What is EBSD analysis?
Electron backscatter diffraction (EBSD) is a SEM (scanning electron microscopy) technique which can be used to investigate the structure of crystallographic materials [1-4]. It is known as a "surface" technique, because diffraction of the backscattered electrons occurs only within tens of nanometers of the sample surface. Therefore, to obtain EBSD patterns, the sample surface should be crystalline and free of any damage or contamination from preparation.
Challenges of EBSD sample preparation
Normally to prepare a specific region of a material, such as a cross section, for analysis, methods like focused ion beam (FIB) are used. However, these methods typically do not allow accurate and reliable analysis of “mixed” or composite materials when using techniques like EBSD. The problem is deformation below the surface which leads to a curtaining effect [5]. This effect is especially problematic for multi-phase composite materials, as each material has different properties and milling behavior [5]. FIB preparation can result in varying thicknesses for the various materials and cause the resulting sample surface to have lines or be irregular and rough (curtaining effect).
Methods
Materials
The samples analyzed were from a M4 CPU (central processing unit of a computer) with gold (Au) wires embedded in a silicon (Si) matrix with tungsten (W) (refer to figure 1a) and a composite material composed of aluminum (Al), diamond, and graphite (C) (refer to figure 1b) [6]. Cross sections of the materials were prepared with the methods described below.
Cross sectioning by slope cutting with broad ion-beam milling
Sample cross sections were prepared by first using sawing, mechanical milling, grinding, and polishing, which was performed with the EM TXP (refer to figure 2a), to reach the area of interest in a very short time [5]. Then, broad ion-beam milling was done, using the EM TIC 3X (refer to figure 2b), to attain a cross section with a high-quality surface which is ready for EBSD analysis [6,7].
Imaging and analysis
SEM imaging and EBSD analysis of the sample cross sections were done with an ARGUS FSE/BSE (forward/back scattered electron) system.
Results
Electronic component: CPU
The EBSD analysis was done only on areas of the CPU’s Au wire where it was highly deformed (refer to figure 3a). FSE images reveal that there is a bit of curtaining effect, however, none of the map data, specifically, the EDS hypermap and misorientation average and kernel map, show any appreciable curtaining, i.e.., there is no structure which follows the curtaining effect (refer to figure 3 below). So the map data indicate that broad ion-beam milling enables a high-quality cross-section surface to be prepared, as it did not cause noticeable sub-surface damage.
Composite material: aluminum/diamond/graphite
The Al/diamond/C composite cross section was examined using BSE imaging, EDS (energy dispersive x-ray spectroscopy), and EBSD with phase and inverse pole figure (IPF) maps along the X axis. The results reveal a high-quality surface preparation for the Al matrix, graphite flakes, and diamond grains with no appreciable curtaining effect (refer to figure 4 below).
Conclusions
Although the focused-ion-beam (FIB) technique is often used for site-specific preparation of material samples, it usually prevents successful EBSD analysis from being done due to the introduction of sub-surface deformation and curtaining. This fact is especially true for multi-phase composite materials. Here, it was demonstrated that broad ion-beam milling allows a high-quality preparation of both hard and soft material simultaneously. The combined use of the EM TXP and the EM TIC 3X allows users to prepare high-quality, large-area samples from very challenging “mixed” or composite crystalline materials in a short time. When such samples are analyzed with EBSD, useful results are obtained.
Acknowledgements
We would like to thank Andi Kaeppel and Roald Tagle for contributing the photo of the gold-wire-bonded M4 CPU processor shown in figure 3a.
