Revealing the 3-dimensional shape of atom probe tips by atomic force microscopy.

For the very first time, atomic force microscopy is used to determine quantitatively the 3-dimensional shape of an atom probe tip, which is key towards improved accuracy and understanding of artefacts in atom probe tomography. We have successfully measured by atomic force microscopy the apex and shank region of 3 different atom probe tips, of which two show (severe) deviations from a hemisphere due to either non-uniform laser light absorption or the presence of two different materials. Clearly, our method which overcomes the challenge of aligning two very sharp tips on top of each other, offers new pathways to study physical mechanisms in (laser-assisted) atom probe. It represents an important step towards improved reconstruction algorithms as the image formation in atom probe tomography is based on the intricate link between the tip shape (down to the atomic level), the electric field distribution and the ions' flight path towards the detector. Further on, present reconstruction algorithms solely account for a hemispherical tip shape, which does not hold true for most applications and results in complex artefacts. Therefore our method is an attractive novel approach to assess the 3D tip shape.