Month: October 2014

High Resolution EBSD – Seeing More of the Details

(Note from the Editor) The use of the term HREBSD in this post needs clarification. The term High Resolution EBSD has had several different meanings since the advent of automated EBSD indexing. As indexing algorithms have improved it has referred to the improvement in angular precision of the orientation measurements. Others have used the term to refer to strategies for improving the spatial resolution of the technique. However, in recent years the term HREBSD is commonly being used to refer to the use of cross-correlation methods for precise calculation of elastic strains and small angle misorientations as pioneered by Angus Wilkinson (Wilkinson, A. J. (1996) Ultramicroscopy 62(4): 237-247.). It should be noted that in the results presented in this post, no cross-correlation calculations were performed, rather careful use of the standard Hough Transform and Triplet Indexing procedures were used.

Travis Rampton, Applications Engineer EBSD/EDS EDAX

I recently saw an article on a social media site that showed how a particular telescope was able to look into the dark of space for a long stretch of time and see with impressive resolution thousands of galaxies. The image itself was quite amazing, but what was interesting to me was the comparison of the technique used to capture this image with that of what we call high resolution EBSD.

High resolution EBSD (HREBSD) is a method within the framework of EBSD that seeks to extract improved angular resolution from an EBSD scan. The current angular resolution with standard EBSD is ~0.1o. This allows scans the ability to show plastic deformation within a sample which is caused by small measurable rotations of crystal lattices. HREBSD, however, has more than ten times the resolution of traditional EBSD methods at ~0.006o [1]. This additional resolution allows measurement of additional materials characteristics including elastic strain and dislocation content. In order to obtain this information high quality EBSD patterns are required.

Much as in the example of the telescope, high quality images/patterns generally require long collection times to get a high signal to noise ratio. Often frame averaging is suggested to reduce the noise in patterns. Long collection times per point often mean fewer points are collected in a given dataset. The issue is often ignored and lower resolution images are collected to save time or the technique is altogether neglected. When this happens the details that can be obtained from HREBSD are lost. With this blog post I hope to show the potential good data can offer to the extraction of high quality HREBSD results and also that with modern hardware high quality data can be collected much more quickly than previously thought.

Figure 1

Figure 1: SiGe single crystal (left) and Ni standard used for EBSD calibration (right) IPF maps.

The above inverse pole figure (IPF) maps seem relatively uninteresting, just a couple of colors. With high quality EBSD patterns we can start to see more details than the IPF maps show. One of the basic metrics we use in EBSD to quantify and show deformation is the kernel average misorientation (KAM). This measurement shows plastic deformation based on local changes in orientation typically caused by dislocations. For the SiGe sample we wouldn’t expect much, if any, deformation. Small rotations can occur in SiGe though due to mismatch with the underlying Si substrate. One line of misorientation is faintly seen in the SiGe sample shown here. The KAM map of the Ni standard shows much more detail of the deformation that is present in the material. The structures that produce the deformation can clearly be seen as the higher intensity green, yellow lines. The Ni standard also shows pile up of deformation at the grain boundaries. While I can’t definitively state that the high intensity lines we see are dislocations, many researchers have used KAM to correlate with dislocation density and level of deformation. To obtain these values, the KAM of each map is charted and average values are used.

Figure 2: KAM maps of SiGe and Ni standard. An arrow is shown to illustrate the deformation structure in SiGe which aligns with an edge of the overlaid crystal wireframe.

Figure 3

Figure 3: KAM bar charts of SiGe and Ni standard. The x-axis on the SiGe sample goes up to 0.1o while the Ni standard x-axis extends to 1.0o. The average KAM for the SiGe is 0.0066o and for the Ni standard it is 0.11o.

In the case of the Ni standard KAM chart there is a lightly distinguishable Gaussian shape running into the smallest misorientation angle bin. This is a result of the deformation structures in the material, whereas the SiGe chart is mostly strain free and thus all the misorientations are very small. The small KAM associated with the SiGe might be the real angular resolution of standard EBSD. Even if the angular resolution shown in the SiGe sample were to match that of HREBSD, there remain measurements that currently require the aid of HREBSD. As mentioned earlier in this post, HREBSD has the ability to extract the elastic strain of a sample and dislocation content, while standard EBSD only provides general information about the plastic strain.

In addition to the KAM, grain reference orientation deviation (GROD) maps help illuminate some of the deformation in the samples studied. GROD maps show both misorientation angle and axis relative to the average orientation of a grain. This information helps show areas in the microstructure as they are partitioned by the deformation. The SiGe sample shows a diagonal trend of misorientation which is often due to extremely small variations in the pattern center calibration across a scan. (Pattern Center is used to properly index a material orientation).

Figure 4

Figure 4: SiGe GROD-angle map (left) and GROD-axis map (right). The black area in the GROD-axis map is an area of zero misorientation and thus has no axis associated. The range of the GROD-angle map is 0-0.44o.

Figure 5

Figure 5: Ni standard GROD-angle map (left) and GROD-axis map (right). Grain boundaries are drawn in black to distinguish the microstructure. The range of the GROD-angle map is 0-5.48o.

One of the benefits of HREBSD is the ability to break deformation information into slip systems. With standard EBSD we currently measure the axes of rotation which are related but not the same. An example of the axes of misorientation is shown below. Each deformation structure lines up with a given axis of rotation. If the EBSD patterns are not of sufficient quality the axis measurement will be incorrect.

Figure 6

Figure 6: Ni standard KAM map (grayscale) with misorientation axes overlaid.

With the data collected and shown in this post, good quality HREBSD results are more likely, however the time required to collect such data can be overwhelming. In collecting HREBSD data it is important to ensure a high signal to noise ratio. This can be achieved by either longer exposure times with the EBSD camera or increased signal per time. In an SEM, increased signal per time is related to the amount of current going into the sample. For EBSD it is typical to use about 1 nA of current at 20 kV to collect data. These kinds of settings have resulted in collection times of less than one pattern per second for a full resolution EBSD pattern without using any frame averaging. Modern SEMs are capable of putting out >100 nA of current, much greater than the 1 nA typically used, and thus collection times can be greatly shortened. For this study ~50 nA was used to collect the data. In addition to the modern SEM capabilities the EDAX Hikari camera is able to run at 200 frames per second when using the full resolution of the camera. For this study the camera was run at 40 fps and a frame average of 8 was used, which resulted in a pattern collection time of 0.2 fps (5x faster than 1 fps an no frame averaging). With this speed the datasets of about 20,000 points were collected in about an hour. An example of one of the patterns used is shown below.

Figure 7

Figure 7: SiGe pattern used in high quality EBSD scan. The camera settings used to collect this pattern were an exposure of 25 ms and a frame averaging of 8 (Best Pattern setting in the TEAM™ software).

 

[1] A. J. Wilkinson, G. Meaden, D. J. Dingley. (2006) High Resolution Mapping of Strains and Rotations Using Electron Backscatter Diffraction. Materials Science and Technology 2006 22:11, 1271-1278

 

TRANSMISSION-EBSD – Taking Transparency to a New Level.

Dr. René de Kloe, Applications Specialist, EDAX

EBSD on electron transparent samples is quickly becoming the next big thing in orientation measurement techniques. In the literature you may find it described as TKD (Transmission Kikuchi Diffraction) or t-EBSD and there is actually a bit of a debate going on about what the proper name is for the technique, because calling a forward scattered transmission technique by a backscattered diffraction acronym may seem a bit incongruous. Personally I like the name transmission-EBSD best as that name describes exactly what the technique does: getting diffraction patterns in a transmission geometry that may be analyzed with a conventional EBSD system. And I think that a good descriptive name that people will recognize is worth a little incoherence. TKD for me points a little too much to a TEM imaging mode that many people may not recognize as a SEM-based technique.

Figure 1: schematic t-EBSD measurement geometry

Figure 1: schematic t-EBSD measurement geometry

Anyway, knowing what a name means does not necessarily describe what you can actually do with a technique. A few years ago t-EBSD started to gain popularity due to the improved spatial resolution that it offers. For standard EBSD on bulk samples the lateral resolution is mainly determined by the formation depth of a virtual point source in the sample combined with the effective beam diameter. Typically the resolution ranges from about 15 to perhaps 75nm. One of the parameters that affects the resolution in standard EBSD is the 70 degree tilt of the analysis surface as that causes the intersection of the beam with the sample to become an ellipse that is three times higher than wide. When you collect EBSD patterns in a transmission geometry the parameters that govern the effective lateral resolution are different: the accelerating voltage, the specimen foil thickness, and material density, all of which together define the effective beam diameter at the point of exit from the sample. And that corresponds to the obtainable lateral resolution. The beam shape is less important. When the sample is mounted horizontally the beam-sample intersection is circular, but even at a -40 degree tilt, the aspect ratio is still only 1 : 1.3. And that is a nice bonus as the intensity distribution on the EBSD detector is much more homogeneous at -20 to -40 degree tilt which allows for excellent indexing performance.

figure 2 - copper t-EBSD pattern figure 2 - HR t-EBSD mapping
Figure 2: Copper t-EBSD pattern and high resolution mapping result with 2.5 nm steps, scalebar is 100 nm

The smaller information volume will give the user a significant resolution improvement which allows successful orientation analysis of previously inaccessible nanomaterials and extremely deformed samples. But that is not all you can do with t-EBSD. It also opens up a host of combined applications where electron transparent samples are involved. The most obvious to me is looking at TEM specimens before doing any detailed work in the transmission electron microscope.

figure 3 - SE image of STEM sample

Figure 3: SEM SE image of an electro-polished TEM foil of AlTi3-TiAl3 alloy. The grain microstructure is not easily recognizable. (specimen courtesy of Klemens Kelm – DLR Cologne, Germany).

Many imaging modes in materials science TEM analysis such as weak beam dark field dislocation analysis depend on specific grain orientations. Finding these orientations or crystallographic zone axes required for certain diffraction conditions in the TEM can be very time consuming, and thus expensive. Furthermore not all grains that you try to orientate may actually have the desired zone axis within the tilting range of your specimen holder. And when that happens all the work and time spent to find the orientation for that particular grain has been wasted. This is normally not too bad for highly symmetric cubic materials, but it will become a challenge on for example hexagonal Ti and Mg alloys and many low symmetry ceramics and minerals that have a limited number of “usable” zones.

And this is where t-EBSD can help as identifying orientations is exactly what EBSD is designed to do for you and rapidly too. Just imagine that you can analyze your sample with low resolution t-EBSD to prepare a full orientation map of all electron transparent areas so that you know which grains are usable for your diffraction analysis before going to the TEM.

figure 4 - overview TEM sample figure 4 - -low mag t-EBSD map
Figure 4: SEM image of the electro-polished TEM foil and corresponding EBSD IPF crystal direction map of the entire electron transparent area

When you want to do this there are a few things to keep in mind. First, the EBSD pattern quality varies strongly with specimen thickness and you will need some special background processing to obtain indexable patterns.

figure 5 - sample with raw patterns figure 5 - sample with processed patterns
Figure 5: TEM sample overview with raw and processed patterns over a range of foil thickness

Also, EBSD patterns are projected more or less sideways onto the EBSD detector and not straight down as in the TEM. So if you want to use the EBSD indexing results to identify specific zone axes that would be accessible in the TEM you cannot just look at the crystallographic zones that are visible in the pattern. You always need to rotate a simulation of the measured pattern to the TEM geometry to identify the zones within a certain angular distance to the optical axis.

 figure 6 - t-EBSD geometry  figure 6 - TEM diffraction direction
figure 6 - AlTi3 t-EBSD simulation figure 6 - AlTi3 TEM simulation
Figure 6: Actual EBSD pattern after special background processing with indexing result and pattern simulation. The corresponding TEM projection can then be obtained by rotating the simulation. When the tilting range of the used TEM specimen holder is now superimposed on the rotated simulation, the zones that are within tilting range may be determined together with the required rotation angles.

When the required zones are not generally accessible in all grains, you can use the full EBSD map of the sample and generate a crystal orientation or crystal direction map to identify which grains can be used for your TEM diffraction analysis.

An important thing that still remains is the correlation between the SEM and the TEM images. And for that purpose a series of dark-field STEM images may be collected using the PRIAS imaging mode. From this sequence of images a custom combination can be selected to display the best grain contrast to help navigating the sample in the TEM.

Figure 7: Left: PRIAS center image of the analysis area displaying strong orientation contrast. Right: Superimposed crystal direction map of the AlTi3 phase only showing grains that have the [111] direction within 20°of the optical axis

The application described above is just one example of the use of t-EBSD to assist another technique and I am sure that many more hybrid t-EBSD applications will be found. So if you know of other applications where EBSD or t-EBSD may be used to facilitate or improve materials analysis on other instruments please send me a short message so we can all save a bit of time. I cannot help but wish that I had known about this one when I was doing my own TEM diffraction analysis for my PhD thesis. It might have saved me quite a few weekends being hidden away in a dark room …