Techniques

• Field Ion Microscopy
• Atom Probe Tomography
• Specimen preparation

 

Field Ion Microscopy

 

In Field Ion Microscopy (FIM) a needle-shaped specimen with an end radius smaller than 100nm is cooled down to cryogenic temperature in a ultra-high vacuum chamber. Subsequently, a small amount (1/100,000,000 of an atmosphere) of an inert gas such as helium is admitted into the chamber, and the specimen is subjected to a high voltage of a few kV. The high electric field induced at the specimen apex is enough to ionise the gas. The positively charged gas ions are projected away from the positively charged specimen and hit a phosphor screen. The very high curvature of the specimen generate a highly magnified image, typically 1 million times, and gives a picture of the positions of individual atoms on the specimen surface. Example of FIM images can be found in image gallery section.

Atom Probe Tomography

Atom probe tomography (APT) also requires a needle shaped specimen and exploits the effect of a high electric field. When the electric field reaches a critical value, the atoms of the surface of the specimen are ionised and desorpted in a combination called field evaporation. The ions generated are subsequently accelerated away from the specimen surface by the highly divergent elecric field surrounding the specimen, giving also rise to a highly magnified projection of the ions. APT can be considered as being a point projection microscope that enables the observation of single atoms ionised and desorbed from the surface of the specimen via the application of a very intense electric field. Coupled to a time-of-flight mass spectrometer and using a position sensitive detector (PSD), the atom probe becomes a nano-analytical microscope capable of mapping the distribution of the atoms in three dimensions. Being constrained by the transmission of a high voltage pulse of a few nanoseconds through the specimen, the field of application of the atom probe was initially limited to metals, metallic alloys or highly doped semiconductors. However, the recent implementation of fast pulsed lasers on atom probes has greatly broadened the field of application of the instrument to include very brittle or poorly conducting materials.

Details of the atom probe microscopes available in the FIM Group are available on the facility page, and example of results are shown in the image gallery section.

Specimen preparation

Specimen fabrication for FIM and Atom Probe Tomography used to be prepared by electro-chemical polishing.

During the first stage of electropolishing the specimen in the form of a wire or rod (0.5mm x 0.5mm x 15mm) is moved up and down through a thin layer of electrolyte (e.g. 25% perchloric acid 75% acetic acid). As material is removed a necked shape is formed. The second stage of polishing uses a weaker electropolish to slowly remove material uniformly until the specimen fractures at the neck under its own weight. This produces two sharp needle-shaped specimens with an end-tip radius of approximately 50nm.

These techniques are, however, not suitable for site specific investigation or in the case of poorly conducting materials. APT specimen preparation techniques using Focused Ion Beam (FIB) were thus adapted from TEM specimen preparation. For example, using a roughly-electropolished needle, a tip suitable for analysis in APT can be prepared on a specific site, shown here is a specimen preapred along a grain boundary in a steel. Annular masks are used to progressively sharpen the needle, reducing its diameter down to below 100 nm.

Otherwise, using a combination of a micromanipulator and a deposition source within the FIB microscope, it is possible to perform so-called lift-out of a small beam of material that can then be simply mounted on a grid and directly sharpened. This beam can also be cut into small wedges mounted on an array of pre-sharpened tips, producing several specimen from a single beam. The wedges are finally sharpened using annular milling to form a tip.

 

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