Scanning Electron Microscopy and X-Ray Microanalysis

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29.1.3 Conditions for Detecting Electron

Channeling Contrast

Specimen Preparation

Channeling contrast effects are created in a shallow near-

surface layer that is 10–100 nm in depth, depending on the

incident beam energy and the material, where the incident

beam has not yet undergone sufficient elastic scatter to

destroy the initial beam collimation. Thus, the condition of

the sample surface is of extreme importance for a successful

channeling contrast measurement. The low level of channel-

ing contrast, 2–5 %, also requires eliminating other compet-

ing sources of contrast, especially surface topography. Thus, a

typical sample preparation will involve grinding and polish-

ing to achieve a flat, topography-free surface. However, the

inevitable surface damage layer produced in many materials,

especially metals, by mechanical abrasion, including the

“Beilby layer” that remains after the final polishing with the

finest scale abrasives, must be removed by chemical or elec-

trochemical polishing or low energy ion beam sputtering.

(Note that high energy ion sputtering can produce sub-

surface damage that can destroy electron channeling

contrast.)

Instrument Conditions

Detecting channeling effects that produce weak contrast in

the range 2–5 % in an SEM image requires careful choice of

operating conditions. The low contrast creates a requirement

for a high threshold current, so that typically a beam current

of 10  nA or more is needed when scanning at rapid frame

rates to find features of interest. After such features have been

located, a smaller beam diameter can be used to improve

resolution at the inevitable cost of a lower beam current,

which then requires a longer frame time to maintain contrast

visibility. Because a small convergence angle is also desirable

to maximize channeling contrast, a high electron source

brightness is important to maximize the beam current into a

focused beam of useful size. A high beam energy in the range

from 10 to 30 keV is desirable both to increase source bright-

ness and to create high energy BSEs for efficient detection.

Channeling contrast is carried by the high energy fraction of

the backscattered electrons, and to maximize that signal, a

large solid angle BSE detector (solid state or passive scintilla-

tor) is the best choice with the specimen set normal to the

beam. The positively biased Everhart–Thornley (E–T) detec-

tor, with its high efficiency for BSEs through capture of the

SE 2 and SE 3 signals, is also satisfactory. Because of the weak

contrast, the live image display must be enhanced by careful

adjustment of the “black level” and amplifier settings to

spread the channeling contrast over the full black-to-white

range of the final display to render channeling contrast visi-

ble. Post-collection image processing by various tools, e.g.,

CLAHE in ImageJ-Fiji, can be very effective at recovering

fine scale details.

Crystallographic contrast by electron channeling pro-

vides images of the crystallographic microstructure of

materials. For many applications in materials science, it is

also important to measure the actual orientation of the

microstructure on a local basis with high spatial resolu-

tion of 1 micrometer laterally or even finer. The technique

of electron backscatter diffraction (EBSD) patterns pro-

vides the ideal complement to channeling contrast

microscopy.

29.2 Electron Backscatter Diffraction

in the Scanning Electron Microscope

An understanding of the crystallography of a material is

required to fully describe the structure property relation-

ships that control a material’s physical properties. By linking

the microstructure to the crystallography of the sample, a full

picture of the sample can be developed. The development of

electron backscattering diffraction (EBSD) equipment and

its placement in commercial tools for phase identification

and orientation determination has provided new insights

into microstructure, crystallography and materials physical

properties.

EBSD in the SEM has been developed for two different

purposes. The oldest application of EBSD is for the mea-

surement of texture on a grain by grain basis. Texture deter-

mined in this way is called the microtexture of the sample

(Randle 2013 ). An example of this is. Fig. 29.9, where the

point-by- point orientation of an assembly of ZnO crystals is

shown where the color coding indicates the orientation of

the crystal. The other use of EBSD is for the identification or

discrimination of micrometer or sub-micrometer crystal-

line phases (Michael 2000 ; Dingley and Wright 2009 ).

. Figure 29.10 is an example of this, showing a dual-phase
steel with both ferrite (body-centered cubic) and austenite
(face-centered cubic) present. EBSD can easily discriminate

50 μm

. Fig. 29.8 Electron channeling contrast from grains in polycrystal-
line Ni deformed by a diamond indentation placed in a single grain;
E 0 = 20 keV; BSE detector

Chapter 29 · Characterizing Crystalline Materials in the SEM