Scanning Electron Microscopy and X-Ray Microanalysis

XIII

Measuring the Crystal Structure

An electron beam incident on a crystal can undergo
electron channeling in a shallow near-surface layer
which increases the initial beam penetration for
certain orientations of the beam relative to the
crystal planes. The additional penetration results in
a slight reduction in the electron backscattering
coefficient, which creates weak crystallographic
contrast (a few percent) in SEM images by which
differences in  local crystallographic orientation
can be directly observed: grain boundaries, defor-
mations bands, and so on (e.g.,. Fig.  8 ).
The backscattered electrons exiting the speci-
men are subject to crystallographic diffraction
effects, producing small modulations in the intensi-
ties scattered to different angles that are superim-
posed on the overall angular distribution that an
amorphous target would produce. The resulting
“electron backscatter diffraction (EBSD)” pattern
provides extensive information on the local orienta-
tion, as shown in. Fig. 8b for a crystal of hematite.
EBSD pattern angular separations provide mea-
surements of the crystal plane spacing, while the
overall EBSD pattern reveals symmetry elements.
This crystallographic information combined with
elemental analysis information obtained simultane-
ously from the same specimen region can be used to
identify the crystal structure of an unknown.

Dual-Beam Platforms: Combined

Electron and Ion Beams

A “dual-beam” instrument combines a fully func-

tional SEM with a focused ion beam (FIB), typi-

cally gallium or argon. This combination provides

a flexible platform for in situ specimen modifica-

tion through precision ion beam milling and/or

ion beam mediated material deposition with

sequential or simultaneous electron beam tech-

nique characterization of the newly revealed

specimen surfaces. Precision material removal

enables detailed study of the third dimension of a

specimen with nanoscale resolution along the

depth axis. An example of ion beam milling of a

directionally solidified Al-Cu is shown in. Fig.  9 ,

as imaged with the SEM column on the dual-

beam instrument. Additionally, ion-beam

induced secondary electron emission provides

scanning ion microscopy (SIM) imaging to com-

plement SEM imaging. For imaging certain speci-

men properties, such as crystallographic

structure, SIM produces stronger contrast than

SEM.  There is also an important class of stand-

alone SIM instruments, such as the helium ion

microscope (HIM), that are optimized for high

resolution/high depth-of-field imaging perfor-

mance (e.g., the same area as viewed by HIM is

also shown in. Fig.  9 ).

a b

BSE MAG: 400 x HV: 20.0 kV WD: 11.0 mm

40 mm

. Fig. 8 a Electron channeling contrast revealing grain boundaries in Ti-alloy (nominal composition: Ti-15Mo-3Nb-3Al-
0.2Si); E 0 = 20 keV. b Electron backscatter diffraction (EBSD) pattern from hematite at E 0 = 40 keV

Scanning Electron Microscopy and Associated Techniques: Overview