About the technique
Atom Probe Tomography
Atom Probe Tomography is a microanalysis technique with the ability to reconstruct the positions and chemical identities of individual atoms within a sample. Volumes of material up to 100s of nm in size may be visualised in three dimensions, with sub-nanometre resolution and equal sensitivity across all elements. The data is reconstructed as an atomic point-cloud, with individual atoms identified and positioned in 3D space.
How it works
Specimens for the atom probe take the form of sharp needles, approximately 200nm across at the tip. A high voltage of several kV applied to these needle specimens creates a sufficient electric field at the apex to induce ionisation and field evaporation of the surface atoms. In ‘laser mode’ a UV laser pulse is targeted at the specimen tip and triggers the evaporation process at a well-defined time. The evaporated ions are accelerated by the field and projected on to a two-dimensional detector. The recorded impact information is sufficient to determine the original location and mass-to-charge ratio of each atom, resulting in an atomic-scale 3D reconstruction of the original sample.
Specimen preparation
The sharp needle specimens may be prepared by standard electropolishing techniques in the case of conductive materials. However, for most samples analysed in the GAP facility a focussed-ion beam (FIB) milling technique is used. In this process, employing a FIB/SEM and in-situ micro-manipulator, a small volume (a few cubic µm) is removed from a polished sample surface and mounted on a pre-fabricated post. This is then sharpened to form an atom probe needle specimen by further ion-beam milling. Almost any material type can be prepared by this means. It also has the significant advantage of allowing targeted preparation by positioning a particular feature or region of interest within the specimen apex.
Workflow
Samples are typically prepared and loaded into the atom probe in batches (5-15 tips), which take a day of preparation and a day or more of analysis. The time required for reconstruction and analysis of the results depends on the complexity of the data and the information required from the sample.
Atom Probe Tomography is a microanalysis technique with the ability to reconstruct the positions and chemical identities of individual atoms within a sample. Volumes of material up to 100s of nm in size may be visualised in three dimensions, with sub-nanometre resolution and equal sensitivity across all elements. The data is reconstructed as an atomic point-cloud, with individual atoms identified and positioned in 3D space.
How it works
Specimens for the atom probe take the form of sharp needles, approximately 200nm across at the tip. A high voltage of several kV applied to these needle specimens creates a sufficient electric field at the apex to induce ionisation and field evaporation of the surface atoms. In ‘laser mode’ a UV laser pulse is targeted at the specimen tip and triggers the evaporation process at a well-defined time. The evaporated ions are accelerated by the field and projected on to a two-dimensional detector. The recorded impact information is sufficient to determine the original location and mass-to-charge ratio of each atom, resulting in an atomic-scale 3D reconstruction of the original sample.
Specimen preparation
The sharp needle specimens may be prepared by standard electropolishing techniques in the case of conductive materials. However, for most samples analysed in the GAP facility a focussed-ion beam (FIB) milling technique is used. In this process, employing a FIB/SEM and in-situ micro-manipulator, a small volume (a few cubic µm) is removed from a polished sample surface and mounted on a pre-fabricated post. This is then sharpened to form an atom probe needle specimen by further ion-beam milling. Almost any material type can be prepared by this means. It also has the significant advantage of allowing targeted preparation by positioning a particular feature or region of interest within the specimen apex.
Workflow
Samples are typically prepared and loaded into the atom probe in batches (5-15 tips), which take a day of preparation and a day or more of analysis. The time required for reconstruction and analysis of the results depends on the complexity of the data and the information required from the sample.