Techniques, hints and tips for maximizing EBSD data quality

1. Oxford Instruments Techniques, hints and tips for maximizing EBSD data quality

2. Oxford Instruments Agenda • Pre-SEM setup • Assessing prep quality • Placement in holder, sample orientation • Charging prevention • SEM stage setup • Minimizing drift, choosing a working distance • The speed / precision balance • Setting SEM conditions • kV, probe current • Setting up the EBSD system • Camera settings • Acquisition software settings: Bands, reflectors, Hough • Phase ID strategies • Mapping strategies

3. Pre-SEM set-up: Assessing prep quality • Sample preparation critical to data quality • Intrinsic strain and prep-introduced damage difficult to distinguish • Prep-induced damage is strain • Although, usually cold work • Little recovery, except in some cases • Effect on EBSD: Cold-worked vs. recovered • Cold worked: Imperfect crystal lattice = fuzzy EBSPs • Recovered defect microstructure: Lattice orientation change within grains = EBSP rotation • Recovered defect microstructure usually not due to sample preparation • Except in certain materials with relatively high room-temperature diffusion rates

4. Pre-SEM set-up: Assessing prep quality Methods of assessing surface quality • Orientation-contrast electron imaging (BEI at zero tilt, FSEI at high tilt) • The same phenomenon that generates a diffraction pattern generates crystallographic orientation contrast in imaging

5. Pre-SEM set-up: Assessing prep quality Methods of assessing surface quality • Orientation-contrast electron imaging (BEI at zero tilt, FSEI at high tilt) • The same phenomenon that generates a diffraction pattern generates crystallographic orientation contrast in imaging • Strong orientation contrast = high quality EBSPs • If lots of intrinsic strain, varying grayscale within grains, but still high contrast

6. Pre-SEM set-up: Assessing prep quality Methods of assessing surface quality • Orientation-contrast electron imaging (BEI at zero tilt, FSEI at high tilt) • The same phenomenon that generates a diffraction pattern generates crystallographic orientation contrast in imaging • Strong orientation contrast = high quality EBSPs • If lots of intrinsic strain, varying grayscale within grains, but still high contrast • Look for scratches at higher magnification • EBSP inspection • Are EBSPs sharp or fuzzy? • Are the EBSPs sharper towards grain centers? • SEM resolution settings

7. Pre-SEM set-up: Assessing prep quality Methods of assessing surface quality NOTE • Fuzzy patterns may also be due to the presence of an oxidation film on some metals • If samples of susceptible materials have been exposed to air for a significant length of time, best to perform a short re-polish with colloidal silica prior to the EBSD work • Remember to also be careful to remove the colloidal silica residue

8. Pre-SEM set-up: Sample orientation • If the sample shape is elongated, make the long direction parallel to the tilt axis • Stage movement at high tilt is generally safer and more flexible in the tilt axis-parallel direction • If grains are elongate and the job is pushing spatial resolution limits, make the smaller dimension of the grains parallel to the tilt axis • Spatial resolution is lowest in the downhill direction • When mounting a sample in epoxy (or Probemet, etc.), offset the sample, so that at tilt it is the closest item to the pole piece • Allows maximum flexibility in WD (except longest) • Safer for the pole piece

9. Pre-SEM set-up: Sample orientation • Beware of excessively tall samples – the stage may run out of travel before the polished surface is brought under the beam at 70° stage tilt • Try a pre-tilted holder

10. Pre-SEM set-up: Charging prevention Conductive sample in non/semi-conductive mount • Cover/mask insulating material (including sides) • Easiest: If edges of sample not important, mask surface with carbon tape +/- Al foil • Carbon/metal coat • Thin (~2-5 nm) with no masking • Thicker okay if sample can be masked (try broken glass cover slip) • Carbon/silver paint non mount surface • Be sure it’s dry before placing in SEM • Break-out, if possible • Bottom surface parallel to top or top-referencing holder • Be sure to have lead from sample itself to ground (holder) • Best: carbon/Ag paint • Metal tape • Scrape off some of the glue on part touching sample

11. Pre-SEM set-up: Charging prevention Non-conductive sample • Coat • Carbon: 2-5 nm; Metal: “couple of nm”, depending on Z (of coating) & intended e-beam kV • If too thick • Patterns get “soft” • Individual patterns look like patterns from damaged surface • Try increasing kV if this is the case • Mask all parts not involved in job, if possible • Carbon tape • Al foil with carbon tape • Carbon/Ag paint • Be sure to mask cross sections of mount that are not in the tilted plane, such as tops of the mounts or thin sections • Be sure to have lead from sample itself to ground (holder) • Best: carbon/Ag paint • Metal tape • Scrape off some of the glue on part touching sample • Do nothing! • On a particularly flat polished surface on ceramics (low res work) • Grain size large (e.g. 100s of mms) Images courtesy J.R. Michael

12. Pre-SEM set-up: Charging prevention Non-conductive sample (cont’d) • Miscellaneous tricks: • Try lowering kV and probe current if charging remains an issue • On samples with cracks/topographic grain boundaries (such as geologic materials), try gold coating before final polishing step • Makes conductive network in cracks • Tilt sample before turning beam on

13. Pre-SEM set-up: Charging prevention On SEMs with low-vacuum capability • Operate at lowest pressure that prevents charging, given sample/beam conditions • Try 15 Pa to begin, increase if charging apparent • Typical range: 15 – 30 Pa • Compromises: • Pattern slightly dimmer • Spatial resolution somewhat worse • EDS resolution worse • Conventional E-T detector inactive • Do the benefits outweigh the drawbacks? Often • Easier than coating, no chance of over-coating • No line-of-sight issues as with coating (topographic samples) • But, there are instances where coating is better • E.g., finer grained materials where X-ray data is important • Be sure to use maximum probe current for the sample/job resolution conditions • Use smallest reasonable WD and detector-sample distances

14. SEM stage set-up: Mitigating drift • Most stages possess considerable weight and will settle under gravity when first tilted • In general, give the stage ~ 1 hour to settle if higher resolution jobs are desired • Try also tilting slightly too far then untilting to 70° to remove play • Pre-tilted holders are useful if drift minimization is a primary concern • Make sure the holder is at a height that will allow the EBSD camera to miss the untilted stage • Excessively tall pretilted holders can transfer vibrations • Preilt drawbacks: • Z-motion necessary to keep surface in focus when stage moved orthogonally to tilt axis • Inability to rotate sample in tilted plane for alignment with features of interest – need to eyeball the alignment when mounting

15. SEM stage set-up: Mitigating drift • Ensure stage grounding is robust • Beware of insulating wires in the path of backscattered electrons near the tilted sample • Keep chamber as clean as possible! • Excessive contamination buildup & charging can produce non-mechanical drift unless under LV conditions • Method of holding sample in holder can affect drift • Carbon tape: Bad • Much stronger in shear than tension • May deform under vacuum • Carbon or silver paint: Good • Mechanical bolting: Best • E.g., set screw

16. SEM stage set-up: Choosing a working distance • Smaller WD advantages: • Better spatial resolution • Longer WD advantages • Safer operation when stage movement necessary • Larger gap between sample and pole piece • Larger depth of focus • For manual work on powders & fracture surfaces • When the sample surface may not be exactly parallel to the stage • Optimal WD for EBSD or EBSD+EDS • A range of WDs will be optimal for the position of the EBSD detector, in acquiring the brightest & highest quality part of the pattern • This varies with sample density, detector-sample proximity, etc., but in general, where the pattern center is about 2/3 – 3/4 from the bottom of the screen • Don’t be afraid to deviate from these WDs when desired • Especially for high resolution work: Keep the WD small (<10 mm), even if the pattern center is high on the screen

17. Speed / precision balance • Speed vs. precision is probably the primary consideration in setting up the EBSD system for a job • Higher speed: More data collected in session time available • Smaller step size for better map resolution in a given area • Larger area of sample covered • Higher precision: Orientations more precise! • Often better phase discrimination • Higher accuracy/precision grain boundary characterization • Better resolution of substructure (less orientation “noise”) • Generally, the analyst decides on what the goals for the job are vs. the time available in the EBSD session • The EBSD settings should reflect these goals and efficiently fill out the session time • Often, single-setting standard procedures are inefficient • Test maps are important for new materials and/or job goals • Sometimes, the requirements are not obvious • E.g., for jobs that require extreme resolution (<20 nm step size), the analyst needs to keep the probe current relatively high for fast acquisition rates when drift is a factor

18. Speed / precision balance Many SEM & EBSD settings affect the speed / precision balance: • SEM • kV • Probe current • EBSD system • Band detection • Reflectors • Hough settings • Camera binning • Frame averaging These should all be considered when preparing to collect EBSD data • Don’t be intimidated! You’ll find settings that work well for a particular sample/job and be able to re-use with minimal preparation • Some settings are insensitive beyond certain limits, i.e., you won’t need to explore lots of range for each setting, probably just a couple when working with a new sample or job type

19. Setting SEM conditions: kV • Higher kV advantages: • Higher electron yield from gun • More efficient scintillation effect on phosphor screen • Crisper/more well defined patterns • Brighter & narrower bands may be easier detect • Lower kV advantages • Better spatial resolution • Sample may charge/damage less readily • For phase identification, broader bands will more clearly show subtle crystallographic differences between candidate phases • Typical settings: • Range: 10 - 30kV • Blind setting: 20kV • Large grained sample: 30kV • High resolution needed: 10kV • Except for single grain-thick thin films with ~amorphous substrate, then go with high kV

20. SEM set-up strategy: Probe current (Pc) • Higher Pc advantages: • Faster acquisition • Shorter necessary camera exposure time • May be important when drift is a factor • Higher precision acquisition • Higher quality camera settings possible while maintaining reasonable cycle time • Higher X-ray yield • Lower Pc advantages (via condenser/aperture controls) • Better spatial resolution • Better depth of focus • Less charging & sample damage, if applicable • General strategy & typical settings: • Essential strategy: Use as high a probe current as possible while maintaining the spatial resolution required for the sample (grain/crystallite size) • Range: 0.5 to 25+ nA • Large grained sample: As much as possible, within reason (e.g, don’t flood the EDS if simultaneous EBSD+EDS is desired) • Fine grained sample: ~~2 nA • But, try using reduced WD and kV first at a higher probe current

21. Setting up the EBSD system: CCD binning • EBSD cameras use CCD chips comprised of light sensitive pixels in an ~1000 X 1300 array • “Superpixels”: Group of pixels that are effectively used as a single pixel • Using superpixels through binning allows more light to be captured, at the expense of image resolution • E.g., 4X4 binning yields a 4X brighter image (-> lower CCD exposure time) than 2X2, but the image is 4X more pixilated

22. Setting up the EBSD system: CCD binning • Advantages of higher CCD binning: • Brighter pattern • Shorter exposure time (-> faster data acquisition) • Smaller image size from CCD, quicker to process • Advantage of lower CCD binning: • Less pixilated pattern • Higher precision indexing • Better differentiation of different orientations and different phases • Nicer-looking pattern (e.g, for publication) • Generally better indexing of difficult-to-index materials • General strategy • 8X8 binning: Highest speed for higher symmetry phases • 4X4 binning: General “happy compromise” setting • 2X2 binning: Best for Phase ID or for highest precision indexing • Note: For fast cameras, divide binnings listed here by 2

23. Setting up the EBSD system: CCD binning Remember If the job/sample allows, use more probe current before compromising EBSD system settings

24. Setting up the EBSD system: Hough Resolution • Advantages of high Hough resolution: • Higher precision orientation determination • May increase number of successfully indexed points • May improve phase discrimination • Better performance on lower symmetry phases • Advantages of low Hough resolution • Higher indexing speed • Typical settings (Channel): • 60-70 for low symmetry materials, and where precision is more important • 50-60 for most jobs • 40-50 for high speed jobs on cubic materials

25. Setting up the EBSD system: Number of detected bands • Advantages of a higher number of detected bands • Less chance for mis-indexing, if pattern quality good (bands all correctly detected) • Less chance for pseudo-symmetric indexing • Better indexing on lower symmetry materials • BUT: Too many bands can lead to reduced hit rate • Advantages of fewer detected bands • Higher overall hit rate • Faster indexing • BUT: Too few detected bands can lead to mis-indexing • The number of bands should be tailored to the material • Common settings (Channel): • 5 max for single phase cubic metals • 6 to 8 max for multi-phase materials in general • 7 to 8 for low symmetry phases

26. Setting up the EBSD system: Number of reflectors • The number of reflectors should be customized to suit the phase(s) being indexed • Basic strategy: The simulated EBSP should contain all bands that will be detected in the actual EBSP, not more or fewer • Too many: Greater chance for mis-indexing • Too few: Reduced hit rate • In general, one can use default values for at least reasonably good performance, e.g.: • 50 is a good starting number • 25-32 for FCC, BCC metals • 50-75 for lower symmetry materials • The number can be correctly customized by looking at the simulated pattern. Determine the minimum number of reflectors that captures all bands in the actual EBSP likely to be detected

27. Setting up the EBSD system: Frame averaging • Advantages of a larger number averaged frames • Higher quality EBSPs • Better indexing on difficult-to-index phases • Advantages of fewer averaged frames • Higher indexing speed • In general, use the minimum amount of frame averaging that still yields high quality data • Use small test runs to determine this • The Hough transform allows robust band detection even in seemingly noisy patterns, so try 1 or 2 frames first • Remember, saved patterns will be only as high quality as the pattern quality settings allow, thus will affect re-indexing • Common settings: • Phase ID • As many as necessary to reach high quality “plateau”, usually 6+ • Mapping • High symmetry materials with high quality patterns, 1-2 frames • Normal starting setting: 2-3 frames • For highest quality indexing, 3+ frames

28. Setting up the EBSD system: Other • Area of EBSP Hough transformed • Normally, most of the phosphor screen will contain pattern of reasonable contrast unless the detector is particularly close to the sample • Be sure to set the area transformed to match the area of the EBSP with reasonable contrast • The farther apart detected bands are in diffraction space, the higher quality and precision the indexing • Number of phases • In general, the more phases, the slower the indexing process • Consider indexing some of the phases offline on saved EBSPs, if the sample contains many phases • Indexing can be customized to best settings for those particular phases • Pattern saving • Slight hit on cycle time but usually worth the bother • Simultaneous EDX collection • Generally, need 100-300ms per point minimum for quality EDX data (SiLi…SDDs need less dwell time) • If perform simultaneous collection, be sure change the EBSD indexing settings to highest accuracy that fits within the EDX time delay

29. General strategies: Phase Identification • Use high quality/precision conditions • Speed is totally unimportant here • Suggested settings (Channel settings in parentheses): • Low binning (2X2, low gain) • Lots of probe current, if grain not too small • Lots of frame averaging (5+) • High Hough resolution (70+) • High number of detected bands, to fit pattern quality (7+) • Maximize the area of diffraction space captured • (Maximize Area Of Interest) • (Set detector position to fill screen with high quality pattern) • (Use “Advanced Fit”, level 4) • Don’t be afraid to load up match unit list; Flamenco is good at phase discrimination

30. General strategies: Mapping Each new type of material or job goal will have optimal settings, especially in tuning the speed / precision balance • In this case, try running small test jobs before the main job, to determine the best run parameters • Play primarily with the following settings: • Frame averaging • # of bands detected • # of reflectors • Hough resolution • AOI (area involved in Hough Transform) • Try making adjustments on the fly, and noting the effect on indexing quality and speed • This process usually takes a few minutes to half an hour for particularly challenging samples • (Channel) Settings will be saved in current profile, so the optimal settings will be ready the next time you run a similar job • Once optimal settings are established, note cycle time • This will allow the actual run design to make most efficient use of session time available