Highlights

Optimized Sample Fabrication for Atom Probe and Electron Tomography with Dual-Beam FIB

Scientific Achievement

We have developed optimized procedures on the Zeiss 1540 XB dual-beam FIB for the production of high-quality, high-yield atom probe tomography (APT) specimens of thin films grown on wafer substrates.  A highly adjustable annular mill routine was developed to control the final tip morphology, and this methodology allows us to efficiently make a series of specimens and choose the best set of parameters for a given sample.  The ability to use high resolution SEM imaging while ion milling in the Zeiss XB FIB allows us to correctly position the apex of the tip at the region of interest, which has greatly improved the quality of our APT specimens with  yields of successful APT analyses of up to 80%.  In addition, we have demonstrated that a final cleanup using 5 kV ion beam energy dramatically improves the APT mass resolution in comparison to a 15 kV cleanup.  The 5 kV cleanup results in less Ga ion damage and creates a larger tip shank angle near the tip apex, which allows the tip to cool more quickly during laser pulsing, improving the mass resolution.  These improved FIB methodologies have enabled us to make detailed investigations of magnetic tunnel junctions (MTJs) with APT, for example, on the effect of annealing (360°C for 2hrs) on the distribution of B in CoFeB/MgO MTJs.  Prior to annealing, B segregated preferentially to the MgO/CoFeB interface.  After annealing we found that B had diffused away from the MgO, improving the performance of the MTJ.  Comparison with results from other groups has allowed us to show that the annealing temperature plays a vital role in the resulting B distribution and thus the MTJ performance. 

These FIB procedures have also been adapted to produce site-specific specimens for TEM tomography.  Electron tomography benefits from a cylindrically shaped, electron transparent sample – very similar to an APT tip.  The milling routine has allowed us to produce these specimens and control the desired tip diameter to ±10 nm accuracy at the feature of interest.

Significance

Three-dimensional, nanoscale structural characterization is needed to truly understand the fundamental behavior of MTJs and other oxide heterostructures.  Previously, the use of APT and electron tomography to analyze these structures has been limited, in part, by the difficulty in preparation of reliable samples.  The FIB procedures developed in this work have drastically improved both the quality and reproducibility of the data collected by these three-dimensional techniques through precise control of the tip milling process.

Performers