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.
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.
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.
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.
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.
(a) The pattern quality map shows clearly the fine grain size, with a few areas with significantly poorer quality patterns.
(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.
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.
Click to read the full application note: TKD with AZtec - the application of EBSD to Nanoscale.