Stanford Synchrotron Radiation Laboratory
The Stanford Synchrotron Radiation Laboratory (SSRL), one of four DOE-supported
national synchrotron radiation sources and a division of SLAC, is located
on the SLAC site. SSRL annually attracts over 1300 experimenters from around
the world. Graduate students and researchers from Stanford and other universities,
industry, and government laboratories use electromagnetic radiation, known
as synchrotron radiation, to study the structural and electronic properties
of gases, liquids, and solids. These studies are conducted in a variety
of scientific contexts ranging from the basic physics of
materials, to the molecular structures of proteins and viruses, to
spectroscopic studies of pollutant elements like lead and plutonium in
complex environmental materials. The radiation produced by electrons in
the 3 GeV SPEAR storage ring is continuous from below the visible to the
hard x-ray region and is 5 to 6 orders of magnitude more intense than that
produced by a conventional sealed-tube x-ray generator. With energies matched
to the binding energies of electrons and wavelengths matched to the distances
between atoms, synchrotron radiation is a powerful and unique tool for
measuring both the electronic and atomic structures of materials.
Students are involved in all phases of research at SSRL, as well as
in the development of instrumentation, with over 350 Ph.D. theses completed
from more than 30 universities. Graduate students from over 80 universities
are currently involved in research at SSRL. About 20 Stanford faculty and
85 Stanford graduate students are presently using SSRL to study a wide
variety of problems.
Physics
of Materials
Electronic Structure of Materials: Photoemission and X-ray Absorption
Spectroscopy. Photoemission and x-ray absorption are very powerful
and direct probes of the electronic structure of a material, and are widely
used at SSRL for fundamental and technological studies of semiconductors,
superconductors, magnetic materials, fullerenes, metallic clusters, etc.
Because these studies are conducted in the 10–3000 eV energy range, they
are in general rather surface sensitive (5–1000 Å) and are performed
in ultra-high vacuum systems. In photoemission spectroscopy, an incident
photon of energy 10–1500 eV ejects an electron from one of the occupied
electronic states of a material. This “photoelectron” is collected and
its energy analyzed. The energy distribution of the photoelectrons can
then be mapped back to determine the density of occupied states of the
material. This information can then be used to study both the valence band
electronic structure as well as chemical bonds in a material.
The complementary technique of x-ray absorption spectroscopy is used
to probe the conduction band states of materials as well as to determine
surface atomic structures. If the conduction band states are exchange split
due to magnetism, then the absorption coefficient of circularly polarized
light is dependent upon the spin of the conduction electrons. Such an experiment
is called Magnetic Circular Dichroism (MCD) and is very powerful for determining
magnetic information at the atomic scale. MCD measurements on materials
that may be used in magnetic storage devices in future generations of computers
is currently one important area of research at SSRL.
Examples of research in the physics of materials in which graduate students
may participate include the following:
• Measurements performed at SSRL have provided key information about
the symmetry of the superconducting order parameter. Experimenters are
able to measure the superconducting energy gap as a function of crystal
momentum. It was found that, unlike conventional superconductors, the gap
in the high-temperature superconductors is very anisotropic. The specific
form of the anisotropy of the gap measured is suggestive of d-symmetry
superconductivity.
• The coupling of magnetism across non-magnetic spacer layers enables
the engineering of advanced magnetic storage media. The origin and nature
of this coupling is not understood. By studying the Cu and Co edges individually
with circularly polarized radiation, researchers at SSRL are able to determine
that the Cu atoms in the multi-layers obtained a small magnetic moment,
in contrast to the situation in pure Cu metal.
• In the area of chemical sciences, researchers have made use of valence
band photoemission and chemical shift scanned energy photoelectron diffraction
to identify molecular species and the structure of the surface bond for
self-assembled alkyl layers on silicon surfaces, which may ultimately be
used as chemical receptors in sensor applications.
Structural Studies of Materials. X-rays
can also be used to study long- and short-range order in solids, liquids,
and gases. Scattering, such as diffraction, can be used to determine the
structure of thin films on surfaces, to explore nuclear magnetic structure,
and to determine the structure of newly discovered compounds such as high-temperature
superconductors. X-ray absorption spectroscopy (XAS) is useful for determining
the near-neighbor environment of an atomic species where no long-range
order exists. Researchers use diffraction at SSRL because of the vastly
greater intensity and lower divergence of the synchrotron beam relative
to a laboratory x-ray source. In addition, this synchrotron source has
sufficient intensity over a wide photon energy range (10–30,000 eV) to
make feasible new types of scattering studies, including anomalous x-ray
scattering and anomalous small-angle x-ray scattering, which provide unique
information on the local atomic arrangements around individual elements
(x-ray scattering) and on nanoscale domains (small-angle x-ray scattering)
in amorphous materials.
Examples of research in structural studies of materials include the
following:
• X-ray tomography is being used at SSRL to study internal bone structure
and how it changes with advancing osteoporosis. The results are clinically
relevant for quantitative evaluation of the effects of different therapies.
• Time-resolved, in situ, grazing-incidence x-ray scatter-ing is being
used to study the growth of epitaxial GaAs by chemical vapor deposition.
Because these films are grown in high-temperature, near-atmospheric pressure
conditions, electron-based probes are ineffective. Optical probes can measure
electronic structure, but only x-rays can measure atomic long-range order.
X-ray scattering is used to determine growth rates in situ and to map out
regions where layer-by-layer growth occurs and the crossover to step-flow
and three-dimensional growth.
• The extra brightness of the synchrotron means that diffraction from
resonant nuclei is possible. The resulting diffracted beam has the energy
resolution of a Mössbauer transition, that is,?E/E of 10 –8 , and
has the potential of being a powerful new probe of matter.
• Molybdenum-germanium alloys, depending on the ratio of the constituents,
can show insulating, metallic, and superconducting behavior. Small-angle
x-ray scattering has been used to observe the presence of phase separation
in these films in the composition region around the metal-insulator transition.
Using the tunability of the synchrotron source, the chemical species of
the two separated regions have been identified, which helps to explain
the observed electrical properties.
Powder Diffraction Studies of Materials. In
1997 SSRL commissioned a new experimental station with dedicated instrumentation
for x-ray powder diffraction. Powder diffraction is one of the most widely
used materials characterization methods. Over the last 50 years it has
been routinely used for crystalline phase “finger-printing.” Recently,
cheap and powerful computers and dedicated second- and third-generation
x-ray synchrotron sources have transformed powder diffraction into a very
powerful structural tool. Powder diffractometry projects the three-dimensional
reciprocal lattice into one-dimensional space. Such projection causes (partial)
overlap of peaks with similar lattice spacing. The peak overlap loses structural
information and sometimes makes the task of structure solution difficult.
Nevertheless, the partial peak overlap is very useful in investigations
of subtle symmetry breaks which are often missed in routine single crystal
measurements. Further, the compaction of the entire reciprocal lattice
into one dimension accelerates data collection. With a bright radiation
source and appropriate detector, many structural changes can be investigated
in real time. Because of the tunability of synchrotron x-rays over a wide
energy range, one can now conduct x-ray scattering in the vicinity of an
absorption edge where it is modified by resonance processes. Such resonance
effects can be utilized in deciphering complex structures. Also, at resonance
conditions some of the magnetic scattering cross-sections are significantly
enhanced, allowing magnetic structural investigations using x-ray scattering.
The SSRL powder diffractometer can be tuned between 4 and 10.5 keV, covering
many transition-metal absorption edges. Research utilizing this new SSRL
resource is just getting underway.
Impurities on the Surfaces of Semiconductor
Materials. SSRL is the world leader in the application of synchrotron
radia-tion to total external reflection x-ray fluorescence (TXRF) for the
measurement of ultra-low levels of metal contamination on silicon wafer
surfaces. Ultra-clean silicon wafer surfaces are critical to the fabrication
of ULSI circuits for microprocessors and memory. As a result, integrated
circuit manufacturers expend significant resources to develop wafer-cleaning
processes to make possible the next generations of circuits with smaller
and smaller feature sizes. In order to develop advanced cleaning processes
to remove a wide variety of metals from silicon wafer surfaces, the semiconductor
industry has long realized that equally powerful metrology tools are needed
to quantify the quality of the cleaning processes. Currently, the semiconductor
industry uses in-house TXRF systems employing rotating anode x-ray sources
or chemical methods, which involve removing the impurities from the wafer
surface and measuring them separately. While these conventional methods
continue to improve, the state-of-the-art cleaning capability is often
better than these methods are capable of measuring. As a result, in collaboration
with Hewlett-Packard and Intel, SSRL began a program to evaluate the use
of synchrotron radiation TXRF (SR-TXRF) for the detection of transition
metal impurities and aluminum on silicon wafers several years ago. This
work has shown that the detection limits of TXRF could be improved by more
than an order of magnitude, resulting in a detection limit of 3 ×
10 8 atoms/cm 2 for Ni, compared to that of the best conventional systems
of 6 × 10 9 atoms/cm 2 . This project has also included efforts to
understand the processes that limit the sensitivity of SR-TXRF as well
as developing techniques for high-sensitivity detection of light elements.
Given this success, Sematech placed synchrotron radiation metrology
on its “roadmap” of critical metrology technologies and is sponsoring a
trial to evaluate the applicability of SR-TXRF to industrial applications
within the semiconductor industry. This trial has involved the extensive
participation of scientists from the Sematech member companies to evaluate
and use SR-TXRF. The industrial participation has allowed SSRL to benchmark
the technique for a wide variety of semiconductor process wafers, as well
as to obtain feedback on the practical aspects of the method. Companies
involved in this study include AMD, Applied Materials, Balazs Labs, DEC,
Hewlett-Packard, IBM, Intel, Kevex, Lucent, Motorola, National
Semiconductor, and Texas Instruments.
The Sematech trial has offered the opportunity to perform a number of
real industrial applications. This interest by the Sematech member and
related companies has been heightened by both improvements in the technique
and hardware upgrades made possible by two cooperative research and development
agreements between Hewlett-Packard and SSRL. These improvements, including
contamination-free wafer handling and wafer mapping, have made possible
industrially relevant applications. The work during the past year has included:
comparison of SR-TXRF with other techniques, including VPD-ICPMS (vapor
phase decomposition-inductively coupled plasma mass spectrometry) and HIBS
(heavy-ion backscattering); evaluation
of manufacturing process lines at several companies; evaluation of
new and repaired process equipment; evaluation of as-received wafers (vendor
qualification); evaluation of chemical bath life; evaluation of new cleaning
chemistry, including more dilute baths (for reduction of chemical usage);
cross-contamination evaluation of process steps with new process materials
(such as polymer coatings); and characterization of ion-implanted materials.
These data have demonstrated to industry that synchrotron radiation
brings TXRF sensitivities into a regime which can impact ULSI process development.
We are now at a point where the focus of the work can turn toward application
of the technique. At this time, the needs of the semiconductor industry
regarding the SR-TXRF capabilities are being evaluated through the continued
participation of the scientists from the Sematech member companies. One
of the goals of this evaluation process is to develop an understanding
of the practical issues of accessibility, throughput, and cost, which may
ultimately determine the long-term impact of SR-TXRF on semiconductor research
and development. In addition, SSRL is exploring the options for a dedicated
service provider who might efficiently perform the measurements. Finally,
future technical advances are being explored, including new detectors and
undulator beam lines on the proposed SPEAR3, which could further improve
throughput and sensitivities (by an order of magnitude). Work at SSRL continues
to attract considerable interest from a wide international audience including
new industrial users as well as visitors from other synchrotron facilities
interested in setting up an SR-TXRF capability at their facility. It seems
likely that this interest will continue into the future.
Structural
Molecular Biology
The goal of understanding life has evolved into a large interdisciplinary
effort that integrates information extending from experimental studies
at the atomic and molecular levels to studies of organelle, cellular, and
tissue organization and function. There is strong interplay among scientific
fields including chemistry, structural biology, biochemistry, genomics,
bioinformatics and the computational sciences. Atomic-level information
can provide an
understanding of biological function and malfunction that leads to
disease. Structural Molecular Biology (SMB) provides the vast majority
of information on an atomic level and will play an even greater role in
this regard in the future. The unique properties of synchrotron radiation
(tunability of the energy, high intensity, and high degree of collimation)
make it a powerful source of x-ray radiation for biological structural
studies.
SSRL has a strong program in SMB with a focus on cutting edge research
in science and technology, and providing world-class facilities to
Stanford faculty and students as well as to a large community of outside
research groups.
The techniques developed and used at SSRL include protein crystallography
(PX), x-ray absorption spectroscopy (XAS), and small-angle x-ray scattering/diffraction
(SAXS/D). These techniques provide the basis for an integrated approach
to studying the structure and function of biological macromolecules to
unravel life processes. Protein crystallography uses x-ray dif-fraction
from single crystals to determine the three dimensional, atomic-level structures
of macromolecules. Scientific developments are focused on the multiple
wavelength anomalous dispersion (MAD) method for structure determination
and very high-resolution structures for atomic detail. X-ray absorption
spectroscopy (XAS) can be divided into two regions: the edge spectrum,
and the extended x-ray absorption fine structure (EXAFS). XAS edge studies
provide the means to determine electronic structure of atoms in biological
molecules, including properties such as oxidation state, spin state, and
covalency for specific atomic sites. EXAFS, on the other hand, provides
a tool for determining local radial structure, and can yield particularly
accurate bond lengths. XAS thus provides an important means to probe the
active sites of metallo-proteins. SAXS/D is used to study proteins and
larger molecular assemblies in non-crystalline
forms such as solutions or fibers, and provides structural insights
at the molecular level, thus bridging the knowledge
gap between the atomic scale of individual molecules and more complex
systems. Time-resolved studies are routinely per-formed
to investigate proteins in action. Small-angle single crystal diffraction
provides a means of solving the phase prob-lem
for high-symmetry crystals of large molecular assemblies, such as virus
particles, when used in combination with high-resolution
diffraction.
Examples of research in structural and molecular biology in which graduate
students may participate include the following:
• Structure and function of biological macromolecules with focus on
ultra-high resolution and MAD tech-niques in protein crystallography. State-of-the-art
facilities allow for leading-edge research to be performed where details
of enzymatic processes can be studied.
• Technological developments in the data collection facilities, instrumentation,
and software crucial for studying very large bio-molecular assemblies and
very small, weakly diffracting crystals. These “next generation” experiments
will begin to unravel complex processes such as viral activation.
• New theoretical approaches to the analysis of multiple-scattering
EXAFS have recently expanded the traditional role of EXAFS to include bond-angle
determination in some special cases, including metal ligands such as -CO,
-CN, and -NO.
• Time-resolved x-ray scattering studies on viral assembly and protein
folding; biophysical studies on proteins of clinical importance; methodological
developments for single-crystal diffraction technique and time-resolved
studies.
Molecular
Environmental Science
Molecular Environmental Science (MES) describes the multidisciplinary fields
of research concerned with elucidating the molecular-scale speciation and
chemical processes affecting environmental pollutants in earth materials
and anthropogenic materials, and their interactions with the hydrosphere
and biosphere. Synchrotron-based techniques, particularly x-ray absorption
spectroscopy (XAS), have become essential tools in MES because of their
unique capabilities for analyzing environmental materials. XAS provides
quantitative chemical and structural information (e.g., oxidation state,
local coordination environment, molecular structure, physical form, and
host-phase identity) for pollutant metal ions and organic molecules in
complex natural materials and material assemblages, including amorphous,
poorly crystalline, and/or microcrystalline materials, adsorbates on solids,
and aqueous species. XAS measurements are being used to determine the speciation
of toxic and radioactive metals in soils, groundwaters, and man-made waste
forms. The transformations of these species to more or less toxic forms
and the rates of these reactions are also being studied using XAS. No other
spectroscopic technique provides this combination of structural and chemical
information. Other synchrotron-based methods at SSRL, including x-ray scattering,
x-ray standing wave measurements, and photoemission spec-troscopy, provide
information about the geometric and electronic structures of solid surfaces,
the changes in electronic and geometric structures of these surfaces after
reactions with small molecules such as H2O, and the distribution of ions
in solutions in contact with solid surfaces. One of the most chal-lenging
areas of MES research involves probing the structure and composition of
the solid-water interfacial region, where some of the most important environmental
reactions occur. This can be done using a combination of grazing-incidence
XAS and x-ray standing wave measurements. The molecular and atomic-scale
knowledge gained from these MES studies is
critical for the development and optimization of safe and cost-effective
contaminant remediation and disposal technologies, understanding contaminant
transport in contaminated water supplies, and predicting present and future
risks to vulnerable populations. Synchrotron-based MES was pioneered at
SSRL in the late 1980s/early 1990s. Since then, the amount of beam time
provided for MES research has grown 40-fold.
Examples of research in molecular environmental science in which graduate
students may participate include the following:
• Speciation of arsenic and lead in mine wastes and sediments. The
chemical and physical forms of these metal ion contaminants in mine wastes
from Leadville, Colorado (lead), and Trona, Jackson, and Marysville in
California (arsenic) were determined using XAS in combination with
electron microscopy techniques. Arsenic was found to be present as As(0)
and As(V), both in precipitates and adsorbed on mineral surfaces.
The more toxic and mobile As(III) species was not detected. Lead was
found to be predominantly Pb(II), up to 50% of which was adsorbed on mineral
surfaces. Since these species are sources and sinks for arsenic and lead,
this information is requisite for predicting arsenic and lead transport
in groundwater over time and developing strategies to remediate the contamina-tion.
Photocatalyzed oxidation of As(III) to As(V) on TiO2 particles in clay
minerals is also being studied with XAS and is found to be a very rapid
process that can naturally transform arsenic from the more toxic to the
less toxic form.
• Speciation and oxidation state of selenium in soils at Kesterson
National Wildlife Refuge. Soils from this California Central Valley wildlife
refuge are being studied by a collaboration of scientists and graduate
students from Stanford University and Lawrence Berkeley National Laboratory.
The oxidation state of Se affects its mobility in natural waters and thus
controls its dispersal in the environment. Synchrotron-based XAS studies
of the contaminated soils provided the first direct evidence of elemental
Se in soils. In addition, laboratory-based XAS studies have shown that
aqueous Se can be reduced from the highly toxic and soluble Se(VI) form
to the relatively non-toxic and insoluble Se(O) form by interaction with
natural Fe(II)-containing solids.
• Fundamental structures and compositions of oxide surfaces and water
at oxide surfaces. A combination of single-crystal grazing-incidence XAS,
x-ray standing wave, and photoemission spectroscopy measurements are being
used to characterize the fundamental physical and chemical aspects of solid-water
interfaces, including metal-ion adsorption reactions and the distribution
of ions in the electrical double layer. Knowledge of the fundamental reactions
that occur at solid-water interfaces would enhance a host of more applied
MES research efforts, ultimately leading to more efficiently parameterized,
generalizable, and hence more accurate predictive models of solute-oxide
interactions.
• Adsorption of uranium and co-contaminants on oxides. XAS is being
used in combination with Fourier Transform Infrared (FTIR) spectroscopy
and macro-scopic solution measurements to study the molecular structures
and compositions of uranium (VI) – co-contaminant reaction products on
oxide minerals. Ternary interactions between these species are common in
contaminated groundwaters and play major roles in the fate and transport
of uranium and co-contaminants (e.g., EDTA, NTA, citrate, phosphate, and
carbonate) in aquifers and the vadose zone. Knowledge of the adsorbate
species is necessary in order to predict their mobility in affected groundwaters,
assess the risk posed to humans and wildlife, and remediate the contamination.
Polymer Science
Soft-materials research conducted at SSRL has focused on the structure
and processing of polymers. The main synchrotron method used in these studies
is small-angle x-ray scattering (SAXS). A key element in the ability to
study the processability of polymer blends, composites, and block copolymers
is the use of a temperature-controlled chamber where materials can be heated
or cooled at prescribed rates. This capability has been used to investigate
the phase transitions occurring in polymer blends, polymerizing copolymers,
and ionic block copolymers. The phase transitions observed at the macromolecular
length scale through time-resolved SAXS significantly influence the material
rheology and, consequently, its behavior in processing. Of particular technical
interest is the ability, by using SAXS,to follow the bulk polymerization
of a microphase separating copolyurethane with a small reaction injection-molding
machine to meter, mix, and inject reactants into a thermostated cell at
an appropriate beam line at SSRL.
The melting transition in rigid-rod polymers is critical to the production
of ultra-high strength materials such as Kevlar, used in bullet-proof vests.
This transition, along with related processes in ionomeric membranes, have
been focuses of recent studies at SSRL.
Another important polymer processing issue involves the removal of solvent
from polymeric materials. Researchers at SSRL have recently examined the
shape transitions occurring in highly concentrated block copolymer
solutions as solvent is removed. An important feature in these studies
is the use of a new two-dimensional CCD array detector that allows fine
resolution of anisotropic structures developing in the material.
Accelerator
Physics
Graduate Student Training in Accelerator Physics.
Traditionally, the accelerator physics graduate student program is carried
out jointly with the Applied Physics Department of Stanford University
through joint faculty appointments. The gradu-ate student program in accelerator
physics concentrates on the production of coherent and incoherent electromagnetic
radiation in various forms, including synchrotron radiation and transition
radiation from particle accelerators.
The SUNSHINE facility, located on the Stanford campus, is a student-operated
30 MeV linear accelerator where research is aimed at producing and utilizing
femto-second electron bunches. Such short bunches emit coherent radiation
at wavelength equal and longer than the bunch length. Radiation is generated
in form of transition-, stimulated transition- and undulator-radiation.
The photon brightness achieved so far is 4 to 8 orders of magnitude higher
than conventional sources in the far infrared at wavenumbers of up to 100
cm -1 . During their research program, students engage in theoretical and
computer simulation studies as well as hands-on experience in the development,
operation, and optimization of particle accelerators.
The LCLS Project.
Accelerator scientists from SLAC, in collaboration with scientists from
LBNL, UCLA, LANL, and LLNL, are developing a plan for using the SLAC linac
to drive a linac coherent light source—an x-ray laser operating at wavelengths
down to the Å range. While the LCLS operates on the principle of
the free-electron laser, a single-pass approach with no optical cavity
will be used. This makes it possible to reach shorter wavelengths than
would be possible with a conventional free-electron laser.
The LCLS offers significantly higher brightness, coherence, and peak
power than any other existing or planned x-ray source. With present technology,
an LCLS could be built to operate at wavelengths down to about 20 Å.
With improvements to technical components, it should be possible to extend
this to wavelengths as short as a few Å. The high coherence and ultra-short
pulse capabilities of the LCLS would allow fundamentally new types of research
to be carried out in chemistry, materials science, and structural biology.
Time-resolved studies of crystal lattice motions and fast chemical reactions
would be possible. Enough coherent photons would be available to study
nonlinear optical properties of materials. Opportunities exist for graduate
students to join research and development efforts on the various technical
components of this project (rf photocathode guns, bunch length compressors,
precision long undulators, simulation studies, beam line optics, experimental
stations), as well as on the applications of this unique source.
SLAC