide variety of materials are being grown with excellent lateral thickness
uniformity, high density and low interfacial roughness. These films are
characterized in-house by Grazing Incidence X-ray Reflectivity (GIXR) which
provides film thickness and interface roughness to 0.1 nm resolution and
density to a few percent resolution. Films on
7.5 cm × 7.5 cm float-glass and 7.5 cm diameter
silicon substrates are currently being shipped to both internal and industrial
partners. (J. Pedulla)
-
Recertification of Si SRM-640b Powder-Diffraction Reference Material.
X-ray powder diffraction is a widely used analytical method for which NIST is
the world's principal supplier of powder-diffraction reference materials
(SRMs). Current inventory and previous certification accuracy are inadequate
to future need. In collaboration with the Materials Science and Engineering
Laboratory, a major effort has been undertaken to produce and certify a new
generation of these reference materials. Diffractometer calibrations and
uniformity tests on 30 kg of single-crystal silicon material were
completed. This material was crushed and sized to form the new silicon
SRM-640c; its packaging and certification await selection of a surface
stabilization process. Meanwhile, we have re-certified the previous material,
silicon SRM 640b, with a relative uncertainty close to
1 × 10-6 Å. (J.-L. Staudenmann,
L. Hudson, and R. Deslattes)
- Picometer Heterodyne Interferometery Demonstrated. Heterodyne
Michelson interferometry is the most widely used technique for accurate
displacement measurements. Its accuracy has traditionally been limited to a
few nm by well-known periodic systematic errors arising from optical
crosstalk. Brute-force improvement of a traditional interferometer in our
laboratory brought the amplitude of the periodic error down to 500 pm in
1992, but in order to go beyond this level, new techniques are required. We
have recently developed and demonstrated two new schemes for doing heterodyne
interferometry in which the amount of optical crosstalk can be greatly
reduced. In one such scheme, the residual periodic error has an amplitude of
20 pm. More recent results with the second scheme suggest that the
periodic error is even lower, and in fact beyond our ability to measure. In
addition, we have demonstrated a new digital phase meter with a 10 kHz
bandwidth capable of splitting optical fringes by a factor of 32,000.
(C.-M. Wu, J. Lawall, and R. Deslattes)
|
