1995/1996 Technical Highlights
- Calibrations and Instrumentation Development at the NIST/ARPA National
EUV Reflectometry Facility. The NIST/ARPA National EUV Reflectometry
Facility at SURF II, the only such facility in the U.S. open to all
members of the EUV community, entered its fifth year of operation. Over 180
calibrations were performed in 1995-1996 on a variety of mirrors, gratings,
photocathodes, and photographic emulsions for collaborators in industry,
national laboratories, and universities. Fabrication of a new reflectometer
is well underway and should be completed this summer. The new reflectometer,
which we expect to install in 1997, will be able to accommodate optics up to
35 cm in diameter and 40 kg in mass. This capability, which will be
unique, is necessary for the characterization of large optical components
required for EUV projection systems. (T.B. Lucatorto, C.S. Tarrio,
and R.N. Watts)
- First Demonstration of Figuring by Multilayer Deposition. As part of
our continuing effort to support the semiconductor industry's interest in EUV
lithography as a possible technology for future generation devices, we have
begun studies to determine the limits of using multilayer deposition as a
post-polishing technique for the fabrication of precision EUV optics. Thus far,
traditional polishing has not been able to simultaneously produce optics with
the figure and finish requirements for such advanced applications. Our
previous experience with making EUV multilayer mirrors has given us confidence
that the multilayer technique has the potential to make large changes in
surfaces (such as aspheric corrections to high-quality spheres) with virtually
no addition of roughness. During this period we have demonstrated the method
for rotationally symmetric errors. We are currently investigating the limits of two-dimensional
corrections. (C. Tarrio, E. Spiller, T.B. Lucatorto)
Figure 1. Figuring by deposition: a
radially symmetric error was polished into a glass test piece. The solid line
is the error measured using phase-measuring interferometry. A mask was
constructed from this, and a Co-C multilayer was deposited through the
stationary mask onto the rotating sample. The corrected figure (dashed line)
shows almost a factor of ten improvement.
- Extreme Ultraviolet Microscope Project. In
collaboration with H. Milchberg of the University
of Maryland, we have recently begun an effort to
construct an extreme ultraviolet Schwarzschild
microscope with the eventual goal of imaging live
biological specimens. The ultimate success of the
NSF-sponsored program rests on two unique
capabilities: the correction and aspherization of
the optical figures (see above) to unprecedented
accuracy, and the generation of high-intensity,
ultra-short EUV radiation using high-harmonic
generation. The optical design for a first-generation
prototype using spherical optics at a
wavelength of 13 nm has been completed and
construction is underway. The final microscope is
anticipated to use at least one aspherical surface
and work in the water window (2.5 nm to 4.4 nm).
(C. Tarrio, E. Spiller, and T.B. Lucatorto)
- SURF III Upgrade. The present SURF II facility,
built in 1974, is soon to undergo a major overhaul
designed to improve its performance for
radiometry and other scientific applications. In
1994, a contract was let with the University of
Wisconsin (UW), which built SURF II, to study the
available options leading to improved performance.
Following that report, delivered in July
1995, a second contract was let with UW to
provide a detailed design for the selected
approach. That report was delivered in May 1996,
and a construction contract with UW was let on
September 27, 1996.
The most important aspect of the SURF III
concept is improved radiometric accuracy. The
magnitude and angular distribution of the flux
radiated by SURF III will be much more accurately
characterized through a better knowledge of
the electron energy and trajectory in the storage
ring and improved methods of determining the
electron current. The improved accuracy of the
electron orbit will be achieved by more stringent
mechanical tolerances on the magnet, better steel,
and optimized magnet design. Thus the SURF II
magnet, iron and coils, will be replaced, and the
control system will be overhauled. As a result of
the improved iron in the magnet, a significant
gain in field strength will also be realized,
allowing the maximum achievable energy of the
electrons in the storage ring to be increased from
the 300 MeV for SURF II to close to 400 MeV for
SURF III. This will allow radiometry and scientific
applications to be carried out down through the
water window to a wavelength of about 2.5 nm
(~500 eV). Changes to the storage ring itself are
planned to increase the number of beamlines
available by two, and to increase the solid angle of
collection of the infrared beamline to 90 µrad.
This will be an important enhancement for this
new application of SURF III, a source which will
be competitive with the best in the world in the infrared spectral region.
The assembly of the SURF III magnet system is scheduled to begin in
October 1997, with commissioning complete on April 1, 1998. If this
schedule holds, we will cease normal operations at SURF II on about
July 1, 1997 to begin dismantling the SURF II magnet system and
prepare the site for the new one. (A. Hamilton, L. Hughey, and
R. Madden)
Figure 2. Electroreflectance
spectrum of cytochrome-c immobilized on an evaporated
gold electrode modified with N-acetyl cysteine,
as obtained on the UV beamline at SURF II.
Signal is relative change in reflectance in units
of 10-6, so that the maximum change exhibited
here is about 1.5 × 10-4. Solid line shows expected spectrum.
- Protein Electroreflectance Studies at SURF II.
Adolfas Gaigalas of the Biotechnology Division
has set up an apparatus on beamline 5 at SURF II
for the measurement of electroreflectance from
metal surfaces with adsorbed proteins. This
technique requires the measurement of the
change in reflectance induced by a sinusoidally
varying applied potential. The bare metal surface
has a characteristic reflectance signature in the
250 to 400 nm spectral region due to electron
plasma oscillations. However, adsorbed species
can alter the signal dramatically. This is especially
true if the adsorbed molecule is a protein
with a metal site which can be reduced and
oxidized by electrons from the electrode. The
photon absorption tends to be very different in the
two redox states leading to a large reflection
modulation amplitude, which can be used to
determine electron transfer rates between certain
metaloproteins and electrodes. Adolfas is finding
that synchrotron radiation from SURF II is highly
stable, continuously tunable, and gives him
greatly improved signal-to-noise over other
available laboratory sources. He observes intrinsic
intensity noise at a level of 10-5 in the 10 Hz to
100 Hz region, of paramount importance for
observing electroreflectance modulations of
typical magnitude 1 part in 104. These are results
obtained in his first experiment, performed in
September/October 1996, on cytochrome-c
immobilized on an evaporated gold electrode. (R. Madden)
- Activity at the SURF Spectrometer Calibration
Facility. During 1995 and 1996 there were 26
instruments calibrated by seven user Groups at the
Spectrometer Calibration Facility at SURF II.
Users of the facility included Lawrence Livermore
National Laboratory, Naval Research Laboratory,
National Institute of Standards and Technology,
University of Southern California Space Sciences
Center, NASA Goddard Space Flight Center, and
the National Center for Atmospheric Research
High Altitude Observatory (NCAR/HAO).
Lawrence Livermore researchers calibrated
their SPRED UV spectrometer with multiple
gratings in the wavelength range 10 nm to 200 nm.
The instrument is used to characterize impurities
and impurity transport in fusion experiments at
the DIII-D tokamak at General Atomics in San Diego.
NCAR/HAO scientists calibrated a number of
detectors for two NASA experiments. The XUV
Imaging of the Solar Corona experiment will
provide helium and hydrogen abundances in the
solar corona. The TIMED/Rocket Solar EUV
experiment will explore the energy balance of the
upper atmosphere above 60 km. (M. Furst and R. Graves)
- Activity in Transfer Standard Detector Calibrations.
New detection devices suitable for standards use in the far ultraviolet
are being explored. Filter radiometer photodiodes useful in solar
physics and in plasma diagnostics, for example, have been extensively
characterized in collaboration with industry and academia. A new
form of semiconductor detector, a platinum
silicide Schottky barrier silicon photodiode, has
been studied and looks very promising. Silicon
photodiodes with hardened oxide have been
developed in collaboration with industry, and are
now routinely issued as NIST calibrated transfer
standard detectors for the far ultraviolet, from 5 nm to 250 nm.
Special calibrations of filter radiometer
instruments designed to monitor the He II
30.4 nm emission line were made at the SURF II
facility in collaboration with industry, academia,
and NOAA. A primary flight instrument is
successfully monitoring the solar irradiance
aboard the SOHO international spacecraft,
launched in 1995. Several similar underflight
instruments have also been characterized.
Sixty-three calibrations of transfer standard
detectors were performed during 1995-1996 for
applications in astronomy, aeronomy, solar
physics, and plasma diagnostics. A number of
special radiation filters were also characterized in
research collaborations. (L.R. Canfield and R. Vest)
- Chemical Identification of Alloys on the Atomic
Scale. As part of our research on magnetic
multilayers, we have identified alloying that
occurs in the Cr/Fe(001) system with atomic scale
resolution using scanning tunneling microscopy.
Our work on magnetic multilayers is motivated by
the fact that these systems exhibit phenomena of
exchange coupling and giant magnetoresistance
that have technological application in areas such
as magnetic recording and non-volatile memory
storage. Research and development in this area
has proven challenging because magnetic
properties are strongly influenced by structural
details which can be difficult to characterize and
control. In this regard, much progress has been
made by studying epitaxial Fe/Cr/Fe structures
where growth can be controlled to a large extent;
however, some of the magnetic properties of this
system have remained anomalous. The alloying
which we have identified at the Cr/Fe interface
may in part be the cause of these anomalies.
Scanning tunneling microscopy measurements
after submonolayer deposition of Cr on
Fe(001) at 300 °C show the formation of single
atomic step islands on the surface (Fig. 3). If
alloying did not occur, the islands would be pure
Cr on top of the Fe(001) substrate. A high resolution
image of the surface showing the substrate
(central region in Fig. 3b) and island levels
(regions surrounded by a thick black line in
Fig. 3b) indicates that both levels are not chemically
uniform but are an Fe/Cr alloy. The white
dots are the individual alloyed Cr atoms surrounded by
Fe. The imaging contrast is due to the
electronic difference between the Fe and the Cr
and leads to the perceived small height contrast
between the elements in the STM image (Fig. 3c).
(A. Davies, J.A. Stroscio, D.T. Pierce, and R. Celotta)
Figure 3.STM images of Cr growth on Fe(001).
- Reflection in Magnetic Multilayers.
In magnetic multilayers, electrons in one material can
reflect from interfaces between the two materials. This
reflection contributes to two important effects,
oscillatory exchange coupling and giant
magnetoresistance. Exchange coupling between
the magnetizations of magnetic layers is the
coupling that is mediated by the electrons in a
non-magnetic spacer layer which separates them.
For some systems it oscillates in sign as a
function of the spacer layer thickness and for
particular thicknesses gives antiparallel alignment
of the magnetizations in neighboring
magnetic layers when there is no applied field.
The giant magnetoresistance is the change in
resistance when the relative orientation of the
magnetizations is switched by applying a magnetic
field. Devices based on the giant magnetoresistance
effect have been proposed as magnetic
field sensors and read heads in magnetic disk storage.
To better understand such systems, we have
calculated reflection probabilities for a series of
noble metal spacer layers and lattice-matched
ferromagnetic layers from first principles. These
calculations show strong spin dependence for all
systems considered. The strong spin-dependence
results because the bands of the spacer layer
match well with the majority bands of the ferromagnetic
layers, but poorly with the minority
bands. The calculated reflection probabilities lead
to predictions of the coupling strength that will
be measured in these systems as the quality of
growth continues to improve. The predictions are
much larger than measured values, but the
measured strengths continue to increase as better
experiments are done. These results also suggest
that the contribution to the giant magnetoresistance
from a process called channeling
can be quite substantial, particularly in the
Fe/Au(100) and Fe/Ag(100) systems. (M.D. Stiles)
- Magnetic Hysteresis in Ultrathin Films. The
performance of devices based on ultrathin magnetic
films depends on the films' coercivity, i.e.,
the field required to reverse the magnetization. In
most magnetic systems, defects reduce the
coercivity below the value predicted by simple
models based on uniform rotation of the magnetization.
Understanding the effect of defects on
coercivity will lead to the ability to predict and
control the magnetic behavior of ultrathin films.
Figure 4. A typical configuration of spins
in an ultrathin film in remanence. The square in the center is a magnetic
island on the film. Due to the anisotropy at the step edge, the
magnetization is starting to reverse.
In collaboration with scientists at the Georgia
Institute of Technology, we have theoretically
modelled magnetic hysteresis in ultrathin films.
We have shown that for ultrathin films, defects as
small as single atomic steps can determine the
coercivity. Even the best ultrathin films have step
edges associated either with the perimeter of
monolayer-height islands that nucleate during
growth or with the steps of an unavoidably miscut
crystal substrate. Because the steps have reduced
crystallographic symmetry, the magnetic
anisotropy at steps can be large compared to the
intrinsic anisotropy of the flat surface. This large,
local anisotropy leads to non-uniform
magnetization reversal. In particular, rotated
domains, which are nucleated at the step edges,
start the reversal at fields much lower than the
field required for uniform reversal. Situations
where steps control the magnetization reversal
have the feature that the properties of the steps
can be readily measured. In these situations, it
will be possible to make a stringent comparison of
theory and experiment. (M.D. Stiles)
- SEMPA Observation of Large Magnetic Domains in Magnetoresistive Granular
Metals. Using the new high resolution SEMPA facility, Electron Physics
Group researchers showed that large (100 nm) magnetic domains exist in
cobalt-silver granular metals (Fig. 5). Few researchers anticipated large
magnetic domains in granular Co-Ag: the microstructure of these materials was
thought to limit the magnetic domains to sizes comparable with the particle
size (less than 10 nm). The presence of large domains is particularly
noteworthy because these materials exhibit the giant magnetoresistance effect
(GMR), which has many potential applications. The presence of large domains
implies that a significant fraction of the cobalt in these materials does not
contribute to the giant magnetoresistance.
Figure 5. SEMPA image of magnetic domain structure in
Co0.35Ag0.65. Typical dimension of magnetic domain is
300 nm to 600 nm.
In collaboration with researchers at The Johns Hopkins University, members of
the Electron Physics Group have investigated the
composition and fabrication parameters that lead
to the presence of large domains, and have
suggested two alternate models for their origin.
The domains may represent correlations among
large numbers of isolated cobalt particles, or they
may be due to residual cobalt in the silver matrix.
A report of this work was published in Applied
Physics Letters. (M.H. Kelley and A. Gavrin)
- Laser-Focused Deposition of Chromium
"Nanodots." Building on our earlier work on
making chromium "nanoline" by focusing atoms
in a laser standing wave, we have succeeded in
making an array of chromium "nanodots" on a
silicon surface. While the earlier experiments
used a single standing wave grazing across the
surface of a silicon wafer to make a one-dimensional
pattern, the new work employs two laser standing-waves at 90° to each
other. At the intersection of the two beams a two-dimensional
optical standing wave is created, whose nodes act
as atom-optical lenses for chromium atoms being
evaporated onto the surface. The atoms are
concentrated at the nodes, making an array of
dots on the surface that is essentially a "contact
print" of the optical wave. The dots, shown in an
atomic force microscope image in Fig. 6, are
approximately 80 nm wide and 13 nm high, and
are spaced on a square lattice at exactly
212.78 nm, as fixed by the laser wavelength.
Figure 6. Atomic force microscope image of chromium nanodots formed
by laser-focused atomic deposition.
The research represents another step in the
development of a wide range of extensions and
applications of nanostructure fabrication by laser
focusing of atoms. A major advantage that this
technique has over other methods such as electron
beam lithography is the efficient, parallel
nature of the fabrication an entire square
millimeter can be patterned in about 10 min. In
addition, theoretical calculations show that the
ultimate feature size could be as small as 10 nm
or less. Eventual applications may include the
fabrication of nanostructured materials or devices
for microelectronics or micromagnetics, and the
fabrication of length standards on a microscopic
scale. (R. Gupta, J. McClelland, and R. Celotta).
Figure 7. Computed excitation frequencies for the three lowest
modes of the JILA 87Rb BEC vs. number of condensate atoms;
experimental results displayed as points. These are the frequencies of the free
oscillations of the BEC that can be induced by modulation of the confining
potential.
- Quantitative Modelling of Atomic Bose-Einstein Condensates.
A new theoretical program for modelling the properties of zero-temperature,
dilute atomic Bose-Einstein condensates (BECs) was initiated in the autumn
of 1994, in collaboration with Groups at Georgia Southern University and
Oxford University. Its initial focus was on developing practical methods
for solving the nonlinear Schrödinger equation (NLSE) that describes the
properties of a condensate in the mean-field approximation, and its scope has
expanded to treat time- and temperature-dependent phenomena. Codes were
developed to solve the NLSE for systems of up to a million atoms confined in
the magnetic traps of experimental interest. Experiments at JILA first reported
the first observation of BEC in the summer of 1995, and within the following
year, the first experimental investigations of specific BEC properties had
begun. The most detailed comparison of this theory with experimental data is
displayed in Fig. 7, which shows the excitation spectrum of an
87Rb condensate in the JILA trap. Our mean-field calculations
predicted a maximum attainable condensate size of ~1500 atoms for the case of
7Li, which appears to have been confirmed by subsequent measurements
made at Rice University. Current effort is directed at solving mean-field
theory at finite temperature and describing time-dependent BEC evolution, to
provide general tools for modelling the "atom laser."
(M. Brewczyk, K. Burnett, C.W. Clark, R.J. Dodd,
M. Edwards, W.P. Reinhardt, and K. Rzazewski)
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