Upload
others
View
8
Download
0
Embed Size (px)
Citation preview
Atom‐by‐atom in 3D: Atom Probe Tomography
Stephan Gerstl, Roger Wepf
Scientific Center of Optical & Electron Microscopy, ETH Zürich
History and Introduction
Considering the time that has passed since Erwin Müller and his students were the first to view
individual atoms by Field Ion Microscopy (FIM), atom probe tomography (APT) has grown
significantly from the days of FIM. The history of APT’s development is likely older than most would
believe, considering the characterization technique identifies individual atoms in 3‐dimensions with
nanometer resolution. It is actually a fascinating story including recognizable names such as Gustav
Hertz (Müller’s PhD advisor), Max Knoll, and Ernst Ruska; the latter two gentlemen were working on
an ‘electron microscope’ at about the same time in the 1930’s at the same institute in Berlin. [1]
In brief, we learned early‐on that like charges repel each other and move as far apart as possible –
think of iron powder on the end of a bar magnet. The iron powder grains in this state follow field
lines, spreading radially outward from either end of the bar. Now replace the bar magnet with a
sample material of interest, then shrinking its size ca. 100,000 times and shaping it into an extremely
sharp needle, fig.1a. When applying high voltages to this needle – creating intense electric field lines
– gas atoms near the needle point (apex) become ions. These ions travel approximately along field
lines (visualized with the bar magnet example above) towards a 2D detector resulting in the very
first images of individual atoms by Müller and colleagues in 1955. This atomic scale imaging was
called Field Ion Microscopy.[2] By applying short voltage pulses to the apex via a counter electrode
approximately 30 µm away (fig.1b), the needle’s outermost atoms become ionized, following a
trajectory in ultrahigh vacuum from the apex towards the 2D detector (arrows in fig.1b). In this way
the specimen materials’ atoms can be individually detected, with their relative positions on the
detector and mass‐to‐charge states being recorded by measuring their time of flight (TOF).
Repeating this process billions of times effectively erodes the specimen needle literally atom‐by‐
atom, then successively digitizes the positions of the atoms laterally (x,y) – with the z‐dimension
encoded by their sequential arrival – on the detector. With the modern automation in Local
Electrode Atom Probe (LEAP©) tomographs, millions of atoms can be collected in an hour and with
the computing power available today, we can reconstruct the specimen from its millions of atoms
into a 3‐dimensional image, or as they can be referred to: an ‘atom map.’
Due to the high electric fields the samples are subject to during data collection (upwards of 10
V/nm!), intense field induced stresses of GPa can occur in the top tens of nanometers of the
material. Here the electrical and/or thermal conductivity of the material is often an advantage.
There is intense development ongoing to enable new materials classes to be analyzed e.g. using a
pulsed UV‐laser instead of an electrical pulse to ionize atoms at the apex. The following examples
present an idea of the tremendous insights gleaned from applying APT to a variety of material
systems.
Figure 1: To give perspective: (a) a photomicrograph comparing an APT specimen (left) with an
ordinary needle (right) captured at the same magnification [2]. (b) An APT specimen needle aligned
to the local electrode aperture, which is responsible for ionizing atoms on the specimen’s apex.
These ions then fly to a multichannel plate (arrows) where their coordinates & masses are detected
depending on their takeoff angle and time they traveled (TOF), respectively.
Contemporary Applications
Although the regions of interest revealed through APT are relatively small, the high sensitivity across
the whole periodic table alongside the high spatial resolution, makes it an increasingly sought after
characterization technique, exemplified by the following applications.
A classic application for APT is in the study of metallic alloys to investigate their micro‐ and nano‐
structural & chemical development with various processing treatments. One example recently
published by ETH involving Al‐Mg‐Si based alloys, demonstrates the high level of detail APT can
reveal about a potentially large scale application.[3] It was found that by adding a trace amount of
Sn to the alloy, negative effects of natural pre‐aging are hampered. Natural pre‐aging occurs
commonly and on a massive scale: such as the room temperature (RT) diffusion occurring while
automobile parts are stored in a warehouse awaiting further treatment. Due to the minor addition
of Sn, the structures can safely be stored because the Sn binds to quenched‐in vacancies, restricting
RT diffusion of Mg and Si (which would otherwise form unwanted clusters). During the impending
higher temperature artificial heat treatments, the vacancies are then released and diffusion remains
unimpeded, which is essential to develop the strengthening precipitates within the alloy (fig.2).
Figure 2: Atom probe tomography data, in the form of atom maps (note each colored dot is the
location of one atom), showing the ultra‐fine structure of a specially designed Al alloy – potentially
to be used for automobile parts. [3]
Particularly in industries where nanometer size features and structures are more and more in focus,
characterization of materials near the atomic scale is essential. Here APT beside Electron
Microscopy became an indispensable tool to characterize metal alloys, semiconductors, and other
nano‐structured matter. Point‐ and line‐defects particularly in semiconductors – better known as
vacancies and dislocations, respectively – can (and will increasingly) have serious influences on the
operation of our daily computers and smartphones, which are soon using structures below 20nm on
their chips. This is demonstrated in the measured segregation of phosphorus to such defects in Ge
grains (fig. 3).[4] Note that each purple dot in figure 3 represents the position of one P atom (the
matrix atoms are hidden for clarity). Data analysis methods allow us to draw iso‐concentration
surfaces (dark purple shades) enabling the quantification of these local concentrations, revealing a
jump of two orders of magnitude within the defect.
Figure 3: Atom maps of Phosphorus atoms segregating to 0‐ and 1‐dimensional defects within Ge. [4]
The efficiency of record breaking solar cells may also be better understood by enabling the
quantification of atomic species at particular grain and phase boundaries (fig.4). EMPA has used the
LEAP to investigate localized chemistry at grain boundaries in their highly efficient multi‐layer solar
cell structures.[5] The quantification of elements at these critical locations within the structures aid
in improving the solar cell’s yield.
Figure 4: A grain boundary within a solar cell structure, revealing localized segregation of both
Chlorine and Sulfur.[5] Such grain boundaries are largely responsible for effective charge transfer
within the material.
Volumes that are analyzed will depend on the ROI and material type, but typically range from 10’s of
nm laterally and 50 nm in depth to 200 nm laterally and 100’s to 1000 nm in depth. These ROIs are,
for the most part, localized & extracted using FIB/SEM techniques, so targeted harvesting of samples
is the regular atom probe tip preparation process.[1] Conductive materials that express their ROI in
high number densities can also be electropolished into sharp wires.
Figure 5. An atom map (red: Ge atoms) overlayed on electron microscope images of the original APT
needle specimen and a TEM image of the Si‐Ge based multi‐layered quantum well structure showing
alternating thick and thin layers.
Conclusion
We’ve shown that APT expands analytical structure research into a new dimension and can resolve a
variety of materials atom‐by‐atom in three dimensions. Nevertheless, it has limitations like any
technique, and thus correlating APT with other techniques enables more comprehensive views of
the specimen material (also over larger length scales). An ideal synergy exists between electron
microscopy (EM) and APT: EM delivers morphology, crystallography, and possibly bonding
information, whereas APT provides the localized high resolution chemical information in 3‐
dimensions. The relatively undistorted morphology from 2D and 3D EM provides information for
improved reconstruction protocols of the APT data. Such a synergy is demonstrated in fig. 5., where
the Ge‐layers commonly appear arched after the first reconstruction cycle, with the exact
morphology such arching can be avoided.
This correlative imaging and analytical approach is one of the focuses currently in the APT field and
at ETH, to be able to describe selected ROIs on the atomic scale with highest precision. Furthermore
improving APT ionization methods to reproducibly yield ions is a current area of research in APT
technology as well as adapting sample preparation techniques to cryo‐EM methods to one‐day
enable the analysis of frozen‐hydrated materials as “needles” in the LEAP located at ETH Zürich (fig.
6).
Everyone is welcome to contact us for consultation and discussion for their applications in
characterizing their materials, atom‐by‐atom in 3‐dimensions.
Figure 6. Instrument layout of the unique local electrode atom probe (LEAP) instrument with its
auxiliaries at ETH Zürich.
References
[1] D.J. Larson et al., Local Electrode Atom Probe Tomography, Springer (2013).
[2] E.W. Mueller, Atoms Visualized, Sci. Amer., vol. 196, issue 6 (1957).
[3] S. Pogatscher et al., Diffusion on Demand to Control Precipitation Aging: Application to Al‐Mg‐Si
Alloys, Phys.Rev.Lett. 112, 225701 (2014).
[4] R. Geiger et al., Excess carrier lifetimes in Ge layers on Si, Appl.Phys.Lett. 104, 062106 (2014).
[5] L. Kranz et al., Tailoring Impurity Distribution in Polycrystalline CdTe Solar Cells for Enhanced
Minority Carrier Lifetime, Adv.Energy Mater., 4 (2014).