New Methods for Measuring Chemistry and Temperature Using Scanning Ion and Electron Beams

Abstract

We describe two new methods we have developed for measurement of local chemistry and tempertaure using scanning ion and electron beams.In the first method, we implement ion-induced Auger Electron Spectroscopy (AES) using a focused ion beam (FIB), with the eventual goal of developing a new high spatial resolution and high chemical sensitivity tomographic technique. While FIB tomography has emerged as a powerful and widely-used method for recreating 3D reconstructions of the internal structure of materials ranging from length scales of tens of nm to tens of microns, corresponding quantitative chemical methods are lacking. Integration of secondary ion mass spectroscopy (SIMS) with FIB is hampered by the fact that secondary ionization yields with Ga + ion beams in the FIB are very low (10 -5 or less) for many elements [e.g. 1,2]. This is further accentuated by the relatively low transmission factors of standard SIMS detectors such as quadrupole mass spectrometers, although higher transmission factor detectors such as time-of-flight (TOF) systems have been successfully applied [e.g. 2,3]. These factors compromise the ability to generate chemical information from small volumes of material. The fundamental advantage of coupling AES to the FIB is that Auger electron yields per incident ion can be relatively high, of order 10 -1 [4]. Coupled with the high transmission factors of Auger electron detectors, such as hemispherical analysers, this provides the potential for extracting high sensitivity chemical information from small volumes of material. We have successfully integrated an Orsay Physics Cobra mass-selecting FIB column into an ULVAC-PHI VersaProbe system, aligning the focal points of the FIB and of the detector optics with the necessary 3D precision. We use an Au-Si alloy source separated into Si + , Si 2+ and Au + , Au 2+ beams. Depending on the atomic number of the target specimen, this is necessary to ensure that the majority of the Auger transitions are from the atoms of the sample rather than in the primary beam. Example spectra for multiple elements with a primary 60 keV Si 2+ ion beam are shown in Figure 1. Strong Auger peaks are observed for each element, but for the Mg, Al and Si samples, additional extremely sharp peaks are observed (arrowed). These peaks arise from Auger emission from atoms that have been sputtered from the surface before Auger decay, and thus are much more spectrally sharp than for Auger emission from the typical case of atoms that remain in the near surface region of the sample. The fact that these free atom peaks are only observed in a subset of the samples can be understood in terms of the product of the kinetic energy of the sputtered excited atoms and the substantially longer vacancy state lifetimes, and thus probability of escape of the target atom from the surface, for the relevant elements and vacancy states. In our experimental geometry and conditions, we observe an average Auger yields of 0.06 for Cr and 0.09 for Al per incident 60 keV Si 2+ ion. This compares very favourabl...