Technical Highlights
- X-Ray Nanotomography of Integrated Circuits. Three-dimensional
Three-dimensional imaging of buried structures with sub-micrometer resolution
represents an important tool for analyzing modern engineered structures such
as those found in microelectronics, quantum wells, and multilayer magnetic
devices.
We have begun a program to perform x-ray microtomography of buried structures
in integrated circuits, in collaboration with the Intel Corporation,
Rensselaer Polytechnic Institute, and Argonne National Laboratory. We have
used a 60 nm to 150 nm x-ray microprobe at 1.6 keV to
1.8 keV photon energy at Argonne’s Advanced Photon Source to produce
several series of two-dimensional microradiographs with different views of
both aluminum and copper pairs of microelectronic interconnects.
Tomographic algorithms were then applied to these microradiographs to
synthesize a three-dimensional image of the buried circuit. With a capability
of resolving features as small as 140 nm, this work represents the
highest resolution tomography ever achieved at a photon energy above
1 keV (S. Grantham, Z.H. Levine, and T.B. Lucatorto)
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Figure 1. X-ray tomographic image of an integrated circuit interconnect,
obtained by use of Bayesian algorithms. A manipulable 3-D image is in VRML format at
physics.nist.gov/ppg.
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- DUV and EUV Transfer Standard Detectors
Activity. After the upgrade of SURF, the Detector Radiometry Beamline
(BL-9) was recommissioned. This system is used to perform absolute calibrations
of NIST working standards and relative
calibrations of transfer standards in the 5 nm to 50 nm spectral
region. In addition, an upgrade was made to the laboratory facility used to
perform absolute and relative calibrations in the 50 nm to 254 nm
spectral range. New instrumentation, computer hardware, and software were
installed. Measurements were made of the stability of avalanche photodiodes
from Advanced Photonix, Inc. (API), under exposure to 161 nm radiation.
Despite nitridation of the passivating oxide layer, these avalanche photodiodes
are not as radiation hardened as the Si transfer standard photodiodes. API is
continuing to make improvements to the design and manufacturing process. Several
GaN and AlGaN devices, essentially solar-blind vacuum-ultraviolet photodiodes,
from SVT Associates were investigated. SVT Associates has been awarded an
SBIR
Phase II contract from the Department of Energy to improve AlGaN
photodiodes, and the Photon Physics Group has agreed to perform sensitivity and
lifetime measurements as part of this SBIR program. The effect of exposure to
254 nm radiation was determined for several types of Si photodiodes. In
general, no device was stable, but photodiodes with a nitrided passivating
oxide layer were more stable than standard ultraviolet photodiodes. Twenty-six
calibrations were performed in 1999 for applications in solar physics,
astronomy, aeronomy, plasma diagnostics, and electron beam generation.
(R. Vest and T. Lucatorto)
- SURF III Upgrade. The SURF III electron
storage ring gave its first light in December 1998, and all key operations were
brought back on line during 1999. Operation of SURF III has become routine
at the energy of 331 MeV, vs. the former SURF II energy of
284 MeV. The SURF main magnet has been shown to be capable of providing
for operating energies as high as 405 MeV. However, a major new component
of the radio frequency cavity system is required before operation will be
possible above 331 MeV. This device, an adjustable phase shifter, is
scheduled to be delivered late in 1999.
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BL-1 has been instrumented with an EUV conversion microscope, which is being
developed as a tool for mask inspection for DUV lithography. (See
figure 2.)
The Spectrometer Calibration Beamline (BL-2) is fully reconnected and
operational, and serviced its first calibration customer in July, 1999. The
new radiometric beamline (BL-4) is in regular operation, measuring UV optical properties needed for semiconductor
lithography. (See figure 3.)
The EUV Optics Characterization Facility (BL-7) resumed operation in November
1999, and is engaged in measurements of new yttrium/ruthenium mirrors recently
fabricated in the Photon Physics Group. The VUV Photodiode Beamline (BL-9) has
resumed operation for calibration of transfer-standard photodiodes.
The principle objectives of the SURF III project are to improve the
accuracy of the SURF electron storage ring as a primary standard source of
spectral irradiance and to extend its spectral range. Work towards these
objectives is ongoing, with considerable effort being made to understand beam
dynamics and criteria for stable operation. (A. Farrell, E. Fein,
M. Furst, R. Graves, L. Hughey, A Hamilton, and
R. Vest) |
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Figure 2. Test image acquired by the EUV conversion microscope on
SURF BL-1, demonstrating a 25 µm field of view and 100 nm spatial
resolution. |
- Matter-wave Talbot Effect in Bose-Einstein Condensates. The optical
manipulation of a Bose-Einstein condensate (BEC) has possible applications
ranging from nanolithography to quantum computing. A key step towards realizing
such applications lies in understanding the interaction of laser light with a
BEC. When a high-intensity, short-time, standing-wave laser pulse is applied to
a BEC, as was done in a recent experiment in the Atomic Physics Division, small
pieces of the cloud are produced that travel away from the condensate. The
momenta given to such cloudlets are integral multiples of twice the laser
photon recoil momentum. If the laser intensity is sufficiently high, application
of a single standing-wave pulse can result in the complete breakup of the
condensate.
If, however, two pulses are applied, separated by half of the so-called
"Talbot time," there is no effect at all on the condensate. At other
delay times, many cloudlets appear displaying a rich variety of momentum-space
structures.
We reproduced the data in the NIST experiment using a simple model of
interfering, indistinguishable, quantum pathways as shown in
figure 4.
This figure displays the fraction of the original condensate having zero
momentum after two pulses have been applied with a variable delay (represented
by the horizontal axis) between them and exhibits the good agreement between
theory and experiment. The agreement between the simple model and the data
provides insight into light/condensate interactions and a step towards future
applications. (M. Edwards and C.W. Clark)
- Irreversible Magnetic Field Dependence of the Magnetic Structure in
GMR
Co/Cu Multilayers. Polarized neutron reflectivity (PNR) and scanning
electron microscopy with polarization analysis (SEMPA) were successfully used
to resolve a controversy involving the giant magnetoresistance (GMR) in Co/Cu
multilayers. The GMR in these multilayers makes them potentially useful in
magnetic-field sensor applications. The PNR and SEMPA measurements revealed the
magnetic domain structure and the layer-by-layer alignment of the magnetization
responsible for the GMR.
In general, the magnetoresistance of a GMR multilayer dramatically decreases
when an external field reorients the in-plane magnetizations of the
ferromagnetic layers parallel to each other. The magnetoresistance is largest
for systems in which the low-field resistance is associated with antiparallel
alignment of adjacent ferromagnetic layers. For Co/Cu multilayers with thick Cu
layers, the magnetoresistance for the as-prepared multilayer is often larger
than the maximum obtained after cycling the magnetic field. The magnetic
structure corresponding to this potentially useful, large magnetoresistance
state has been a puzzle.
Co/Cu samples from Michigan State University were analyzed by the NCNR
scientists using PNR and the Electron Physics Group using SEMPA. PNR and SEMPA
are powerful, complementary tools for the investigation of magnetic structures
in materials with buried magnetic layers. PNR probes the order of the entire
sample and provides a depth profile of the magnetization, while SEMPA produces
a direct image of the magnetic domain structure within one magnetic layer at a
time.
Figure 5. Magnetic domain structure in two adjacent Co layers of
Co/Cu/Co multilayer.
The PNR and SEMPA
measurements reveal that as-prepared samples show a strong
antiparallel alignment of the ferromagnetic Co domains across the non-magnetic
Cu interlayers. This antiparallel magnetic state is irreversibly destroyed by
the application of a magnetic field. After field cycling, the smaller residual
peaks in the magnetoresistance are then associated with the presence of small,
randomly oriented Co domains.
Our investigation shows that it would be possible to enhance the performance
of these GMR multilayer
sensors by stabilizing the initial, antiparallel,
magnetic state. We have also demonstrated that PNR and SEMPA are powerful and
complementary tools for the analysis of magnetic thin films. NIST is probably
the only institution in the world that maintains the capabilities needed for
such a comparison (J. Unguris, in collaboration with J. Borchers, NIST
Center for Neutron Research)
- Calculations of Giant Magnetoresistance; Testing the Relaxation Time
Approximation. The highest recording densities in commercial magnetic hard
disks are achieved using read heads based on an effect known as giant
magnetoresistance (GMR). Use of GMR read heads promises to allow the recording
density to be doubled each year in the foreseeable future. GMR is also the
basis of magnetic sensor devices. This effect is observed in thin magnetic
multilayers in which magnetic and non-magnetic layers alternate. If the
magnetic layers are in an antiferromagnetic configuration in the absence of an
external magnetic field, a magnetic field can align them. The electrical
resistivity of the system decreases in the aligned state and thus the magnetic
field can be detected by measuring the change in resistance of the structure.
The exact mechanism of the giant magnetoresistance is still controversial.
To optimize the GMR, extensive theoretical and experimental studies are being
carried out to determine the mechanism. The theoretical work is generally
based on using the Boltzmann equation to describe electron transport. This
equation is almost always solved by means of the relaxation approximation, an
approximation never tested in the present context. We have devised a method of
solution that does not require the relaxation time approximation and thus have
been able to test its validity. We find that the relaxation time approximation
is valid within 10% for metals with simple electronic structure. We are
extending our study to more complex materials, which are highly anisotropic
and are less likely to be properly described by the relaxation time
approximation. (D.R. Penn and M.D. Stiles)
- Chromium Atoms Trapped. Building on expertise developed over the
past few years on laser manipulation of chromium atoms, researchers in the
Electron Physics Group have recently demonstrated for the first time the
cooling and trapping of chromium atoms in a magneto-optical trap. Using laser
light tuned to the 7S3 – 7Pş4
Cr atomic transition at 425 nm, they were able to cool and collect over
106 atoms in a sub-millimeter sized cloud. Because Cr has an
intermediate metastable 5D level, extra care was needed to make the
cooling and trapping effective. Two extra laser frequencies, generated by laser
diodes at 654 nm and 663 nm, had to be introduced and locked on a
transition to pump atoms back into the cooling process. Once the atoms were
trapped, observations of the trap lifetime revealed interesting results:
non-exponential, density-dependent trap loss was seen, indicating a
surprisingly high level of excited-state collisions. Applications of this work
include forming a highly coherent source of atoms for optically-controlled,
atomic nanofabrication, generating a precursor to creating a quantum degenerate
gas in a system with either fermionic (53Cr) or bosonic
(52Cr) particles, or providing a new arena for studying ultracold
collisions. (J.J. McClelland)
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Figure 6. Image of trapped Cr atoms.
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Imaging Nanoscale Magnetization Dynamics. We have shown that Scanning
Electron Microscopy with Polarization Analysis (SEMPA) can be used to image
domain dynamics on the nanoscale. In general, imaging how magnetic structures,
such as domains and domain walls, respond to an applied magnetic field is
essential for understanding how most magnetic storage devices and sensors work.
Traditionally, magneto-optic imaging methods have been used to image
magnetization dynamics. However these methods are limited to optical
resolution. On the other hand, electron microscope methods, such as SEMPA,
have much higher spatial resolution, but the electron imaging can be distorted
by stray magnetic fields.
By using a specially designed sample holder with the sample configured in a
closed magnetic circuit, we have found that the applied magnetic fields could
be confined to the sample and SEMPA could successfully image the magnetization
dynamics without loss of resolution. The initial measurements examined domain
wall motion in amorphous metal ribbon materials used for transformer cores.
Although these measurements used low frequency magnetic fields, the images
clearly showed domain wall interactions with defects and the different
mobilities of various domain walls. The SEMPA images also showed that much of
the domain wall motion was not reproducible after cycling the magnetic field.
This irreproducibility is a serious problem for imaging of the magnetization
dynamics at high frequencies, since most high speed imaging methods use
stroboscopic techniques which, in turn, rely on reproducible domain motion.
(J. Unguris and A. Gavrin)
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