Atomic Physics Division
- The strategy of the Atomic Physics Division is to develop and apply
atomic-physics research methods to achieve fundamental advances in measurement
science relevant to industry and the technical community, and to produce and
critically compile physical reference data.
|
GOAL: To determine
atomic properties
and explore their
applications |
| Strategic Focus Areas: |
| |
First |
Light-Matter
Interactions and Atom Optics - to advance the physics of
electromagnetic-matter interactions and to explore new applications for laser
cooled and trapped atoms. |
| Second |
Plasma and X-Ray
Measurement Methods - to develop advanced optical and x-ray
measurement techniques for applications involving laboratory and space plasmas,
thin-film structures, and nanoscale devices. |
| Third |
Nanoscale and
Quantum Metrology - to advance measurement science at the
atomic and nanometer scale, focusing on ultraprecise length-displacement
measurements, x-ray and gamma-ray precision metrology, and nanooptics and
nanosystems modeling. |
| Fourth |
Critically Evaluated
Atomic Data - to produce reference data on atomic structure and
to critically compile reference data for scientific and technological
applications. |
Plasma and X-Ray Measurement Methods:
to develop advanced optical and x-ray measurement
techniques for applications involving laboratory and space plasmas, thin-film
structures, and nanoscale devices.
INTENDED OUTCOME AND
BACKGROUND
This strategic element focuses on the use of atomic radiation as an
efficient, noninterfering probe of plasmas and industrially important materials.
Our research includes sizable plasmas used for etching semiconductor wafers,
small-scale plasmas confined in electromagnetic traps, and nanoscale plasmas
induced on surfaces by individual, highly charged ions. Information is obtained
by measuring the emitted photons and massive particles using a variety of
instruments, including visible, ultraviolet, and x-ray spectrometers,
microcalorimeters, mass spectrometers, submillimeter wave detectors, and
spatial imaging systems. In addition, surface effects are analyzed at the
atomic level using a scanning tunneling microscope and an atomic-force
microscope.
Our plasma diagnostic research includes collaborations with industry,
university, and government partners. For example, Intel and International
SEMATECH have requested that the NIST electron-beam ion trap be utilized to
assist in the development of EUV lithography light sources.
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Figure 3
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We work with x rays since their weak interactions make them a nearly ideal
penetrating probe. The primary goals of this research are the development and
application of high-resolution x-ray scattering techniques, the production of
reference samples of thin-film and multilayer structures, and the understanding
of the microstructure of thin-film and multilayer structures appropriate to the
semiconductor industry.
Following on our experience fielding a high-energy, curved crystal spectrometer
at the OMEGA laser facility at the University of Rochester, we are presently
producing a more ambitious, broadband, multichannel x-ray spectrometer system
for the National Ignition Facility (NIF). These types of instruments will serve
the plasma-diagnostic community by providing information about such things as
the hot-electron energy distribution and the plasma temperature.
Figure 3. Reflection-geometry spectrometers constructed as a
National Ignition Facility plasma diagnostic (top). Four convex, curved
crystals [bent to five inches (11.7 cm) radius of curvature] together
provide cascading high-resolution x-ray spectral coverage from 1 keV to
20 keV. The four crystals shown (bottom) are Potassium Acid Phthalate,
Quartz, Silicon, and Germanium.
Accomplishments
Optical Properties in Support of UV Lithography
Optical-lithography technology is growing to more rely upon deep- and
vacuum-ultraviolet radiation in order to further extend the performance of
advanced integrated circuits. This necessitates the use of crystalline fluoride
materials for the refractive optics, though little was known of the optical
properties of these materials at these short wavelengths.
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Figure 4. Directional dependence of the spatial-dispersion-induced
birefringence in calcium fluoride and related cubic materials, first calculated
and measured as part of this project. The figure shows the previously-unknown
heptaxial behavior (seven nonbirefringent axes) for this cubic system. |
In May 2001, we uncovered a very large, completely unexpected intrinsic
birefringence in calcium fluoride at short wavelengths. Its index of refraction
depends on the polarization of the incident light. Since then, the 157 nm
lithography community has been struggling to find ways to design around this
aberration.
The NIST team continues to work on developing a new crystalline material that
does not exhibit birefringence at 157 nm as one approach to ameliorate
this problem. This material is based on mixed solid solutions of CaF2
and SrF2, which separately have opposite signs of the effect.
Working with industrial partners for material growth, this new material is
being grown and characterized.
To establish the properties of this and other UV materials, NIST has developed
a unique phase-shifting interferometer that can measure the index homogeneity
and birefringence at wavelengths ranging from the visible through 146 nm
(in the vacuum ultraviolet). It has enabled a determination of the dispersion
of these properties in this complete wavelength range.
An effort to characterize many materials and samples is being pursued to see if
a scaling factor can be established which reliably provides the 157 nm
values from the much more easily measured visible wavelength values.
Ion-Gas Recombination Studies
Motivated by a growing interest from the fusion-energy and astrophysics
communities, we have expanded the NIST Electron Beam Ion Trap facility to allow
ion-gas collision studies. We have installed a gas-jet target and additional
detectors on our ion beamline to study the photons that are emitted as the ions
traverse the target.
The new apparatus has yielded measurements on a sequence of charge states of
krypton, starting at Kr27+ and extending through Kr36+.
The study reveals systematic changes in x-ray wavelength and intensity ratios
as a function of ion charge state. Particularly dramatic is an abrupt increase,
by a factor of two, in the L /M ratio
as the L-shell changes from single vacancy to multiple vacancy. A
time-dependent collisional-radiative model of the excited-state population
distribution confirms that the origin of this shift is the formation of
metastable states.
Detection of Chemical Contamination in a Semiconductor Plasma-Etching
Reactor
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Figure 5 (© H. Mark
Helfer). |
High resolution, submillimeter wavelength, linear-absorption spectroscopy has
been developed as a tool for monitoring chemical contamination in
plasma-etching reactors. Contamination may arise from feed-gas impurities,
vacuum-chamber leaks, and incomplete chamber cleaning. The submillimeter
methods are particularly sensitive to contamination originating from water
vapor, which has an intense rotational transition at 557 GHz.
We have measured this rotational transition in a Gaseous Electronics Conference
(GEC) reference cell, installed at a commercial company, under three different
reactor conditions prior to initiating a fluorocarbon plasma. The apparatus has
demonstrated sensitivity levels less than 10 nmol/mol. Levels less than
1 nmol/mol should be possible.
Figure 5. The GEC RF Reference Cell, a standardized plasma source
designed to create plasmas similar to those found in commercial semiconductor
etching reactors.
A New Powerful Probe for High-Efficiency Lighting
There are an estimated one billion plasma light sources in service in the
United States, consuming an estimated 2 exajoules (600 billion
kilowatt-hours) of electrical energy annually. These sources are principally
fluorescent lamps and metal-halide discharge lamps. In the past, metal-halide
lamps were used mainly for high-intensity lighting of large spaces. Now,
because of their high brightness and energy efficiency, they are also being
developed for automobile headlights and interior lighting.
As a result, there is a growing interest in increasing their luminous
efficiency through a better understanding of the processes that govern their
operation.
These processes are so complex that they have defied attempts at predictive
modeling or even the development of scalable design rules.
Figure 6. Temperature distribution in a high-pressure lighting discharge
as revealed by x-ray absorption imaging.
As part of a cooperative program with the Electric Power Research Institute,
NIST researchers have used x rays from the Advanced Photon Source at
Argonne National Laboratory to develop a new generation of diagnostic methods
for metal-halide lamps. Both x-ray absorption and x-ray fluorescence were used
to map the distribution of the temperature and the various elemental components
in a production-style lamp.
The use of these techniques will enable lighting scientists and engineers to
develop a more complete understanding of metal-halide arc lamps, leading to
improved design rules, advanced production methods, and eventually, more
energy-efficient lighting.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2002" - Table of Contents |
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