An upgrade on the site-specific FIB preparation of atom probe tips using a combination of STEM and TKD inside a dual beam microscope

Session

PSA.2 - Metals & Alloys

Authors

Dr.-Ing. Andreas Landefeld (1), Dr.mont Christina Hofer (1), Prof. Dr. Ronald Schnitzer (1)

Affiliations

1. Department Materials Science, Montanuniversität Leoben

Keywords

APT, FIB, Rotatable sample holder, Sample preparation, STEM,TKD

Abstract text

An advanced rotatable sample holder is designed in order to combine site specific focused ion beam preparation of atom probe tips with the complementary assessment of the milling process via TKD and STEM inside a dual beam microscope.

The improvements in atom probe tomography (APT) are closely linked to the possibilities in preparing samples in the shape of needles with a radius below 100 nm. In the early days, only electrically conductive materials could be investigated by APT and the approach to prepare a really defined region of interest was not possible or at least highly challenging. The development of laser-pulsed APT and sample preparation via focused ion beam (FIB) extended this exceptional technique to a wide range of materials and the defined preparation of specific features. Using FIB, typically a lamella is lifted-out of the region of interest, partitioned on multiple posts and shaped into the final tip geometry via annular milling. This method is suitable also for non-conductive materials and a limited quantity of material or unusual shape. 

A major challenge still represents the preparation of e.g. surface layers, very small features or grain boundaries which have to be exactly positioned in the tip apex for APT investigation. Transmission Kikuchi diffraction (TKD) enables to quickly obtain crystallographic information of the APT tip in between the individual milling steps and thus greatly facilitates preparation of grain boundaries or specific phases. The small excitation volume and image formation by the 10-20 nm of the surface facing the electron backscatter diffraction (EBSD) camera allow a spatial resolution down to 5 nm since grain overlaps do not pose a particular problem. However, rotation of the tip is required to get a clearer picture of the overall 3D structure. A minor shortcoming of TKD is that the outer region responsible for pattern formation is usually not completely detected in the atom probe due to its limited field of view. By contrast, a scanning transmission electron microscopy (STEM) detector inside the SEM can provide information on the interior of the APT tip since it represents a transmission image of the complete tip. 

A combination of these two techniques can greatly support site-specific sample preparation. Therefore, a new sample holder was designed which is i) suitable to be used for STEM and TKD in between the FIB milling steps, ii) able to take up different APT sample holders such as conventional tubes and half grids and iii) rotatable in order to align boundaries and obtain information from various sides of the specimen tip. 

Different examples from advanced steel research will present how the complementary information supplied by TKD and STEM further improves the site-specific preparation of APT samples inside a dual beam microscope.