Field ion microscopy (FIM)

About this technique

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Field ion microscopy (FIM) permits inspection of the location of atoms on the surface of a needle-shaped specimen tip with a radius of around 50 nm. It was the first technique, developed in 1951, that permitted clear observation of atoms. The imaging is achieved by backfilling an ultra-high vacuum analysis chamber with an imaging gas, such as helium or neon, which adsorbs to the specimen tip, and then desorbs through field-ionisation and is directed to a detector by an electric field. The specimen is kept at a temperature of between 20 and 80 K during observation depending on field-ionisation conditions. The FIM instrument is a point-projection microscope – the desorbing gas atoms project in a near perpendicular direction from their original point upon the surface of the specimen tip. As a result, atomic resolution may be clearly discerned, corresponding to a magnification of a few million times. FIM is typically applied to crystalline and amorphous metal alloys and semi-conductor devices. It has selective applicability to polymers and biological specimens and these sample types are at the frontier of research in this area.

The projected image from FIM can qualitatively communicate atomic structure and chemical environments. Select in-situ observations may also be made, for example, of corrosion or catalysis chemical reactions. Field-ionisation of the specimen surface atoms may be performed in parallel to observing the surface using the imaging gas. Voltage or laser pulses are used to field-ionise the specimen atoms for conductive or semi-/non-conductive materials, respectively. The ionisation of the specimen's surface atoms reveals more of the internal structure of the material for inspection. Applications for FIM typically include investigating the variables involved in treatment or processing parameters (thermal annealing time, composition alterations, etc.) and identifying the physical and chemical structures that relate to a material's behaviour (e.g. mechanical or electrical).