Technique Development - 3D FIM

Field Ion Microscopy (FIM) is a 2-dimensional imaging technique based on the process of field ionization, which has the capacity to map the configuration of atoms on the surface of the specimen with atomic resolution. Much like in the atom probe the specimen takes the form of a very sharp needle. It is placed inside a vacuum chamber cooled to a cryogenic temperature into which a small amount of inert imaging gas such as Ne of He is admitted. A high voltage (several kV) is then applied to the specimen, causing polarized atoms of imaging gas to be attracted and absorbed to its surface. Given a sufficiently high field, the gas atoms are then ionised, before being accelerated by the electric field away from the tip, towards a micro channel plate/ phosphor screen, creating a highly magnified image of it surface. The FIM image is constructed of bright points, each representing an atom on the surface of the tip. The intensity of the spots is influenced directly by the topography of the surface of the tip, as more protruding atoms will generate a higher field around them, causing more gas atoms to be absorbed in their proximity

 

Conventionally, FIM is a 2D imaging technique since only the surface of the needle- shaped specimen is imaged. However, by sufficiently increasing the voltage applied on the tip during imaging, it is possible to field evaporate constituent surface atoms from the specimen. This enables FIM to operate in a mode such that the surface of the specimen is continually evolving as the next layer of atoms is progressively uncovered. The result of this procedure is a series of 2D FIM images that can be tomographically stacked to create a highly resolved 3D view of the crystal lattice. This method is known as 3DFIM. The development of an accurate 3D reconstruction procedure for 2D FIM images is one of the group's goals, and is currently a work in progress. 3DFIM is currently employed in our group to study atomic-scale radiation damage in candidate materials for future fusion reactors

 For more details on this project, please contact Michael Moody.