Selection of Candidate Crystallographic Phases

EBSD requires the possible phases that will be analyzed to be selected before the analysis is started. Generally, EBSD is conducted on samples that are already well characterized with respect to the phases that are present. Modern systems are capable of sorting through a large number of phases to match with the experimental patterns, but the operator should try to keep this list to a minimum to allow maximum speed of acquisition. There are many databases that provide

29 crystallographic data and it is also possible for the operator to input specific descriptions of unit cells.

Microscope Operating Conditions and Pattern Optimization

It is difficult to recommend specific operating conditions for EBSD of all samples, but there are starting conditions that should allow the system to be set up efficiently. It is suggested that 20 kV and a beam current of a few nanoamperes is a good starting point for EBSD analysis. The quality of EBSD patterns can be rapidly assessed under these conditions. For faster acquisition higher beam current is always better, as long as the resolution is consistent with the microstructural length scales that are to be studied. Higher operating voltages are also sometimes useful with coated samples, and lower voltages may be used to provide an improved spatial resolution at the expense of acquisition speed. The operator should strive for clear patterns. In most commercial systems, the operator has a choice of the pattern resolution that is to be collected. For many orientation studies, the largest number of pixels is almost never needed and the EBSD detector is binned to produce larger pixels. For example, a typical EBSD detector may have a maximum pixel resolution of 1600 × 1200, but one would not use the full resolution and would select to bin the result; so, for example, 4 × 4 binning would result in an EBSD pattern with 400 × 300 pixels. Binning helps with pattern quality as larger pixels collect more signal and thus increasing the S/N of the pattern. Binning of the detector allows higher speed acquisitions to be achieved. Additional increases in pattern quality can be achieved at the expense of collection speed.

Once the detector settings have been determined it is also necessary to select the background removal method. Modern EBSD systems have many methods for background removal while older systems will be limited. Correct background correction is important to maximize the signal content of the patterns while suppressing the high background contribution that is always present. For polycrystalline samples, it is easy to scan a large representative region of the sample which collects the average background levels without the sharp diffraction features. Other methods that utilize a software blurring algorithm may also be utilized and can be better than the collected back ground method.

Now that the sample and the detector position are set and beam conditions that provide useful EBSD patterns are established, it is now time to calibrate the system. Calibration on

modern systems is entirely automatic provided a suitable match unit has been specified. It is important that once a calibration has been established that the sample to detector geometry not be altered or a new calibration will need to be determined.

Selection of EBSD Acquisition Parameters

Successful orientation mapping will depend on careful selection of the mapping parameters and the most important of these is specifying the step size or the spacing between individual measurement points. A selection of a spacing that is too large risks missing the important microstructural features and a spacing that is too small will require longer acquisition times with little gain in information. A good starting point is to plan on between four and ten pixels or measurements across the smallest features to be studied. This sampling will provide quality images and data while not wasting time acquiring redundant information.

At this time it is useful to acquire electron images of the region of interest. Secondary electron imaging may show some surface features but imaging of highly polished samples may not provide useful information. The use of forescattered detectors is recommended as there is a good signal level and forescattered images often show surprising high grain contrast.

Collect the Orientation Map

Now that the experimental conditions for the EBSD acquisition have been selected it is often best to collect a small map to determine if the parameters selected are capable of producing a quality result. One of the most common ways to judge a quality result is to look at the number or the fraction of pixels in the map that have been successfully indexed to a certain level of confidence. Modern systems on well-polished samples can be capable of indexing 95 % or more of the pixels. Of course second phase fractions and grain size can influence the number of indexed patterns. It is not always necessary to have 95 % of the pixels indexed to obtain a useful result. If a high fraction of the pixels are not indexed it is important to understand the reasons. If the correct phase or phases have been selected then it may be that the system was not calibrated adequately. If the selected phases are correct and the calibration is correct then it is possible that sample preparation was not optimal or the sample is heavily deformed, leading to the low fraction of indexed pixels. Once a satisfactory indexing rate is achieved in the test map it is now reasonable to select a larger area for orientation mapping and proceed with mapping.

References

Brewer L, Michael J (2010) Risks of ‘cleaning’ electron backscatter data. Microsc Today 18:10

Britton T, Jiang J, Guo Y, Vilalta-Clemente A, Wallis D, Hansen L, Winkelmann A, Wilkinson A (2016) Tutorial: crystal orientations and EBSD – or which way is up. Mater Charact 117:113

References

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Dingley D, Wright S (2009) Phase identification through symmetry determination in EBSD patterns. In: Schwarz AJ, Kumar M, Adams BL, Field DP (eds) Electron backscatter diffraction in materials science, 2nd edn. Kluwer Academic/Plenum Publishers, New York, p 97

El-Dasher BS, Torres SG (2009) Electron backscatter diffraction in low vacuum conditions. In: Schwarz AJ, Kumar M, Adams BL, Field DP (eds) Electron backscatter diffraction in materials science, 2nd edn. Kluwer Academic/Plenum Publishers, New York

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Kamaladasa R, Picard Y (2010) Basic principles and application of electron channeling in a scanning electron microscope for dislocation analysis. In: Mendez-Villas A, Diez J (eds) Microscopy: Science, Technology, applications and education. Formatex: Spain

Keller R, Geiss R (2012) Transmission EBSD from 10 nm domains in a scanning electron microscope. J Microsc 245:245

McKie D, McKie C (1986) Essentials of crystallography. Blackwell Scientific Publications, Boston

Michael J (2000) Phase identification using EBSD in the SEM. In: Schwarz AJ, Kumar M, Adams BL (eds) Electron backscatter diffraction in materials science. Kluwer Academic/Plenum Publishers, New York, p 75

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30.1\ Introduction

The use of focused ion beams (FIB) in the field of electron microscopy for the preparation of site specific samples and for imaging has become very common. Site specific sample preparation of cross-section samples is probably the most common use of the focused ion beam tools, although there are uses for imaging with secondary electrons produced by the ion beam. These tools are generally referred to as FIB tools, but this name covers a large range of actual tools. There are single beam FIB tools which consist of the FIB column on a chamber and also the FIB/SEM platforms that include both

30 a FIB column for sample preparation and an SEM column for observing the sample during preparation and for analyzing the sample post-preparation using all of the imaging modalities and analytical tools available on a standard SEM column. A vast majority of the FIB tools presently in use are equipped with liquid metal ion sources (LMIS) and the most common ion species used is Ga. Recent developments have produced plasma sources for high current ion beams. The gas field ion source (GFIS) is discussed in module 31 on helium ion microscopy in this book.

This chapter will first review ion/solid interactions that are important to our use of FIB tools to produce samples that are representative of the original material. This discussion will then be followed by how FIB tools are used for specialized imaging of samples and how they are used to prepare samples for a variety of SEM techniques.

30.2\ Ion–Solid Interactions

It is important to understand some of the ion-solid interactions that occur so that the user can appreciate why certain methods and procedures are followed during sample preparation. There are many events that occur when an energetic ion interacts with the atoms in a solid, but for the case of SEM sample preparation and ion imaging we are mainly interested in sputtering, secondary electron production and damage to

the sample in terms of ion implantation and loss of crystalline structure. Sputtering is the process that removes atoms from the target. Secondary electron production is important as images formed with secondary electrons induced by ions have some important advantages over electron-induced secondary electron imaging. Finally, it is important to realize that it is impossible to have an ion beam interact with a sample without some form of damage occurring that leaves the sample different than before the ion irradiation.

A schematic diagram of the interactions is shown in

.  Fig. 30.1. Here an energetic ion is injected into a crystalline sample. The ion enters the sample at position 1. The ion is then deflected by interactions with the atomic nuclei and the electron charges. As the ion moves through the sample it has sufficient energy to knock other atoms off their respective lattice positions as shown at position 2. The target atoms that are knocked off their atomic positions can have enough energy to knock other target atoms off their atomic positions as shown at position 3. Some of the atoms that have been knocked from their atomic positions may reoccupy a lattice position or may end up in interstitial sites. There can also be lattice sites that are not reoccupied by target atoms and are left as vacancies. Both interstitials and vacancies are considered damage to the crystalline structure of the sample as shown in position 4. Most of the time, the original beam ion will end up coming to rest within the sample. This is termed ion implantation and is shown at position 5. Ion implantation results in the detection of the ion beam species in the sample. Many of the collision cascades will eventually reach the surface of the sample. Sufficient energy may be imparted to knock an atom from the surface into the vacuum. This process is called sputtering and results in a net loss of material from the sample as shown in position 6. At the same time when the ion is either entering or leaving the sample, secondary electrons are generated that are useful for producing images of the sample surface scanned by the ion beam. It is important to remember that scanning an energetic ion beam over the surface of the sample will always result in some damage to the sample. Understanding the interaction of ions

. Fig. 30.1  Schematic of some of the important ion–solid interactions that can occur