Why high sensitivity is essential for unlocking the power of CMOS-based EBSD detectors

The launch of the complementary metal oxide semiconductor (CMOS)-based Symmetry electron backscatter diffraction (EBSD) detector in 2017 has changed the way researchers utilise the EBSD technique. For the first time ever there is no need to compromise data quality when analysing at high speeds: Symmetry delivers good quality, distortion-free EBSD patterns at exceptional speeds in excess of 3000 patterns per second (pps) and with sufficient pixel resolution (156 x 128) to ensure good indexing and high angular precision even on the most challenging samples.

However, there is little benefit in this high speed performance if the detector has low sensitivity and if very high electron beam currents or high beam energies are needed to achieve such high speeds. This short technical bulletin demonstrates the necessity of high sensitivity in an EBSD detector and shows the performance of the Symmetry CMOS EBSD detector using standard electron beam conditions.

Beam current - why less is more

The Symmetry CMOS detector has a guaranteed top speed of >3000 pps at a specified beam current of only 12 nA (with >99% indexing). At this current, modern field emission gun scanning electron microscopes (FEG SEMs) can achieve nanometre-scale beam resolution, ensuring optimum EBSD data quality and minimum scanning distortion. This sensitivity is achieved using groundbreaking fibre-optics and a customised CMOS sensor, developed exclusively for Oxford Instruments' EBSD detectors. However, the question remains: why is sensitivity so important?

The following images demonstrate the difference between using 5 nA and 50 nA beam current to analyse a deformed martensite-ferrite steel sample using a modern FEG SEM.

Forescatter detector (FSD) using 5 nA beam current

Orientation map with low angle boundaries in red collected using 5 nA beam current. Scale bar marks 5um.

Forescatter detector (FSD) using 50 nA beam current.

Orientation map with low angle boundaries in red collected using 50 nA beam current. Scale bar marks 5 um. Arrows mark examples of missing information relative to the 5 nA map.

It is clear from the electron images that the resolution at high beam currents is severely compromised, and this will have a significant effect on the EBSD results. The 2 EBSD orientation maps look broadly similar, but closer inspection highlights key differences. Numerous small grains and the martensitic packets are missing from the 50 nA analysis, resulting in an apparent increase in mean grain size from 817 nm to 1029 nm – a 26% error.

Further discrepancies are clearly visible in the distribution of low angle boundaries. Many boundaries are missing from the 50 nA map, whereas others appear to be broader. The larger beam diameter at high beam currents results in overlapping EBSD patterns, effectively smoothing the misorientation values of low angle boundaries across several pixels. An example is shown in the transect below: using a 5 nA beam current, the misorientation across a low angle boundary is measured as 5.4° across a single measurement step, whereas at 50 nA it is smoothed across 4–5 pixels and the maximum value is only 1.5°. This would result in false interpretations of the deformation processes and physical properties of this steel sample.

Part of the orientation map shown above (5 nA beam current), showing the position of a misorientation transect across a low angle boundary (yellow line). Scale bar marks 1 um.

Misorientation profile across the low angle boundary shown to the left using 5 nA and 50 nA beam currents.

The Symmetry CMOS detector's ultra-high sensitivity enables guaranteed EBSD performance at beam currents as low as 250 pA, with speeds still in excess of 250 pps. For beam sensitive and non-conductive materials, analyses are possible using very low electron doses, such as in the following example from an uncoated ceramic sample analysed using only 37 pA current.

Orientation map and example EBSP collected from an uncoated Zirconium based ceramic using only 37 pA beam current. Scale bar marks 10 um.

Conclusion

CMOS-based EBSD detectors enable extremely fast rates of microstructural characterisation, but without the drawbacks associated with high levels of pixel binning. However, if high beam currents of many 10s nA are required to achieve these speeds, then the EBSD analysis will not give a true representation of the microstructure. Small grains will be missed, boundaries will be smoothed across several pixels and deformed structures, such as martensite, will not be effectively characterised. Resulting conclusions about the material's key physical properties such as strength, fatigue resistance and ductility will be incorrect.

Therefore a CMOS-based EBSD detector must have high sensitivity in conjunction with the native high speed of the CMOS sensor. The Oxford Instruments Symmetry EBSD detector benefits from the exceptional efficiency of fibre optics (the first commercial EBSD detector to utilise a fibre optic lens system) as well as a fully customised CMOS sensor, enabling maximum speeds of >3000 pps at beam currents of 12 nA or less, ensuring accurate results and dependable materials interpretation.

Note: The other Oxford Instruments CMOS-based EBSD Detectors, C-Swift and C-Nano, also benefit from optimised optics and sensor design, ensuring similarly high sensitivity, albeit with lower maximum analysis speeds. For further information about measuring detector sensitivity and the importance of fibre-optics, please refer to the accompanying technical bulletin "High-sensitivity EBSD detectors - DQE and the advantage of high-gain fibre optics".