I always liked the commercial that asked,” How many licks does it take to get to the center of a Tootsie Pop?”. I like contests where you estimate the number of M&Ms in a jar. Taking the concept away from delicious treats and moving towards something more technical, I’ve also enjoyed looking at the number of grains we need to measure with EBSD to get a good idea of the texture of a material.
Recently I’ve been working with our new Clarity Direct Electron Detector for EBSD. It’s the first commercial EBSD direct detector and will be launching soon. Traditionally, EBSD patterns are captured when the diffracted electrons strike a phosphor screen, where energy is converted into light photons, which are focused through a lens onto an imaging sensor, where the light photons are then converted back to electrons. However, a direct electron detector is just that, it captures the diffracted electrons directly. This allows us to count the electrons in an EBSD pattern directly.
Figure 1. EBSD pattern collected with Clarity with an average of 5,000 electrons per pixel.
Take the EBSD pattern collected from a nickel superalloy using the Clarity shown in Figure 1. For an EBSD pattern like this, remember that it has been background corrected to flat-field the image and improve the contrast. This is because the actual live EBSD pattern does not have a uniform intensity across the sensor, as shown in Figure 2. In this example, a background collected while imaging many grains was collected and subtracted from the live signal to produce the image in Figure 1. The background image has the spatial information for a specific orientation removed, while retaining the overall intensity gradient that is a function of the material of interest and the sample geometry. Note that the Clarity uses four direct electron detectors that are coupled together. The cross-hair image visible in Figure 2 shows the location of the seams between the detectors. These can be masked out of the image if desired but are quickly minimized with this background correction.
Figure 2. EBSD pattern from Figure 1 prior to background correction.
For Figure 1, a pixel at the center of the signal intensity contained approximately 10,000 electrons, and the average counts for all pixels was approximately 5,000 electrons. After background subtraction, I drew a line across the image, and the intensity profile across this line is shown in Figure 3. This profile shows that the final processed EBSD pattern has a dynamic range of about 1,700 electrons.
Figure 3. Line profile across the EBSD pattern in Figure 1 showing the dynamic range of the EBSD signal.
Figure 4. EBSD pattern with an average of 10 electrons per pixel.
Now seeing that I could count the number of electrons in an EBSD pattern, I wanted to know how many I needed to get a usable EBSD pattern. I could decrease the exposure time, decrease the beam current, or do both. In this case, I continually decreased the exposure time to find where the EBSD pattern indexing started to fail. Figure 4 shows an EBSD pattern where the maximum number of electrons is 20 and the average number of electrons is 10. Even with this small amount of a signal, I was still able to index it with a confidence index of 0.92 and a fit of 0.6°, which indicates a good orientation solution. Talk about doing a lot with a little. This performance is enabled by the single electron sensitivity and zero readout noise of the detector, which makes this camera very exciting for low beam dose applications for beam-sensitive materials. I look forward to sharing more later.
Figure 5. Indexing solution for the pattern in Figure 4 with a confidence index of 0.92.
One of the joys of my job is troubleshooting issues and ensuring you acquire the best results to advance your research. Sometimes, it requires additional education to help users understand a concept. Other times, it requires an exchange of numerous emails. At the end of the day, our goal is not just to help you, but to ensure you get the right information in a timely manner.
For any sort of EDS related question, we almost always want to look at a spectrum file. Why? There is so much information hidden in the spectrum that we can quickly point out any possible issues. With a single spectrum, we can quickly see if something was charging, tilted, or shadowed (Figure 1). We can even see weird things like beam deceleration caused by a certain imaging mode (Figure 2). With most of these kinds of issues, it is common to run into major quant related problems. Any quant problems should always start with a spectrum.
Figure 1. The teal spectrum shows a strange background versus what a normal spectrum (red) should look like for a material.
This background information tells us that the sample was most likely shadowed and that rotating the sample to face towards the detector may give better results.
Figure 2. Many microscopes can decelerate the beam to help with imaging. This deceleration is great for imaging but can cause EDS quant issues. Therefore, we recommend reviewing the spectrum up front to reduce the number of emails to troubleshoot this issue.
To save the spectrum, right-click in the spectrum window, then click on Save (Figure 3). From there, save the file with a descriptive name, and send it off to the applications group. These spectrum files also include other metadata, such as amp time, working distance, and parameters that give us so many clues to get to the bottom of possible issues.
Figure 3. Saving a spectrum in APEX is intuitive. Right-click in the area and a pop-up menu will allow you to save the spectrum wherever you want quickly.
The actual image file can also help us confirm most of the above.
Troubleshooting EBSD can be tricky since the issue could be from sample prep, indexing, or other issues. To begin, it’s important to rule out any variances associated with sample preparation. Useful information to share includes a description of the sample, as well as the step-by-step instructions used to prepare the sample. This includes things like the length of time, pressure, cloth material, polishing compound material, and even the direction of travel. The more details, the better!
Now, how do I know it is a sample prep problem? If the pattern quality is low at long exposure times (Figure 4) or the sample looks very rough, it is probably related to sample preparation (Figure 4). That being said, there could be non-sample prep related issues too.
Figure 4. This pattern is probably not indexable on its own. Better preparation of the sample surface is necessary to index and map this sample correctly.
Indexing problems can be challenging to troubleshoot without a full data set. How do I know my main issues could be related to indexing? If indexing is the source, a map often appears to be very speckled or just black due to no indexing results. For this kind of issue, full data sets are the way to go. By full, I mean patterns and OSC files. These files can be exported out of TEAM/APEX. They are often quite large, but there are ways available to move the data quickly.
As for camera set up, this is a dance between the microscope settings, operator’s requirements, and the camera settings. In general, more electrons (higher current) allow the experiment to go faster and cover more area. With older CCD based cameras, understanding this interaction was key to good results. With the newer Velocity cameras based on CMOS technology, the dance is much simpler. If you are having difficulty while trying to optimize an older camera, the Understanding and Optimizing EBSD Camera Settings webinar can help.
So how do you get your questions answered fast? Bury us with information. More information lets us dive deeper into the data to find the root cause in the first email, and avoids a lengthy back and forth exchange of emails. If possible, educate yourself using the resources we have made available, be it webinars or training courses. And always, feel free to reach out to my colleagues and me at firstname.lastname@example.org!