The atom probe is a point projection microscope capable of resolving the chemical
identity and position of individual atoms in 3D with atomic resolution in the
z direction and subnanometer lateral resolution. The tool usually requires a
needle shaped sample of solid conducting material.
The atom probe is capable of both analyzing a sample with sensitivities that
no other system can match and providing a positional accuracy of individual
atoms that is unmatched. The analytical information is obtained by counting,
identifying and spatially locating individual atoms.
Other competitive analytical tools function by either impinging the surface of a sample with a species and obtaining information from the reflection or transmission of the species (XRD, SEM, TEM) OR impinging the surface of a sample with a species and driving off the constituent atoms of the sample (sputtering) in order to identify the chemistry of the material present (RBS, SIMS).
Compared to the atom probe, these methods are limited in their lateral and spatial resolution - the minimum length scale they can analyse is defined by the size and depth penetration of the incident beam. They determine concentrations and compositions of atoms present by analysing the signal from a minimum volume defined by the beam dimensions and characteristics. But they cannot be used to determine the exact 3D position of the atoms within the sample (XRD, RBS, SIMS, TEM, SEM).
What the atom
probe uniquely does is to evaporate the atoms themselves from their positions
in the lattice and then chemically identify and spatially locate them within
the sample. As the atoms are removed form the surface, layer-by-layer, it is
possible to get a true 3D image of the sample - UNIQUE.
Due to continued technological advance such as the growth in nanotechnology,
IC development and metallurgical development, critical features are now on the
nanometre scale. These features can be completely resolved by the atom probe
and the analytical information required approaches the exact strengths of the
atom probe. For example, on a project for Seagate it was necessary to chemically
identify and spatially locate the individual atoms in order to resolve a manufacturing
problem. Only the atom probe could provide the required analytical capability.
Similar analytical challenges exist with alloy steel (nuclear reactor pressure
vessels), superalloys, IC's and, of course, nanotechnology.
10exp(19) Atoms/cc doped Silicon (Blue), Reconstructed Volume 20nmx20nmx5nm,
Boron Atoms in Green (11 total), Courtesy
of Professor Alfred Cerezo, Oxford University
The basic principle of the 3DAP is very simple. A specimen of material, in the form of a sharp needle, is held at cryogenic temperature (<100K) in an ultra-high vacuum chamber (approximately 10-10 mbar). High DC voltage is applied, plus nano-second long pulses of high voltage. These raise the field at the tip of the specimen such that individual atoms are ionised and removed. The ions travel to the detector via a reflectron energy-compensating lens, which increases the mass resolution to M/DM = 500 (full-width at half maximum). At the detector, the time-of-flight and the spatial location of each ion is recorded providing the chemical identity of each atom and it’s 3-dimensional position to single atom depth resolution and sub-nanometre lateral resolution.
Extremely Accurate (are you correct / how closely you match a standard?) - Ability to correctly identify atoms present. Unlike the X-Ray and SEM techniques, which are limited by the actual physical size of the analysed volume, the atom probe evaporates individual atoms and chemically identifies each. It is therefore extremely accurate.
Very Precise (are your results tightly distributed / repeatable?) - the SD on the composition of a sample is a function of the number of atoms counted: to a first approximation, with very low concentration of dopant species, SD(n) = {(n)^1/2}/N where n is the number of atoms detected and N the total number of atoms counted. From a sample size (N) of 1e6 atoms, the 3DAP can routinely provide compositional information with a SD of +/- 0.0001
Very Sensitive (how small a concentration can you detect?) - depending on the setup, the 3DAP can achieve sensitivities of 10ppm. Sensitivity is a function of both the mass resolution and the number of atoms counted. This will improve significantly with the next generation tools, which will have improved mass resolutions and which will be able to collect and analyse an order of magnitude more atoms.
Positional Sensitivity (where were these atoms located within the sample?) - Because of the way they are evaporated and collected, the positions of the individual atoms within the sample are obtained with subnanometer lateral resolution and the depth resolution of a monolayer.
No other tool can currently match the above combination of detailed information. All this matters to any materials manufacturer or designer who is developing or working with advanced materials for sensitive applications. Advanced materials are those in which performance critically rests on the position of individual atoms at the nanoscale. A sensitive application is one where the costs of non-performance are very high.
Superalloys and Nuclear Reactor Pressure Vessel Steels:
Ongoing advances in materials have revealed that many of the critical features
such as obstacles to dislocation movement in metals must be introduced and manipulated
on the nanoscale. Consequently modern Aluminium alloys and high strength steels
are designed with high densities of nanometre-sized particles. The size and
location of these particles are critical to the lifetime performance of these
materials. Until recently much of the development and failure analysis of these
materials required a process of trial and error due to lack of nanoscale analytical
techniques. The atom probe is the only tool with the analytical capabilities
to identify and spatially locate these individual particles. The atom probe
provides a unique window into this otherwise hidden world enabling design and
manufacturing optimization to proceed much more efficiently saving millions
in development costs.
Semiconductors:
In the Semiconductor industry, development is driven by adherence to Moore's
Law. The performance improvements are achieved by reduction in feature sizes
and are increasingly sensitive to nanoscale variations in the materials. The
continued advances requiring increasingly detailed metrology and analytical
capabilities. Anticipated shortfalls in analytical capabilities are identified
in the International Technology Roadmap for Semiconductors (ITRS). This document
records the areas for concern if development is to continue at the current rapid
pace dictated by Moore's Law. The atom probe is particularly well suited to
the analysis of the interfaces in these materials. The ability to reveal interface
roughness and layer thickness is particularly unique. Other tools cannot distinguish
between a rough surface and an inter-diffuse interface. The atom probe can.
This rests on its ability to chemically identify and locate all atoms in 3D.
The ITRS roadmap reports a number of areas of concern such as thin film metrology
for which the atom probe is uniquely suited.
Nanotechnology:
In nanotechnology where materials are manipulated on the nanoscale in order
to influence material properties there is a huge need for appropriate analytical
techniques with which to support and drive this development. The atom probe
finds unique application in any nanoscale spatial and chemical analysis of conducting
material. The terminology nanotechnology includes any manipulation at the nanoscale
hence the above alloys and semiconductor examples are also examples of nanotechnology.
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