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EBSP: Introduction

What is Electron Back-Scatter Diffraction?

Electron Back-Scatter Diffraction (EBSD) or Backscatter Kikuchi Diffraction (BKD) is a diffraction technique for obtaining microtextural information from bulk samples or thin layers in the scanning electron microscope (SEM). It provides information on the orientation of crystals with a spatial resolution of a few microns in our system. With special hard- and software interfaced to the SEM, the Electron Back-Scatter Patterns (EBSP's) of individual grains can be acquired easily and can be compared to the standard SEM images, allowing crystal structure information to be coupled to the crystal image.

EBSP's consist of symmetrically arranged bands (Kikuchi bands) of slightly higher intensity with respect to a non-uniform background. This contrast is due to the diffraction of the backscattered electrons by the crystal. Those electrons, generated by an incident electron beam in the SEM, spread beneath the specimen surface in all directions. This produces a divergent source of electrons within an interaction volume in the sample, which will diffract with the crystal planes according to the Bragg condition. Because the electrons travel from the source in all directions, for each set of planes for which the Bragg condition is satisfied, the diffracted beams lie on the surface of a cone whose axis is normal to the diffracted plane. Those cones intersect with a phosphor screen placed in front of the specimen and give rise to the pattern (see Figures below).

Schematic of the diffraction cone and pattern formation

EBSP set up at the Dartmouth Electron Microscopy facility

How did we build the detector ?

The pattern generated on the phosphor screen is detected by viewing the screen through a view port with a CCD video camera which is configured for on-chip integration. On-chip integration is required because the signal (glowing of the phospor screen) is so weak that the CCD readout noise exceeds the detected signal if video rates are used. There are a number of good commercial systems available (see below). Money is always in short supply, so we have chosen to use readily available components to build our own EBSP system (parts are listed below). We made a port cover for the right -hand port of the DSM-962 SEM. The cover is fitted with a 2" diam view window. Mounted on the cover is a Cohu 4915 CCD camera with manual gain and offset and wired to permit on-chip integration. It is connected to a Scion LG-3 frame grabber card in a PowerMac 7200/75. For camera control and image acquisition, we use NIH Image which permits triggering of the on-chip integration and transfer of a single readout for the image.

For pattern acquisition it is essential that the video camera control be set on manual and gain and offset are both at minimum. The sample is then imaged so grains can be see. This can done two ways. In the secondary mode most grains can be seen when the sample is tilted to +70 degrees. If this does not give a good image, a specially designed holder allows samples to be imaged at 0 tilt with the BSE signal and at +70 tilt for EBSP. The patterns are acquired by placing the beam on a single grain with the spot mode. The image is acquired in NIH Image using the on-chip integration mode. An integration for 15-30 seconds will generally yield a good image.

Parts:

  • Phosphor screen -- 2" scintillator screen from Grant Scientific Corp.
  • Cohu model 4915-4000/ER2523D video camera and control box from Cohu, Inc.
  • Scion LG-3 PCI bus frame grabber and cable for Cohu camera on-chip integration.
  • PowerMac 7200/75 (used because it is what we had).
  • NIH Image.
  • Machine shop time for port cover and screen holder.

What can I do with it ?

The contrast in the images can be enhanced by subtracting the background intensity electronicaly. The geometrical arrangement of Kikuchi bands in the EBSP provides information on crystal symmetry, crystal orientation, as the acquired pattern is indexed to determine grain orientation, grain to grain misorientation and even crystal deformation. Distortion due to projection of these bands onto a phosphorous screen make the patterns very difficult to analyse "by hand". Matlab software has been written to facilitate the indexing of EBSP's in the cubic system, as well as further analysis of the orientation data. Instructions can be found here.

For copies of the MATLAB files, please contact Charles Daghlian.
It is a good idea to look at the instruction manual first, then download the files.

Sample preparation

The sample surface must be crystalline and without excessive plastic deformation. In the case of metals, this is generally obtained by electropolishing the surface of the specimen after polishing.

For poorly-conductor material, sample preparation consists of providing a means of shunting the electron beam current to avoid surface charging. Carbon coating may be required for insulators and semiconductors, but coatings must be minimized (less than 100 Angstroms) to ensure sufficient signal to noise ratios for usable patterns.

More information on the formation of the EBSP, as well as on specimen preparation can be found in the references given below.

Oxford Instrument EBSP Web Page

D. J. Dingley, K. Z. Baba-Kishi, V. Randle, Atlas of Backscattering Kikuchi Diffraction Patterns. Institute of Physics Publishing, Bristol and Philadelphia, (1995)

V. Randle, The measurement of grain boundary geometry, Institute of Physics Publishing, Bristol and Philadelphia, (1993)

Last modified on 2/5/97.

Last Updated: 4/1/13