Interface orientation dependent field evaporation behavior in multilayer thin films
In general, atom probe reconstruction algorithms assume a constant evaporation field across the surface of the specimen. In reality, chemical inhomogeneity modulates the evaporation field at the specimen surface, which introduces reconstruction artifacts and degrades the spatial resolution of the atom probe tomography (APT) technique. Multilayer thin films provide ideal specimen geometries to measure and quantify these artifacts. Thin films can be deposited with near atomic layer precision and can exhibit large planar surfaces with degrees of intermixing across the interfaces. Quantifying and rectifying interfacial compositional differences in atom probe data sets is critical, as such information can be used to understand the growth and intermixing of species in nanolaminate devices, such as giant magneto-resistance multilayers. A series of Fe/Ni and Ti/Nb multilayers featuring a bilayer repeat unit of equal thickness and repeat distance of approximately 4 nm have been sputter-deposited onto n-doped Si  substrates. The multilayers were focus ion beam (FIB) milled with an annular milling geometry into the required needle-shaped geometry for the APT analysis. Specimens were prepared with the film interfaces oriented with the chemical modulations for a given bilayer spacing parallel and perpendicular to the specimen apex to compare field evaporation behavior at these limiting geometries. For the Fe/Ni multilayers, the 4 nm bilayer films exhibited Fe intermixing within the Ni layers. The Electron Energy Loss Spectroscopy (EELS) based compositional profiles were used in order to cross-correlate across the two techniques using a chemical comparison. These profiles were acquired using aberration-corrected scanning transmission electron microscopy (STEM) with an approximate 100 pm electron probe. The slope of the compositional gradients between the layered interfaces showed very little differences between the EELS and atom probe data sets for Fe/Ni. In addition, little difference was observed between the parallel and perpendicular field evaporation limiting geometries. This has been contributed to Fe and Ni having a similar elemental evaporation field strength of 33 and 35 V/nm. The Fe/Ni results were compared to data obtained from a Ti/Nb multilayered thin film with a bilayer spacing of approximately 4 nm, prepared in similar parallel and perpendicular evaporation orientations as those discussed above. Compositional EELS profiles were collected using an aberration-corrected scanning transmission electron microscopy (STEM) with an approximate 100 pm electron probe. The slope of the compositional gradients between the layered interfaces showed very little differences between the EELS and atom probe data sets for Ti/Nb. In the perpendicular evaporation orientation, it was found that the layer thicknesses for both the Ti and Nb elemental layers were measured at values closer to the actual specimen, while the perpendicular orientation contained reconstruction artifacts that compressed the layer thicknesses. The EELS based compositional profiles were used in order to cross correlate across the two techniques using a chemical comparison. The compositional gradient across the interfaces was also closer to the true value in the parallel orientation, while the perpendicular orientation contained artifacts that altered the composition across the compressed interfaces. This study showed in the atom probe data there was upwards of 20 atomic % Nb was intermixed in the Ti layers. This finding was verified by the EELS compositional profiles.