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A personal and practical guide to the history, installation and future of the Electron Back-‐Scattered Diffraction (EBSD) system.
David Mainprice*
Géosciences Montpellier UMR CNRS 5243, Université Montpellier 2, 34095 Montpellier, France.
* corresponding author tel: +33-‐467143283; fax: +33-‐467143642
email: [email protected]‐montp2.fr
Submitted to 10th European Microbeam Analysis Society (EMAS) regional meeting in Padova, Italy, in May 2012 and draft subject to review and revision, not currently submitted manuscript.
Abstract
1. Introduction
2. A brief history of the development of EBSD
3. Sample preparation
4. Generic SEM Requirements for EBSD
4.1 Introduction
4.2 SEM
4.3 Basic requirements
4.4 Stability
4.5 Pollution
4.6 Diffraction Geometry
4.7 SEM Geometry
4.8 Camera and Screen
4.9 Operational considerations
5. Reliability of EBSD measurements
5.1 Sampling – point to point measurement
5.2 Quantitative estimate of the ODF
5.3 Indexation
6. Future of EBSD
6.1 Introduction
6.2 Electron signals
6.3 X-‐ray and spectroscopic signals
6.4 In-‐situ stages
6.5 Software
7. Conclusions
Acknowledgements
References
Figures
A personal and practical guide to the history, installation and future of the Electron Back-‐Scattered Diffraction (EBSD) system.
David Mainprice*
Géosciences Montpellier UMR CNRS 5243, Université Montpellier 2, 34095 Montpellier, France.
* corresponding author tel: +33-‐467143283; fax: +33-‐467143642
email: [email protected]‐montp2.fr
Abstract
Since 1993 Electron Back Scattered Diffraction (EBSD) has experienced a tremendous technological evolution associated with the passage from analogue to digital video acquisition of the diffraction pattern, increasing computer power and evolving camera speed. In my personal overview I emphasize the importance of sample preparation as the interaction volume, which is the source of diffracted electrons for EBSD, is restricted to a few tens of nanometres for most materials commonly studied with EBSD. A number of key issues are discussed for the successful choice and installation of an SEM for EBSD. The geometric constraints resulting from the rather unique operating regime for EBSD with the sample stage tilted at 70° are discussed and illustrated in detail. Problems of stability and pollution related to the long operating time of EBSD maps and the relatively high probe current, which is necessary for EBSD are underlined along with other operational considerations.
The fact that EBSD is single orientation measurement made typically on a predefined grid means that the irradiated surface area is very small compared to tradition volume fraction weighted measurements like X-‐ray and Neutron diffraction pole figures used for texture analysis. It is show that this problem is compounded at small grid spacing by overlapping of the interaction volumes between adjacent grid points. Recent studies confirm that fully quantitative texture analysis, for example for estimating the orientation distribution function, can be achieved with about 10,000 grains, and perhaps less for strong textures.
It is obvious that although great efforts has been made to have faster acquisition rates and better spatial resolution, the limiting factor for reliability remains the quality and signal to noise ratio of the diffraction pattern. However few recent developments have been made in the direction of greater camera sensitivity, which would allow better band detection and more reliable indexing. Future directions for the development of an EBSD environment that fully encompasses all the possibilities of the signals resulting from the electron beam’s interaction with the specimen are outlined. The importance special in-‐situ stages and open source software are discussed.
1. Introduction
In metallurgy and material science in general the word ‘texture’ represents the crystal preferred orientation (CPO) and the microstructure of a crystalline sample. In the past the subject has been dominated by application to metallurgical problems and summarized in the classical text book by H.-‐J. Bunge ‘Quantitative Texture Analysis’ published in its English version in 1982. Bunge’s book is mainly devoted to the X-‐ray or Neutron diffraction pole figure inversion method of calculating the orientation distribution function (ODF) as a complete description of the CPO, relatively few pages treat the microstructural aspects. The new texture methodology based on a combined Scanning Electron Microscope (SEM) and electron back-‐scattered diffraction (EBSD) had not been invented in 1982. Even the more recent textbook by Kocks, Tomé and Wenk (1998) did not devote a full chapter to EBSD. The combination of SEM and EBSD was called ‘Orientation imaging’ by Adams et al. (1993), presented the advantage of measuring orientation pixels within grains in statistical way. For the first time we had access not only to pole figures, inverse pole figures and ODFs, but also information about misorientation within grains and between grains, with access to some grain boundary parameters as well. Because the SEM has the possibility to zoom the magnification from the cm to nm scales the exploration of the microstructure could begin in statistical and digital way over seven orders of magnitude from 10-‐2 to 10-‐9 m.
The fact the EBSD-‐SEM is more closed linked to the microstructure than techniques like texture diffraction pole figure goniometry that previously dominated the texture world, new communities in Earth, Ceramic and Biological sciences with a strong tradition of optical microscopy became attracted to EBSD. The SEM was already a very well established instrument in 1993 that was accessible at almost all university sites. The interaction of an electron beam with a bulk sample had been extensively studied as various signals resulting from this interaction (e.g. X-‐ray photons) could be combined with EBSD to give a more complete knowledge of the microstructure over the same over seven orders of magnitude in scale.
2. A brief history of the development of EBSD
EBSD is characterized by the formation of an electron diffraction pattern with a stationary electron beam, even if in today’s applications the displacement beam is only stopped for a faction of a second. The earliest publish papers on electron diffraction were by Kikuchi (1928), Nishikawa and Kikuchi (1928) who reported diffraction patterns from mica and calcite respectively. A series of papers followed in the 1930s, perhaps the most remarkable because of the high equality of their diffraction patterns was by Finch and Wilman (1937). The patterns contained Kikuchi lines and a few diffraction spots, indicating that the diffraction geometry with a very low angle of incidence was close to thin film limit where Bragg diffraction spots appear. In the 1950s there was an isolated report by Alam et al. (1954). In the 1960s the advent of the first commercial Scanning Electron Microscope (SEM) called the ‘StereoScan’ made by Cambridge Scientific Instrument Company in 1965 provided a new environment for the study of electron diffraction. In 1967 Coates described a new diffraction technique called electron-‐channelling patterns (ECPs) produced by rocking the electron beam about a point on the specimen surface using the modified scanning coils. The ECP became very popular from the 1970s (e.g. Joy, 1974) until the early 1990s as several commercial SEMs integrated special rocking coils for routine ECP work. In the 1970s some progress
was made towards a practical geometry need for EBSD in an SEM for the characterization of crystal orientation was made by Venables and Harland (1973). In the 1980s Dingley and co-‐workers (Dingley et al., 1984, 1989) made the first steps towards the on-‐line acquisition of EBSD. The use for ECP continued to be used for CPO worked, but the indexing of diffraction pattern was still manual using spherical (or globes) maps (Lloyd and Ferguson, 1986) and the number orientations was limited to a few hundred at most.
EBSD as we know it today started in 1990s with three benchmark papers from Adams and co-‐works (Wright and Adams, 1992; Adams et al.,1993; Kunze et al.,1993). These papers introduced a system featuring an SEM controlled by mini-‐supercomputer, low light level sensitive camera, video acquisition card for on-‐line diffraction pattern acquisition and in-‐house software for automatic indexing. The system proposed by Adams and co-‐workers was designed essentially for characterization of the microstructure and CPO in the metallurgical tradition. Almost at the same time another major development occurred although its impact would take longer to be recognized, this was the first ‘true’ phase identification system, combining a high quality EBSD pattern with chemical information from X-‐ray spectrometers, either semi-‐quantitative energy dispersive (EDS) or quantitative wavelength dispersive (WDS) by Michael and Goehner (1993, 1994). The fact that EBSD could be combined with EDS and WDS would eventually result in the manufactures of these X-‐ray spectrometers taking a leading role in commercialization of EBSD systems. I call the Michael and Goehner system a ‘true’ phase identification as the analysis is based on reconstructing the unit cell from the very high resolution diffraction patterns and using the chemical data to restrict possibilities from an extensive database of structures. Even today the majority of commercially proposed systems are phase ‘verification’, that is the user has some idea of the phases present in the sample and a simple matching of composition and known structures is required from an off-‐line database. In 1996 the development of an image correlation method for measuring very small differences between diffraction patterns opened new perspectives for greatly improved relative orientation measurement, but also the direct measurement of elastic strain (Wilkinson et al., 1996). Image correlation would be ignored by commercial EBSD systems, which will take a different route dictated by the need for higher speed acquisition and processing rates required for classical tasks like CPO and mapping at increasingly smaller spatial resolution. Developments of hardware (e.g. CCD cameras) and software continued to be proposed by academic groups (e.g. Schwarzer , 1997). Krieger Lassen (1998) proposed a more refined analysis of the patented Hough (1962) transform used to automatically detect the Kikuchi lines. From 2000 onwards a number of improvements would be proposed to increased speed, reliably of indexing etc. The Hough transform, so essential to automatic EBSD, was identified as special case of the more general Radon (1913) transform (Schwarzer and Sukkau, 2003), which can take into account additional information of the band profile and intensity. Wilkinson et al. (2006) introduced a more complete and higher resolution strain analysis of EBSD diffraction patterns and the determination of elastic distortion. Although it was generally accepted that kinematic diffraction theory was sufficient to simulate the intensity of Kikuchi bands (e.g. Kogure, 2002; Zaefferer, 2007), the need for a dynamical theory was clearly needed to understand the finer details present in the diffraction patterns. Winkelmann et al. (2007) produced the first dynamical theory simulation of EBSD diffraction patterns showing remarkable detail around the zones axes where the dynamical effects are strongest and higher order Laue zones (HOLZ) rings are present. A 3D Hough transform to take into account of the curved nature of the
Kikuchi diffraction cones was developed by Maurice and Fortunier (2008) to allow high-‐accuracy automatic detection, it also provided the first fully automatic indexing of X-‐ray Kossel patterns. Deal et al. (2008) explored the possibility of reducing the noise level in diffraction patterns by using an energy-‐filter. In the same year high-‐speed EBSD cameras started to appear with frame rates as high as 700 patterns per second (Søfferud et al., 2008). The high-‐speed cameras have a trade-‐off between high-‐speed in general resulting in lower sensitivity. Hence high-‐speed cameras are best suited to materials that strongly diffract electrons like metals, or special cases where speed is an advantage such as in-‐situ deformation or heating experiments. The existence of high-‐speed cameras has opened a new debate on the pertinence of on-‐line indexing and processing versus off-‐line (Schwarzer and Hielen, 2010). Among the reasons that may favour off-‐line analysis of recorded diffraction patterns or Hough transforms is the likelihood of reducing several problematic issues common in EBSD mapping, note that measurement time is not the only issue here,
a) Time – reduced SEM time, time associated with indexing removed, shorter measurement times reduces the likelihood of beam instability (e.g. Tungsten filament SEMs) and specimen charging (e.g. non-‐conducting materials), and provides uniform acquisition time all measurements with predictable run times.
b) Unexpected -‐ presence of unknown phases, factors causing variable background (e.g. electrical charging, phases with very different diffraction intensities), need for variable Hough settings.
c) Off-‐line -‐ Indexing can repeated several times, reliability of indexing most likely improved over automatic by cyclic analysis of the complete data set, data analysis can be reproduced independently using different indexing protocols for quality control.
d) Disadvantages -‐ requires optimised hardware-‐software-‐PC system, for example high-‐speed disc transfer buffer, high-‐speed disc (e.g. solid state memory disc), high capacity disc (e.g. storage of diffraction patterns) and specialized post-‐treatment software with optimized data management for large files. For ultra-‐fast acquisition high beam currents are required to maintain the signal to noise ratio, causing potential problems of charging, beam damage and enhanced pollution due to the high temperature in the region of the probe.
Many other developments are in progress, many with the object to reduce the resolution at the nanometre scale, for example by using lower accelerating voltages (e.g. Steinmetz and Zaefferer, 2010).
3. Sample preparation
Before discussing the requirements need for EBSD it is important to mention the problems of sample preparation. The back-‐scattered electrons that are diffracted in crystalline materials to form diffraction patterns come from the near sub-‐surface of the sample, typically the first 20 nm or less. Hence preparation of the sample surface is critical for the success and quality of diffraction measurements. Five minutes additional polishing can transform a non-‐diffracting or poorly diffracting sample into a high quality sample, whereas no SEM, no high-‐resolution EBSD camera, no indexing routine, and no amount of post post-‐processing will transform a non-‐diffracting crystal surface into a high quality diffracting crystal surface, only sample preparation can do this. Despite the
obvious fact that sample preparation is vital for the success of any EBSD project, it is often the last item in the budget, and sometimes completely forgotten.
Samples for EBSD are typically slices of various thicknesses with long and short dimensions of (1 x 1) to (2 x 1) cm or smaller. The objective of sample preparation is; a) to reduce surface relief to a minimum, essential for samples tilted at 70°, and b) to remove any residual surface damage, either introduced during sample selection (e.g. scratches) or sample surfacing and polishing procedure. The specific procedures used to obtain the best results for a given crystal will depend on many factors and will often require optimization for a given sample. Good sources of information and application notes for EBSD samples are available on-‐line from some of leading companies specializing in metallographic or materialographic specimen preparation. For rocks and minerals the sample preparation used by our research group are available in French written by Christophe Nevado at (http://www.gm.univ-‐montp2.fr/spip/spip.php?article1048) and another site for again for rocks and minerals by Rachael Beane, Bowdoin College, Maine, USA at (http://serc.carleton.edu/research_education/geochemsheets/ebsd_sample_preparation.html). A generic sample preparation suitable for rocks and minerals is as follows with washing between each step;
a) Impregnate sample with resin (in vacuum to achieve best results) to avoid holes and reduce surface relief. For rocks and ceramics this is important as micro-‐cracks are almost always present.
b) Lap with 220 grit silicon carbide until flat. c) Lap with 500 grit silicon carbide for 5 mins. d) Polish with 9 micron diamond for 15 mins. e) Polish with 3 micron diamond for 15 mins. f) Polish with 1 micron diamond for 15 mins. g) Chemical-‐mechanical polishing (CMP) is used extensively for the final
preparation of EBSD samples in Earth Sciences. CMP is optimum when the sample surface is flat with no relief. SYTON is a commercial fluid originally developed for CMP of semiconducting single crystals. SYTON is 10 nm silicon particle saturated colloidal NaOH alkaline polishing fluid typically used for polishing times between minutes to 20 hours. The alkaline nature of fluid some times attacks certain compositions like clay minerals, which reduces the possible polishing times using SYTON. Best results are obtained by optimizing the fluid flow rates of the chemically active SYTON with mechanical polishing rates determined by the rotation speed of the polishing lap (e.g. Fynn and Powell 1979; Steigerwald et al. 2004). An additional parameter to be optimized is the mechanical ‘hardness’ of the polishing cloths, mats or pads now proposed by private companies, and the specific chemical properties of the one mineral or several minerals present in the sample. Some success in improving the percentage of indexed EBSD patterns using an argon ion beam polishing system for final surface preparation of the mineral antigorite has been reported by Van de Moortèle et al. (2011). 4. Generic SEM Requirements for EBSD
4.1 Introduction
EBSD is characterized by the recording of diffraction pattern formed by the interaction of the stationary collimated electron beam with a 20° angle of incidence with specimen surface and a specimen tilted at 70° from the horizontal. Hence EBSD measurements require an SEM with similar technical characteristics to an electron micro-‐probe. Additional factors are required due to the tilted sample geometry and extended measurement times due to mapping, particularly at small grid intervals. One of most common problems for EBSD systems is when they placed on a shared SEM facility, as EBSD increasing involves high-‐resolution mapping that can last many hours, hence it vital evaluate the time you require before installing your EBSD system on a shared SEM.
4.2 SEM
If your EBSD project is a combined EBSD-‐SEM acquisition then there are several parameters to determine. Before buying the SEM it is important to know where it will be installed and if you have the choice where it should installed! Ideally, like any high-‐resolution electron microscope, the SEM should be placed in a room with controlled temperature, stable power supply, mechanically stable environment (e.g. stable floor, not on 10th floor of tower block that sways in the wind), controlled humidity (e.g. not in the basement of a tower block where all pipes with fluids exit via the basement and are leaking). Today some SEM manufactures provide a built-‐in Universal Power Supply (UPS), this avoids problems of high frequency voltage variations and power-‐cuts up to typically one hour. Protecting your SEM is especially important for SEMs with Field Emission Gun (FEG), as these run continuously for at least 18 months and switching off and on the FEG generally degrades their performance and reduces their lifetime. I strongly advice anyone with a FEG-‐SEM to install an UPS to protect their SEM as there price has reduced significantly in recent years due to the increased demand from computer servers. The choice of the SEM gun between tungsten (W), lanthanum hexaboride (LaB6) or FEG will control the ultimate theoretical spatial resolution of the EBSD-‐SEM, the highest resolution being possible with a FEG, with LaB6 intermediate and tungsten being the lowest. Beware all commercial SEMs are quoted with their highest resolutions for samples in horizontal position (0° tilt), whereas for EBSD the sample will be tilted at 70° and the resolution will be lower. In fact EBSD resolution most often limited by the imperfect nature of the sample and its intrinsic diffraction properties rather than by the SEM. In the literature you will find statements like ‘a field-‐emission microscope is almost always the best choice. Only users assessing textures with very large grains or identifying phases that always occur as large crystals – in short, almost none of us – would be as well off with a thermionic filament’ Eades (2000). Well apart from the high cost of a FEG-‐SEM and its relatively delicate nature, requiring expert personnel (typically form the SEM manufacturer) to change the FEG tip. In comparison a robust thermionic W-‐filament can be changed by almost anyone with the appropriate training, and of course at a small fraction of the cost. The FEG produces a finely collimated beam, which results in high beam current in a small interaction volume in the near surface of the sample. For metals the FEG produces good quality and strongly contrasting diffraction patterns, which results in high spatial resolution typically below 20 nm (e.g. Humphries et al., 1999). The FEG high beam current in a small interaction volume can cause damage in certain non-‐conducting minerals (e.g. quartz) and generally produces poorer quality and less contrasting diffraction patterns than typically produced by thermionic W-‐filament. However, a W-‐filament SEM will have a much
larger interaction volume at high probe current, which typically limits EBSD resolution to about 1 μm in many cases, although a resolution (60 nm) has been reported for aluminium alloys (Humphries et al., 1999). When considering the initial cost, servicing costs and spatial resolution have to balance against the type of materials to be studied and technical experience of the people involved with the day to day running the SEM when comparing housing FEG-‐SEM verses a W-‐SEM in your facility. Another useful SEM option when buying an SEM for EBSD work is the low vacuum (LV) or pressure variable (PV), which allows the sample chamber pressure to be raised to several hundred Pa, although 5 to 10 Pa is usually enough to stop charging during EBSD mapping of non-‐conducting samples.
4.3 Basic requirements
There are certain basic requirements that SEM has to meet to be able host an EBSD camera, stage and beam external EBSD control system (Figure 1). Most modern SEMs are computer controlled and have communication interfaces for external control. Firstly, SEM requires a chamber port that has a larger enough diameter to allow the access of the EBSD camera (typically between 50 to 75 mm depending on the conception of the optics and the sliding vacuum seal or bellows associated with the camera system), secondly the port also has be placed at convenient azimuth in horizontal plane so that the sample can be tilted 70° from the horizontal to face the camera (e.g. the tilt axis of stage has to be ideally at 90° azimuth from the center of the EBSD camera port)(Figure 2), and thirdly the port should be at suitable height so that the center of camera screen is slightly below the tilted sample (see Day,2009 his figure 5.3 for an example of a dysfunction SEM for EBSD with custom made solution). When the specimen stage is tilted in this orientation it should not touch any of the detectors (e.g. Secondary electron detector, Back scattered electron detector), or the detectors should be retractable (e.g. X-‐ray EDS) and of course it should not touch the pole piece! The specimen stage should be eucentric to allow the point where the beam touches the specimen to pole piece (working distance, WD) to remain constant while displacing along the X and Y axes while the stage is tilted at 70°. The SEM must have at least two horizontal (X and Y) stage axes motorized and controllable by an external EBSD System PC (Figure 3). It is also important the X (or Y depending of specific configuration of your SEM) axis can be alignment horizontal when the stage is tilted, by a rotation around the normal to the specimen surface using Z-‐axis of the stage (typically called the ‘vertical’ axis at 0° tilt) (Figure 2), so that EBSD maps have co-‐ordinates with their orthogonal axes that are parallel to specimen directions. As the EBSD is conducted with specimen tilted at 70° the electron beam has an angle of incidence of only 20° from the specimen surface plane and the SEM image has a strongly distorted appearance. To produce an image with corresponds to the ‘classical’ situation where the beam is 90° to the specimen surface an option typically called ‘tilt corrected image’ is proposed, this very useful for positioning on the sample before running an EBSD map for example. In past some of the ‘tilt corrected image’ options only worked up to a certain angle of tilt (e.g. 45°), hence it is important to verify it works up to 70° for EBSD applications. In some cases where it is not possible to tilt the sample correctly with specimen stage to 70°, for example one of the fixed detectors such as the secondary electron detector is touched by the stage when tilted and you cannot use an alternative SEM port. The easy solution to such a problem is to mount the specimen on a mounted block with an inclined surface at 70° degrees, in this case mapping requires the vertical axis (Z) must also be motorized. However the solution with X-‐ and Y-‐axes motorized and eucentric stage tilted at 70° is strongly
preferred as the WD is constant which avoids calibration problems and the additional positioning time needed for the Z-‐axis. So that the beam remains focused on the specimen inclined at 70° while beam scanning it is essential that the SEM has dynamic focus.
4.4 Stability
As the most common practice these days is to run EBSD maps for hours, over-‐night, or even several days the question of stability becomes very important. Mechanical, thermal, and electrical stability of the SEM and the room have been mention above. There are additional factors concerning operational conditions. For example electrical charging of non-‐conducting samples is very complex subject as the charge can accumulate over a period of hours and suddenly discharge in an unstable manner, even if the specimen and specimen stage is well grounded. Very thin carbon coating is the traditional way to reduce charge accumulation, but this also reduces EBSD pattern intensity. The alternative is to use LV with a low pressure of 5 to 10 Pa and this is very effective. W-‐filament SEMs are in general not as stable by their very nature as FEG-‐SEMs and LV-‐FEG-‐SEMs.
4.5 Pollution
There a number of types of hydrocarbon pollution can reduce EBSD signal by ‘coating’ the sample or even the EBSD camera screen as it is only 20 mm from the specimen and is bombarded by a large number of high energy (close to accelerating voltage) diffracted electrons. The sources of hydrocarbons in a SEM are; a) diffusion pumps are often used in entry level SEMs that contain heavy hydrocarbon based oils, that may also cause pollution of the sample chamber if the operating temperature is too high, for example due to insufficient cooling water, this may vary with the time of day, b) carbon conducting tape is used grounding the non-‐conducting samples. However, either by evaporation in the vacuum or decomposition under the electron beam such tape is also a potential source of polluting hydrocarbons, c) the human hand from users not using gloves when mounting specimens on the holder or touching anything in the sample chamber can also cause pollution.
4.6 Diffraction Geometry
As the name ‘EBSD’ suggests the basic measurement of an EBSD system is a diffraction pattern. The formation of Kikuchi diffraction cones observed in EBSD is governed by the classical Bragg’s law, where significant diffracted intensity is only observed when the path difference between parallel beams if some integer multiple of the wavelength of the electrons (Figure 4). Electrons at the accelerating voltages typically used for EBSD (10 to 20 kV) have a wavelength between 12.25 to 8.66 x 10-‐3 nm respectively, hence the Bragg angle will be typically less than 1°. The consequence of the small Bragg angles involved in electron diffraction is that the Kikuchi diffraction cones appear to be planes and the planes intersect the phosphor screen to give rise to nearly straight lines, commonly called Kikuchi lines (Figure 5). The geometry near the source region illustrates that the formation of a pair Kikuchi lines is quite a complex two-‐stage process; a) an inelastic (or quasi-‐elastic) interaction with a small energy loss resulting in a radical change in propagation direction of incident electrons to form an effective point source that scatters the electrons all directions, which involves a thermal diffuse scattering mechanism according to Zaefferer (2007), b) scattered electrons that
have incident angles equal to Bragg angle with specific lattice planes undergo coherent Bragg reflection. The geometry of the Kikuchi lines in a diffraction pattern can be calculated quite simply using equations given by Young and Lytton (1972), slightly modified by Mainprice (1981) (Figure 6). From Figures 5 and 6 it is obvious that spacing of the pair of Kikuchi lines is proportional to 2θ Bragg angle and the spacing between planes with the Miller indices and in the general case in Figure 5.
4.7 SEM Geometry
The SEM geometry requirements for EBSD are illustrated in Figure 7. The best know parameter to all SEM users is the working distance (WD). The most general definition of WD would be the distance from the bottom of the objective lens pole piece and the electron beam impact point on the sample surface (S). The probe-‐forming lens can focus the beam at various WDs typically from 5 to 30 mm, but most SEMs are optimized for a recommended WD. The probe diameter increases with probe current, although this effect is much stronger in thermionic tungsten-‐filament SEM than in a Cold or Schottky FEG-‐SEM (e.g. Goldstein et al., 2003) and decreases with increasing accelerating voltage. However, as the sample is tilted 70° the probe has a projected elliptical shape on the sample surface with a short dimension (Xprobe) parallel to the X tilt axis (e.g. X in Figure 7) and a long dimension (Yprobe = Xprobe /cos70° = 2.92 Xprobe) parallel to Y axis. Hence the effective resolution when limited by probe size will be different in X and Y translation axes of the SEM stage, this also emphasizes that SEM resolution and image quality will always be inferior for a sample tilted at 70° compared with a sample tilted at 0°, hence SEM constructors do not quote for the resolution of a tilted sample.
A horizontal line from sample beam impact point (S) intersects the EBSD phosphor screen at point called the pattern center (PC). The sample to screen distance (SS) is also called the ‘camera length’ by analogy with the identical geometry for Kikuchi line formation in transmission electron microscopy. The pattern center is above the screen center because of the forward scattered nature of the angular distribution of diffracted electrons results in peaked distribution below the pattern center. To emphasis the importance of placing the pattern center Figure 8 shows situation with PC near top of the screen and near the bottom of the screen using short and long working distances. With the PC near top of the screen the diffraction intensity is uniform and the Kikuchi lines are in focus. When the PC is in the middle of the screen the diffraction intensity is stronger near the bottom of the screen and the Kikuchi lines are diffuse near the top of the screen where intensity is weak. It is interesting to note that the recommended accelerating voltage for EBSD is typically around 20 kV, where as a recent study by Steinmetz and Zaefferer (2010) has shown high resolution EBSD can be achieved at 7.5 kV. Using their measurements (Figure 9) the plot of position for best contrast diffraction patterns on the screen as a function of kV can be made, it shows that in general the take-‐off angle of the diffracted electrons is always much greater than 20° and the angle is highest at low kV.
When the appropriate uniform pattern with PC near the top of the screen has been achieved you need to be able to calibrate your ‘camera length’ (SS) and PC. The simplest way to do this is my using a silicon wafer as reference sample. A small rectangular piece of oriented silicon can be obtained from a standard (001) silicon wafer by breaking it along the (110) cleavage plane (Figure 10). The (110) cleavage plane can
h k l h k l
conveniently be aligned with specimen tilt axis (e.g. X axis) and suitable position for PC would the [114] direction. Most EBSD software provides tools to doing the calibration.
A major concern when installing an EBSD system is avoiding a collision between the motorized specimen stage when tilted and the SEM pole piece. The essential geometry is shown in Figure 11. The triangular relationship between the beam exit on the pole figure (BE) and the lowest beam position on the specimen (LBS) will govern the touch point (TP) of stage on the pole piece, when the Y-‐translation is operated. One might think the with the range of SEM pole piece geometries available from different manufactures that it would easy to find one that is idea for EBSD. In Figure 12 we see there is no problem when move Y-‐translation to the top of the sample (lower row of figures). On the other hand, despite a range of geometries there is at the moment not an ideal pole piece available for EBSD work. Clearly these problems are very acute for samples with large Y dimension, and will not affect people working on small grain size homogeneous materials.
4.8 Camera and Screen
The choice of camera screen size and the distance screen to sample, which will determine the solid angle you can detect on your screen as shown in Figure 13. Most EBSD camera screens are designed detected a solid angle of about 90° so that several low index zones are visible to facilitate indexing. Moving the screen farther way from the sample will result in a smaller solid angle, but sometimes this is useful to have a higher resolution image about a given zone axis for crystal distortion measurements (e.g. Wilkinson et al., 2006). Note as the screen is moved further from the sample the signal will also decay requiring a more sensitive low light level camera or increased integration time.
A final point is camera screen is technically called a scintillator, which converts electrons into photons. The most common scintillators are composed of phosphor deposited onto a glass substrate. The glass substrate is coated with indium tin oxide, which is optically transparent and electrically conductive to reduce electron charge accumulation. Phosphor produces light in the visible spectrum and emission spectrum of the specific phosphor (e.g. P22G ZnS:Cu,Al,Au, maximum emission peak at 540 nm) should match the wavelength sensitivity of the CCD (typically around 550 nm) of camera for optimum results. Phosphor screens typically have maximum electron conversion efficiency for 20 kV electrons, which is why 20 kV is often recommended for EBSD (e.g. Baba-‐Kishi, 2002). The alternative is a single crystal cerium-‐doped yttrium aluminium garnet or YAG-‐Ce (Y3Al5O12(Ce)), which has a maximum emission at 550 nm. YAG has the advantages of high electron conversion efficiency, which increases with the total energy of the electron beam, high thermal conductivity (13 W m k-‐1), very long lifetime and is mechanically robust, but such crystals can have growth line defects (e.g. Day, 2009) and are very expensive.
4.9 Operational considerations
Although for classical SEM operation is often dictated by conditions that allow high quality imaging with secondary electrons, such as small probe diameter on the surface of the sample can be achieved by using low probe current and high voltages. SEM operation for EBSD has more in common with an electron probe. Whereas high probe currents of several nano-‐amperes or more depending on the material are typically
needed for EBSD, either because of the intrinsic poor yield of diffracted electrons of the material or due to the need for high yield at short exposure time to maintain a high signal to noise ratio for high-‐speed EBSD cameras (e.g. Søfferud et al., 2008; Chen et al., 2012). Note to that very high beam current can also cause contamination problems due to local heating and even beam damage in non-‐conducting materials. The probe is no longer restricted to the surface, there is three dimensional pear-‐shaped interaction volume, which is well known as the source for x-‐rays, which can have origins much deeper in the specimen (ca 1000 nm) as x-‐rays can escape to the surface from such depths as they have 10 to 100 times the electron penetration distance. Factors that become essential to generate forward-‐scattered diffracted electrons are similar to those that result in high yields of back-‐scattered electrons. The following characteristics originally established for back-‐scattered electrons are important in the context of EBSD; a) they have their origin in the first few hundred nanometres of the sample (e.g. 200 nm for copper at 10 kV,Newbury and Yakowitz, 1976), b) the electron signal increases with atomic number (Z) for elements or weighted mean atomic number for compounds in non-‐linear way, being nearly linear for elements to Z = 35 and less sensitive at higher Z (e.g. Lloyd,1987), but is nearly independent of accelerating voltage between 5 and 50 kV (e.g. Goldstein et al., 2003 their figure 3.9), c) the electron signal is an increasing non-‐linear function of tilt proportional to (1+cos(tilt angle))-‐p where p=9/√Z for elements, increasing very slowly at tilts less than 45° and almost linearly at greater tilt angles favouring high signal (e.g. Arnel et al., 1969), hence a tilt of 70° typically used for EBSD to give reasonable compromise between electron yield and spatial resolution, and d) finally spatial distribution electrons at 20 kV on a sample tilted at 70° has a strong down slope forward scattered distribution inclined at 8° from the surface with very low energy loss (Wells,1974; Newbury and Yakowitz, 1976). The electrons involved in diffraction are high-‐energy electrons with energy close to that of the incident beam. Zaefferer (2007) reports very low experimental energy losses and resulting diffracted electron energies of 99.7 to 97.1% of the incident energy at 15 kV, and hence like back-‐scattered electrons they are less sensitive to ambient charging than the low energy secondary electrons. Estimates of the source depth of diffracted electrons are often based on Monte Carlo modelling of electron trajectories (e.g. Drouin et al., 2007). Using the Monte Carlo model and assuming energy losses of the order of 800 to 1000 eV gives a depth for which 50% of the diffracted electrons arise from a maximum depth of 5.5 nm for iron (Zaefferer, 2007) and 10 nm for copper (Borbély et al., 2008), note this one order of magnitude smaller than values given for backscattered electrons at zero tilt. For iron at 15 kV Zaefferer (2007) measured a value of 2 nm. These values will vary with composition and kV, but taken at face value the depths of between 2 -‐10 nm are very shallow and certainly imply that sample surface preparation needs to optimised, coating either intentional (e.g. carbon coating) or unintentional (e.g. hydrocarbon pollution) should be avoided if possible (e.g. with low vacuum for non-‐conducting materials). Harland et al. (1981) quote depth resolution of 40 nm for silicon. The lateral resolutions measured by Zaefferer (2007) for iron at 15 kV are 35 nm and 90 nm respectively parallel and perpendicular to the tilt axis, this defines the shape of the near surface volume with a depth of 2 nm, which is the source region for 50% of the diffracted electrons Figure 14. So interaction volume for EBSD is not like the classical three-‐dimensional pear-‐shaped interaction volume for x-‐rays, but rather the ‘neck’ of pear with dimensions of 35 x 90 x 2 nm thin elliptical disc shape for Fe (atomic number 26), for lighter elements these values will be larger (e.g. Si 14) and for heavier elements (e.g. Pb 82) they will be smaller.
When talking about the resolution of EBSD and maps generated from EBSD data you have to distinguish between ‘physical’ and ‘effective’ resolutions. Physical resolution is typically measured by loss of diffraction lines in the pattern as the probe moves from one crystal over a high angle grain or twin boundary to another (e.g. Harland et al., 1981). Physical resolution will primarily be determined by the interaction volume for diffracted electrons, but the measurement will also influenced by sensitivity and integration time of the screen-‐camera system as these limit the visibility of the diffraction lines. The effective resolution as observed in an EBSD line scan or map, which integrates factors like the spacing of beam positions (called the step size), Hough transform resolution for the detection of diffraction lines in manual processing and the ability of the indexing software to find a solution in the case of automatic processing. The key factor in either physical or effective resolution is the quality of the diffraction pattern (e.g. Humphreys, 2004). The quality of the recorded diffraction pattern depends on the sample composition (e.g. Z, atomic number) increasing with Z, sample surface preparation, orientation of the crystal, probe current as high current produces more signal, accelerating voltage as high voltage causes more beam spreading in the interaction volume, the sample to screen distance as the signal intensity will fall with distance, the low light sensitivity of the screen-‐camera system, various acquisition settings of the camera system (camera pixel resolution, summing blocks of adjacent pixels ‘binning’, acquisition time with on-‐chip integration, number of patterns for averaging). In addition for effective the step size, Hough transform resolution, and indexing software all contribute to the final resolution. Increasing the probe current increases the effective resolution (e.g. Humphries 2001; Chen et al., 2012). The noise in the line scan or map effects the perceived resolution and techniques of filtering EBSD maps have been introduced to improve the signal to noise ratio and the effective resolution (e.g. Humphries et al., 1999). In the literature the physical, effective and filtered effective values makes it difficult to compare the resolution values as one also has to add the factors of accelerating voltage, probe current, probe size, step size and material mean atomic number to name just a few mention above. It is interesting to note the highest effective resolution (6-‐9 nm) reported in the literature is given by Dingley (2004) for Pt with relatively high atomic number (78) at 25 kV and step size of 3 nm, the high atomic number will favour a small interaction volume.
Mapping has become the most common way of using EBSD. Mapping, that is displacing the electron probe on the sample in a x-‐y grid pattern, can be done either by using the scan-‐coils of the SEM or moving the specimen stage. Mapping using the scan coils is called beam scanning; it has the advantage of being very fast, as it does not involve any mechanical parts. It is essential to have dynamic focus for beam scanning to maintain the focus constant over the tilted sample. However, beam scanning in the tilted x-‐y plane of the sample results is small change of the working distance and specimen to screen distance at each position, and hence slight change in the EBSD calibration, including the pattern center. The calibration changes are small and in general will not adversely affect indexation. Beam scanning to can be done on relatively small areas typically less than 1 mm2 (Figure15). Stage scanning with a eucentric stage the working distant and other calibration parameters will be constant and you can displace over long distances. However, the mechanical nature of the stage will be slower to position than beam scan. You cannot reasonably used stage scanning at very small step sizes, for example less than 1 μm when backlash is taken into account, because of the positioning resolution of typically 0.25 μm and time to do serial port hand-‐shaking communications and positioning requires a few seconds. The solution to mapping in samples for a areas
of mm2 to cm2 is to combine beam scanning of small areas with stage movement to create a mosaic of beam scanned maps that are then merged to form the full-‐scale map (Figure16). The combined mapping strategy has the advantage of the speed of beam scanning and the large scale of stage positioning. The time taken for a give beam and stage scanning measurements is controlled by the chosen pattern acquisition rate for beam scanning, typically between 25 and 700 patterns per second, and the time to move the motorized stage, which depend on the distance moved. Using pattern acquisition rate, beam scan step size and a stage movement time of 3 seconds the time to measure a large sample of 1 cm3 is illustrated in Figures 16 and 18. The time calculated without considering the delay time due motor displacement in Figure 16 is non-‐linear with step size being proportional to step size to power of -‐2. In a double log plot provides linearization of time versus step size. When the time for motor positioning is taken into consideration almost the same relation holds with a small time increase, but there is a further non-‐linearity for large step sizes (Figure 18).
5. Reliability of EBSD measurements
5.1 Sampling – point to point measurement
EBSD is a point-‐to-‐point measurement technique. If measurement were confined to the surface of the sample, then the ‘point’ would be the surface area of the electron beam (i.e. the probe). As discussed above the physically significant region of the sample for EBSD is the interaction volume, which for iron at 15 kV has been measured as 35 x 90 x 2 nm (Zaefferer, 2007). In terms of 2D x-‐y surface map representation of EBSD measurements should look like a series of ellipses with an x:y ratio of 35:90. The interaction area expressed by the ellipse with x:y ratio of 35:90 represents the real area measured by EBSD (dA) and the sampling grid used for mapping represents the area of the sample (A). So the measured area fraction (MAF) can be written as
MAF = dA/A = Area of interaction ellipse / Area of measurement grid = (π a b)/ (k s2)
where a and b are semi-‐axes lengths of the interaction ellipse (i.e. 35/2 and 90/2), k is a constant equal 1.0 for a square grid and 3√3/2 for a hexagonal grid, and s is the step size for square grid and the length of the side for a hexagonal grid. The MAF is a non-‐linear function of step size to the power -‐2 because of the nature of the EBSD two-‐dimensional sampling grid (Figure 19). A MAF of one normally signifies 100 % coverage; a step size of 50 or 30 nm is required for square and hexagonal grids respectively to obtain this value. However, in Figure 19 there are MAF values greater than one, this should be impossible except for the case where the measurement points are overlapping. By examining scale drawing of grids with ellipse centred on grid points (Figure 20) the effect of long axis of ellipse in the normal to the tilt axis (x) will cause over-‐lapping of the ellipses in y-‐direction when the step size (s) is less than the long axis of the ellipse (s<90 nm). In the example grid with s=100 nm the ellipses do not overlap and MFA gives a realistic value. The spacing of the ellipses in the x-‐direction is such that smaller grid spacing along x would result in more homogeneous sampling as suggested by the placement of the blue ellipse in Figure 20. When s<90 nm then overlap occurs first in y-‐direction for 35<s=50<90 nm and then in x-‐ and y-‐direction when s=25<35<90 nm, when overlap occurs the MAF does not give the real value of measured area fraction. However, the correction of the MAF for overlap is not a trivial procedure as the overlap pattern becomes complex as shown for the case with a step size of 25nm in Figure 20. When the interaction ellipse is placed in more realistic position of Figure 13 with the
extreme upper y-‐axis edge of ellipse at the grid point, most all of the ellipse is down slope from the grid point, (Figure 21). The complex overlapping patterns of the 25 nm grid in Figure 21 are highlighted by the use of transparency, deeper red corresponding to multiple overlapping. Two regions of high overlapping occur, band-‐like features occur between grid points in the y-‐direction and in more oval areas off-‐set below the grid points also in y-‐direction. As the overlapping is controlled by the interference between step size and the dimensions and orientation of interaction ellipse, the geometrical interference pattern will have to be investigated for each case as the interference ellipse parameters will also depend on composition, kV and other SEM parameters. A common observation is that reported ‘effective’ resolution being better than the ‘physical’ resolution (e.g. Humphreys, 2004; Zaefferer 2007), Humphreys (2004) suggests this due the software’s resolution of diffraction patterns that overlap from two grains with contrasting intensities. The presence of overlapping of the interaction ellipses at small step sizes could be another contributing factor to the physical and effective resolution, causing more averaging of diffraction intensities than currently admitted.
EBSD systems can also measure microstructural characteristics, their application to metallography (e.g. grain size) was evident and results here have proved to be reliable, if the SEM system is regularly calibrated (e.g. Humphries, 1999; Humphries, 2001; Mingard et al., 2009). Comparisons are made with the standard methods of quantitative metallography and it is shown that in many cases EBSD can produce more accurate and detailed measurements than the standard methods, an additional advantage is the data may be obtained more rapidly. The microstructural characteristics can of course be to linked directly to crystal orientation, and for example the effect of grain size on the strength of texture can be quantified.
5.2 Quantitative estimate of the ODF
When EBSD was first introduced as competitive and automated technique to measure the texture of metals (e.g. Adams et al., 1993) several tests were made on the ability of EBSD to reproduce the pole figures or ODFs classically measured by X-‐ray or neutron texture goniometry (e.g. Heidelbach et al., 2000). Taking into account the results from Figure 19, which suggest that step sizes above 100 nm the MAF for EBSD is very low compared to 100% beam exposure of diffraction techniques like X-‐ray or neutron texture goniometry. However, in the early days of EBSD and to some extent even today for coarse grained materials, a single point per grain measuring scheme is used, that may be manual or automatic beam movement and indexing can provide acceptable data for pole figure and ODF work. Of course, one point per grain measurement should be weighted by the grain area (or volume) to be comparable with volume-‐weighted measurements of X-‐ray or neutron texture goniometry. Note that EBSD mapping provides maps of orientation pixels at each grid point and hence post-‐processing is required to convert the data into grains by imposing an angle of misorientation for the grain cut-‐off and some other software rules. In Earth Sciences comparisons were also made between U-‐stage, ECP or texture goniometry and EBSD. These comparisons revealed that EBSD could reproduce the main features of pole figures or ODFs for the materials that were tested. Some tests were also done comparing results using different commercial EBSD systems (e.g. Pera et al., 2003); these also showed for pole figures the results were very similar. More demanding tests were done more recently on metals with detailed comparison of X-‐ray texture goniometry and a state-‐of-‐the-‐art EBSD systems are given by Wright et al. (2007) and Bozzolo et al.
(2007). The ODF, f(g) is defined by dV/V = f(g) dg which gives the volume fraction of crystals with orientation g within an infinitesimal domain dg (Bunge,1982). Based on the definition of the ODF Bunge (1982) proposed that any ODF estimated based on individual crystal orientations should sample the ODF space in such a way to have about 25 orientations per Euler angle cell (e.g. 5° x 5° x 5° cell) to have statistical relevance (i.e., statistical reliability of 80%). Bunge’s suggestion implies about 10,000 grain orientations for cubic crystal symmetry and orthorhombic sample symmetry (ODF of 90° x 90° x90°), typical of metal sheet rolling problems (Engler and Randle, 2009). The usual comparison made in Materials science is to compare the ODFs derived from X-‐ray pole figure inversion with an ODF calculated from EBSD data. It is assumed that the ODF derived from pole figure inversion is the statistically representative of the sample and is a correctly volume weighted measurement. The calculation of the ODF from single orientation data requires an integration of the individual orientations in the Euler space of the ODF to produce a smooth function. Several possible methods can be used to do this, of which the spherical harmonic method is generally considered the most reliable and when programmed using modern techniques it is also the fastest method (e.g. MTEX by Hielscher and Schaeben, 2008; http://code.google.com/p/mtex/). Typically raw EBSD maps are subjected to data cleaning; filtered to remove spikes, systematic mis-‐indexing data points and non-‐indexed pixels, mostly located along grain boundaries replaced by assigning the orientation of a neighbouring point (Wagner et al., 2002). The cleaned data is firstly processed to converted the 2D-‐ or 3D-‐grid points into grains, the conversions depends what the user considers to be an appropriate cut-‐off angle between grains and way this implemented in the software (neighbour topology etc). Secondly, one needs to define several parameters when using the spherical harmonic method on individual orientation data; a) the radial kernel function, historically Bunge (1982) used a Gaussian-‐like function, b) the half-‐width of the function and c) the maximum degree of harmonic development. Wagner et al. (1981) proposed that the half-‐width of the function should be vary with the number of individual measurements (n), the half-‐width being larger for small n, implying greater smoothing. A typical X-‐ray sample of grain size of 25 μm and surface area of 1 cm by 1 cm represents approximately to 300,000 grains according to Engler and Randle (2009). Wright et al. (2007) studied low carbon steel samples and measured between 134,000 and 30,000 grains depending on grain size. Bozzolo et al. (2007) measured a low-‐alloyed Zr sheet sample with grain size of 5.5 μm containing 83,000 grains. Hence modern EBSD measurements can approach the correct order of magnitude in grain numbers to be compared with X-‐ray data. A question that has often been posed in recent years is how many grains should be measured to have the same statistical significance as X-‐ray data. The mean square deviation (ρ) has been proposed as a global parameter to quantify the difference between an ODF estimated from individual orientations and one obtained by X-‐ray diffraction (Pospiech et al., 1994), this approach suggested relatively small samples of 500 to 1000 grains could characterise the ODF, because of the sharp textures studied and limited ODF space considered (cubic-‐orthorhombic). Another way of posing this question was in terms of the 1/n sampling law proposed by Matthies and Wagner (1996). The 1/n sampling law predicts that as the number of grains (n) measured increases the estimate of the infinite sample is approached in asymptotic manor. For example the J-‐index of Bunge (1982) is scalar measure of the texture sharpness or strength, define in such a way that is J = 1 for random texture. J will decrease from its value for n = 1 asymptotically to approach the value for n = ∞ according to the 1/n law. Estimates made by Matthies and Wagner (1996) suggested that around 2000 grains
were sufficient to estimate the ODF, a factor of two more than the method of Pospiech et al., (1994). The number of orientations is not the only important factor; it quickly became apparent the sharpness of texture was another important factor (e.g. Jura et al., 1996).
Wright et al. (2007) used a number of different parameters to evaluation the dependent on grain numbers and difference between EBSD and X-‐ray values, these included; pole figure densities, ODF densities, ODF differences, pole figures symmetry differences and texture index differences. Wright et al. (2007) concluded that approximately 10,000 grains are required to reproduce the x-‐ray data, the maximum pole figure densities and the symmetry parameters appeared to be the best comparative indicators. Bozzolo et al. (2007) introduced an alternative approached, extending the idea of Pospiech et al., based on using a reference ODF or sub-‐populations of a single to calculated volume fraction difference (VΔ) between ODFs rather than the mean square deviation. Minimizing the value of VΔ serves as the objective function for optimizing the half-‐width as a function of the number of grains. The results show that error parameter VΔ is 10% for 10,000 grains and 8% for 20,000 grains. It is interesting to note that initial suggestion by Bunge (1982) of 25 points per ODF cell, implying about 10,000 grain orientations for metal sheet rolling problems, in good agreement with results of Wright et al. (2007) and Bozzolo et al. (2007).
5.3 Indexation
The process of indexation still relies heavily on operator experience and many factors buried deep inside the software. Hence indexation remains most delicate operation in EBSD, which is strongly dependent on the quality of the diffraction pattern and the detection of the diffraction bands. The settings controlling the Hough transform can have a strong influence on band detection. The number of bands used for indexing have a strong influence on indexing. Different software packages will require a different number of bands, which illustrates the different strategies used by the software. The choice of using the orientation of bands, the orientation and the band-‐width or the orientation, the band-‐width and intensity will strongly affect indexing results and indexing speed. Often the diffraction intensity is just used to select which hkls are retained the database for indexing, rather than to match the relative intensity of each band. The choice of the reference crystallographic data for the crystal structure will also effect indexing, particularly for more complex compositions. The main concerns about reliability of indexing occur when the quality of the diffraction pattern is reduced, for example due to poor sample preparation, fast acquisition low sensitivity camera, low symmetry or weakly diffracting crystal. Some consideration of the objective of the study and quality of data to meet these objectives is essential (Randle, 2009). For example the indexing of poorly diffracting minerals would be too unreliable using a low sensitivity ultra-‐fast camera (e.g. Trimby et al., 2002). In some cases reliable indexing can only be done manually. In all cases initial indexing for each phase present in a sample should be done manually to optimise settings of the Hough transform, number of bands, band setting (line, width etc), wait time etc are required. Then a small test map should be run to check if the settings work reliably, only then should a large-‐scale map be envisaged. The step size should be chosen as a function of the grain size, time available for the acquisition and objective of the study. Various suggestions have been made for the number of steps per grain needed to reliable results; to obtain an accuracy of 10% at
least 5 pixels per grain are required, and for an accuracy of 5%, a minimum of 8 pixels per grain are required (Humphries, 2001), 10 pixels per grain (Engler and Randle, 2009).
6. Future of EBSD
6.1 Introduction
The evolution of EBSD is certainly already guided by the trends developed in recent years, such as increased acquisition speed with ultra-‐fast cameras (e.g. Chen et al., 2012) and increasing computational power. Increased spatial resolution is also clearly defined objective with diffraction physics being limited by interaction volume and possibility better resolutions at lower accelerating voltages (e.g. Steinmetz and Zaefferer, 2010). The integration EBSD and X-‐ray analysis is already available, but could certainly be improved remembering certain limitations due to the interaction volumes, particularly difference in their depth of origin for diffracted electrons and characteristic X-‐ray photons. EBSD will face new challenges due to ever increasing size of data sets. Today the main limitation of EBSD is the pattern quality and the reliability of indexation, especially for low symmetry weakly diffracting crystals. Clearly an effort in the direction of more sensitive EBSD scintillators would automatically guarantee major improvements in pattern quality and therefore the reliability of indexation. The use of sensitive and robust YAG screens, currently very expensive, would be a straightforward option, which would not need a protective aluminium coating. Scintillators with built-‐in image intensifiers may be another avenue to develop more sensitive detectors. Noise reduction by using energy filters should also improve diffraction pattern signal to noise ratios and hence make band detection more reliable. The Use of more sophisticated detection methods such as the 3D-‐Hough or the Radon transform taking into account more parameters, such as diffraction intensities would also improve indexing. More experience is probably needed to judge if a combination of more sensitive detection and recorded hi-‐fidelity patterns with post-‐mortem off-‐line processing is a more reliable solution than the current model of on-‐line processing. The currently marginal areas of EBSD, such as 3D-‐EBSD will certainly progress as the access to such expensive system becomes more frequent. The Crossbeam or Dualbeam SEMs using focused ion beam (FIB) for sample preparation are only suitable for studies of fine-‐grained materials. Future developments for coarse-‐grained using automated serial sectioning methods at the macro-‐to-‐micro scale is through serial sectioning experiments Spowart (2006), including a six-‐axis robot!
6.2 Electron signals
If we just considering electron signals, several recent studies have highlighted the use of EBSD and electron channelling contrast imaging (ECCI) in combined studies of dislocation Burgers vectors, dislocation densities and twins (e.g. Trager-‐Cowan et al., 2007 ;Gutierrez-‐Urrutia et al., 2009; Gutierrez-‐Urrutia and Raabe, 2012), the two methods are quite complimentary with ECPs having a small solid angle less than 20° allowing direct access to fine details. The other type of electron diffraction that could be exploited in an SEM is the grazing incident Reflection High-‐Energy Electron Diffraction (REED) with small penetration depth and a spatial resolution of about 1 μm (Ichimiya and Cohen, 2004). REED diffraction spots or streaks seem to be present in some EBSD patterns with appropriate geometry (Day, 2009) at 5 to 30 kV in GaN, but contamination
is observed after a few minutes. The combined spot and Kikuchi line information in the diffraction pattern may provide additional constraints on indexing solutions. REED would also provide additional information about the surface crystallography not provided by EBSD. REED could be used in in-‐situ heating experiments to monitor atomic layer growth. Clearly prolong use of REED would require very high vacuum conditions.
6.3 X-‐ray and spectroscopic signals
In classical textbooks on the SEM (e.g. Goldstein et al., 2003) the now classical use of secondary, backscattered electrons and X-‐ray microanalysis are presented. Hence it is no surprise that there has been an integration of coupled EBSD and X-‐ray detectors by detector suppliers. The interaction between an electron beam and the sample produces a rich variety other signals that have not as yet been integrated into the EBSD environment. When the diffracted X-‐ray emission stimulated by the electron beam the resulting diffraction pattern is called Kossel pattern, named after Walter Kossel who first observed them in 1924. When the Kossel patterns stimulated by an X-‐ray source it is called pseudo-‐Kossel pattern. Kossel patterns would also be very complimentary to EBSD as the patterns can be used for the analysis of unknown lattice constants with precision of 10-‐5 and high-‐resolution residual stress mapping (Langer and Däbritz, 2010). The introduction of the 3D Hough transform by Maurice and Fortunier (2008) has provided the first means of automatically indexing Kossel patterns. Some problems remain for integration of Kossel patterns in an EBSD system; these include an X-‐ray source if all compositions are going to be analysed. All patterns from three different methods Kossel diffraction, pseudo-‐Kossel diffraction and the EBSD, can be recorded by a single scintillator-‐CCD camera system in the SEM (Langer et al., 2001). Perhaps a simpler technical problem is the integration of cathodoluminescence (CL) detector into an EBSD system. Although commercial CL systems exist that can map the hyper spectral (or colour) CL, they have not been directly integrated into an EBSD system and until such systems are as expensive as a complete EBSD system. The recent development of compact CCD spectrometers that allow the visible, near infra-‐red and near ultra-‐violet spectra to be recorded in digital or analogue form directly to a computer paves the way to an extended wavelength ‘CL’ system than could be recorded using a light-‐pipe or parabolic mirror at the same time as the EBSD mapping. As the optical CL spectrum contains signals related to point and line defects, as well as impurities at the trace element level (e.g. Parish and Russell, 2007; Edwards and Martin, 2011), it provides additional information that cannot be detected by EBSD or X-‐rays. Finally, the coupling of micro-‐Raman spectroscope associated with a laser, which has a spatial resolution of about 1 μm could compliment EBSD and X-‐ray microanalysis. It can also be used for chemical mapping using a database of spectra to identify minerals and other compounds. In addition Raman scattering by an optically anisotropic crystal gives information on the crystal orientation.
6.3 In-‐situ stages
Another area of future development is the use of special stages for in-‐situ work. Use has been made of liquid nitrogen cold stages for preserving ice Ih, the hexagonal structure commonly observed on the Earth’s surface in Polar Regions. Sub-‐grain structures have been measured in naturally deformed ice (e.g. Piazolo et al., 2008; Weikusat et al., 2010). Heating stages have also been used for in-‐situ annealing of cold rolled alloy where the crystallization mechanism was identified as strain-‐induced
boundary migration (e.g. Hurley and Humphreys, 2004). A novel heating stage using laser-‐powered heater has been developed for EBSD work on phase transformations (Kirch et al., 2007). Deformation stages have also been developed for EBSD work (e.g. Bjerkaas et al., 2006) to study texture evolution with strain.
6.4 Software
Finally, but not least is the development of academic open source software for EBSD. The academic community provides an import driving force for new developments in the area of EBSD, for hardware and software. Given the complexity of the parameterization of acquisition software and the post-‐processing software it is important the independent systems are developed by the academic community. I will just indicate two important recent developments in this area. One is the MTEX open source MatLab toolbox for texture analysis initiated by Ralf Hielscher. The first reference publication about the MTEX was published by Hielscher and Schaeben (2008) and more recent one by Mainprice et al. (2011) on physical properties (see http://code.google.com/p/mtex/). The second is an open source EBSD acquisition software initiated by Philippe Pinard and recently described by Pinard et al. (2011) (see http://ebsd-‐image.org/).
7. Conclusions
In this review I have tried to emphasise from the technical details that are important for the successful operation of SEM-‐EBSD system based on my own experience at Montpellier and experiences with dysfunctional systems elsewhere. Firstly, I emphasis the need for top quality sample preparation. The technology has greatly advanced since 1993, mainly directed it is true to needs of the community working on metals; fast acquisition speed and improved spatial resolution below 50 nm. Many practical problems associated with EBSD are due to the tilted sample geometry at 70° and the high probe currents. The long acquisition times needed for EBSD results in the need for great stability of the SEM system and the shallow depth of the interaction volume results in EBSD measurements being very sensitive to any form of surface pollution, a problem that is enhanced due to the high probe currents that are generally necessary. The reliability of EBSD measurements is strongly dependent on the quality and high signal to noise ratio of the measured diffraction pattern. However very little progress has been made in the last 10 years on the improving the sensitivity of the scintillator-‐camera system, despite the fact the greater sensitivity would directly improve band detection, indexing reliability, and allow lower probe currents.
Future developments will I hope include more sensitive camera systems. As the electron beam specimen interaction generates various X-‐ray, electron and optical signals, many of these to be combined with EBSD to give much more powerful synergy that presently used. Specialized in-‐situ stages will allow the SEM and EBSD to realize its true potential to study a wider range of materials and dynamic processes. The incubator that open source software represents must be encouraged by the academic community if we want to explore new developments that have no immediate commercial impact and are motivated by accuracy, reliability and reproducibility.
Acknowledgements
The author is grateful to the organisers of the 10th European Microbeam Analysis Society (EMAS) regional meeting in Padova, Italy, specifically Anna Fioretti and Richard Spiess, for the opportunity to share his practical experiences of EBSD systems. I thank Luc Van't dack, EMAS Secretary, for his patience and editorial assistance. Over the years conversations and e-‐mail exchanges has helped refine my view of EBSD systems, these include Austin Day, Florian Heidelbach, Karsten Kunzer, Geoff Lloyd, Joe Michael, David Prior, Niels-‐Henrik Schmidt and Robert Schwarzer. I also thank my present and past colleagues in Montpellier for providing the continuous scientific stimulus to push EBSD to new limits, these include Pierre Azïs, Fabrice Barou, Guilhem Barruol, Walid Ben Ismaïl, Benoit Ildefonse, Luiz Morales, Andrea Tommasi, and Alain Vauchez. Special thanks to Christophe Nevado, without his careful sample preparation none of this would be possible.
References
Adams B.L., Wright S.I., Kunze, K., (1993) Orientation imaging: The emergence of a new microscopy. Met Trans 24A: 819–831.
Alam M.N., Blackman M., Pashley D.W., (1954) High-‐angle Kikuchi patterns. Proc Roy Soc London A221,224–242.
Arnel, F., Verdier, P., Vincinsini,P.-‐D., (1969) Coefficient de retrodiffusion dans le cas d’électrons monocinétiques arrivant sur la cible sous une incidence oblique. Compt. Rend. Acad. Sci. (Paris), 268, 1526-‐1529.
Baba-‐Kishi, K.Z., (2002) Review Electron backscatter Kikuchi diffraction in the scanning electron microscope for crystallographic analysis. J. Materials Science, 37, 1716-‐1746.
Bjerkaas, H., Fjeldbo, S.K., Roven, H.J., Hjelen, J., Chiron, R., Furu, T., (2006) Study of microstructure and texture evolution using in situ EBSD investigations and SE imaging in SEM. Mater Sci Forum 519–521, 809–814.
Bozzolo, N., Gerspach, F., Sawina, G., Wagner, F. (2007) Accuracy of orientation distribution function determination based on EBSD data -‐ A case study of a recrystallized low alloyed Zr sheet. Journal of Microscopy, 227, 275–283.
Bunger, H.-‐J., (1993) Texture Analysis in Materials Science, Cuvillier Verlag Göttingen, 1993, ISBN 3-‐928815-‐81-‐4
Borbély, A., Maurice, C., Driver, J.H. (2008) Rotation axis analysis of deformed crystals by X-‐rays and electrons. J. Appl. Cryst. 41, 747-‐753.
Chen, Y., Hjelen, J., Gireesh, S.S., Roven, H.J. (2012) Optimization of EBSD parameters for ultra-‐fast characterization. Journal of Microscopy, 245, 111–118.
Coates, D.G., (1967) Kikuchi-‐like reflection patterns obtained with the scanning electron microscope, Philos. Mag. 16, 1179-‐1184.
Day, A.P., (2009) Spherical Kikuchi Maps and Other Rarities. In Schwartz, A. J., Kumar, M., Adams, B.L. and Field, D.P. (Editors) (2009) Electron Backscatter Diffraction in Materials Science (2nd Edition), ISBN 978-‐0-‐387-‐88135-‐5, Kluwer Academic / Plenum Publishers, 65-‐80.
Dingley, D.J., Mackenzie, R., and Baba-‐Kishi, K.Z., (1989) Application of backscatter KIkuchi diffraction for phase identifcation and crystal orientation measurements in materials in : Microbeam Analysis 1989 P.E. Russell ed., San Francisco Press, San Francisco Press.
Deal A., Hooghan T., Eades A ., (2008) Energy-‐filtered electron backscatter diffraction. Ultramicroscopy 108,116–125.
Dingley, D.J. (2004) Progressive steps in the development of electron backscatter diffraction and orientation imaging microscopy, J. Microsc., 213, 214–224.
Drouin, D., Couture, A.R., Joly, D., Tastet, X., Aimez, V. & Gauvin, R. (2007) CASINO V2.42-‐A fast and easy-‐to-‐use modeling tool for scanning electron microscopy and
microanalysis users. Scanning 29, 92–101. http://www.gel.usherbrooke.ca/casino/index.html
Eades, A., (2000) EBSD : Buying a system. In Schwartz, A. J. , Kumar, M. and Adams, B.L. (Editors) (2000) Electron Backscatter Diffraction in Materials Science, ISBN 0-‐306-‐46487-‐X, Kluwer Academic / Plenum Publishers,123-‐126.
Edwards, P.R., Martin, R.W., (2011) Cathodoluminescence nano-‐characterization of semiconductors. Semicond. Sci. Technol. 26 064005 doi:10.1088/0268-‐1242/26/6/064005
Engler, O., Randle, V., (2009) Introduction to Texture Analysis Macrotexture, Microtexture, and Orientation Mapping. CRC Press, Taylor and Francis Group, Boca Raton ,USA
Finch G.I., Wilman H., (1937) The study of surface structure by electron diffraction. Erg Exakt Naturwiss 16, 353–436.
Fynn, G. W., and Powell, W.J.A., (1979) The Cutting and Polishing of Electro-‐optic Materials, Adams-‐Hilger, London, UK.
Goldstein, J., Newbury D., Joy, D.C., Lyman, C.E., Echlin, P., Lifshin, E., Sawyer, L., Michael, J.R. (2003) Scanning Electron Microscopy and X-‐ray microanalysis 3rd edition. Springer Science & Business Media Inc., New York.
Gutierrez-‐Urrutia, I., Zaefferer, S., Raabe,D., (2009) Electron channeling contrast imaging of twins and dislocations in twinning-‐induced plasticity steels under controlled diffraction conditions in a scanning electron microscope. Scripta Materialia, 61, 737–740.
Gutierrez-‐Urrutia, I., Raabe,D., (2012) Dislocation density measurement by electron channeling contrast imaging in a scanning electron microscope. Scripta Materialia, 66, 343-‐346.
Heidelbach, F., Kunze, K., Wenk, H-‐.R. (2000) Texture analysis of a recrystallized quartzite using electron diffraction in the SEM. Journal of Structural Geology, 22, 91-‐104.
Hough, P.V.C., (1962) Method and means for recognizing complex patterns. US patent 3,069,654.
Harland, C.J., Akhter, P., and Venables, J.A., (1981) Accurate microcrystallography at high spatial resolution using electron back scattering patterns in a FEG-‐SEM. J. Phys. E14, 175-‐182.
Hielscher, R., Schaeben, H., (2008). A novel pole figure inversion method: Specification of the MTEX algorithm. J. Appl. Crystallog. 41, 1024–1037.
Humphreys, F.J., (1999) Quantitative metallography by electron backscattered diffraction. Journal of Microscopy, 195, 170-‐185.
Humphreys, F.J., Huang, Y., Brough, I., & C. Harris, C., (1999) Electron backscatter diffraction of grain and subgrain structures -‐ resolution considerations. Journal of Microscopy, 195, 212-‐216.
Humphreys, F.J., (2001) Review grain and subgrain characterisation by electron backscatter diffraction. J. Mater. Sci. 36, 3833–3854.
Humphreys, F.J., (2004) Characterisation of fine-‐scale microstructures by
electron backscatter diffraction’, Scr. Mater., 51, 771–776.
Hurley, P.J., Humphreys, F.J., (2004) A study of recrystallization in single-‐phase aluminium using in-‐situ annealing in the scanning electron microscope. Journal of Microscopy, 213, 225-‐234.
Ichimiya, A., Cohen, P. (2004) Reflection high energy electron diffraction. Cambridge University Press, Cambridge, UK, ISBN 0-‐521-‐45373-‐9.
Joy D.C., (1974) Electron channelling patterns in the scanning electron microscope. In: Holt DB, Muir MD, Boswarva IM, Grant PR (eds) Quantitative scanning electron microscopy. Academic Press, New York.
Jura, J., Pospiech, J., Gottstein, G. (1996) Estimation of the minimum number of single grain orientation measurements for ODF determination. Z. Metallkd. 87, 476–480.
Kikuchi, S. (1928) Diffraction of cathode rays by mica, Proc. Imp. Acad. 4 , 354–356.
Kirch, D.M., Ziemons, A., Lischewski, I., Molodov, D.A., Gottstein, G., (2007) A novel laser powered heating stage for in-‐situ investigations in a SEM. Materials Science Form, 558-‐559, 909-‐914.
Kocks, U.F., Tomé,C., N. and Wenk,H.-‐R. (1998). Texture and Anisotropy, Cambridge University Press.
Kogure, T. (2002), Identification of polytypic groups in hydrous phyllosilicates using electron backscattering patterns. American Mineralogist, 87, 1678-‐1685.
Krieger Lassen, N., (1998) Automatic high-‐precision measurements of the location and width of Kikuchi bands in electron backscatter diffraction pattern. J. Microsc 190,375–391.
Kunze, K., Wright, S.I., Adams, B.L., Dingley, D.J. (1993) Advances in automatic EBSP single orientation measurements. Textures and Microstuctures, 13, 41-‐45.
Langer, E., Däbritz, S., (2010) 75 Years of Kossel patterns – past and future. Materials Science and Engineering 7 012015 doi:10.1088/1757-‐899X/7/1/012015
Langer, E., Däbritz, S., Schurig, C., Hauffe, W., (2001) Lattice constant determination from Kossel patterns observed by CCD camera. Appl. Surf. Sci. 179, 45-‐48
Lloyd G.E., (1987) Atomic number and crystallographic contrast images with the SEM: a review of backscattered electron techniques. Mineralogical Magazine, 51, 3-‐19.
Lloyd G.E., Ferguson, C.C., (1986) A spherical electron-‐channelling pattern map for use in quartz petrofabric analysis. J. Struct. Geol. 8, 517–526.
Mainprice, D. (1981) Experimental deformation of quartz polycrystals. Ph.D. these Australian National University.
Mainprice, D., Hielscher, R., Schaeben, H. (2011) Calculating anisotropic physical properties from texture data using the MTEX open source package. In: Prior, D.J., Rutter, E.H., Tatham, D. J. (eds) Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy. Geological Society, London, Special Publications, 360, 175-‐192.
Maurice, C., Fortunier, R., (2008) A 3D Hough transform for indexing EBSD and Kossel patterns. J. Microsc. 230, 520–529.
Matthies, S., Wagner, F., (1996) On a 1/n law in texture related single orientation analysis. Phys. Stat. Sol. B 196, K11–K15.
Michael, J.R., Goehner, R.P., (1993) Crystallographic phase identification in the scanning electron microscope: Backscattered electron Kikuchi patterns imaged with a CCD-‐based detector. MSA Bull 23,168.
Michael, J.R., Goehner, R.P., (1994) Advances in backscattered electron Kikuchi patterns for crystallographic phase identification. In: Bailey G.W., Garratt-‐Reed A.J. (eds), Proceedings of the 52nd annual meeting of the microscopy society of America, San Francisco Press, pp 596–597.
Mingard; K., Roebuck, B., Bennett, E., Gee, M., Nordenstrom, H., Sweetman, G., Chan. P., (2009) Comparison of EBSD and conventional methods of grain size measurement of hardmetals. Int J Refract. Met. Hard. Mater., 27, 213–23.
Nishikawa S., Kikuchi S., (1928) The diffraction of cathode rays by calcite. Proc Imperial Acad. (Japan) 4, 475–477.
Newbury, D.E. Yakowtiz, H.(1976). National bureau of standards special publication 460. Eds. Hienrich, K.F.J. ,Newbury, D.E., Yakowtiz, H., Washington, DC, p.15.
Randle, V. (2009) Electron backscatter diffraction: Strategies for reliable data acquisition and processing. Material Characterization, 60, 913-‐922.
Parish, C.M., Russell, P.E., (2007) Scanning Cathodoluminescence Microscopy, Advances in imaging and electron physics, 147, 1-‐135.
Pera, E., Mainprice, D., and Burlini, L., (2003) Anisotropic seismic properties of the upper mantle beneath the Torre Alfina area (Northern Apennines, Central Italy). Tectonophysics, 370, 11-‐30.
Piazolo, S., Montagnat, M., Blackford, J.R., (2008) Sub-‐structure characterization of experimentally and naturally deformed ice using cryo-‐EBSD. J. Microsc. 230, 509–519.
Pinard, P.T., Lagacé, M., Hovington,P., Thibault,D., Gauvin, R. (2011) An Open-‐Source Engine for the Processing of Electron Backscatter Patterns: EBSD-‐Image. Microscopy and Microanalysis, 17, 374-‐385. doi:10.1017/S1431927611000456
Pospiech, J., Jura, J., Gottstein, G. (1994) Statistical analysis of single grain orientation data generated from model textures. Mater. Sci. Forum 157–162, 407–412.
Radon, J. (1917) Über die Bestimmung von Funktionen durch ihre Integralwerte längs gewisser Mannigfaltigkeiten. Ber Verh Sächs Akad Wiss Leipzig Math-‐Naturw Klasse 69, 262–267.
Schwartz, A. J., Kumar, M., Adams, B.L. (Editors) (2000) Electron Backscatter Diffraction in Materials Science, ISBN 0-‐306-‐46487-‐X, Kluwer Academic / Plenum Publishers,350 pp.
Schwarzer, R.A. (1997) Automated crystal lattice orientation mapping using a computer-‐controlled SEM. Micron 28, 249-‐265.
Schwarzer R.A., Sukkau, J. (2003) Automated evaluation of Kikuchi patterns by means of Radon and fast Fourier transformation, and verification by an artificial neural network. Adv Eng Mater 5,601–606.
Schwarzer,R.A. and Hjelen,J. (2010) High-‐speed orientation microscopy with offline solving sequences of EBSD patterns. Solid State Phenomena 160, 295-‐300.
Søfferud, M., Hjelen, J., Karlsen, M., Breivik, T., Krieger Lassen, N.C., Schwarzer, R. (2008) Development of an ultra-‐fast EBSD detector system. In: Luysberg, M., Tillmann, K., Weirich, T. (eds) Proceedings of the 14th European microscopy congress, EMC2008, Vol. 1: Instrumentation and methods. Springer-‐Verlag, Berlin, pp 623–624.
Spowart, J.E., (2006) Automated serial sectioning for 3-‐D analysis of microstructures. Scripta Mater., 55, 5–10.
Steigerwald, J.M., Murarka, S.P., Gutma, R.J., (2004) Chemical Mechanical Planarization of Microelectronic Materials WILEY-‐VCH Verlag GmbH & Co
Steinmetz, D.R., Zaefferer, S., (2010) Towards ultrahigh resolution EBSD by low accelerating voltage. Materials Science and Technology, 26, 640-‐645.
Trager-‐Cowan,C., Sweeney,F., Trimby, P.W., Day, A.P., Gholinia, A., Schmidt, N.H., Parbrook, P.J., Wilkinson, A.J., Watson, I.M., (2007) Electron backscatter diffraction and electron channeling contrast imaging of tilt and dislocations in nitride thin films, Physical Review B: Condensed Matter 75, 085301.
Trimby, P., Day,A., Mehnert, K., Schmidt, N.-‐H. (2002) Is fast mapping good mapping? A review of the benefits of high-‐speed orientation mapping using electron backscatter diffraction. Journal of Microscopy, 205, 259–269. J. Microscopy, 239, 245-‐248.
Van De Moortèle, B., Bezacier, L., Trullenque, G., Reynard, B., (2010) Electron back-‐scattering diffraction (EBSD) measurements of antigorite lattice-‐preferred orientations (LPO). J. Microscopy, 239, 245-‐248.
Venables, J.A., Harland, C.J., (1973) Electron back-‐scattering patterns—A new technique for obtaining crystallographic information in the scanning electron microscope. Phil Mag 27,1193–1200.
Wagner, F., Wenk, H.R ., Esling, C. and Bunge, H.J., (1981). Importance of odd coefficients in texture calculation for trigonal-‐triclinic symmetries . Phys . Status Solidi, A67: 269-‐285.
Wagner, F., Bozzolo, N., Dewobroto, N., Gey, N. (2002) Some remarks about the processing of automatic EBSD orientation measurements in view of texture determination. Mater. Sci. Forum 408–412, 143–148.
Weikusat, I., De Winter, D.A.M., Pennock, G.M., Hayles, M., Schneijdenberg, C.T.W.N., Drury, M.R., (2010) Cryogenic EBSD on ice: preserving a stable surface in a low pressure SEM. Journal of Microscopy, 242, 295-‐310. doi: 10.1111/j.1365-‐2818.2010.03471.x
Wells, O.C., (1974), Scanning electron microscopy, McGraw-‐Hill, New York.
Went, M.R., Winkelmann,A., Vos,M. (2009) Quantitative measurements of Kikuchi bands in diffraction patterns of backscattered electrons using an electrostatic analyzer. Ultramicroscopy 109,1211–1216.
Wilkinson, A.J., Meaden, G., Dingley, D.J., (2006) High-‐resolution elastic strain measurement from electron backscatter diffraction patterns: New levels of sensitivity. Ultramicroscopy 106,307–313.
Winkelmann, A., Trager-‐Cowan, C., Sweeney, F., Day, A., Parbrook, P. (2007) Many-‐beam dynamical simulation of electron backscatter diffraction patterns. Ultramicroscopy 107, 414–421.
Wright, S.I., Adams B.L., (1992) Automatic analysis of electron backscatter diffraction patterns. Metall Trans 23A, 759–767.
Wright, S.I., Nowell, M.M.,Bingert, J.F., (2007). A comparison of textures measured using X-‐ray and electron backscatter diffraction. Metal. Mat. Trans. A, 38A, 1845-‐1855.
Young, C.T., Young, J.L. (1972) Computer generation and identification of Kikuchi Patterns. J. Appl. Phys. , 43, 1403-‐1417.
Zaefferer, S., (2007) On the formation mechanisms, spatial resolution and intensity of backscatter Kikuchi patterns. Ultramicroscopy 107,254–266.
Figures
Figure 1 Generic SEM-‐EBSD system showing all the elements essential for EBSD. Motorized eucentic specimem stage with external stage controller. EBSD camera system comprising of ; phosphor screen, forward scattered dectector and associated amplifier, protective lead glass window, lens, low light level integrating CCD camera and control unit. External x-‐y beam controller. PC computer with a frame grabber card to record the diffraction pattern and interface cards for beam and stage controllers. Software for EBSD acquisition (diffraction pattern and forward scattered image) and data processing (indexing to mapping and pole figures etc).
Figure 2. Generic SEM chamber with EBSD camera system mounted on the port at 90° from the tilt axis, which is parallel to the X translation direction. The specimen stage is tilted to 70° from the horizontal, with the specimen (obscured) facing the EBSD screen. The alignment of X-‐axis translation of the tilted specimen with the horizontal axis of the SEM can be made by a rotation about the Z-‐axis. Note the EBSD camera system can move in towards the sample or be retracted to change the sample to screen distance, also know as a camera length.
Figure 3. Comparison between a non-‐eucentric and eucentric stage. To keep the EBSD system well calibrated when using the Y-‐translation of the specimen stage when tilted the eucentric stage is essential because the working distance is constant. The illustration is for 70° tilt.
Figure 4. Bragg’s law the basic equation governing the presence of Kikuchi lines in an EBSD diffraction pattern. Note the diffraction angle theta θ will be very small because the wavelength of electrons with a typical SEM accelerating voltage of 20 kV will be 8.66 x 10-‐3 nm.
Figure 5. Illustration of the 3D nature of the Kikuchi cones that project on the EBSD phosphor screen as nearly straight lines due to the very small Bragg angles for electrons at typical SEM accelerating voltages. The detailed geometry near the source region is shown at the top right.
Figure 6. Detailed geometry of the projected geometry on the EBSD camera screen and the relative intensity across a Kikuchi band composed of the excess, deficient lines and relatively high intensity between them. Parameters of band slope and band contrast can be derived from the Kikuchi band profiles, which can provide useful images of the diffraction contrast associated with the microstructure. An averaged intensity profile taken from Went et al. (2009) using a silicon single-‐crystal at 25 to 40 kV.
Figure 7. Relative positioning of the SEM pole piece with respect to the tilted sample and EBSD camera screen. The ideal geometry is show where the pattern center (PC) is near the top of the screen, this requires that the screen center (SC) and hence the axis of SEM port is slightly below the touch point of the beam on the sample (S) for the chosen working distance (WD). Note the probe is elliptical on the tilted sample.
Figure 8. The relationship between pattern quality and pattern center with working distance.
Figure 9. The position of the best EBSD pattern contrast and accelerating voltage. The data are taken from Steinmetz and Zaefferer (2010).
Figure 10. The silicon wafer calibration sample.
Figure 11. A common problem when installing an EBSD system is to define the relative positions of the tilted sample, camera screen and SEM pole piece for an acceptable working distance and a good pattern center, while avoiding that specimen touches the pole piece during Y-‐translation and you can access an acceptable area of the sample!
Figure 12. The effect of the pole piece profile on sample positioning. When buying an SEM for EBSD the profile of the pole piece could be a factor for getting detectors close to sample.
Figure13. The screen geometrical parameters showing variation of solid angle with screen to specimen distance as a function of screen diameter. The shaded in grey area is where most commercial EBSD screens occur, with circular screens of diameters of 34 and 40 mm, as well as rectangular screens (vertical V=30 and horizontal H=40 mm).
Figure14. The shape of the probe on the surface of tilted (70°) sample is an ellipse with short axis parallel to the tilt axis (X-‐axis) and long axis parallel to the Y-‐axis. The interaction volume between diffracting electrons and sample make disc-‐like shape with thickness of 2 nm elongated ‘down slope’ in the forward scattered direction parallel to Y-‐axis. The dimensions of ellipse and depth (2 nm) are taken from Zaefferer (2007) for iron at 15 kV. The purple region is the source region for 50% of the diffracted electrons. Note the figure is not to scale.
Figure15. Beam and Stage scanning.
Figure16. The combined beam and stage scanning method for sampling large specimen area.
Figure17. The relation between step size and time in the linear and log-‐log plots
Figure18. Log-‐Log plot of step size versus time
Figure19. The measured fraction area (MAF) of the interaction ellipse as measured for iron at 15 kV by Zaefferer (2007) of the area between EBSD measurement grid points defined by the step size. The region MAF >1 in grey is due to overlapping interaction ellipse, but overlapping also occurs below MAF =1. (See text for details)
Figure 20. The EBSD square grids are marked in green with the interaction ellipses in red, the x-‐ (tilt) and y-‐axis of the specimen stage are shown. The top row shows the grids and ellipses at relative true scale with step sizes from 100, 50 and 25 nm. The bottom row shows the 50 and 25 nm step size magnified to illustrate the overlap using semi-‐transparent ellipses on the left-‐hand side. In the top right-‐hand example (step size =100 nm), the effect of having a shorter step size in the x-‐direction is shown by the blue ellipse to produce a more homogenous sampling.
Figure 21. The EBSD square grids are marked in green with the interaction ellipses in red, the x-‐ (tilt) and y-‐axis of the specimen stage are shown for a step size of 25nm. The first row of grid points is marked by a black dot. The interaction ellipses have their topmost position on the grid points, in the same geometry as shown in Figure 14. Each ellipse has transparency of 85% so that the complex overlaps can be clearly seen. High overlapping regions from bands (B) and oval (O) shapes are marked by black boxes.