SEM and EPMABeyond the BasicsStereo SEMExample: porous sandstoneStereo SEM DetailsEDS of particlesParticles - 1Particles - 2Particles - 3PowerPoint PresentationEDS X-ray Mapping-1EDS X-ray Mapping-2EDS X-ray Mapping-3X-ray Mapping and the clockContinuum ArtifactContinuum and Atomic NumberHowever…Data CubeSpectral (“hyperspectral”) ImagingOne of the issues with SEMs…Multivariate statistical analysisSlide 22Needle in a haystackX-ray MappingFeature Sizing, Chemical Typing1. Set the BSE intensity and then set the range to select one phase2. Select the elements needed to find the phase3. After SEM done, check the grains the software suggests are the ones you want“What will they think of next?”TTL “Through-the-lens Detectors”Energy filtering of SEsElectron Backscatter DiffractionFocused Ion Beam InstrumentsSlide 34FIB-EBSDSlide 36“Personal SEM”SEM and EPMA Some Advanced TopicsRevised 2/18/2014Beyond the Basics• Stereo SEM• EDS of particles: beware!• TTL Detectors• X-ray mapping• Feature Sizing, Chemical Typing• EBSD • FIB• and on and onStereo SEMWhy? Graphical demonstration of 3D shapes of rough or complex objectsHow? Very generally …..1. Acquire two SEM images of the same object at the same magnification but by tilting the stage slightly. 2. Color each image red or green in Photoshop.3. Then in Photoshop overlay the two images, while wearing red-green stereo glasses, shift them slightly apart until you have the correct 3D effect.4. Show downstairs in 2nd floor visualization lab for maximum effect…Exact detailed instructions on wall in room 308Example: porous sandstoneStereo SEM DetailsFor rough objects, use 3-7° tiltFor smooth objects, use more, 7-15°Use less tilt if higher magnificationIf you need to refocus, do NOT change objective lense setting, rather move sample stage Z up or down.Cited reference in literature: Heuser, 1989, Protocol for 3-D visualization of molecules on mica via the quick-freeze, deep-etch technique, Journal of Electron Microscopy Technique, vol 13, p. 244-263.EDS of particles… is easy…. Maybe too easy? --> And easy to make mistakes!Particles - 1• Mass effect/error: electrons escape from sides of small particles if E0 is large enough, so quantitative analysis will be in error because different elements (with different binding energies) will be affected differentlyGoldstein et al, 1992, p. 479, 481Particles - 2 Absorption effect of non-flat upper surface: different path length from normal flat geometry and the “normal” way we do quantitative analysis is to use FLAT polished standards for calibration -- so we could have “too much” x-ray intensity for particlesGoldstein et al, 1992, p. 479, 481Particles - 3•Variable effect of geometry of trajectory between beam impact area on non-uniform surface and the location of the detector -- so we get different results from the same material, depending upon where we place the electron beam. Goldstein et al, 1992, p. 479, 481Results from 2006 777 student project on EDS of rough samples with VP-SEMThe first column shows the actual chemical composition, followed by the average composition of 20 points on different grains, followed by the variation (=standard deviation) of those individual measurementsEDS X-ray Mapping-1BSE imaging provides rapid distinction of some/many/most chemically distinct phases in a multiphase sample. However, in many cases, having explicit 2D chemical information is value. This is where “x-ray mapping” comes into play.X-ray mapping has changed tremendously since it was first introduced in 1956 as a combination of WDS (crystal diffraction spectrometry) and SEM: in those days it was called “dot mapping” as literally dots would be painted onto the CRT. To capture it, a photograph would have to be snapped of the screen. It generally was pretty grainy. And only 1 or 2 or 3 elements could be acquired simultaneously (depending on how many spectrometers were on the electron probe).EDS X-ray Mapping-2EDS was developed in the late 1960s and soon began to be used for X-ray mapping.For the next 30 years or so, users would select “regions of interest” (ROIs), essentially the peak areas of particular elements -- sometimes limited to a finite value like 8 or 12 -- which would then be used to “color in” a 2D area over which the beam would scan, either one very long and slow scan, or many faster scans that would be averaged. Note: To this point, all the above maps included both the characteristic X-ray being chosen AND the background/ continuum under the characteristic peak.EDS X-ray Mapping-3Things have changed in the 40 years since EDS came on the scene:1. Digital pulse processing have taken over from the older analog processing, making shorter time constants and thus higher count rates possible (e.g. 30,000 cps vs 3,000 cps before) which create many more opportunities -- larger areas, shorter times.2. And then the development of SDD (Si-Drift Detectors) have boosted the count rates to the hundreds of thousands of cps.X-ray Mapping and the clockDue to the low count rate of detected X-rays, dwell times in the past generally needed to be hundreds of milli-seconds. A 512x512 X-ray map at 100 msecs took ~8 hours to acquire. The improvements to EDS systems with improved digital processing throughput allows 1-10 msec dwell times, dropping the x-ray mapping times down to the hour or less range, in many casesReed, 1996, Fig 6.1, p. 102Continuum ArtifactGoldstein et al, 1992, Fig 10.6, p. 535Here is an example of false compositional contrast, an artifact of the background being a function of Z (atomic number, or mean at. nu., MAN, for compound). Specimen is Al-Cu eutectic; X-ray maps are (a) Al, (b) Cu, (c) Sc. The contrast in (c) suggests Sc is present in the Cu-rich phase. However, there is no Sc, only the background in the Cu-rich phase is elevated relative to the background in the Al-rich phase. Thus one needs to be aware whether the background is or is not subtracted from X-ray maps, esp. when looking at minor elements where the continuum is a major component. Many published maps do not state if bkg-subtracted, so assume they aren’t.False impressionCorrect impressionAl Ka mapCu Ka mapSc Ka mapSc Ka background substracted mapContinuum and Atomic NumberAt a given energy (or ), the intensity of the continuum increases directly with Z (atomic number) of the material. This is of critical importance for minor or trace element analysis, and also
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