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 170
calibrations were performed in 1994 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 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)
Figure 1. A 25 µ period copper TEM grid illuminated by 13 nm
radiation and imaged using the nanodetector. The magnification is about 370X
and the preliminary resolution is about 0.5 µ. The bright spot is a tear
in the 20 nm thick carbon film coating the grid and the vertical lines are
an artifact due to the CCD frame grabber used to capture the
image.
- First Images from the Nanodetector. The "nanodetector," a
unique EUV/X-ray microscope for high resolution imaging, has produced its first
images. It is a conversion microscope which converts an EUV or X-ray image into
a photoelectron image which is then magnified by a low-energy electron
microscope. We have designed this device for several possible uses. In EUV
lithography it could be used to test and align the projector optics and for
mask inspection. In x-ray (proximity) lithography it could be used for mask
inspection. In a more conventional microscope configuration, it could be used
to image biological structures, to provide elementally resolved images of
microstructured composites, and to provide mapping of magnetic domains.
(R.N. Watts, C.S. Tarrio, and T.B. Lucatorto)
- Fabrication of Ultraprecise Reference Surfaces by Thin Film
Deposition. The technology developed for the fabrication of EUV multilayer
mirrors includes the capability of depositing multilayer stacks up to a micron
thick without degrading the smoothness of the surface from that of the
substrate. Present reference surfaces, which are fabricated by mechanical
polishing, cannot be made reliably ultraprecise and ultrasmooth simultaneously.
We plan to fabricate ultraprecise reference surfaces by first taking
state-of-the-art smoothness surfaces and mapping their figure. The figure maps
will then be used to create free standing 1-to-1 masks that correlate
transmission through the mask with deviation of the surface from the geometric
ideal. (High spots, less transmission and vice versa.) For this program we have
established a cooperative research agreement with IBM and Eberhard Spiller, the
father of EUV multilayer optics, in which Spiller's entire thin film deposition
apparatus has been donated to NIST and Spiller will act as collaborator and
consultant. The initial phase of the program will concentrate on empirically
tailoring our multilayer deposition techniques to optimize their application to
the fabrication of geometrically "perfect" surfaces. Subsequently,
our goals will turn towards more fundamental studies of growth formation for
amorphous and microcrystalline thin films. (C.S. Tarrio, R.N. Watts,
and T.B. Lucatorto)
- Electronic and Chemical Structure of Metal-Semiconductor Interfaces.
In collaboration with a group at the Forschungszentrum Juelich (Germany), we
have begun an application of ab initio quantum molecular dynamics
to study the growth of boron films on silicon substrates, and vice versa, with
particular attention to the dependence upon growth conditions of chemical
bonding at the interface, interface sharpness, and interlayer stress. These
are critical factors in the ultimate performance of B/Si multilayer optics now
being developed for extreme ultraviolet imaging.
At present we are concerned specifically with the interaction of
B clusters with the surface of amorphous Si. The electrons are treated
within the framework of density-functional theory, with a pseudopotential
approximation for the valence-core interaction and a plane-wave orbital basis.
A bulk amorphous Si sample has been prepared by heating and annealing a 64-atom
system with periodic boundary conditions; an amorphous Si surface is then made
by doubling the size of the supercell in one direction, and annealing. A study
of the "reconstruction" of this surface is in progress.
The structure of B clusters has been investigated by the same approach,
and comparison made with all-electron calculations and mass-spectrometric
measurements. In agreement with recent Japanese work we find that the
B12 icosahedral structure is metastable; in addition we find that
it can withstand heating up to temperatures of about 8,000 K.
(C.W. Clark)
- Calibration Services and Device Development for Far-Ultraviolet
Radiometry. Absolute calibrations of NIST working standards for the far UV
were carried out using the new dual-grating monochromator system on the
SURF II instrument calibration beamline. These calibrations utilize the
calculable flux from the SURF II electron storage ring as the absolute
standard, and form the basis for NIST detector radiometry in the 116 nm to
254 nm spectral region. In addition to lending further confidence to the
NIST far UV detector calibration base, a problem was identified and corrected
concerning the filter material originally procured for use in this system.
Several NIST standards were calibrated during these sessions.
A collaborative effort with industry, described in Appendix F, has led to
major advances in far UV silicon photodiode technology.
Thirty-one calibrations of transfer standard detectors were performed in FY94,
for applications in aeronomy, plasma diagnostics, solar physics, and astronomy.
In addition to these calibrations, a number of special-purpose detectors and
filters were characterized as research collaborations. An extreme ultraviolet
solar monitor planned for a 1995 launch aboard the SOHO international satellite
was characterized at the SURF II detector calibration facility, both on
the instrument calibration beamline, and on the detector calibration beamline
(L.R. Canfield and R.E. Vest).
- The Spectrometer Calibration Facility at SURF II. During 1994
there were 16 instruments calibrated by 9 user groups at the
Spectrometer Calibration Facility at SURF II. Users of the facility
included 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. The University of Southern California used three
different SURF II facilities to calibrate a unique instrument for the
upcoming Solar Heliospheric Observatory (SOHO) mission. The Solar EUV Monitor
consists of a high density (500 lines/nm) gold transmission grating and
three XUV photodiodes. The instrument will provide on-board cross calibration
for other spectrometers aboard the spacecraft by providing spectral
measurements of the singly-charged helium ion transition at 30.4 nm with
an 8 nm bandpass. Characterization of the transmission grating was
performed on the Soft X-Ray Reflectometry Beamline, the relative spectral
response of the entire instrument was measured on the XUV Diode Calibration
Beamline and the absolute wideband spectral response of the entire instrument
was measured on the Spectrometer Calibration Beamline. (M. Furst,
R. Graves, and R.P. Madden)
- The NIST Parallel Applications Development Environment. Efforts on
developing scalable algorithms for electronic structure problems began in 1994
with support from the NIST High Performance Computing and Communications
initiative. This work takes place largely in an environment of networked
workstations of different types, which are joined together to form a parallel
"virtual machine." Interest in this sort of computational parallelism
is growing rapidly due to the development of "message-passing
library" software, such as the PVM program produced at the Oak Ridge
National Laboratory, which enables independent Unix workstations to act as a
virtual machine, communicating with each other using standard network
protocols. However, developing robust code for such a virtual machine is still
somewhat of a pioneering exercise. In a typical application, one must maintain
independent sets of files on each machine; perform separate procedures for
compilation, execution and debugging; and manage input and output streams and
communications between processors. In collaboration with staff of the Computing
and Applied Mathematics Laboratory, we have developed a program, the Parallel
Applications Development Environment
(PADE) (Figure 2, is now obsolete) which provides a graphical console
for the parallel virtual machine. PADE centralizes the management of files and
allows all actions required of remote processors to be handled by simple
console commands. A "tree view" representation of the virtual
machine, which exhibits the processors and their file systems in a
two-dimensional map, enables files to be duplicated or moved between processors
by familiar "click and drag" actions.
(M. Edwards, J. Turner, and C.W. Clark).
- Laser-focused Deposition of Chromium Nanostructures. In an ongoing
effort to explore the application of atom optics to nanotechnology, we have
been examining a process which uses an optical standing wave generated with a
laser to focus chromium atoms as they deposit onto a surface. Each node of the
standing wave acts as a lens for the atoms, focusing them into lines or dots
with nanometer-scale dimensions spaced by half the optical wavelength, or
213 nm.
Having achieved the first demonstration of the growth of Cr nanostructures last
year, we have been concentrating recently on improving our understanding of the
atom-optical properties of the standing wave laser field and increasing the
resolution of the process. A major step forward in our understanding of the
process has arisen out of our recent analysis of the focusing in terms of a
particle-optics analogue. We have arrived at a paraxial solution to the
equation of motion of the atoms in the standing-wave node, which has allowed us
to view the focusing process in a completely optical sense, with well-defined
focal lengths, principal planes and aberrations. The result is an ability to
quickly make predictions on the best way to improve the focusing process.
To test our new understanding in the laboratory, we have carried out some
depositions with a more tightly-focused laser beam. The paraxial theory
predicts that this is the simplest way to improve the resolution of the
focusing process. Figure 3 shows the results of this experiment. We see
that the linewidth of the deposited chromium lines is now 46 nm, a
significant reduction from its earlier value of 65 nm. In addition, the
region between the lines is significantly flatter than it was in previous
depositions.
Figure 3. Atomic force microscope image of narrow chromium lines
deposited by laser-focused atomic deposition.
Work on the focused deposition of chromium continues in the new fiscal year,
with anticipated progress in depositing even narrower lines, forming
two-dimensional structures, and exploring other expansions of the technique.
(R. Gupta, Z. Jabbour, J. McClelland, and
R. Celotta)
- SEMPA Measurements of Magnetic Exchange Coupling Through V, Al, and Cu
Thin Films. The magnetic exchange coupling between ferromagnetic films
separated by nonmagnetic spacer layers is of intense technological interest,
because of the potential use of exchange coupled multilayers for magnetic
recording heads and as magnetic field sensors. Researchers in the Electron
Physics Group have studied the exchange coupling by using SEMPA to image the
multilayer magnetization. The SEMPA measurements were combined with in situ
scanning electron microscope and reflection high-energy electron diffraction
measurements in order to correlate the magnetic exchange coupling properties
with the film's structural properties.
In the current work, the exchange coupling between Fe layers separated by V,
Al, and Cu spacer films has been investigated. These films differ markedly from
the previously studied Ag, Au, and Cr interlayer films which grew epitaxially,
one monolayer at a time, on the Fe(100) substrate. In general, the growth of V,
Al or Cu begins in a layer-by-layer fashion, but after a few layers the film
growth becomes rough and three dimensional. In the smooth films the exchange
coupling oscillates between ferromagnetic and antiferromagnetic as the spacer
thickness varies, but the roughening dramatically changes the nature of the
oscillatory coupling. For example, in the case of Al, the coupling through the
rough film does not oscillate and is orthogonal to the
ferromagnetic/antiferromagnetic direction. SEMPA was also used to investigate
the influence of roughness induced pin-hole defects on the exchange coupling.
This work should be useful in understanding the exchange coupling in the
rougher multilayers more commonly used for practical devices. (J. Unguris,
D.T. Pierce, and R.J. Celotta)
- Statistical Properties of Islands during Thin Film Growth.
Ultra-thin metal films have become increasingly important in today's smaller
devices. To improve performance of thin films, scientists in the Electron
Physics Group studied the process of film growth from the earliest stages, when
single atoms come together to form islands to the later stages of thin film
formation. In the growth process, atoms that land on the substrate undergo a
two-dimensional random walk. Pairs of migrating atoms collide randomly over the
surface and may bind to form dimers. The dimers may or may not dissociate
thermally before other atoms diffuse to join them. The size at which the island
forms a stable base for growth is called the critical size. Since the diffusion
of the deposited atoms depends on temperature, the critical size will vary with
temperature. The diffusion rate can vary orders of magnitude with a change in
temperature of only 100 °C. The number and size of the islands that form
during growth therefore also depend strongly on growth temperature. We
discovered that an analysis of the size distributions of the growing islands
show self-similar properties in that different size distributions can be
rescaled to fall on a single universal curve, which is independent of all
material properties, except for the critical island size. The island size
distributions show self-similar properties as the atom diffusion rate is
increased over four orders of magnitude by changing the growth temperature by
250 °C. This property is similar to the scaling of coastlines and other
fractal objects which show similar shapes and outlines independent of the
magnification (see Figure 4). The critical nucleus size of the islands at
a given growth temperature can be extracted from a comparison of the measured
universal scaling curve with theoretical models. These measurements are the
first to confirm theoretical predictions of scaling in this growth regime. This
work allows a test of the statistical growth theories which promise a more
general and powerful description of thin film fabrication. (J.A. Stroscio
and D.T. Pierce)
Figure 4. STM images, 100 × 100 nm, of single layer Fe
islands (white) on the Fe(001) surface (black). Sample temperatures during
growth are (a) 20 °C, (b) 108 °C, (c) 163 °C,
(d) 256 °C, (e) 301 °C, and
(f) 356 °C.
- Roughness and Pattern Formation in Thin Film Growth. Controlling
roughness in thin film growth is a major technological challenge as present and
future technological specifications call for films with thickness fluctuations
as small as a single atomic layer. Understanding the origin of roughness in
film growth has been a stimulating theoretical challenge as well. Two schools
of thought have developed to explain the origin of roughness in film growth.
One theory describes roughness as originating from the random fluctuations in
the impinging flux leading to self-affine surfaces that exhibit dynamic
scaling. The second theoretical effort has focused on the importance of
microscopic processes leading to roughness, such as energy barriers to the
diffusing atoms in going down step edges. Scientists in the Electron Physics
Group have recently shown experimentally and theoretically that roughness in
the growth of Fe films on Fe(001) whiskers is controlled by the second
mechanism, i.e., step edge barriers for atom diffusion, which leads to pattern
formation in the thin film (Figure 5(a)). The pattern spacing is initially
set by the initial nucleation of islands (see above). Subsequent atoms that
land on the islands recoil from the energy barriers at the step edge leading to
a greater probability of nucleation of new daughter islands on the incomplete
parent islands. Repeated application of this scenario leads to a wedding-cake
or mound structure in the film as shown by the experimental measurements
(Figure 5(a)). To gain a full theoretical understanding of the growth
process, we simulated the film growth with a continuum approach. The
simulations produced excellent agreement with the properties of the measured
films (Figure 5(b-c)) shedding light on important processes in ultra-thin
film growth. (J.A. Stroscio, D.T. Pierce, and M.D. Stiles)
- Giant Magnetoresistance. Giant magnetoresistance in multilayers has
been the subject of intense theoretical study, but significant questions still
remain. The role of spin dependent scattering at interfaces has been a
particular problem. Utilizing a semiclassical Boltzman approach previously
introduced and improved by various workers, we have added surface roughness in
a more realistic manner than has been done previously. We included varying
interfacial geometric roughness with no lateral coherence, correlated
quasiperiodic roughness, and varying chemical composition of the surface. The
interplay between these three aspects of the interfaces was found to enhance or
suppress the magnetoresistance depending on whether it decreased the asymmetry
in the spin-dependent scattering of the conduction electrons. Numerical
calculations were carried out for Fe/Cr and Fe/Cu multilayers.
(D.R. Penn)
- Superconductivity. We have attempted to get a better understanding
of the role of complex electronic structure in superconductivity by using the
dielectric function. The dielectric function is of fundamental importance in
understanding many properties of solids: the response to an external field, the
elementary excitations of the solid, and screening in the solid. The total
dielectric function includes not only screening by the electrons but also the
polarizability of the lattice. Over-screening by the lattice results in an
attractive total pairing interaction that may lead to superconductivity. We
studied the total dielectric function, which we derived in previous work, and
focused on the total interaction between two electrons. The formalism includes
lattice effects and thus includes the coupling between transverse phonon modes
and electrons. In simpler treatments, only coupling to longitudinal modes is
taken into account. The additional electron-phonon coupling is important for
superconductivity.
Figure 5. (a) STM image of 10 monolayers of Fe grown on an Fe
whisker at 20 C. The image is 100 × 100 nm. Note the
pattern formation of mounds. (b) Contour map of a smaller
20 × 20 nm region from the left bottom corner in (a). Solid
lines denote equi-height contours with the heavy line at the mean height.
(c) Contour plots of the calculated surface height during growth using a
continuum growth equation model.
A necessary condition for an attractive interaction between electrons is that
the total dielectric function have at least one negative eigenvalue. We showed
that there is always one negative eigenvalue for each dimension of the system.
In addition it is required that a solid be stable. The condition for this is
that eigenvalues of the total dielectric be negative or greater than one. This
leads to the condition that the solid will be stable if the electronic and
lattice portions are individually stable. It is also possible to have a stable
system if the electronic portion is unstable but the lattice is stable. If the
lattice is unstable it is not possible to have a stable solid. Because small
negative eigenvalues and hence superconductivity occur in solids that are near
to being unstable, stability conditions are important. We also derived a new
sum rule for the dielectric functions that augments the well known acoustic sum
rule. Unlike the acoustic sum rule, the new sum rule is important for the case
of a metal as well as for an insulator and places restrictions on the
electronic portion of the dielectric function. We carried out numerical
calculations for a one dimensional model dielectric function in order to make
the formalism more concrete and to demonstrate the importance of the sum rules.
(D.R. Penn)
- Imaging of Magnetic Domains and Domain Wall Dynamics in AlliedSignal
METGLAS Ribbons. Researchers from the Electron Physics Group used the SEMPA
facility to image the magnetic domain structure of METGLASTM
ribbons. These are amorphous ferromagnetic ribbons produced by AlliedSignal
Inc. for use as a magnetic core material in power distribution transformers.
The domain structure is of great importance in determining the efficiency of
these transformers. Ribbons were observed in both the as-cast state and after
annealing in a longitudinal magnetic field, allowing the effects of the
annealing process to be evaluated.
We are particularly interested in the interactions between the domain walls and
surface defects; SEMPA is well suited to this task because the magnetic
structure and the surface topography are imaged simultaneously. Electron
Physics Group researchers developed a SEMPA sample holder capable of clamping a
short length of METGLASTM ribbon into a loop and applying a magnetic
induction around the loop. This system can image the domain walls while an ac
or dc induction is applied, and without distortion of the SEMPA image, which is
sensitive to stray magnetic fields. This capability has provided the first
SEMPA images of METGLASTM domain walls in motion and under static
magnetic inductions.
Figure 6. SEMPA measurements of the magnetic domains under four static
magnetic inductions. The central hysteresis loop, which was recorded under ac
excitation illustrates the magnetic state corresponding to each image. The
small, relatively immobile domains at the upper right of the images are pinned
by a surface defect.
Figure 6 shows the magnetic state near a surface defect observed under four
static magnetic inductions. The hysteresis loop, which was recorded at power
line frequency, illustrates the magnetic state corresponding to each of the
images. Of particular note are the small, irregularly shaped domains at the
upper right in these images. These domains change far less than the large
domain in the central region, due to the presence of a surface defect which
pins the domain walls in its vicinity. (A. Gavrin, J. Unguris,
D. Pierce, and R. Celotta)
- SEMPA Observation of Extended Magnetic Domains in Magnetoresistive
Granular Metals. Using the new high resolution SEMPA facility, Electron
Physics Group researchers demonstrated the existence of large (100 nm)
magnetic domains in several cobalt-silver granular metals; these domains
persist over a wide range of compositions and processing conditions.
Researchers at several institutions (IBM, UC San Diego, Johns Hopkins
University) have shown that granular ferromagnets such as Co-Ag exhibit the
giant magnetoresistance effect. Such materials have a broad range of potential
applications in the magnetic recording industry and in products which rely on
the sensing of magnetic fields, e.g., antilock brakes. However, the basis for
this phenomenon is not fully understood. Few researchers anticipated large
magnetic domains in granular Co-Ag alloys: the microstructure of these alloys
was thought to limit the magnetic domains to sizes comparable with the particle
size (less than 10 nm). The presence of large domains suggests that a
significant fraction of the cobalt in these materials does not contribute to
the giant magnetoresistance. In collaboration with researchers at The Johns
Hopkins University, members of the Electron Physics Group have investigated the
material composition and fabrication parameters which 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. This
work is being continued in an effort to ascertain the origin of the domains.
Figure 7. Vector map of the magnetization at the surface of a granular
Co0.35Ag0.65 film, as recorded by SEMPA. The heavy arrows
indicate a magnetic "source" and
"drain."
Figure 7 shows a vector map representation of several domains at the surface of
a thick Co-Ag film. It is apparent in this figure that there are several
"sources" and "drains" of magnetic flux; the heavy arrows
point to one of each. It is possible that this pattern represents the closure
domains capping an underlying domain structure which is perpendicular or canted
to the film plane. (A. Gavrin and M.H. Kelley)
- Exposure of Resists with Metastable Atoms for Nanolithography. In a
recent patent application, members of the Electron Physics Group proposed a
lithographic process in which a resist is exposed by metastable rare-gas atoms.
The advantage of this exposure mechanism is that all the newly developing tools
of atom optics, with their inherent high resolution and potential for massive
parallelism, can be applied to the lithographic fabrication on
nanostructures.
In a collaboration involving the Electron and Optical Physics Division, the
Atomic Physics Division, and groups from the Harvard University Physics and
Chemistry departments, the first stage of this process has recently been
demonstrated using self-assembled monolayers as a resist on gold. A series of
exposures have been carried out, using both helium and argon metastable atoms,
and a dose-response curve has been measured. In the near future, this work will
be extended, utilizing atom optical techniques to realize the potential for
nanometer-scale resolution and massive parallelism.
(J. McClelland)
- Anomalously Cold Temperatures in a Laser-Cooled Chromium Beam. In
our efforts to understand the mechanisms involved in producing a highly
collimated atomic beam, we have made a systematic study of polarization
gradient cooling of chromium atoms. In recent years, the use of optical forces,
to damp the atomic motion and hence collimate atomic beams, has been actively
studied by various groups with a wide variety of atomic species. However, this
has led to some surprising results in the first study made with chromium atoms.
Current laser-cooling models predict that at low intensities and/or large
detunings (when the excited state fraction is small), the temperature depends
only on the light shift (AC stark shift) which is proportional to the light
intensity and inversely proportional to the laser detuning from the atomic
resonance. In our transverse cooling experiments, we have found two interesting
regimes. At low light shifts, a minimum temperature corresponding to a few
times the recoil energy can be obtained for a small fraction of the atomic
beam. At high light shifts, in a region that has been largely unexplored, the
temperature unexpectedly remains essentially constant with the light shift and
up to 85 % of the atoms are cooled.
While it has been suspected that conventional models might not be applicable to
situations with high light shifts and an appreciable excited state fraction,
there have been no prior definitive tests. The unexpectedly low temperatures we
have obtained in this regime should provide incentive for detailed
calculations, for example taking account of the excited state population in
this regime. (R. Gupta, J. McClelland, and R.J. Celotta)
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