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There is a continuing drive to make nano scale materials, and nano scale components. This is driven by increased performance and efficiency. Materials with nano scale grains typically exhibit very different properties to a large grained bulk material. This is linked to the Hall-Petch relationship which predicts material strength to be inverse proportional to the square root of the grain size.

As grains and materials are engineered to be smaller and smaller it is increasingly important that we can characterise these materials on the nano scale. The requirement to improve the spatial resolution has an effect on the EBSD hardware as well as on how the samples needs to be prepared.

Improving the spatial resolution with bulk samples

Traditionally to achieve nano scale performance in the SEM, requires lower acceleration voltages (kV), smaller probe current and shorter working distance. To successfully collect EBSD under these conditions requires the detector position to be optimised for data acquisition at a short WD and ideally the detector needs to be optimised for sensitivity. This is because when the probe current or acceleration voltage is reduced, the intensity of the diffracted signal is similarly reduced. High detector sensitivity is required to compensate for the reduction in signal. If the detector is not sensitive enough then the acquisition speed will be significantly reduced.

EBSD Detector Sensitivity

The NordlysNano EBSD detector is an example of an EBSD detector optimised for sensitivity. It has a customized optics design optimising light throughput to the sensor. In addition the sensor has high quantum efficiency making it the detector of choice for analysis at low beam currents, of beam sensitive samples, and for the identification or discrimination of difficult phases.

This means that with the NordlysNano it becomes possible to collect mapping data at low kV without compromising the acquisition speed.

Click to see more information of NordlysNano Sensitivity.

Figure 1 Typical EBSPs of Iron Pyrite collected.

figure 1a

(a) At 20kV excellent detail can be seen within the pattern.

figure 1b

(b) At 5kV, the patterns remain clear and are readily indexed.

Figure 2 Higher spatial resolution requires low kV analysis.
The example of nanocrystalline Nickel displays small grains, in the order of 0.5μm, surrounded by much larger grains.
Raw data shown with 92% hit rate at 101Hz. Beam conditions 2nA at 5kV.

figure 2a

An EBSD study of mollusc shells illustrates the benefits of working at lower beam energies to both improving spatial resolution and to prevent beam damage on beam sensitive material.

Figure 3 EBSD was required from a Mollusc shell sample at low kV to reduce the beam damage and enhance the resolution using NordlysNano detector.
The sample contains Calcite and Aragonite layers. EBSD results shown here were from Aragonite layer.

figure 3a

(a) EBSD IPFZ coloured plus bandcontrast and grain boundary map from the Aragonite layer. Thick black lines represent grain boundaries >10o misorientation, while the thin black lines are >2o misorientation.

figure 3b

(b) A typical EBSP from the aragonite phase, acquired at 8 kV.

This has for the first time successfully shown low kV EBSD mapping of aragonite.

Click to read the full application note: Characterisation of a mollusc shell with low kV EBSD using AZtec HKL and Nordlys Nano.

Developments to improve spatial resolution - EBSD on electron transparent samples

The spatial resolution in conventional EBSD is inherently limited by the pattern source volume, to resolutions in the order of 25-100nm. This is insufficient to accurately measure truly nano structured materials (with mean grain sizes below 100 nm), as illustrated on the figure below.

Figure 4 Pattern Interaction volume modelled at 25kV from...

figure 4a

(a) Traditional EBSD with Bulk Ni sample, tilted at 70o and...

figure 4b

(b) Thin 50nm thick Ni sample at TKD geometry with 0o tilt. Red regions correspond to electrons which have more than 93% of incident beam energy. It shows reduced scattering and minimal beam broadening in transmission mode (TKD).

A new approach to SEM-based diffraction has received a lot of interest; it applies conventional EBSD hardware to an electron-transparent sample. The technique, referred to as transmission EBSD (t-EBSD: Keller and Geiss, 2012) or transmission Kikuchi diffraction (TKD: Trimby, 2012) has been proven to enable spatial resolutions better than 10 nm. This technique is ideal for routine EBSD characterisation of both nano structured and highly deformed samples.

TKD samples are prepared in the standard way for transmission electron microscopy (TEM). The sample thickness is critical: best results are achieved using relatively thin samples, in the range of 50 nm to 150 nm.

The samples are typically mounted horizontally in the SEM chamber, at a level above the top of the EBSD detector’s phosphor screen.

Figure 5 The geometry of a system set up for TKD. Electron transparent samples are mounted horizontally in the SEM chamber and positioned towards the top of the EBSD detector's phosphor screen.

figure 5a

The geometry for TKD allows a short working distance (e.g. 5-10 mm), depending on the position of the EBSD detector. This geometry maximises the opportunity to achieve the best spatial resolution by reducing the working distance as well as by reducing the sample tilt.

Figure 6 In this example a duplex stainless steel sample has been deformed at room temperature by high pressure torsion, resulting in significant grain size refinement and intragranular deformation. The ferrite (BCC) phase develops a final grain size below 100nm, whereas the austenite (FCC) is even finer grained, with high resolution TEM imaging indicating a mean grain size below 10nm. TKD mapping with a step size of 4nm was used to characterise the sample, with the results shown here.

figure 6a

(a) The pattern quality map shows clearly the fine grain size, with a few areas with significantly poorer quality patterns.

figure 6b

(b) The phase map shows that the poorer patterns are from areas of the FCC phase, in which the TKD technique can only resolve the larger grains.

figure 6c

(c) The cleaned orientation map illustrates the lack of texture in this sample, but also the deformation within the larger grains (>100 nm) exhibited by substantial intra-grain orientation variations.

Although the geometry when using TKD is very different from the geometry when using reflective EBSD, modern systems can deal with this. The most notable differences to consider when working with TKD patterns are:

  • Wider than normal bands at the lower part of the pattern
  • A non symmetric intensity across these wide bands
  • A pattern centre located above the screen

If these effects are not handled correctly then it can cause problems for the band detection, resulting in an associated drop in hit rate and reduced orientation accuracy. Modern systems are able to deal with these issues, making it possible to do TKD analysis using a conventional EBSD system, thereby improving the spatial resolution of the analysis.

Figure 7 A typical TKD pattern from Aluminium. Broad bands can be seen at the lower part of the pattern.
The intensity across these broad bands is non symmetric resulting in the band edges being very bright or very dark.

figure 7a

Click to read the full application note: TKD with AZtec - the application of EBSD to Nanoscale.

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