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ATOM PROBE
ANALYSIS
The atom probe microscope provides
three-dimensional compositional
and structural analysis at the atomicand near-atomic scales.
Amy A. Gribb
Thomas F. Kelly
Imago Scientific Instruments Corporation
Madison, Wisconsin
W
hen introducing the atom probe microscope, it is important to first address a common misconception that
the atom probe microscope is related to the more widely known atomic force microscope (AFM), which operates by scanning a
sharp tip across surfaces and provides atomicscale imaging. In contrast, the atom probe microscope operates by removing and analyzing individual atoms.
The atom probe’s unique atom-by-atom analysis
provides a map of the elemental and isotopic identity and position of individual atoms in volumes of
material of up to 100 nm in diameter (parallel to
specimen surface) and 100 nm in depth (normal to
specimen surface). Thus, the basic differences between the atom probe and AFM are:
• The atom probe analyzes three-dimensional
volumes of material, whereas the AFM analyzes
surface features only.
• The atom probe provides both imaging and
chemical analysis, whereas AFM provides imaging
only.
In this article, the operation and analytical output
of the atom probe is compared with the more
widely known analytical techniques AFM, TEM,
and SIMS. An atom probe study of buried interfaces in multilayer thin metal films is presented as
an example application of the technique.
that the atom probe microscope can provide.
An analytical tool that can provide the equivalent of a ball-and-stick model of atomic structure
with elemental and isotopic identification may seem
too good to be true, and certainly too good to be
relatively obscure. It might be anticipated that the
above-mentioned “notable exceptions” will draw
the atom probe back into familiar analytical
regimes. However, although the exceptions introduce deviations from the ideal analytical results
embodied by a ball-and-stick model, the atom
probe comes closer than any other technique to
combining chemical composition with structure at
the atomic scale.
Analytical output
Having established the atom probe’s distinctness from AFM, it is now worth restating the
type of analytical information available by
atom probe microscopy. Consider a ball-and-stick
model of the atomic structure, in which different
elements and isotopes are represented by balls
of different colors. Now extend the model to
include up to tens of millions of atoms. With
two notable exceptions, this is the information
Atom probe development
The precursor to the atom probe microscope was
the field ion microscope (FIM), which was originally developed by Erwin Müller in the 1940s.
During FIM analysis, gas atoms adsorbed to the
surface of a sharply pointed specimen are ionized
by means of a strong electric field. They are then
accelerated toward a phosphor screen or other position-sensitive detector placed at some distance
from the specimen. The image created by the gas
Fig. 1 — FIM image of tungsten specimen. Bright spots indicate individual tungsten atoms on the specimen surface.
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High voltage
Position-sensitive detector
Needle-shaped
specimen
Fig. 2 — Schematic of atom probe operation. Individual atoms undergo sequential
field evaporation. Position-sensitive detector records time and position of impact.
atoms is a projection of the atomic-scale structure
of the specimen surface.
In 1955, Müller and a colleague used FIM to collect the first-ever images of individual atoms. Figure
1 shows a FIM image of a tungsten specimen. The
bright spots in the image represent individual tungsten atoms.
After years of working with FIM, Müller and his
group took the technique several steps further by
evaporating the specimen atoms themselves from
the surfaces of materials, and analyzing these atoms
by time-of-flight mass spectrometry. Drs. Müller,
Panitz, and McLane published this new process of
field evaporating and analyzing actual specimen
atoms in 1968, and it was in this paper that the term
“atom probe” was first used to describe the new
technique.
How the atom probe works
The basic components of the atom probe are
shown schematically in Fig. 2. The atom probe operates by cycling a high voltage pulse between a
needle-shaped specimen and an opposing electrode, which in this case is also a position-sensitive
detector. The significance of the needle geometry
of specimens is twofold:
• The sharp needle makes it possible to create
the high electric fields at the specimen surface
needed to induce field evaporation of atoms.
• The highly divergent electric field emanating
from the needle is the basis of the projection magnification of the image. For a specimen with a 100
nm tip radius, approximately 10 kilovolts creates
sufficient electric field at the specimen tip to pull
atoms from the surface in the form of positively
charged ions.
During atom probe analysis, the magnitude of
the electric field is carefully controlled so that one
atom at a time leaves the specimen. Note that atoms
are ionized prior to evaporation from the specimen
surface, such that atoms analyzed during atom
probe microscopy are more precisely designated
as ions. The process of field evaporating specimen
atoms (as positively charged ions) continues until
the specimen fails or until the specimen tip becomes
too blunt and the applied voltage is insufficient to
induce field evaporation.
32
The typical number of ions per analysis varies
with the configuration of the atom probe, specimen
material type, specimen shape, and other factors.
The largest atom probe data sets have been collected on metal, which is the most easily analyzed
material type due to its high inherent electrical conductivity. For metal specimens, state-of-the-art atom
probe microscopes routinely collect data sets containing more than 50 million ions.
After field evaporation, specimen ions follow the
electric field lines out to a position-sensitive detector
that records both the time and the position of impact. The identity of the atoms (ions) is determined
by measuring their flight time, which depends on
their mass; and mass determines chemical identity.
For this technique to be effective, atoms must be
evaporated in a pulsed mode so that the departure
time is known. By pulsing the voltage, the time of
departure of an ion is known, and so the total time
of flight of the ion from the specimen to the detector
can be measured.
This atom-by-atom mass spectrometry enables
the atom probe to analyze composition at the
atomic-scale.
As previously mentioned, the unique power of
atom probe microscopy lies in its ability to tie compositional information to structure. The atom probe
achieves this by recording positional information
in addition to time-of-flight for each ion analyzed.
Positional information collected during atom probe
analysis includes the two-dimensional hit position
of the ion on the detector, and a one-dimensional
sequence number.
In a manner analogous to the FIM, ions removed
from the specimen surface during atom probe
analysis create a highly magnified projection of the
atomic-scale structure of the specimen surface on
the detector. Thus, the ion’s position in x,y (the plane
parallel to the specimen surface) in the original specimen can be calculated from the two-dimensional
hit position of the ion on the detector.
Further, because atomic layers erode predictably
from the specimen surface, a sequence number
can be used to calculate an ion’s position in z (direction normal to the specimen surface) with high
precision.
One of the exceptions to the ball-and-stick model
as an analogue for atom probe results should now
be apparent. The source of positional information is
the two-dimensional projection from the curved
surface of the ion’s position in x,y. However, this
two-dimensional projection is subject to an error
that can reduce lateral (x,y) resolution to several
atomic diameters. If the specimen surface were perfectly smooth, then the projection could be arbitrarily accurate. In reality, the surface is atomically
rough and may have facets or grooves at certain
crystallographic orientations or microstructural defects. It is these imperfections that displace the ions
from a perfect projection.
The other notable exception to ideal analytical
output results from the fact that currently available
position-sensitive detectors are based on microchannel plate (MCP) amplifiers as the first line of
detection. MCPs detect about 60% of all atoms that
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Depth
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0.2
a. Ideal
0.4
0.6
Composition
b. TEM
c. SIMS
0.8
d. Atom probe
Fig. 3 — Schematic representations of material structure and analytical results. 3a shows actual structure and hypothetical
ideal analytical result for a two-component material. 3b, c, and d show stylized representation of analytical information available by TEM, SIMS, and atom probe, respectively.
strike them with equal probability. Thus, the atom
probe records approximately 60% of the ions that
evaporate from the specimen. (More detailed explanations of the atom probe technique may be
found in texts by Miller et al.)
Comparison with TEM and SIMS
Although the atom probe microscope does not
yield a perfect atomic-scale model of materials, it
comes closer to achieving this ideal than other techniques. To illustrate this point, consider a hypothetical material comprised of two elements, as
shown in Fig. 3a. Simplified representations of analytical results available from TEM, SIMS, and atom
probe analysis of the material are presented to point
out the basic differences in analytical output.
As shown in Fig. 3b, TEM can provide accurate
atomic-scale structure in projection, but only average composition. The TEM’s ability to resolve
crystal structure derives from its mode of imaging,
namely by electron diffraction from an intact specimen. Compositional analysis may be combined
with TEM by ancillary techniques such as electron
energy loss spectroscopy (EELS). EELS and other
indirect compositional analysis techniques provide
average composition.
SIMS utilizes mass spectrometry for chemical
identification of ions, and so provides compositional analysis superior to the indirect techniques
used in conjunction with TEM (Fig. 3c). However,
the improvement in compositional analysis comes
at the cost of structural information. SIMS dislodges
specimen atoms for analysis by sputtering. The
progress of the sputtering can be controlled to determine the position in z (normal to the specimen
surface) of analyzed ions to about 10 atomic layers,
but the sputtering process is such that information
about an atom’s position in x,y is limited by the spot
size of the primary ion beam to about 100 nm.
Like SIMS, the atom probe uses mass spectrometry for chemical analysis of specimen ions. Unlike
SIMS, the atom probe removes specimen ions in a
controlled manner that preserves positional information for individual ions in three dimensions. Spatial resolution of the atom probe can fairly be said to
be as good as 0.2 nm in z (normal to the specimen
surface) and less than 0.5 nm in x,y (parallel to the
specimen surface) in general analyses. Significantly
better spatial resolution has been demonstrated in
special cases. As illustrated in Fig. 3d, the atom
24 nm
a
16
12
Depth, nm
atom.qxd
Cobaltiron
Copper
8
4
b
0
0.2
0.4
0.6
0.8
Concentration, at. fraction
1
Fig. 4 — Results of atom probe analysis of multilayer thin film stack of copper
(red) and cobalt-iron (blue) layers.
probe provides more detailed structural information than SIMS, and more detailed compositional
information than TEM.
Figure 4 shows actual results of atom probe
analysis of a multilayer metal thin film stack. Many
repeats of copper with cobalt-iron layers approximately 2 nm thick are shown in the 3-D reconstruction of atom probe data. Each dot in the image
represents an individual atom. Copper atoms are
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The atom
probe’s
unique
atom-byatom
analysis
provides a
map of the
elemental
and
isotopic
identity
and
position of
individual
atoms in
volumes of
material of
up to 100
nm in
diameter.
34
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shown in red, and for clarity, both and cobalt and
iron are shown in blue. The copper layers are clearly
visible and distinct from the copper-iron layers.
Thus, the atom probe emerges as demonstrating
unique analytical capabilities that come closest to
replicating the ideal represented by a ball-and-stick
model of materials when both the compositional
and structural components of the model are considered.
Atom probe limitations
Current limitations of atom probe microscopy
are the requirements that specimens possess a certain minimum level of electrical conductivity and
have the capacity to be formed into the needleshaped geometry. These limitations increase the
difficulty of analyzing low-conductivity materials.
Alternatives to voltage pulsing as the means of inducing pulsed field evaporation and improved
methods for specimen preparation are under investigation to resolve these limitations. However,
the unique information available by atom probe
microscopy can provide new insights into structure-property relationships at the atomic- and near■
atomic scales.
For more information: Thomas F. Kelly is Chairman,
Founder, and Chief Technology Officer at Imago Scientific
Instruments, 6300 Enterprise Lane, Madison, WI 53719;
tel: 608 / 274-6880; fax: 608 / 442-0622; e-mail: tkelly@
imago.com; Web site: www.imago.com.
Acknowledgements
The work shown in Figure 4 was done in collaboration
with Peter F. Ladwig and Y. Austin Chang of the University of Wisconsin Madison and David J. Larson,
Martin C. Bonsager, Bharat B. Pant, and Allan E. Schultz
of Seagate Technology, Inc.
Bibliography
1. Z. Naturforsch, by E. W. Müller, Vol. 11a, 1956, p, 88.
2. J. Appl. Phys., by E. W. Müller, Vol. 27, 1956, p. 474.
3. “The atom probe field ion microscope,” by E.W.
Müller, J.A. Panitz, and S. B. McClean: Rev. Sci. Instrum.,
Vol. 39, 1968, p. 83.
4. Atom Probe Tomography, by M.K. Miller: Kluwer Academic/Plenum Publishers, New York, 2000.
5. Atom Probe Field Ion Microscopy, by M.K. Miller, A.
Cerezo, M.G. Hetherington, and G.D.W. Smith: Oxford
Science Publications, New York, 1996.
6. “First data from a commercial local electrode atom
probe (LEAP),” by T.F. Kelly, et al., Microscopy and Microanalysis, accepted for publication, 2004.
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