Technical Highlights
- Kinetic-Energy-Enhanced Etching of Silicon. The conditions required
to sustain high rates of semiconductor etching typically consist of thermal
heating, ion bombardment, and a combination of radical and neutral chemically
active species. Ion bombardment serves to erode the surface and eject product
molecules, providing fresh material for additional chemical attachment, as well
as producing the anisotropy in the materials removal process. However,
potentially damaging effects of the dry process plasma etching steps will be a
critical issue when gate oxides of metal oxide semiconductor (MOS) devices
shrink to 5 nm thicknesses. Thus there is considerable recent interest in
the possibility of supplying energy for the etching process in the form of
neutral-species kinetic energy.
In NIST laboratories, the effects of kinetic energy enhancements of neutral
chlorine species are studied in a beam-scattering arrangement. A novel laser
vaporization source produces a pulsed beam of chlorine molecules with kinetic
energies from near-thermal to 6 eV. This source was used recently to study
the scattering fraction of chlorine on clean and chlorinated silicon, and
etching products were detected as a function of chlorine coverage during
exposure to the hyperthermal beam. SiClX products are observed that result from
sustained etching of Si(100) at room temperature, especially when a significant
fraction of the hyperthermal beam has energy above 3 eV. The
SiCl3 product species exhibits an enhanced yield for room
temperature silicon and is ejected from the substrate surface with
>0.5 eV kinetic energy, suggesting a mechanism of direct release of the
SiCl3 species by nearby impact of an incident high energy
species.
- Laser Flux Monitor for GaAs Growth. With increasing demand for more
complex semiconductor devices, there is a need for new methods to monitor the
film growth process to provide better feedback control. Optical sensing is
desirable because of the noninvasive nature of the detection and the ease to
derive feedback control signals. One such NIST invention is the laser single
photon ionization probe method to monitor the gaseous fluxes during molecular
beam epitaxy (MBE) growth and processing. This method has been applied to the
layer by layer growth of GaAs, and will soon be applied in a collaboration with
a company to silicon MBE. The method utilizes the 9th harmonic of a
commercially available Nd:YAG laser at 118 nm to ionize the gaseous
species. The ions are extracted into a time-of-flight mass spectrometer, and
the incident or scattered fluxes are monitored by integrating the signals from
the desired masses on each laser pulse. Real-time detection with
0.1 second response time is possible.
Recently, the laser probe method was coupled with Reflection High Energy
Electron Diffraction (RHEED) measurements of the layer-by-layer growth of GaAs
to provide an excellent measure of the correlation between the growth rate and
the real time observations of the arsenic incorporation. The uptake of arsenic
is monitored with the laser probe by summing the changes in the signals from
both arsenic dimer and tetramer, and the RHEED spectrometer is used to
simultaneously monitor the rate of layer-by-layer growth. The correlated
measurements permit the precise layer-by-layer growth rate to be assigned
in situ from noninvasive laser determinations for the first time. In a
new collaboration with SVT Associates, the laser flux monitor method will be
developed for commercial feedback control of silicon epitaxy.
- Scanning Tunneling Microscope Measurements. The development of
photovoltaic and microelectronic devices based on thin films of hydrogenated
amorphous silicon (a-Si:H) is progressing rapidly. While
a-Si:H films are grown by a variety of techniques in research
laboratories, most devices are made by plasma-enhanced chemical vapor
deposition (PECVD). The electrical properties of these films vary substantially
depending upon the growth technique and system used. While in many cases growth
systems are optimized by electrical measurements of the films produced, models
of the film growth should help improve film quality. The inhomogeneity in
a-Si:H films may be one of the key issues in understanding the
properties of these films, including their degradation.
A scanning tunneling microscope (STM) has been used to study the topology of
the surface of device-quality, hydrogenated amorphous silicon deposited by rf
discharge from silane or "hot wire" CVD. The STM provides very high
resolution topological data on specific areas of the film surface, allowing
measurement of slopes and indications of void formation. The a-Si:H
surface grown in a PECVD chamber directly connected to an ultra-high vacuum
(UHV) analysis chamber was studied and compared to device quality samples
produced in different laboratories and transferred in air. Thin films
(10 nm) representing early growth stages appear significantly smoother
than the thicker films. The topology of thick films (>50 nm) has large
variations over individual samples. While many regions can be characterized as
"rolling hills," atomically flat areas are sometimes observed nearby.
In most regions the observed slopes were 10 % or less from the horizontal,
but some steep-sided valleys, indicating incipient voids, are seen. Overall
surface roughness measured on sub-micron areas of the films is very
inhomogeneous. Uniformity of the device quality films grown off site was much
better, although no atomically flat regions were observed.
- Femtosecond Wave Packet Dynamics in Lithium Dimer. The behavior of
molecules prepared by ultrafast lasers and with coherent control represents an
important new area of active manipulation of materials. Recently, for example,
it was demonstrated at the National Research Council in Ottawa that the
direction of photocurrents in semiconductor quantum wells can be manipulated
with coherent light fields. At NIST, a new capability has been established in
ultrafast laser coherent wave packet dynamics measurements, and a first series
of rovibrational wave packet dynamics experiments was performed on a
"shelf" state, in Li2. This shelf state is unique in that
the electronic structure is covalent on the inner turning point but ionic at
the outer turning point. The strong transition from covalent to ionic suggests
the possibility of measuring such novel effects as the ionization probability
versus internuclear separation in real time.
Figure 1. (a) Wave packet recurrences in Li2. (b) Fourier analysis
of a 120 ps scan of the wave packet beats.
The resulting wave packet "beats" are shown in Figure 1 together
with the Fourier spectrum. The complicated beat pattern results from the fact
that at least three vibrational levels, each with two widely spaced rotational
states, are prepared coherently. The Fourier spectrum indicates at least eleven
of the expected beat frequencies. Polarization experiments were performed to
assemble the magic angle results, which permit the vibrational coherences to be
observed independently of the pure rotational and rotational-vibrational
coherences. These results establish a new capability, which is presently being
extended to ultrafast studies in the solid state with scanned probe microscopy
as the detection.
- Evaporative Cooling: A Major Breakthrough in Push Towards BEC. NIST
and JILA scientists are undertaking an experimental and theoretical effort to
observe and understand Bose-Einstein Condensation (BEC). This year they
realized a major scientific advance: for the first time, trapped alkali atoms
were cooled evaporatively.
The advance was made possible by a new kind of magnetic trap invented at JILA -
a Time-averaged, Orbiting Potential (TOP) trap. The TOP trap combines very
tight confinement with very long lifetimes such that the evaporative process
has time to take effect. A radio frequency magnetic field is used to induce
evaporation by driving the hottest atoms out of the trap. The remaining atoms
reequilibrate at lower temperatures. During the course of evaporation the
coordinate-space density increases by a factor of two, and the temperature
decreases by a factor of 80 µK (to 200 µK), for a net phase space
density increase of three orders of magnitude. Further condensation causes the
evaporation to taper off because the collision rate decreases by a factor of
four, so the evaporation rate no longer progresses rapidly enough to outpace
loss due to background gas collisions. Models of the evaporative cooling
process in the TOP trap show that either an increase of the initial collision
rate or a decrease in the background pressure by a factor of two will bring the
system over the threshold of "runaway evaporation." Under these
conditions, evaporation causes the elastic collision rate to increase, which in
turn improves the efficiency of evaporation, leading to a situation of rapidly
accelerating phase space density. The model predicts that the system will
follow a trajectory through density-temperature space which intersects the
predicted BEC transition at 3 × 1012 atoms/cc and
50 nK. The best available estimates for two- and three-body inelastic
processes indicate that at the phase transition boundary the ratio of good to
bad collisions is a comfortable factor of 5000. In short, all indications are
that only modest improvements in the apparatus should be necessary to reach
BEC. The very cold dense atoms attainable by evaporation will be ideal for
improved metrology and novel quantum optics experiments.
- First Atoms Guided by Light Through a Hollow Fiber. Recently,
optical forces were used to guide atoms through a hollow glass fiber. Last
year, in collaboration with scientists in the Atomic Physics Division, the
intensity pattern of the evanescent fields of an optical mode that propagates
in an annulus about the hollow core was calculated. The laser light is tuned to
the blue side of an atomic resonance and repels atoms from the high intensity
evanescent wave along the fiber walls. In this way atoms can pass through a
long hollow fiber without sticking to the walls.
The preliminary experiment on Rb atoms was a variation of the guiding concept.
A red-detuned laser beam was coupled into the 40 µ diameter hollow core,
rather than into the annular ring. The fiber acts like an atom hose, spanning
2 cm of open air to join two vacuum systems. One chamber contains neutral
Rb at room temperature, the other a hot-wire detector in ultra-high vacuum. As
the atoms emerge from fiber into the UHV chamber, they are detected on the
hot-wire. The hot wire was located 1 cm from the opening of the fiber, in
what amounts to the "far-field" region of the emerging atoms. The
hot-wire was translated transverse to the fiber axis to map out the transverse
momentum distribution of the guided atoms. The average transverse momentum was
such that the atoms bounce from the walls an average of 12 times as they
propagate through the 3 cm fiber.
Figure 2. Flux of Rb atoms optically guided through the flexible, hollow
glass fiber, plotted versus detuning of the guiding laser field.
The guided atom flux (Figure 2) shows the expected sharp dependence on
laser frequency, turning on sharply for detunings slightly to the red of
resonance. Even without making use of any transverse cooling, the fiber could
be very useful in nanofabrication applications. Controlled by a three-axis
micromanipulator and operating in a "near-field" mode, the fiber
could deposit atoms directly onto a substrate. If the fiber is drawn down to a
very thin tip similar to those used in near-field scanning optical microscopy,
the lines of atoms "drawn" by the fiber could be suitably thin and
readily tapered in depth, an effect very hard to achieve by conventional
lithographic means.
- Absolute Optical Frequency Measurements of Stabilized Nd:YAG Laser and
Fiber Transport. The Nd:YAG laser can be efficiently doubled into the green
at 532 nm where a number of strong absorption bands of I2 are
located. Use of NIST's patented "modulation transfer" technique has
led to laser stabilization below 10-13 for times as short as
0.1 sec (Figure 3). Of fundamental importance to metrology is the
system's independent frequency reproducibility which has been shown to be
~300 Hz (5 × 10-13), nearly 50-fold better than
that of the ubiquitous HeNe/I2 stabilized "standard"
laser. A joint effort between JILA and NIST Time and Frequency Division
colleagues has recently led to the first frequency measurement for these green
stabilized laser frequencies, yielding f(a10) =
563 260 223.480 MHz ± 70 kHz. This is for the
a10 hyperfine component of the R(56) 32-0 band in
127I2. The method is based on a combination of optical
frequencies in two nonlinear optical crystals which perform the
frequency-doubling and -addition functions. The standards employed were the
HeNe/I2 red laser which was frequency doubled and compared with the
sum frequency of the Nd green and a Ti:Sapphire laser locked onto the
D2 resonance transition in Rb at 780 nm. Inaccuracy of
± 60 kHz in this latter frequency reference limited the accuracy of
the measurement at first to ± 70 kHz. By using a higher microwave
driving frequency (91 GHz) for the Schottky diode mixer, it is now
feasible to measure the stable green reference relative to a well-determined
two-photon transition at 778 nm in the same Rb atomic system. This
transition has been accurately measured (± 5 kHz) by colleagues in
Paris (and does not suffer from the "interesting" phenomena which
affect accurate realization of the Rb D2 reference), so a final
accuracy of ~20 kHz may be possible.
Figure 3. Allan variance per laser from beat between independent
frequency-doubled Nd:YAG lasers.
As a byproduct of considering means to transfer the NIST frequency standard
between the JILA building and NIST, 1 km distant, a new
optical-fiber-laser frequency transfer scheme was invented and demonstrated
which can transfer optical frequencies with milliHertz uncertainty. It was
discovered that 25 m of ordinary sheathed mono-mode polarization-maintaining
fiber can introduce a tremendous amount of noise onto an otherwise phase-stable
light beam: more than 1 kHz of spectral broadening is written onto the
light by phase-noise variations of the fiber's transmission phase. The
perturbations include acoustic noise, micro-bending induced by vibration, and
micro-thermal variations. With a frequency-offset coding at the remote end on
a weak returned beam, and by heterodyne at the source end, two units of the
fiber's insertion phase are obtained. This is used with a phase-locked
voltage-controlled crystal oscillator and digital divide-by-two circuitry to
form a real time servo system using an additional acousto-optic modulator in
the light beam, accurately "pre-cancelling" the phase noise before
the beam enters the fiber. In this way it was possible to reduce the fiber's
output bandwidth from kiloHz to the sub-milliHz level. Patent
disclosures have been submitted on this work.
- Nonlinear Optical Interactions. A recent program focuses on highly
nonlinear interactions between light and matter, leading to a variety of
dynamical effects in space and time such as self focusing and the formation of
cones of light. A major emphasis is on the propagation of intense near-resonant
laser light through atomic vapors, which provides clearcut tests of models for
optical switching and propagation mechanisms that are important for optical
computing and communications. Of particular interest is the generation of
frequencies of light close to the Rabi-sidebands which are initiated by quantum
vacuum fluctuations (spontaneous emission). Such "new" frequencies of
light are generated, for example, in the phenomenon of
"cone-emission." A second issue being studied is the spatial
character and propagation of self-focused light filaments in nonlinear media.
Experimental observations (Figure 4) are being performed on Sr vapor with
an intense, single-mode pulsed laser beam under conditions where diffraction
and self-focusing/self-defocusing are important. Under some conditions the
pulses break up into many self-focused filaments of the type associated with
self-induced transparency, and for other conditions single filaments form.
These may exhibit stable, soliton type behavior, or severe spatial
oscillations. The generation of new frequencies occurs in these
filaments.
Figure 4. Measured diameter (dots) of self-focused filaments observed
at the exit of the nonlinear medium (Sr vapor), compared to the
predictions of steady-state propagation code (solid line). These are plotted
versus the ratio of Rabi frequency (ΩF) in the center of the
filament to the detuning from resonance (Δ).
A related phenomenon is "selective reflection." This refers to the
reflection of near-resonant radiation at the interface between a dielectric
(window) and a dense atomic vapor. At relatively large detunings well outside
the Doppler envelope, this is simply the reflection at the boundary between
two dielectrics, where the vapor index of refraction is used inside the vapor.
But for detunings inside the Doppler envelope, the result is very different due
to a transient response of atoms leaving the window surface in the ground
state, i.e., the normal index of refraction results from steady state
excitation, but an atom leaving the wall suddenly experiences the radiation
field. This causes a sharp reflection feature, with the homogeneous line width,
on resonance. This feature provides a valuable tool for studying collisional
line shapes of high density vapors as well as phenomena such as atom wall
interactions and nonlinear effects such as optical pumping. This is also an
essential issue in any study of how the atomic resonance behaves as densities
are raised toward the region where electron conduction can occur.
- Optical Parametric Oscillator (OPO) Development. NIST research also
focuses on developing injection seeded OPO methods to generate Fourier
transform limited, tunable IR pulses of high spectral brilliance. The method is
based on a four mirror ring beta barium borate (BBO) resonator, pumped by a
single mode tripled Nd:YAG, and injection seeded with a single mode ring dye
laser, onto which the resonator is actively servo-loop locked. This novel
source of high resolution, visible ("signal") and IR
("idler") light is sufficiently intense (5 mJ to 10 mJ,
0.005 cm-1 bandwidth) to nearly saturate vibrational overtones
such as v=3-0 in the OH, NH, CH, and HF stretch manifolds. This light source is
currently being applied to studies of vibrationally mediated photolysis in
quantum state and size selected clusters. Specifically, the OPO is used to
excite single rovibrational states in Ar-H2O clusters, which in turn
are selectively photolyzed by 248 nm UV from the excimer. A comparison of
OH distributions from H2O and Ar-H2O reveals surprisingly
large "one atom cage" effects. For example, the OH rotational
distribution from Ar-H2O photolysis has a mean rotational energy
3-fold greater than for H2O monomer. This would be consistent with
heavy Ar atom "blocking" the light H atom recoil, and thus exerting
greater torque on the OH radical fragment.
- Active Vibration Isolation Systems. An important application to
achieve future performance goals for tools used in semiconductor device
manufacturing is the reduction of vibration due to floor motion, rapid air
movement, and internal mass displacements. Other applications being pursued are
in the automotive and aircraft industries. Most commercial applications are in
the 0.2 Hz to 20 Hz frequency range covered in Quantum Physics
Division research, although some space applications are at lower frequencies. A
six-degree-of-freedom active isolation stage, previously built and tested,
provides factors of 70 to 100 isolation from 2 Hz to 20 Hz for
vertical and horizontal motions. This preliminary stage reduces the vibration
in the building to a level consistent with a quiet site. The mechanical systems
for the two main stages, to be mounted inside a vacuum chamber on the
preliminary stage, have now been designed and constructed, and are shown in
Figure 5. The performance goal for the first main stage is to reach a
motion level of roughly 1 picometer/(Hz)0.5 at all frequencies above
1 Hz. Interferometric motion sensors with low readout noise will be used
to achieve this noise level.
Figure 5. Mechanical system for six-degree-of-freedom isolation
state.
- Simple Long Period Springs. Springs have been used to softly
support and therefore isolate apparatus since the time of Robert Hooke
(1635-1703). Yet springs have changed little since that time. In today's world,
a need exists to lengthen a supported-by-spring system's period, and thereby
its softness, without increasing the length of the isolating spring
system. Using a mechanically highly pre-tensioned simple spring assembly, a
3 sec system period was achieved with a "spring length" of only
20 cm. (A simple extension spring would have to be stretched over
200 cm in order to achieve this same period.) By adding a horizontally
stretched torque-cancelling spring, a 5sec period was achieved using one of the
six "coils" of the pretensioned simple spring. New methods are being
investigated to achieve even longer periods in various spring configurations
which would be suitable for mechanically isolating various types of apparatus
and commercial devices such as recording disks. This development will also
permit improved vertical mechanical isolation within the constraints often
imposed by room size or enclosing vacuum system size. These springs, although
they continue to obey Hooke's law, are contrived to achieve long periods within
the constraint of small extended size.
- The Approach to Solvent Dynamics. The high sensitivity and
resolution of near-IR slit jet absorption methods have allowed unique
spectroscopic access to clusters containing multiple rare gas atoms, such as
Arn-HF, Arn-DF and Arn-CO2, with n
as large as 4. Technological interest in these prototypic systems arises from
the need to understand sequential "solvation" of chromophores by
"solute" atoms, and to test trial potential surface predictions of
structures, solvation induced red shifts, and intermolecular
"van der Waals" energy levels. Furthermore, by comparison with
pairwise additive predictions, these studies provide a powerful probe of
non-pairwise additive (so called "multibody") contributions to the
full potential surface. Such multibody effects are thought to play an important
role in the structure of solids and liquids, but are extremely difficult to
extract from the much stronger, pairwise additive interactions unless the
latter is already well determined. Fortunately, the Ar-HF/DF and Ar-Ar pair
potentials are already known to high accuracy from previous high resolution
spectroscopy, and thus comparison of full quantum mechanical pairwise
predictions with experimental observation in Arn-HF/DF cluster
systems offers a special opportunity for measuring the three body contributions
quantitatively.
The most demanding arena for such comparisons is for the low frequency van
der Waals modes in trimer systems such as Ar2-HF and
Ar2-DF, which have recently been observed both for the in-plane and
out-of-plane "bends" of the HF or DF substituent. These studies
reveal systematic deviations between full quantum close coupled calculations
from the pairwise additive potentials and experimental observation. These
results indicate the presence of surprisingly large repulsive three body terms
in the angular potential which support large amplitude motion of the HF/DF
subunit. From a theoretical analysis by collaborators, these three body terms
are due to the repulsive interaction of the HF with the so called
"exchange quadrupoles" that develop in Ar-Ar from electron cloud
overlap. Interestingly, these three body exchange effects in
Ar2-HF/DF have been shown to be nearly an order of magnitude larger
than the Axilrod-Teller forces conventionally considered as the predominant
multibody term.
- Quantum-State-Resolved Studies in Supersonic Jets. A thrust in the
recent year has been toward developing quantum state resolved tools for
studying UV photophysics and photochemistry in supersonic jets. A UV pump/LIF
probe apparatus has been constructed based on a slit jet and high efficiency
(30%) cylindrical light collection system. The initial system for study has
been 193 nm photolysis of H2O jet cooled into its lowest
000 (para) and 101 (ortho) rotational levels. The 193 nm
photolysis is deliberately chosen to be far "red" of the lowest
singlet excited state absorption band (at 165 nm), so as to sample
Franck-Condon excitation from the extreme wings of the ground state
vibrational wave function. As a result of large differential zero point effects
in asymmetric HOD species, full quantum calculations have theoretically
predicted >100 fold preference for HO versus OD cleavage. In dramatic
contrast to these predictions, the studies of HOD photolysis indicate only a
6-fold preference at 193 nm for cleavage of the OH versus OD bond. These
surprising results have stimulated several theoretical groups toward a
reexamination of this prototypic (and presumed "understood")
photolysis system, now including, for example, possible contributions
due to the underlying triplet surface in the photolysis event.
- Between a Star and a Planet. For many years astronomers have sought
to determine the mass that distinguishes small self-luminous stars from massive
planets such as Jupiter. The former radiate into space using the energy from
hydrogen fusion reactions in their cores, whereas the latter never reach
hydrogen ignition in their degenerate interiors but rather radiate the energy
released by their slow gravitational contraction. Using the Goddard High
Resolution Spectrograph on the Hubble Space Telescope (HST), ultraviolet
spectra have been obtained of the star VB10, thought to be one of the very
lowest mass stars, with a mass of about 9% that of the Sun. This very faint
star shows highly variable ultraviolet emission indicative of flaring, a
property known to indicate the presence of strong magnetic fields. This new
discovery is important because stars with this low mass are thought to be
fully convective rather than having radiative cores, and thus would not have
strong magnetic fields. The usual dynamo mechanism that is thought to amplify
magnetic fields in the interior of the Sun and other stars cannot operate in
the lowest mass stars and theoreticians must, therefore, develop new concepts
to explain the magnetic fields in such low mass stars.
- Measuring Electron Densities and the Rate of Mass Loss in Stars. The
HST has been used to obtain ultraviolet spectra of the bright star Capella with
extremely high spectral resolution and signal/noise. These data are being used
to model the plasmas in the outer atmosphere of the star at temperatures of
104 - 2 × 105 K, but they will also be useful
in determining the accuracy of some important atomic physics parameters. The
spectra contain spin-forbidden (intersystem) lines of Si III, C III,
O III, O IV, O V, and S IV formed at temperatures of
3 × 104 -2 × 105 K. Many of these
lines have never before been observed in the spectrum of any star other than
the Sun. These lines are important because their line ratios can be used to
quantify the electron density of the plasma, and all of the line ratios should
yield consistent densities. Intercomparison of these ratios will identify which
ones may be predicting spurious densities, which will identify which
collisional and radiative rates are likely in error.
The rate at which stars lose material is an important factor in determining how
stars age and how much chemically enriched material will be available in space
from which new stars can emerge. While stellar mass loss through high speed
winds is known to exist, there are few accurate measurements of the rate at
which this occurs for most stars. This information is obtained by modelling
high-resolution spectra of the Mg II resonance lines (near 280 nm)
observed with the HST in the spectrum of the star αTrA. These spectra are
of great interest because the gas in the stellar wind is seen as absorption in
the blue wings of these lines and re-emission in the red wings. Detailed
modelling of the absorption and re-emission of photons in the stellar wind has
led to accurate estimates of the rate in which this star is losing matter and
the speed of the wind.
- Atomic Collisions Data Center. The Data Center received notification
in August 1994 that Standard Reference Data (SRD) would cease funding the work
at JILA halfway through FY-95. Good progress was made, despite the disruption,
on the two Alignment and Orientation reviews. After a visit by authors
N. Andersen and K. Bartschat the first week of January, 1995,
Vol. III on Spin-dependent Effects was completed. In the remaining weeks
of January, the voluminous graphs for Vol. II will be put in final shape
and shipped to the authors for completion of the final sections of the
manuscript. After four years of development, the Atomic Collisions Database has
doubled in size and is accessible through a sophisticated graphical user
interface, allowing bulk and interactive data loading and publication-quality
tabular and graphical output. Although SRD has expressed no interest in the
database with this interface, Oak Ridge, Sandia and Livermore National Labs
have. An agreement has been reached to transfer the database to the Atomic Data
for Fusion Data Center at Oak Ridge and to Livermore. The Data Center director
spent a week in December installing the INGRES database management system at
Oak Ridge and assisting the Fusion Data Center in building their bibliographic
database. No word was received from SRD on the disposition of the pc-based Gas
Laser Database developed at JILA's Data Center and delivered in the Summer of
1993, suggesting that legal obstacles to an agreement between CRC Publishing
and SRD were not surmountable.
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