Invited Paper HI-WeA7
Spectroscopy in the Focused Ion Beam

Wednesday, November 1, 2017, 4:20 pm, Room 7 & 8

We review spectroscopic methods in the focused ion beam (FIB), and introduce the coupled Auger Electron Spectroscopy (AES) – FIB technique. While FIB tomography has become a widely-used method for exploring 3D structure of materials over length scales ranging from tens of nm to tens of μm, complementary high resolution and high sensitivity spectroscopic methods are lacking. Secondary ion mass spectroscopy (SIMS) methods are limited by low ionization yields using conventional Ga⁺ liquid metal ion source (LMIS) species and/or by low detector transmission factors. The anticipated advantage of coupling AES to the FIB is that Auger electron yields per incident ion can be in the few percent range depending on the experimental conditions, improving on Ga⁺ ionization yields by several orders of magnitude for many elements. We have integrated an Orsay Physics Cobra mass-selecting FIB column into a PHI VersaProbe X-Ray Photoelectron Spectroscopy (XPS) system, successfully aligning the focal points of the FIB and of the detector/analyzer optics with the necessary precision in 3D dimensions. Using primary ions with different masses (e.g. from an Au-Si alloy source), we can control the relative proportions of the Auger transitions from the atoms of the target sample rather than from backscattered/implanted atoms from the primary beam. We have studied a set of elemental targets, with strong Auger peaks observed from each. For some elements (e.g. Mg, Al and Si), additional extremely sharp peaks are observed, superimposed on the standard Auger peaks. These are due to Auger emission from atoms that have been sputtered from the surface before the inner shell vacancy is filled. The occurrence of these free atom peaks in a subset of the samples can be understood in terms of the substantially longer vacancy state lifetimes in the core levels of some elements, allowing the target atom to escape from the surface field before Auger decay happens. For example, we observe average Auger yields of 0.06 for Cr and 0.09 for Al per incident 60 keV Si₂⁺ ion. Coupled with the estimated transmission factors and solid angular detection range of the XPS hemispherical analyser employed, this translates into detection of atomic concentrations of order 0.1-1.0 % within a (50 nm)³ voxel. These figures of merit will be compared (favorably) to other spectroscopic methods available in the FIB.