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Tips, Techniques and Guidelines for Collecting High Resolution EBSD Data

General Procedure

CrossCourt uses cross-correlation based techniques to measure the relative shifts between EBSD patterns caused by small lattice rotations or elastic strains. Comparisons are made between a reference pattern, representing a point of zero strain and a number of patterns taken from the area of interest. There are 4 stages of the analysis:

1/ Calibration and Preparation.

In some respects this is the most important section. Good quality data leads to good quality results and it is worth spending time optimising the pattern collection.  The user is referred to the separate manuals issued by EDAX, Oxford Instruments and Bruker-Nano for details on the steps necessary to carry out the data collection.

2/ Measurement of Pattern Shifts.

Once the EBSD patterns are transferred from the data collection software used, i.e. OIM_DC, Aztec or CrystAlign, and loaded into CrossCourt4 the software measures the relative shifts of small selected Regions of Interest (ROI) between each pattern and another selected as a reference pattern. The ROIs are selected either automatically or manually and the shifts are measured in pixels. The results from this are displayed graphically and can also be exported to a BLG Project File (based on an Excel worksheet).

3/ Calculation of infinitesimal relative distortion Tensor.

A minimum of 4 ROIs are needed to calculate the distortion tensor. Normally 20 or more ROIs are used to oversample and the best fit distortion tensor is calculated. Alternatively, an iteratively weighted least squares solution to the distortion tensor can be calculated

The pattern centre values and specimen to screen distance values read from the data collection project files are used to calculate the distortion tensor in the reference axes of the sample surface. The lattice distortion matrix is then split to provide the Strain tensor (Normal & Shear strains) and Rotation tensor (rigid body rotations) following infinitesimal strain theory.

Given further information about the orientation of the crystal and the elastics constants of the materials, the stress tensor is calculated.

4/ Calculation of the finite relative distortion tensor.

In the case of finite strain, infinitesimal strain theory breaks down and finite strain theory must be considered.

Finite strain is can be considered as the situation where the elements of the rotation tensor (as calculated above) are substantially bigger than the elements of the elastic strain tensor.

The approach used in Crosscourt 4 is to remap the EBSD patterns so that the rotations, as measured in stage 3 above, are removed by back rotating them around the pattern centre. These remapped patterns are then cross-correlated with the original reference pattern so that the actual elastic strains can be measured without influence from the large rotational components. These measures are then recombined with the initial rotation tensor in order to create the Finite Distortion Matrix. From this, more accurate Elastic Strains and Stresses can be extracted.

Practical Considerations When Collecting EBSD Patterns

1/ Collection of High Quality EBSD Patterns.

The most important ingredient for successful strain measurement is high quality, high resolution EBSD patterns. These should always be collected at the maximum camera resolution for best results.

Pattern quality can generally be increased by (see below for more details):-

• Use a camera with a detector containing at least 1000×1000 pixels and preferable with 12 bit greyscale resolution.
• Increasing the exposure time of the camera to the point where the brightest point in the image is almost saturated. This would correspond to an intensity level of 4096 in a 12 bit camera. Do not over expose. (Long exposure times however may lead to problems associated with specimen drift and contamination.)
• Having a bright and scratch free phosphor.
• Increasing the electron beam current (this may reduce spatial resolution because of the increased electron probe size.)
• Selecting an appropriate electron beam voltage. Higher voltage improves pattern contrast but increases the sample depth of beam penetration.
• Tilting the Sample to 75º will give stronger EBSD pattern contrast. However, this will increase the electron probe footprint on the specimen surface and hence add uncertainty as to where the pattern originates.

2/ Avoid Scratches on the Phosphor

Any scratches or other blemishes on the phosphor represent areas of static (i.e. unmoving) content when comparing images.  Static image content may cause a true shift between strained and reference images to appear smaller than it really is or even result in a zero shift measurement. Ideally, the phosphor screen should be in perfect condition.

The normal EBSD procedure of removing the background image of the phosphor from all recorded images of the EBSD pattern has several advantages in this respect. It not only boosts the contrast of the patterns but may also remove phosphor defects. However, it is not always successful and good background images may be difficult to obtain, especially if the sample is a single crystal. In certain circumstances (e.g. in the case of measurements near an edge of a sample where electrons emitted from both of the surfaces at the edge contribute to the EBSD pattern) there may be such a change in pattern contrast that erroneous strain measurements can result.

3/ Avoid Over Saturation of the Pattern

If the patterns contain areas with saturated pixels (especially in the background pattern used for background removal), these will also represent static features in the pattern and can cause an incorrect measurement of zero shift just as is the case for phosphor blemishes.

4/ Good Vacuum & Clean Samples

Significant contamination of the sample, by the generation of a thin film of carbon deposit on the surface, drastically reduces both the pattern quality and the ability to make precise and accurate strain measurements.  Using clean samples and anti-contamination devices such as a liquid nitrogen cold finger is recommended.

5/ Number of Pixels in the Pattern

If a pattern is taken with the highest resolution that the camera can provide, then more precise and accurate shift measurements will be obtained. However, patterns taken at a higher resolution take proportionally longer to expose, which means that issues such as sample drift and beam contamination become more apparent. Some compromise between quality and speed may become necessary. Such compromises can be tolerated in cases where the focus of the experiment is not on elastic strain, but on rotations caused by plastic strain.

6/ Calibration

It is necessary to know the EBSD pattern centre and specimen to film distance i.e. the standard calibration data for interpreting EBSD patterns.  This information is stored in the project files for the datasets but must be manually entered in the case where no project file exists.

The calibration routines provided by the EBSD system manufacturers can be used for this. .

The accuracy of the measurements depends on the accuracy of the calibration data provided. The calibration parameters are required to enable conversion of the directly measured shifts of the EBSD pattern within an ROI, which are in units of pixels, to an angular measure made with respect to the EBSD pattern centre.  This calibration is essential for calculation of the strain tensor.

Villert et al (below) show that errors in the PC cancel out to a first order approximation if all patterns are taken from the same scan (i.e. when the systematic error of the PC is the same for all patterns).

Villert, S., Maurice, C., Wyon, C. and Fortunier, R. (2009), “Accuracy assessment of elastic strain measurement by EBSD.” Journal of Microscopy, 233: 290–301. doi: 10.1111/j.1365-2818.2009.03120.x

7/ Pattern Centre Position.

Specifically the value of Y* (i.e. the height of the PC on the phosphor) is important. If the sample is too low with respect to the phosphor screen then the spread of the ‘spray’ of back scattered electrons will be centred low down the phosphor screen which means that the signal at the top of the screen will be weak and the results from ROI centred there will be very noisy and will degrade the overall performance of the calculations.
Try and adjust the height of the stage so that the spray of electrons is centred on the centre of the phosphor screen.  This means setting the pattern centre at a height from the bottom of the screen of 2/3rds the screen diameter. The exact Y* position will vary with the atomic number of the specimen.

8/ Choice of the Reference Pattern

The reference EBSD pattern should be taken from a region known to be strain free.  All other patterns will be compared to this pattern so that it should be of as high a quality as possible.  It may not be known precisely if the selected region for the reference pattern is actually strain free.  In this case although the precision of the technique remains unchanged, the strain values measured will not be accurate.  All relative measures will have an accuracy and precision at the stated 2 parts in 10000.

For manual input of the reference point the user may wish to choose a point far away from a known point of strain, e.g. an indentation point, crack or grain boundary triple point in a polycrystalline material.  The selection can be on the basis of the quality of the EBSD pattern, for example: how sharp it is.

For automated reference pattern detection these same criteria can be used, in which case the software will search through data provided in the project files of the EBSD pattern indexing routines of EDAX, Oxford Instruments or Bruker and search for the best pattern(s) as reported using their internal measures. Often the reference patterns found by these routines are not at the centre of grains for example, where the strain might be thought to be least, but closer to a grain boundary.  In this case the user has the option to override the automated choice and manually place the reference pattern.

After strain measurement has been performed strain maps are presented.  Inspection of these maps may then reveal a more likely location for the reference pattern.  For example, at the centre of a region that shows no strain gradients over several microns for all strain components.

9/ Correction for beam movement.

When the electron beam of the SEM is moved across the sample there is an inherent and identical shift of the EBSD pattern across the phosphor screen.  If large enough, generally more than 3 microns, the software will interpret this shift as a rigid body rotation unless it is removed. For this reason the reference pattern should be recorded from an area as close as possible to the area of interest.  If beam movements of more than 3 microns are made, in a line scan or a map for example, then a beam movement correction procedure should be applied by finding the effective pixel size in microns (see the calibration section 6 for further details)

Calibration values calculated with CC3 should still be valid for the same EBSD equipment.

In the case of large area scan, a secondary problem can (in fact almost certainly will) arise.

SEM scans are designed to be perfectly rectangular for a horizontal sample, but in the case of EBSD where the sample is highly tilted, this rectangular scan becomes distorted into a trapezium due to the difference in working distance between the top and bottom of the scan. This means that the actual step size will vary across the scan making calibration impossible and introducing phantom strains.

As such we recommend making sure that the reference pattern is no more than 150µm form the test pattern.

Please note that if large scans (> 150 µm) of single crystal materials are examined, phantom strains will appear.

10/ Keep the Experimental Conditions the Same for All Patterns.

The patterns collected must all be of the same image size and must all be collected under the same microscope conditions.

11/ Specimen to Screen Distance

The best experimental arrangement for taking measurements is to have the phosphor screen close to the sample collecting a wide solid angle of the diffraction pattern.

12/ Data Scan Collection

For the collection of the actual data set to be analysed, we recommend using an automated area scan generated by your EBSD collection software. It is first necessary to initialise the recording of patterns at each point in the video page. When you select the format of the image, always select 12 bit tiff images if possible. In the data collection page select the square grid option if recording an area image.

If available, collecting and averaging more than a single frame at each point in the data set, makes a large difference to the quality of the data collected. This improves the signal to noise ratio in the patterns collected and leads to less noise in the strains calculated.

13/ A note about Euler angle settings

Use the default Euler Angle settings for the EBSD system that you have.

Each of the EBSD manufacturers uses a different default frame of reference for the Euler Angles. Crosscourt automatically rotates these from the standard set used by the manufacturer into the frame of reference used by Crosscourt.

However, if the Euler data is collected in a non-standard frame of reference, Crosscourt will apply the default correction anyway which leads to the mixing up of the normal strains. This is because the Euler angles are only used in the rotation of the Elasticity coefficients which are used to set the traction free condition (i.e. S33 the stress normal to the surface is zero) so that all 9 tensor components can be calculated. If the wrong frame of reference is used, the stress is set to zero in a different plane and the normal strains are not calculated correctly.

14/ Special instructions for Users of Oxford Instruments Aztec Software

Oxford instruments have produced a document explaining how to set up Aztec to collect the best data for use with Crosscourt. We highly recommend that you read it carefully.

We also recommend that you have at least version 3 of Aztec installed as this will allow CrossCourt to perform background removal using the static background stored by Aztec from this version onwards.